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Hubble's 25th Anniversary Symposium
Talk Abstracts

Daniel Apai: The University of Arizona

Extrasolar Storms: Cloud Physics and Atmospheric Dynamics in Brown Dwarfs and Exoplanets

Observations of directly imaged and transiting exoplanets, and brown dwarfs reveal the wide-spread presence of condensate clouds. These clouds profoundly influence the energy transport through ultracool atmospheres and impact their pressure-temperature profiles. Yet, the structure and properties of these cloud layers remain mostly unknown and pose one of the great challenges in understanding ultracool atmospheres. I will show how HST’s unique stability and sensitivity allow us to address this challenge via an exciting new observational approach. With time-resolved spectroscopy and photometry -- rotational phase mapping — we are exploring the longitudinal structure of clouds; with multiple epoch observations we are following the evolution of the cloud cover, opening a new window of atmospheric dynamics. With the powerful combination of simultaneous HST and Spitzer observations we are exploring the vertical structure of the atmospheres, revealing multiple cloud layers. Finally, I will also show how the combination of JWST and HST will allow us to push this technique to new heights - and probe new depths and new physics in ultracool atmospheres.

Martin Barstow: University of Leicester

Testing the White Dwarf Mass-Radius Relation with HST Observations of Sirius-Type Binaries

The presence of a white dwarf in a resolved binary system, such as Sirius, provides an opportunity to combine dynamical information about the masses, from astrometry and spectroscopy, with a gravitational red-shift measurement and spectrophotometry of the white dwarf atmosphere to provide a test of theoretical mass-radius relations of unprecedented accuracy. We demonstrated this with the first Balmer line spectrum of Sirius B to be obtained free of contamination from the primary, with STIS on HST. However, we also found an unexplained discrepancy between the astrometric and gravitational red-shift mass determinations. With the recovery of STIS, we have been able to revisit our observations of Sirius B with an improved observation strategy designed to reduce systematic errors on the gravitational red-shift measurement. We report on the refined precision of the Sirius B mass-radius measurements and extend this technique to a larger sample of white dwarfs in resolved binaries. Together these data provide accurate mass and radius determinations capable of testing the theoretical mass-radius relation and distinguishing between possible structural models. Much of this work relies on a long baseline of astrometric and spectroscopic measurements, many of which will need to extend into the future, beyond the lifetime of HST, to achieve their full potential. We discuss how combinations of observations from Gaia and JWST can support this work and the need for future space observatories beyond these.

Jacob Bean: University of Chicago

Exoplanets in HD

The first detection of an exoplanet atmosphere, which was obtained with HST using the technique of transit spectroscopy, is now more than a decade old. There has been a lot of progress in this area in the intervening years, and detections of many different atoms and molecules have been reported for a number of exoplanets using a wide variety of ground- and space-based telescopes. However, just detecting the atmospheres of these planets is not enough anymore. Revealing the fundamental properties of exoplanet atmospheres to investigate their natures and origins requires high precision spectroscopy. Although HST remains the premier facility for exoplanet atmosphere observations, intense observational campaigns are necessary to achieve this goal. I will present results from the first intensive HST exoplanet atmosphere campaigns (totaling 210 orbits) that aim to advance this new paradigm. I will begin by describing the unambiguous conclusions from a Cycle 20 Large program focussed on the super-Earth archetype GJ1214b. I will then present the outcomes from a Cycle 21 Large Treasury program. The results from this program include the most precise measurement of the water abundance in an exoplanet atmosphere, the first global temperature map of an exoplanet as a function of longitude and altitude, and strong evidence against the ideas of non-solar carbon-to-oxygen abundance ratios and deep thermal inversions in exoplanet atmospheres. I will conclude with a look forward at how HST observations can continue to revolutionize this field in the short-term, and how JWST will ultimately usher in a new era by enabling high-precision measurements of smaller and cooler exoplanets, some of which may resemble Earth.

Sanchayeeta Borthakur: Johns Hopkins University

Escape of Ionizing Radiation from Low-z Starburst Galaxies

In an attempt to understand the mechanism behind the escape of Lyman continuum photons from intense star forming systems that may be responsible for the reionization of the universe, we present a possible local analog that has an escape fraction of ionizing flux of 21%. This is the first direct detection of escaping Lyman continuum in the low-z universe from a star-formation driven galaxy. The galaxy contain a massive yet highly compact star-forming region containing a billion solar masses of stars in: 100pc. Therefore, this galaxy offers us the unique opportunity to explore the physics of starburst-driven feedback creating gaps in the neutral gas enveloping the starburst. In addition, this detection also validates the indirect technique of using residual flux in saturated low-ionization interstellar absorption lines for predicting Lyman continuum escape fractions. This is particularly encouraging as direct detection of ionizing flux is impossible at the epoch of reionization and hence the residual flux technique presents a highly valuable tool for future studies conducted with the upcoming large telescopes to probe the first galaxies!

Marusa Bradac: UC Davis

CLASH and Beyond: Pushing the Frontiers of Galaxy Formation with HST, Spitzer, and Cosmic Telescopes

In the recent years CLASH enabled us to detect galaxies as far as z~11. They are likely beacons of the epoch of reionization, which marked the end of the so-called ``Dark Ages'' and signified the transformation of the universe from opaque to transparent. However very little is known about those galaxies, and a confirmation of their redshift is still out of our hands. Clusters of galaxies, when used as cosmic telescopes, can greatly simplify the task of studying and finding these and lower-z galaxies. With a massive cluster one can gain several magnitudes of magnification over a typical observing field, enabling imaging and spectroscopic studies of intrinsically lower-luminosity galaxies that would otherwise be unobservable, even with the largest telescopes. We are PI's of several large surveys (SURFS UP for Spitzer imaging, GLASS for HST spectroscopy) with the main goal of identifying and studying star formation of galaxies at z=1-11. I will present results from these surveys and show first successful measurements of SFR at z~7 and beyond.

Daniela Calzetti: Department of Astronomy - University of Massachusetts

Nearby Galaxies: What Next?

