STScI Logo

Space Telescope Science Institute
2007 Hubble Fellows Symposium Abstracts

Exoplanet search with the HAT Network

Gaspar Bakos

I will review the current status of the search for extrasolar planets around nearby, bright stars with the HAT Network of small, automated telescopes. I will talk about our discovery, planet HAT-P-1b, and will hint on other candidates we have in the queue.

Relativistic outflows from compact rotators: Pulsar Winds, Pulsar Wind Nebule, and GRBs.

Niccolo Bucciantini

Relativistic flows play are often invoked as the key element in many high energy astrophysical phenomena. Their complexity, and the richness of their interaction with the surrounding medium have stated to be investigated only recently thanks to the improvement in computational techniques, acceleration and outflow properties of winds from compact sources. Theoretical models suggest that acceleration and collimation of the flow are extremely inefficient when the speed is close to ScS, in contrast with many observations. I will present numerical results, concerning the acceleration and collimation of relativistic flows, their comparison with theoretical models and the resulting implications. I will focus on axisymmetric rotators, both in the case of monopolar and dipolar magnetic. Finally I will discuss the interaction of a relativistic wind with the surrounding environment, either a SNR or the progenitor of a GRB.

Galaxy and Quasar Clustering at z=1

Alison Coil

Changes in galaxy and quasar clustering properties with time reflect both evolution in the dark matter halo distribution and galaxy and quasar evolution and are key diagnostics for theoretical models. I will highlight recent results on the color-dependence of galaxy clustering at z=1 using data from the DEEP2 Redshift Survey, discussing the implications of the color-density relation being firmly in place when the Universe was half its current age. I will also show results on the clustering of quasars and X-ray selected AGN at z=1 and compare with galaxy clustering to infer information on their host galaxies.

Formation and evolution of CDM halos and their substructure

Juerg Diemand

Massive supercomputer simulations now allow to follow the formation and evolution of CDM halos and their subhalo population in detail. We have recently completed the "Via Lactea" run: It resolves a Milky Way scale halo with over 200 million particles, over ten times more than the previous largest run. It resolves over 10,000 subhalos, some inside the solar circle, many sub-subhalos and the mass in substructure has still not converged, i.e. it is growing with better numerical resolution. This wealth of small scale structure has implications for direct and indirect dark matter detection, stellar streams, disk heating, gravitational lensing and the Local Group dwarf galaxy population (the "missing satellites problem").

Extrasolar Planets: Testing Models of Planet Formation

Eric Ford

For centuries, knowledge of planetary systems and ideas about planet formation were based on a single example, our solar system. The first discoveries of extrasolar planets revealed a wide diversity of planetary systems, including surprising planets with short orbital periods and large eccentricities. In light of these discoveries, theorists have proposed numerous revisions to the classical model of planet formation and evolution. During the last thirteen years, the discovery of over 200 extrasolar planets has provided new opportunities for testing these models. I will outline some of the outstanding questions about planet formation and discuss how analyzing the extrasolar population can provide insights into such questions.

A Test of the Black Hole/Bulge Paradigm: Host Galaxy Structures of Active Galaxies with Intermediate-mass Black Holes

Jenny Greene

The tight observed correlations between supermassive black hole (BH) mass and host galaxy bulge properties (e.g., the M_BH-sigma* relation) seem to imply that BHs and bulges share a common formation history. One might imagine, then, that bulgeless galaxies ought not contain central BHs. However, there are unambiguous active BHs found in the centers of at least two bulgeless dwarf galaxies, and their BH masses are, correspondingly, the smallest known for supermassive BHs. Such objects are intriguing; they are currently the closest observational analogues of primordial seed BHs. Using the Sloan Digital Sky Survey we have increased the sample of BHs with masses <106 M_sun from two to two hundred. Remarkably, preliminary evidence suggests that these intermediate-mass BHs obey the low-mass extrapolation of the M_BH-sigma* relation, despite very different presumed formation mechanisms for the dwarf hosts. I will discuss new constraints on the structure and formation of our unique sample using HST/ACS imaging.

Shedding (Quasar) Light on High Redshift Galaxies

Joseph Hennawi

I will describe a search technique for finding quasars at distances of < 300 kpc from galaxies with large column densities of neutral hydrogen at high redshift. At these small separations, the ionizing flux from a quasar could be as large as ~ 10,000 times the extragalactic UV background if it illuminates the galaxy. Can proto-galaxies near quasars self-shield against this intense ionizing flux or will they be photo-evaporated? If they survive, Ly-alpha recombination photons should be easily detectable. I will discuss a search for this fluorescent Ly-alpha emission and the things we can learn: it can teach us about the distribution and kinematics of gas in high redshift proto-galaxies, as well as constrain the geometry and radiative histories of quasar emission.