The Hubble Space Telescope has tremendously advanced our knowledge of nearby galaxies and the processes that occur within them. These have been used as benchmarks for teasing out the properties of high redshift galaxies observed with the Hubble, and will remain benchmarks in the James Webb Space Telescope era, as well. Open questions for which Hubble observations over the next several years can pave the road for JWST science include variations (or lack thereof) of the stellar Initial Mass Function and the physics of the outer disk regions of galaxies. Addressing these issues will also shed light on the processes of star formation and on the mechanisms that drive galaxy growth across cosmic times.

Rupali Chandar: University of Toledo

Understanding the Life Cycle of Star Clusters: The HST Era and Beyond

There is now strong evidence that many, and possibly most stars form in groups, clusters, and associations. This means that understanding the life cycles of stellar clusters, from their birth in molecular clouds to their dissolution into the field star population, also tells us about the build up of galaxies in general. Over the last two decades, observations of nearby galaxies taken with the Hubble Space Telescope have revolutionized our understanding of the birth and death of star clusters. I will review some of the key observational results for clusters, both young and old, near and far, and discuss what these results tell us about the process of star formation, and about the build up of the stellar populations of entire galaxies. I will also discuss some of the remaining questions in this field, and potential future work that will help to address them.

Dan Coe: STScI

Lensed High-Redshift Frontiers

Hubble has observed galaxies over 95% of the way back to the Big Bang with sizable samples of candidates and a few confirmed redshifts out to z: 8. At earlier times (the first 600 Myr), we find perhaps fewer candidates than initially expected, including only a dozen or so at z > 9. Those we do find appear no larger than star forming clumps observed in more mature galaxies at later times. The Cluster Lensing And Supernova survey with Hubble (CLASH) yielded magnified views of these early "building block" galaxies bright enough for follow-up study. I will present our efforts to confirm the redshift of the most distant galaxy candidate at z: 11 with Hubble's WFC3/IR grism. Luminosity functions at z > 9 are poorly constrained and inconsistent with extrapolations from lower redshifts. This leaves significant uncertainties in our future abilities to study the first galaxies and in their ability to reionize the universe. This is being remedied by the Frontier Fields program, deep imaging of six strong lensing clusters and six "blank" fields, which we predict may yield as many as 70 z > 9 candidates. I will present our latest results from the Frontier Fields and BoRG programs placing strong new constraints on galaxy evolution and reionization in the first 600 Myr. Looking ahead, I will note a large new Hubble program could discover brighter lensed high-redshift candidates. Longer term, Euclid and WFIRST will yield a larger census of early galaxies including promising candidates for JWST if the missions overlap.

Selma de Mink: Anton Pannekoek Institute for Astronomy

Hubble and the Most Massive Stars

Massive stars are rare and short lived in comparison to their lower mass counter parts. They tend to reside in the most crowded and dusty regions. A large fraction of their radiation is emitted in the UV inaccessible from ground based observatories. The large challenges to obtain large high-quality data sets, crucial to test the theoretical models, have long prevented progress in our understanding of the most massive stars. This is a major concern, since many fields of astrophysics rely directly or indirectly on these stellar models, for example because of the predictions of the radiative, mechanical and mechanical feedback on their host galaxies, because we use their light to study the conditions of star formation nearby and at high redshift, and because they are the closest analogues we have to the very first generation of massive stars. Hubble is playing a major role in changing our understanding of how massive stars live their live. The superb angular resolution allowed to dissect nearby dense star clusters compiling evidence for the existence of stars with birth masses up to 300 solar masses, well in excess of the widely adopted stellar mass limit. Furthermore it allowed to distinguish the massive star progenitors of supernova in pre explosion images. Hubble’s UV spectra have contributed to our understanding of the driving mechanism behind their strong stellar winds. Incredibly precise astrometry is allowing us to find runaway stars, escaping from their birth region even our neighboring galaxies. I will review highlights of recent insights on the theoretical and observational front in the field of massive star evolution and discuss the major open questions where Hubble can still make a major contribution in its remaining years.

Gisella De Rosa: OSU, STScI

Probing AGN Structure on Microarcsecond Scales: The Space Telescope and Optical Reverberation Mapping Program

Reverberation mapping (RM) is a powerful technique that is used to study the innermost structure of AGNs and determine the mass of the central black hole. Variations in the fluxes of the Doppler-broadened emission lines are driven by flux variations in the ionizing UV continuum from the central source, but with a delay that represents the light-travel time across the line-emitting region. By tracing how the emission lines “reverberate” in time and Doppler velocity, we can determine the kinematics and structure of the line-emitting region and determine its possible role in the accretion process. In 2014, we used HST/COS for an RM program for which we obtained 170 UV spectra of the Seyfert 1 galaxy NGC 5548 at a near daily cadence. These data and contemporaneous observations with Swift and ground-based telescopes make this the most intensive RM program ever undertaken. An early major result is that the continuum variations at longer wavelengths follow those at shorter wavelengths, demonstrating that the continuum source is larger than generally supposed and underscoring the importance of UV monitoring for future RM programs.

Drake Deming: University of Maryland

Water in Exoplanetary Atmospheres

Hubble is the premier observatory for measuring the presence of water vapor in the atmospheres of exoplanets. Initial tentative detections of water vapor began in 2007, using STIS and NICMOS. More recently, the advent of spatial scan mode for WFC3 spectroscopy has produced unequivocal measurements of water vapor in the atmospheres of exoplanets ranging in size from Neptune to Jupiter. The transiting exoplanets amenable to HST observations are hot, well above habitable temperatures. At these high temperatures, the abundance of water vapor primarily reflects the abundance of heavy elements in the exoplanetary atmospheres, i.e. their metallicity. A fundamental prediction of the core accretion theory of planetary formation is that small planets should have greater proportions of heavy elements relative to hydrogen than do giant planets. We already know that this is true for the bulk composition of exoplanets, based on their masses and radii. An open question that we hope to answer using WFC3 is to what degree the atmospheric metallicity of exoplanets correlates with their bulk composition, and how common are clouds and hazes that block our view of exoplanetary atmospheres in transmission.

Steven Finkelstein: The University of Texas at Austin

Our First Look Into Galaxy Formation with Hubble

Over the past quarter decade, the Hubble Space Telescope has revolutionized nearly every area of astrophysics. Among the most prevalent since the final servicing mission is the explosion of our knowledge of galaxy formation and evolution in the first billion years after the Big Bang. The number of candidates for galaxies now known at redshifts greater than six has grown to over 1000. This allows us to move beyond mere counting of galaxies, to endeavor to understand the detailed physics underlying the growth of galaxies in the early universe. I will review the recent progress our group in Texas has made in this arena using the exquisite datasets from the CANDELS, Hubble Ultra Deep Field, and Hubble Frontier Fields programs. Specifically, our detailed new measurements of both the evolution of the stellar mass function and rest-frame UV luminosity function now allow us to probe the effect of feedback on low-mass galaxies, the star-formation efficiency in high-mass galaxies, and the contribution of galaxies to the reionization of the universe. I will conclude with remaining open questions which Hubble can solve over the remainder of this decade, including the rise of massive and bright galaxies in the early universe, the escape of ionizing photons from galaxies, and the downfall of star-formation in the universe.

Ryan Foley: University of Illinois

Supernova Progenitors & Explosions

Knowing which stars (and star systems) create which kind of supernova is critical for understanding the physics of the explosion, connecting stellar evolution to supernovae (and their remnants), the metal enrichment with cosmic time, and the connection between these stars and compact objects. Furthermore, a detailed knowledge of which stars produce Type Ia supernovae and how the observations change with different progenitor metallicity will indirectly improve our understanding on a variety of cosmological parameters from the Hubble constant to the dark energy equation-of-state parameter. I will present results from several programs where we use Hubble observations to constrain the progenitor systems and explosions of a variety of supernovae. I will show how deep high-resolution images provide a precise measurement of the mass function for Type II supernovae, how ultraviolet spectra of Type Ia supernovae probe the progenitor metallicity, and the first discovery of a thermonuclear progenitor system. I will conclude with remarks on how Hubble, in combination with JWST and future facilities, can improve our understanding of supernova progenitors and explosions over the next decade.

Andrew Fox: STScI

A Tail of Two Galaxies: Hubble Pinpoints the Origin of the Magellanic Stream

A central theme of modern galaxy evolution is the manner in which gas flows cycle in and out of galaxies. Nowhere can this process be studied in as much detail as around the Milky Way. The most prominent and massive gas flow in the Galactic halo is the Magellanic Stream, an extended network of gas clouds stripped out of the Magellanic Clouds and now accreting onto the Galaxy. Using recent ultraviolet spectroscopy from HST/COS, we have mapped the metallicity and ionization of the Stream as a function of position, finding that two principal strands of the Stream exist, one with an LMC abundance pattern and one with an SMC abundance pattern.This is consistent with kinematic H I studies and shows that the Stream has a dual origin. We have also found that the Stream's mass inflow rate onto the Milky Way is comparable to the Galactic star formation rate, implying the Stream can sustain our Galaxy's star formation into the future. This study allows us to characterize the feeding habits of a large spiral galaxy.

Wesley Fraser: Herzberg Institute of Astrophysics, Queen's University, Belfast

The Binarity, Compositions (and a Few Sizes) of Kuiper Belt Objects: Revealing the Solar System’s Formation

In 1992, the discovery of the first small Kuiper Belt Object (KBO) revolutionized our understanding of the outer Solar system. It led to the recognition that Pluto is not a lone object, but rather a member of a population of planetesimals beyond the orbit of Neptune. In the 23 years since the discovery of 1992 QB1, more than 1400 objects belonging to the Kuiper Belt have been detected. The field has matured from early discovery efforts to characterizing the physical properties of the belt, its sub-populations, and individual objects within them. These properties have revealed many startling facts about the formative history of the outer Solar System. I will review the critical discoveries about the Kuiper Belt in which the Hubble Space Telescope has played a key role, including: • common size distributions shared by the Jupiter Trojans and most KBOs • a large population of binary (and higher multiplicity) KBOs • the existence of multiple primordial compositional families throughout the Kuiper Belt Region Together, this research has uniquely illuminated the origin and history of the outer Solar System. We now know that the early Solar System originated in a much more compact configuration, with the gas-giant planets occupying a region interior to Neptune’s current orbit. The protoplanetary disk was compositionally homogenous throughout the entire region from the gas-giant planets to its edge at roughly 45 AU. Through strong dynamical interactions between the gas-giants, most of the surrounding protoplanetesimal disk was dispersed, with a small fraction of those objects being emplaced on stable orbits in what we now call the Kuiper Belt. I will also present more recent work from the Hubble Space Telescope, including the discovery of a second target for the New Horizons mission, and the detection by WFC3 of a never before seen spectral feature in the reflection spectra of a certain KBO, diagnostic of that object’s composition. These recent discoveries demonstrate well Hubble’s current role amongst modern ground based telescopes, and the critical role it will take alongside future missions such as the JWST and Whipple

Andrew Fruchter: STScI

The Brightest Bursts are the Best Behaved

Long duration Gamma-Ray Bursts are beamed relativistic outflows produced by the collapsing core of a massive star. Whether a black hole or a highly magnetized neutron star (magnetar) is powering the burst is an important issue not only for the field of Gamma-Ray Bursts, but also for our understanding of the evolution of the most massive stars. The rotational energy of a neutron star is limited at about: 10^52 ergs. However, the GRB gives us a way to measure the energy of the burst, through the "jet-break" in its light curve -- an increasingly rapid drop in the brightness of the burst as it slows to a relativistic speed of gamma~1. Here we present the light curves of three extremely powerful Fermi-LAT bursts observed by HST and Chandra and show that their strict power-law decays go on for surprisingly long times (at least months to in one case more than a year) severely testing the magnetar model of GRB formation.