Globular Clusters: From the nearest to the most distant

Jason Kalirai

We present one of the deepest optical images ever taken with the Hubble Space Telescope Advanced Camera for Surveys, a 126 orbit integration of a small field within the second nearest Galactic globular cluster NGC 6397. The data set allows us to construct a very clean, deep (V = 30) color-magnitude diagram with proper motion selection. We analyze the lowest mass hydrogen burning stars within the cluster and characterize the complete white dwarf cooling sequence. Fortuitously, our ACS field also intersects a large elliptical background galaxy, with its own globular cluster population. We present an analysis of these systems, the furthest discovered to date at a distance greater than 1 billion light years.

Lyman-alpha Emission from the Intergalactic Medium

Juna A. Kollmeier

The intergalactic medium (IGM) is a key probe of both cosmology and galaxy formation. Most of our knowledge of the IGM, comes from absorption line measurements, 1-dimensional skewers along the way toward bright background sources. In the era of hydrodynamic cosmological simulations, this information represents just a small fraction of the full 3-dimensional information encoded in current theoretical models for the properties of the IGM. In the era of large telescopes, it also represents only a fraction of the information that is observationally accessible. It is now possible to make predictions for the Lyman alpha emission from the IGM, that provide spatial and kinematic information about the IGM. I will discuss the theoretical advances that have made this possible and compare to current observations, as well as highlight future possibilities for understanding the distribution of neutral gas at high redshift as well as the physical processes that result in Lyman alpha emission.

From Massive Cores to Massive Stars

Mark Krumholz

The physical mechanism by which massive stars form is one of the outstanding problems in astrophysics, but one that has seen much recent progress thanks to an influx of new data. In the last few years, millimeter interferometers have revealed a population of compact (r <~ 0.1 pc), massive (M ~ 100 Msun) gas cores that could be the direct progenitors of massive stars. I discuss the evolution of these objects, focusing on the formation of binaries and multiple systems out of massive cores. Based on a combination of analytic modeling and simulations, I argue that massive cores are indeed the precursors of massive stars, and that many of the observed properties of young star clusters can be understood as direct imprints of the properties of their gas phase progenitors.

Constraints on the Prevalence of Starbursts in Dwarf Galaxies

Janice Lee

An outstanding question in galaxy evolution is whether the star formation histories of low mass systems are preferentially dominated by global starbursts or modes that are more quiescent and continuous. In this talk, I will quantify the prevalence of global starbursts in dwarf galaxies at the present epoch, and infer their characteristic durations, frequencies and amplitudes in the past. I will give an overview of the 11 Mpc H-alpha UV Galaxy Survey (11HUGS), the primary dataset used for this analysis, which provides H-alpha and GALEX UV imaging for an approximately volume-limited, statistical sample of star-forming galaxies within 11 Mpc of the Milky Way. Our approach is to directly tally the number of bursting dwarfs in a complete local sample, and to compute the fraction of star formation that is concentrated in these systems. The resulting starburst number and mass fractions (6% and 22% respectively) are then combined with integrated B-V colors, H-alpha EWs, and stellar evolutionary synthesis models in order to place constraints on the average starburst duty cycle.

Oxygen isotope anomalies of the Sun and the original environment of the solar system

Jeong-Eun Lee

Two recent solar wind oxygen isotope measurements have yielded dramatically different results, in one case showing a mass-independent signature of Delta 17O_SMOW ~ -20 permil and in the other case +20 permil for the Sun. In this talk, I present results from a model of oxygen isotopic anomaly production through selective photodissociation of CO within the proto-solar cloud. This study finds that the model produces a proto-Sun with a wide range of Delta 17O values depending on the intensity of the ultraviolet radiation field. A very intense radiation field, consistent with a nearby supernova source for 60Fe in meteorites, results in a proto-Sun with Delta 17O_SMOW ~ -20 permil, and implies birth of the Sun in a large star cluster.

Intrinsic alignments and cosmic shear: current status, future prospects

Rachel Mandelbaum

Cosmic shear, or weak lensing by large-scale structure, has been used to constrain the matter power spectrum normalization around z~1, and future surveys are planned to allow high signal-to-noise measurements of cosmic shear to constrain the equation of state of dark energy. One potential systematic error is the correlation between the intrinsic ellipticities of galaxies and the density field that causes gravitational lensing (GI correlation). I will describe the original detection of GI correlations to 60/h Mpc scales using galaxies from the Main spectroscopic sample of the Sloan Digital Sky Survey (SDSS) around z~0.1, followed by a measurement of the scaling with luminosity and redshift for the red galaxies in the SDSS Luminous Red Galaxy sample around z~0.25 and the 2dF-SDSS LRG and Quasar Survey (2SLAQ) around z~0.55. These results are then used to predict contamination of the cosmic shear signal as a function of survey depth, and to pinpoint what must be done next to ensure minimal intrinsic alignment contamination in future cosmic shear surveys. I will discuss methods for (a) determining the physical cause of the alignments and (b) eliminating this contamination in current and future weak lensing surveys, particularly those with data in multiple passbands that allow for color selection.

High Speed Time Series CCD Photometer Agile

Anjum Mukadam

Agile is a portable high-speed time-series photometer, based on the Princeton Instruments VersArray CCD camera, designed to study blue variables. A time-series photometer requires a precise measurement of both the start time of an exposure and the exposure duration. The CCD frame transfer operation implies that we do not need a mechanical shutter; an exposure ends followed by the initiation of a subsequent exposure using the positive (or negative) edge of a triggering pulse. We use a GPS-based timing card to generate these pulses, ensuring that the exposure intervals are uniform, and determined directly from the clocking hardware. The uncertainty in the timing of this operation is hard to measure, but we certainly expect it is less than 100 microseconds. We expect to read an unbinned full frame in 1.1s using a low noise amplifier operating at 1MHz with a read noise of order 8 to 10 electron RMS. For exposures longer than 11s, the observer can choose to read the image at 100KHz with a read noise of order 4 to 5 electron RMS. The frame transfer CCD has 1024 x 1024 active pixels, each of size 13micron x 13micron. Using a focal reducer at the Nasmyth focus of the 3.5m telescope at Apache Point Observatory (APO), we expect a field of view of approximately 2.6 arcmin x 2.6 arcmin with a platescale of 0.15 arcsec/pixel. The CCD is back-illuminated and has a special UV coating for enhanced blue efficiency, which is greater than 80% in the wavelength range of 450-750 nm.

Transition Region Heating and Structure in M dwarfs

Rachel Osten

The transition region in late-type stellar atmospheres serves to separate the chromosphere from the tenuous corona. It is important for understanding the dynamics and structuring occurring in the outer atmospheres of stars. Transition region emissions are best studied with UV and FUV spectral lines such as those accessible with STIS and FUSE. A comparison of transition region and coronal emissions allows a first step to sleuthing the heating requirements for stellar coronae. The discovery by Hawley & Johns-Krull (2003 ApJ 588, L109) of transition region emission in very low mass stars opens up the possibility of a comparative study of the atmospheric heating and structuring occurring in stars at late spectral type, and an investigation into the enigmatic nature of magnetic activity signatures in ultracool, neutral stellar atmospheres. Here we present a detailed comparison of the UV spectroscopy of several active late-type M dwarfs with very low mass dwarfs, in an attempt to tease out the underlying similarities and differences of their outer atmospheres.

Progress of the Wide-Field Deep Surveys for Galaxies at z=3-9

Masami Ouchi

I will present the progress of our wide-field deep surveys for dropout galaxies and Lya emitters (LAEs) at z=3-9 in the Subaru and GOODS fields. Based on the narrow-band images in the Subaru fields, we have identified 1100 Lya emitters from z=3.10 up to 6.96. Contrary to the evolution of UV luminosity function (LF) of dropout galaxies, Lya LF of LAEs show almost no evolution from $z=3$ to $6$. The evolution of Lya LF can be found only at z>~6, which is probably made by the combination of cosmic reionization and halo evolution. We have found two candidates of z-dropout galaxies at z~7-9 from first-year Subaru/MOIRCS survey data, together with HST/ACS and Spitzer images in the GOODS-N field. I will discuss these sources and introduce our on-going z-dropout galaxy and z~9 Lya emitter searches.

New Estimators of the Galaxy Correlation Function

Nikhil Padmanabhan

I will introduce a new class of estimators of the correlation function that attempt to minimize various systematic effects. These allow a direct comparison with theory, without modeling redshift space distortions and the large scale correlation function. This has a number of advantages over the more traditional w_p estimator, including (i) an insensitivity to large scale structures and the details of the truncation of the line of sight integral, (ii) a compact kernel in xi(r), and (iii) being unbinned.