Boris Gaensicke: University of Warwick

Planetary Systems Around White Dwarfs

The vast majority of all known planet-hosting stars, including the Sun, will eventually evolve into red giants and finally end their lives as white dwarfs: extremely dense Earth-sized stellar embers. Only close-in planets will be devoured during the red-giantphase. In the solar system, Mars, the asteroid belt, and all the giant planets will escape evaporation, and the same is true for many of the known exo-planets. It is hence certain that many of the known white dwarfs were once hosting planets, and it is very likely that a fair fraction of them still have remnants of their planetary systems. The detection of metals in the atmospheres of white dwarfs is the unmistakable signpost of such evolved planetary systems. The strong surface gravity of white dwarfs causes metals to sink out of the atmosphere on time-scales much shorter than their cooling ages, leading unavoidably to pristine H/He atmospheres. Therefore any metals detected in the atmosphere of a white dwarf imply recent or ongoing accretion of planetary debris. Circumstellar dusty and gaseous debris disks have been detected at over 30 white dwarfs, formed from the tidal disruption of asteroids or Kuiper belt-like objects that were stirred up by left-over planets. Determining the photospheric abundances of debris-polluted white dwarfs is hence entirely analogue to the use of meteorites, "rocks that fell from the sky", for measuring the abundances of planetary material in the solar system. High-resolution HST/COS ultraviolet spectroscopy of white dwarfs has played a fundamental role in exploiting this unique window into the bulk-composition of exo-planetary material. We find a wide spread in the relative abundances of Mg, Fe, Si, and O, a constant Al/Ca ratio, and evidence for differentiation in the form of Fe/Ni over-abundances. All of the above is suggestive of thermal and collisional processing similar to the solar system. We have so far not found any evidence for the existence of "carbon planets", i.e. systems where an elevated C/O ratio would dictate a carbon-based rather than silicon-based chemistry. HST has also been the key to the discovery of the first water-rich extra-solar asteroid, and the detection of volatile-rich planetary material originating from beyond the snow line. Over the next decade, this research area will enormously benefit from the facilities that are being developed. With JWST, we will be able to obtain high-quality mid-IR spectra for tens of white dwarf debris disks, which will provide direct insight into the mineralogy of the planetary material. In other words, we will be able to examine the same exo-planetary material both in its composite state (still orbiting the white dwarf, with JWST), and in its elemental components (dissolved in the white dwarf atmosphere, with HST) providing an extremely powerful diagnostic on the formation and composition of rocky planets. Looking further into the future, WFIRST (and EUCLID) will discover hundreds of debris disks around white dwarfs, enabling a detailed statistical assessment of the chemistry of exo-planetary systems. However, full exploitation of this potential will require the possibility of follow-up large-aperture ultraviolet spectroscopy, such as is currently in the context of ATLAST.

Avishay Gal-Yam: Weizmann Institute of Science

Superluminous Supernovae

Supernovae, the luminous explosions of stars, have been observed since antiquity. However, various examples of superluminous supernovae (SLSNe; luminosities >7 × 10^43 ergs per second), have only recently been documented. I will briefly describe the discovery and first studies of these events, their apparent division into sub-groups or types, and will present our current understanding about their physical nature. Studies of the host galaxies of these events using Hubble will be reviewed, as well as potential future avenues of research with JWST and other telescopes.

Cecile Gry: Laboratoire d'Astrophysique de Marseille

The Diffuse Cloud Surrounding the Sun: Peculiarities of an Interstellar Cloud Explored from Within

A new interpretation of the kinematics of UV absorption lines observed by HST in the spectra of nearby stars indicates that the interstellar medium surrounding the Sun can be regarded as a single, coherent cloud if we relax the assumption that a cloud behaves like a rigid body. This outlook permits us to construct a more comprehensive picture of a diffuse interstellar cloud and reveals that it departs from homogeneity in a number of aspects and physical properties: - This local cloud undergoes a deformation related to a compression in the direction of motion and an expansion in perpendicular directions, much like a squashed balloon. - The metal abundances decrease steadily from the rear to the head of the cloud, and this phenomenon does not appear to be related to ionization effects. - The cloud average HI density, estimated toward a number of nearby stars around which an astrophere is detected in Lyman alpha, varies from 0.03 to 0.1 cm-3. The cloud outer boundary inferred from the average density and column densities is very irregular with an average distance to the Sun of 9 +/- 7 pc. - The electron density and the cloud temperature can be derived from the combination of the ionization equilibrium of MgI and the excitation of CII in a restricted number of sightlines where column density is such that MgI and CII* features are strong enough to be detectable without saturating MgII. The treasury program ASTRAL provides a few additional such targets from which we examine the variability of physical conditions inside the cloud. This local cloud dominates the interstellar material in the first 50 pc around the sun. Half of the secondary velocity components, covering half of the sky, evidence a uniform differential velocity of -7 km/s toward the cloud interior, which may be a manifestation of a shock created by a surge in the external thermal pressure in the much lower density medium that surrounds the cloud. The local cloud turns out to be a great laboratory to explore the details of the warm neutral medium (WNM) without the confusion from overlapping absorptions from different regions. Future missions providing UV and FUV spectroscopy at high resolution and high S/N would be very valuable to improve the knowledge of physics of a typical interstellar cloud embedded in hot diffuse gas."

David Harvey: École polytechnique fédérale de Lausanne

Astronomical Particle Colliders

Despite the successes of the cold dark matter paradigm inconsistencies exist between theory and observations, predicting many more satellite galaxies that should form stars and much higher central densities in galaxy clusters. One method to resolve these inconsistencies is to invoke a self-coupling in the dark sector such that dark matter is collisional. The first constraints on the self-interaction cross-section of dark matter were derived using the Bullet Cluster. In this talk I present the first ever study of a sample of 30 merging galaxy clusters to place constraints on the interaction cross-section of dark matter that does not rely on a single system. Using state-of-the-art data from the Hubble ACS camera and the Chandra Xray observatory I measure the centroids of dark matter, X-ray emitting gas and galaxies. Using an innovative method to interpret mass offsets in merging galaxy clusters I gather unequivocal evidence for the existence of a dark mass and place the strongest constraints on the self-interaction cross-section to date.

Mathilde Jauzac: Durham University - ICC

Hubble Frontier Fields : A New Era for Gravitaitonal Lensing

The Hubble Frontier Fields (HFF) initiative constitutes the largest commitment ever of HST time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. This program devotes 140 orbits of HST time to deep imaging observations of each of six cluster lenses reaching m~29 (AB) uniformly in all pass-bands. The targeted clusters are some of the most powerful lenses known, and are all in a highly-disturbed dynamical state. The first two clusters, Abell 2744 (z=0.308) and MACSJ0416.1-2403 (z=0.397), were fully observed during cycle 21, and observations of the third and fourth clusters, MACSJ0717.5+3745 (z=0.54) and MACSJ1149.5+2223 have now started. I will present the new gravitational lensing pictures of these complex systems using this exquisite set of data coming from the HFF program, allowing us to detect more than 150 strongly lensed features in each clusters (with: 200 detected in MACSJ0416). We have demonstrated that we are now able to `weight’ these clusters' cores down to the percent level precision (recently published works), serving our quest for the high-redshift Universe. However, while the depth of these dataset makes these clusters amazing Cosmic Telescopes, it also enables us to get an unprecedented understanding of the cluster physics. Therefore, I will demonstrate the importance of such high-quality data to analyse the merging/dynamical history of the clusters while comparing dark matter, light and gas distributions.

L. Clifton Johnson: University of Washington

Resolving Star Cluster Formation and Dissolution in M31: Results from HST and PHAT

Star clusters are long-lived tracers of star formation that can be identified in galaxies that are tens of Mpc away. Interpreting these populations, however, depends on the understanding of evolutionary processes and physics related to cluster formation and destruction. Until recently, the availability of a high quality dataset that would allow us to build a definitive, foundational understanding of star cluster physics had escaped our grasp. Imaging from the Hubble Space Telescope obtained as part of the Panchromatic Hubble Andromeda Treasury (PHAT) survey, however, serves just that purpose; these data facilitate an unprecedented census of M31's star cluster population and allow a detailed study of cluster formation and evolution. Using innovative citizen science techniques, we assembled a well-characterized catalog of clusters, increasing the known cluster population within the PHAT survey footprint by a factor of: 6. From observations of individually resolved cluster stars, we obtain precise age and mass constraints for a sample of clusters that spans a range of galactic environments within Andromeda. Combined with spatially and temporally resolved star formation histories of unbound field populations, we derive characteristics of cluster formation and dissolution in a full galactic context. We find no evidence for significant cluster dissolution at young ages (~10-100 Myr) and find that only 2-5% of stars are born in long-lived star clusters within M31. The fraction of stellar mass bound in clusters correlates with galactic environment, confirming results from other extragalactic studies. Further, we investigate how galactic environment affects the intrinsic cluster mass function and properties of cluster dissolution. Looking to the future, I will briefly explore how the next generation of space-based observatories will help us continue our exploration of star and cluster formation in the nearby Universe.

John Johnson: Harvard CfA

Hot on the Trail of Warm Planets Orbiting Cool Stars

Just three years ago the prospect of finding temperate, rocky worlds around other stars was still the subject of science fiction: none had been found and reasonable estimates put us years or decades away from such a momentous discovery. All of that has changed very recently on the heels of the extraordinarily successful NASA Kepler mission. By searching for the tiny diminutions of starlight indicative of an eclipsing planet, Kepler has produced thousands of new planet candidates orbiting distant stars. Careful statistical analyses have shown that the majority of these candidates are bona fide planets, and the number of planets increases sharply toward Earth-sized bodies. Even more remarkably, many of these planets are orbiting right “next door,” around tiny red dwarf stars. I will describe our multi-telescope campaign to validate and characterize these tiny planetary systems, and present some early, exciting results that point the way to the first detection of the first Earth-sized planets in the habitable zones of nearby stars

Jason Kalirai: STScI

Ultra-Deep HST Imaging of the SMC: The IMF at M < 1 Msun

Knowledge of the stellar Initial Mass Function (IMF) serves as one of the most important pursuits of Astrophysics. The IMF has its origins in the theory of star formation and serves as a key ingredient to translate light from a mix of stellar populations to fundamental properties. Historically, the IMF has been measured by analyzing the distribution of stellar masses near the Sun, conducting large photometric surveys of the Milky Way, and studying star forming regions and star clusters. In this talk, I will present the highest precision measurement of the IMF to date, based on an unprecedented (30th magnitude) ultra-deep, high-resolution Hubble imaging field in the outskirts of the Small Magellanic Cloud (SMC). From exquisite photometry of >5000 stars, I demonstrate that the IMF is well represented by a single power-law form with slope slope alpha = -1.90 ([+0.15] [-0.10]) (3 sigma error) (i.e., dN/dM propto M^(alpha), Salpeter alpha = -2.35) over the mass range M = 0.37-0.93 Msun. This result may have important implications for the metallicity dependence of the IMF, and is now the foundation of several active HST programs.

Nitya Kallivayalil: University of Virginia

Milky Way Halo Kinematics

The Local Group, the regime in which detailed star-by-star studies can be done, is becoming a testbed for the study of the pressing questions governing both cold dark matter theory and galaxy formation studies, such as the distribution of matter on small scales and even the impact of reionization. A better estimate of the total Milky Way halo mass is important for many of these questions. Due to the mass anisotropy degeneracy it is not well determined from radial velocities. Proper motions are required in addition to the (generally known) radial velocities to test halo models. Turning to the baryonic sector, while accretion evidently plays a role in galaxy formation, as attested to by the existence of tidal debris streams, it is not yet excluded that some halo components formed in situ. To disentangle this process in detail, orbits are necessary. I will discuss HST's contributions to obtaining proper motions for a variety of tracers in the Milky Way halo, in order to constrain its shape and total mass, as well as the orbital histories of these tracers.

Heather Knutson: Caltech

Atmospheres of Extrasolar Planets in the Super-Earth Era

Ongoing surveys of nearby stars have revealed an amazing diversity of planetary systems, many of which have characteristics that differ substantially from those of the solar system planets. Perhaps one of the biggest surprises to come out of these surveys was the discovery that "super-Earths" (planets between 1-10 times the mass of the Earth) are in fact the most common type of extrasolar planet. Despite the name we actually know very little about the compositions of these mysterious planets, and it has been suggested that this mass range may include both "water worlds" and "mini-Neptunes" with thick hydrogen envelopes in addition to more Earth-like terrestrial planets. In my talk I will explore both current and potential future studies of the compositions of planets with masses ranging from that of Neptune down into the super-Earth regime, and discuss the corresponding implications for our understanding of planet formation and evolution.

Laurent Lamy: LESIA, Observatoire de Paris

Planetary Ultraviolet Aurorae: The Legacy of Hubble

Hubble has been an active hunter of ultraviolet (UV) planetary aurorae in the past decades. These powerful electromagnetic radiations emanate from the polar regions of magnetized planets throughout the solar system, once activated by collisions between energetic magnetospheric particles and atmospheric species. In the case of giant magnetospheres, the neutral atmosphere is dominated by atomic and molecular hydrogen, whose emission bands extend from 80 to 160nm, covering the extreme to far ultraviolet ranges. UV planetary aurorae sampled by HST therefore offer a unique, remote, probe of the magnetopsheric activity and of its electrodynamic coupling with the planetary ionosphere. This presentation will review our current knowledge of planetary aurorae of Jupiter, Saturn and Uranus, and presents some important remaining objectives to achieve before the HST retirement.

Andrew Levan: University of Warwick, UK

Going Out With a Bang: HSTs Contribution to Gamma-Ray Bursts

I will outline the contributions that HST has made to the studies of gamma-ray bursts (GRBs), from the discoveries of the first host galaxies of long gamma-ray bursts in 1997, through to the unveiling of the progenitors of short GRBs in 2013. Beyond this, I will discuss the major role that HST can continue to play; the significant open questions that remain around the use of GRBs as lighthouses to the distant Universe and probes of exotic stellar death; and the importance of HST studies of extreme transients in the era of large scale transient surveys and gravitational wave detections.

Jennifer Lotz: STScI

The Frontier Fields and Beyond

Exceptionally deep observations of the distant universe with the Hubble Space Telescope have consistently pushed the frontiers of human knowledge. How deep can we go? What are the faintest and most distant galaxies we can see with the Hubble Space Telescope now, before the launch of the James Webb Space Telescope? This is the challenge taken up by the Frontier Fields, a director's discretionary time campaign with HST and the Spitzer Space Telescope to see deeper into the universe than ever before. The Frontier Fields combines the power of HST with the natural gravitational telescopes of high-magnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. These observations will reveal distant galaxy populations: 10-20 times fainter than any previously observed, allowing astronomers to study the early progenitors of the Milky Way and the faint galaxies responsible for reionization. I will review the prospects for studying galaxies at cosmic dawn with JWST, extremely large ground-based telescopes, and future space missions over the next decade and beyond.

Melissa McGrath: SETI Institute

Solar System Satellites

Study of the solar system’s wide array of planetary satellites has been without a doubt one of the crowning achievements of the Hubble Space Telescope in the last 25 years. From the discovery of many new solar system satellites, to the early results on satellites such as Titan and Charon that have presaged major discoveries by missions such as Cassini and the soon to come New Horizons encounter with Pluto, to the major discoveries of tenuous atmospheres and aurora on Jupiter’s Galilean satellites, Hubble’s influence has been powerful and will provide an enduring legacy of scientific achievement. This talk will provide an overview of highlights from the past 25 years, and end with a brief discussion abut future possibilities with JWST.

Eileen Meyer: STScI

The New Proper Motions Frontier: Recent Discoveries on AGN Jets with HST

The long operating lifetime of Hubble has resulted in an increasingly valuable archive of images of AGN jets taken over the last twenty years. With recent advances in state-of-the-art astrometric methods, we can now leverage this archive to measure the motions of the relativistic plasma in these jets in galaxies as distant as 500 Mpc, reaching accuracies of: 10% the speed of light. I will present recent discoveries, including the unwinding of helical kpc-scale structure in M87, as well proper motions and their implications for jet physics for other nearby radio galaxies. I will discuss the advances in technique which have enabled even very short exposures (e.g. early WFPC2 “snapshot” images) to be registered using background galaxies, and discuss future observations which will continue to push the envelope of HST proper-motions science.

Ivelina Momcheva: Yale University

Results from the 3D-HST Survey

3D-HST is a 248-orbit spectroscopic survey with the Hubble Space Telescope designed to study galaxy evolution at z>1. Providing the critical third dimension - redshift - via slitless optical and near-IR grism spectra, 3D-HST opens new possibilities for science and discovery in the deep extragalactic fields AEGIS, COSMOS, GOODS-S and UKIDSS-UDS. We have combined the grism observations with archival data to create an unique dataset which incorporates: 2000 HST orbits. Observations have been completed and all final data products have been made public. I will demonstrate the data products from the survey, present science highlights from recent publications, and discuss how we can use the insights from 3D-HST to inform future work with Hubble and other planned missions.

Mireia Montes: Yale University

The Ghost Light of Abell 2744

Originally thought to explore the high redshift Universe, the extreme depth of the Frontier Fields survey provides also an excellent opportunity to explore with unprecedented detail the faintest component of the galaxy clusters: their intra-cluster light (ICL). In this contribution, we will show our analysis of the ICL of the cluster Abell 2744. The Frontier Fields multiwavelegth dataset have allowed us, for the first time, to explore the age and metallicity of this diffuse component of the clusters to distances as far as 120 kpc from the cluster center. We have found an ICL metallicity compatible with solar metallicity (Z=0.018 +- 0.007) and an ICL age 6+-3 Gyr younger than the average age of the most massive galaxies of the cluster. The fraction of stellar mass in the ICL component comprises at least 6 % of the total stellar mass of the galaxy cluster. Our results are consistent with a scenario where the bulk of the ICL of Abell 2744 has been formed relatively recently (z<1). The properties of the ICL suggest that this diffuse component is mainly the result of the disruption of infalling galaxies with similar characteristics in mass (M~ 3x10^10 Msolar) and metallicity than our own Milky Way. The amount of ICL mass in the central part of the cluster (<400 kpc) is equivalent to the disruption of 4-6 Milky Way-type galaxies. In this talk we will show how the Frontier Fields survey, once will be completed, will revolutionize our understanding of the formation and evolution of this subtle component of the galaxy clusters.

Pascal Oesch: Yale University

Galaxy Build-up at Cosmic Dawn: Hubble's Lasting Legacy

The last servicing mission of the Hubble Space Telescope with the installation of the WFC3/IR camera represented a major milestone to further extend Hubble's legacy in understanding galaxy build-up at the earliest cosmic times. Thanks to HST's WFC3/IR camera we were finally able to efficiently discover large samples of galaxies in the heart of the cosmic reionization epoch at z>6, and to push the observational frontier of galaxies to only: 450 Myr after the Big Bang, to z~10-12. Several large HST surveys with the WFC3/IR camera have allowed us to identify an unprecedented sample of more than 800 galaxies at z~7-8, and to build up galaxy detections at z~9-12. Using HST, we can now directly track the evolution of the UV luminosity function and the cosmic star-formation rate density over 96% of the age of the universe. Additionally, the combination of ultra-deep HST and Spitzer/IRAC datasets has proven to be extremely powerful leading to the first measurements of the cosmic stellar mass density at z~8-10, tracing the buildup of the first generations of galaxies from z~10. This would not have been possible without HST and represents one of Hubble's greatest legacies. In this talk I will highlight recent progress in exploring the build-up of the first generations of galaxies and also discuss the lasting legacy value of Hubble's ultra-deep blue imaging into the JWST era and beyond.

Saul Perlmutter: University of California, Berkeley

Studying Dark Energy with Precision Supernova Measurements, from the Ground and from Space

Supernova measurements of the expansion history of the universe have advanced dramatically over the lifetime of the HST, but they still have not reached the high precision necessary to distinguish the subtle effects of any time variation in the properties of dark energy. New data from ground- and space-based studies point the way to dramatically improving the precision of the expansion measurements, over a wide range of redshifts.

Marc Postman: Space Telescope Science Institute

The Future of UVOIR Space Astronomy: Unveiling the Universe in High-Definition

Understanding the connections between the growth of baryonic structure in the universe, its chemical evolution and the origin of life has emerged as one of the most fundamental endeavors in astronomy. The formation of planetary systems and life are intimately connected to larger-scale physics that controls the formation and evolution of stars and galaxies. We are at the brink of achieving major breakthroughs in astrophysics with the new telescopes coming on line in the next 10 years (e.g., ALMA, JWST, GMT, TMT, E-ELT). But even these impressive facilities will leave very significant gaps in our ability to explore all the processes that govern galactic and stellar evolution and the role they play in establishing environments suitable for life to exist. Nor will any of these telescopes be able to detect and characterize the atmospheres of Earth-like exoplanets in orbit around solar-type stars. An inspirational future for the exploration of the cosmos in the era beyond JWST must include the ability to assess the possibility of life beyond our Solar System by searching, spectroscopically, for biomarker molecules in the atmospheres of Earth-like exoplanets. The facility that can accomplish this must be in space and will also, by design, be able to spatially resolve scales of 100 AU everywhere in the Milky Way, 0.1 parsec everywhere in the Local Group, and 100 parsecs everywhere in the observable Universe. In this talk, we present the compelling science drivers, and their associated observational requirements, that will allow us to begin to answer the question “Are We Alone?” as well as follow the formation and evolution of the star forming regions inside galaxies over the past 10 Gyr, robustly determine the star formation histories in every galaxy within the local volume (10 Mpc), and track the motions of virtually any star in the Milky Way.

Mary Putman: Columbia University

Gas Flows in the Largest Dark Matter Halos

The ways that baryons assemble into dark matter halos is expected to vary by their mass and redshift. I will present the distribution of gaseous baryons at low redshift surrounding the largest coherent dark matter structures, galaxy clusters, and discuss the variations from smaller galaxy sized halos. The talk will include new results from HST's Cosmic Origins Spectrograph and cosmological simulations.

Adam Riess: JHU/STScI

Expansion of the Universe Seen by Hubble

The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. Streamlined distances ladders constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide 3.5% precision and offer the means to do much better. We will discuss a new round of improvements to the measurement of the Hubble constant including additional observations of Cepheids in recent SN hosts and a new technique, Parallel Astrometric Spatial Scanning (PASS), to measure parallax distances beyond a kiloparsec.

Steve Rodney: Johns Hopkins University

A New Lens for Hubble: Highly Magnified and Multiply-Imaged Transients

The Frontier Fields survey is extending the reach of the Hubble Space Telescope by targeting massive galaxy clusters that act as cosmic telescopes: using gravitational lensing to amplify the light of distant objects. This makes it possible to detect the explosions of stars that formed when the universe was less than a few billion years old. It also yields a small but special sample: highly magnified supernovae that serve as sensitive probes of the lensing cluster's dark matter potential. I will describe some of the recent discoveries from this program, including a highly magnified Type Ia supernova, a peculiar fast transient observed twice in a multiply-imaged galaxy, and an ancient supernova being multiply imaged by both a galaxy and a galaxy cluster. I will also look ahead to discuss how HST, JWST and WFIRST can help develop strongly-lensed supernovae into a powerful new tool for studying dark matter and dark energy in the next decade.

Lorenz Roth: Royal Institute of Technology

Probing the Neutral Environment of Jupiter’s Moons with Hubble: Detection of Water Vapor at Europa

With its subsurface water ocean and young icy surface Europa is generally considered a prime candidate in the search for present-day habitable environments in our solar system. Observations of electron excited ultraviolet emissions with Hubble provide an excellent possibility to investigate Europa’s neutral gas environment. In the 1990s observations of auroral oxygen emissions with Hubble’s Goddard High-Resolution Spectrograph detected Europa’s global oxygen atmosphere for the first time. Images of these emissions taken with the Space Telescope Imaging Spectrograph (STIS) show that the oxygen aurora is correlated to the orientation of the local Jovian magnetic field and therefore time-variable. In addition, STIS images taken in December 2012 include coincident surpluses in the hydrogen Lyman-α and oxygen 130.4 nm emissions that are found persistently in the same area above the anti-Jovian south polar limb. The relative brightnesses of these H and O emissions are indicative of electron impact dissociative excitation of H2O and suggest the existence of: 200 km high water vapor plumes. The plumes have not been seen before or after this detection by STIS, so their nature and abundance remains unclear to date.

Amy Simon: NASA Goddard Space Flight Center

Probing Giant Planet Atmospheres with Hubble

This talk will cover the long-term study of atmospheric processes and changes in the atmospheres of the Giant Planets in our Solar System with a focus on Hubble imaging. Timescales for different atmospheric phenomena will be discussed, highlighting how Hubble observations have contributed to our understanding of atmospheric processes and interactions. Past observations will be shown, along with upcoming and planned observations through 2020.

rachel somerville: Rutgers University

What We Have Learned about the Physics of Galaxy Formation from HST

Modeling galaxy formation and evolution in a cosmological context is one of the great open challenges in astrophysics. Although the "double dark" paradigm of cold dark matter and dark energy provides a promising framework, many key physical processes must be modeled schematically, owing to the vast range of physical scales and the diversity of the physics at play. In particular, the details of how stars and supermassive black holes form within galaxies, and how they impact their galactic hosts, remain quite uncertain. I will discuss how observations from HST have given us a unique probe that constrains these most uncertain aspects of our models and simulations, and highlight open questions. In closing, I will propose new observational programs with HST that could address these persistent puzzles.

Tommaso Treu: UCLA

The Grism-Lens Amplified Survey from Space (GLASS): Dissecting Reionization, z~2 Galaxies, and Dense Environments

The Grism Lens-Amplified Survey from Space (GLASS) is a large HST cycle-21 program targeting 10 massive galaxy clusters with extensive HST imaging from CLASH and the Frontier Field Initiative. The program consists of 140 primary and 140 parallel orbits of near-infrared WCF3 and optical ACS grism observations, which result in spatially resolved spectroscopy of hundreds of galaxies; both cluster members beyond the local Universe and faint background galaxies at z>6 made observable by the cluster's lensing magnification. GLASS has three primary science drivers although a wide variety of other science investigations are possible with the public GLASS data (e.g. SN 'Refsdal'). The key science goals of GLASS are to: 1) Use the spectra of galaxies at 6

Todd Tripp: University of Massachusetts - Amherst

Twenty-five Years of QSO Absorption Studies with HST: A Windfall of Missing Baryons, Gastrophysical Diagnostics, and Puzzles

To many astronomers, the first observations from HST were shocking and disappointing -- the now (in)famous optical problems were a devastating setback. However, it is often underappreciated that spectroscopists were able to immediately overcome this problem by placing a narrow aperture on the well-focused core of the HST PSF. Consequently, in the spectroscopic arena, HST unleashed a flood of discoveries and surprises from day 1, and the quality and usefulness of HST spectroscopy only improved (often by leaps and bounds) in the 25 ensuing years. This talk will review some highlights of ultraviolet spectroscopic studies of the intergalactic medium and circumgalactic medium of galaxies. While UV spectroscopy with HST has likely revealed large reservoirs of metal-enriched matter, these observations have also raised a number of interesting questions. Some puzzles will be presented with emphasis on recent work, and the talk will conclude with some comments on how HST might continue to build its legacy in its final years.

Jessica Werk: University of California, Santa Cruz

Where Accretion Meets Feedback: HST/COS Observations Reveal Massive Stores of Gas in Galaxy Halos

Over the last 25 years, simulations and observations have established that galaxies evolve by maintaining an elaborate balancing act among gas supply, consumption, and removal. Many of the baryons involved in this cycle are in a phase that is exceedingly difficult to observe directly via emission -- i.e. diffuse, highly-ionized gas in the extended halos of galaxies known as the circumgalactic medium. Thus, experiments using quasars as illuminating background sources have become the gold standard for probing this elusive medium. In this talk, I will present observations of circumgalactic gas made possible only by the revolutionary sensitivity of HST/COS. These observations have had tremendous success in solving long-standing problems in galaxy formation and in confirming the predictions of hydrodynamical simulations for the importance of recycled accretion in building a galaxy’s observed stellar content. I will discuss several ongoing challenges in developing a consistent physical picture of the gas in the circumgalactic medium and will focus on three open and pressing questions about the role gas flows play in shaping galaxies throughout cosmic time. Finally, I will outline the need for future survey data that will not only address the outstanding questions related to the circumgalactic medium but will ultimately enable a more complete picture of galaxies and the cosmic web in which they reside

Katherine Whitaker: NASA Goddard Space Flight Center

Tracing the Cosmic Shutdown of Star Formation with CANDELS/3D-HST

Over the last couple of decades, observational studies have progressed from the anthropology of nearby galaxies to a direct study of the early Universe, uncovering billions of years of cosmic growth and challenging galaxy formation models. The wealth of data from deep extragalactic surveys have revealed a picture where galaxies follow a relatively tight relation between star formation rate and stellar mass. This observed star formation sequence encapsulates information about feedback in galaxy formation and the evolution of gas density and gas accretion rates over cosmic time. All the while, there exists a growing population of massive “red and dead” (quiescent) galaxies that are no longer actively forming stars, falling far below the observed star formation sequence. The physical mechanisms responsible for quenching star formation and the buildup of the quiescent population remain poorly understood. Moreover, we do not have a cohesive evolutionary theory that ties together the observed structures and stellar populations of star-forming and quiescent galaxies. With a state-of-the-art compilation of imaging and spectroscopy in the CANDELS legacy fields, I will present a self-consistent empirical study of the sizes, stellar populations, and star formation rates of a complete sample of galaxies spanning the last eleven billion years. This novel data set combines deep, high-resolution rest-frame optical imaging from CANDELS with accurate distance measurements of both quiescent and star forming galaxies from 3D-HST, making possible the first detailed studies of the early development stages of massive galaxies. These observations enable us to gain a broad understanding of the physical mechanisms driving the growth of massive galaxies over 85% of the history of the universe, paving the way for future advances in galaxy formation and evolution using JWST and WFIRST.