Dynamical behavior of generic dark energy models

Hiranya Peiris

Much theoretical and experimental effort is currently being devoted to investigating "dark energy", a mysterious component that contributes about 75% of the total energy density in the universe and causes its expansion to accelerate. Virtually nothing is known about the nature of dark energy -- it could be simple vacuum energy (Einstein's cosmological constant), a time-varying, spatially inhomogeneous scalar field, or something even more exotic. Picking a theoretical paradigm (say, scalar field theories), it is often unclear exactly what range of observational signatures this paradigm predicts as a class, because particular examples of that class (say, specific scalar field models) predict only a subset of the signatures predicted by the whole class. I will describe an efficient algorithm to rigorously and exhaustively sift through all of the possible evolution histories of the universe predicted by a particular paradigm of dark energy models. These histories are then compared with cosmological data to find the subset of models within the overall framework that look like our Universe. I will show how, as a by-product, the analysis also yields answers to a diverse range of interesting questions one can ask about dark energy.

Molecular gas in the terrestrial zones of planet-forming disks

Klaus Pontoppidan

The dynamics and chemistry of molecular gas in the inner few tens of AU in proto-planetary disks are crucial for understanding the process of planet formation as well as the initial chemical conditions on the surfaces of Earth-like planets. I will describe a coupled observational and radiative transfer modeling program aimed at using molecular ro-vibrational transitions in the 2-20 micron wavelength regime to trace the conditions of gas in this region in disks around low-mass stars. The observational part of the program uses high resolution (up to 3 km/s) spectroscopy from Keck-NIRSPEC and in particular the new CRIRES spectrometer on the ESO Very Large Telescope with special focus on the CO ro-vibrational fundamental band around 4.7 micron, but other molecules, such as HCN, C2H2, CO2 and OH, are observed as well. The data are interpreted using a new 2D non-LTE radiative transfer code optimized for infrared lines to obtain axisymmetric abundance profiles and search for radial gas motions.

The Dynamical Structure of the Local Interstellar Medium

Seth Redfield

The local interstellar medium (LISM) provides an opportunity to study general ISM processes in great detail and in three dimensions, as well as observe how stellar and planetary systems interact with their surrounding interstellar environment (e.g., the heliosphere). The dynamical structure of the LISM is critical to understanding how warm partially ionized gas in the solar vicinity may have formed, evolves, and influences the heliosphere. High spectral resolution observations are used to derive rigid velocity vectors and determine the shapes of the associated clouds located within ~15 pc of the Sun. Assignment of cloud membership is not solely derived from fitting projected velocity, but also includes comparison of physical parameters (e.g., temperature, nonthermal velocity, and depletions) with nearby cloud members. Several clouds have a filamentary structure, which may be indicative of shocked or interacting material. I will discuss cloud dynamics at boundary regions and search for evidence of shear flows or cloud-cloud collisions, and explore the consequences of such interactions. In addition, I will discuss recent work involving the connection between radio scintillation scattering screens observed toward redshift z~1.3 quasars and these very same LISM clouds within 15 pc (i.e., z~10^-9).

Observations of Dwarf Planets

Scott Sheppard

Several large dwarf planet type objects have recently been discovered beyond Neptune. These objects are important because they are the brightest and thus easiest objects to observe in the Trans-Neptunian region. This allows the use of a variety of observational techniques at various different wavelengths in order to try and understand the surface structure and composition of the objects. I will discuss the recent results obtained for some of the largest known Kuiper belt objects, which give us insight into their physical properties.

The Shadow of Dark Matter

Kris Sigurdson

We discuss a model independent study of resonant photon scattering off dark matter (DM) particles. The DM particle chi_1 can feature an electric or magnetic transition dipole moment which couples it with photons and a heavier neutral particle chi_2. Resonant photon scattering then takes place at a special energy set by the masses of chi_1 and chi_2, with the width of the resonance set by the size of the transition dipole moment. We discuss constraints on the parameter space of the model from stellar energy losses, data from SN 1987A, the Lyman-alpha forest, Big Bang nucleosynthesis, electro-weak precision measurements and accelerator searches. We show that the velocity broadening of the resonance plays an essential role for the possibility of the detection of a spectral feature originating from resonant photon-DM scattering. Depending upon the particle setup and the DM surface mass density, the favored range of DM particle masses lies between tens of keV and a few MeV, while the resonant photon absorption energy is predicted to be between tens of keV and few GeV.

The Eddington Limit in Cosmic Rays: An Explanation for the Observed Faintness of Starbursting Galaxies

Aristotle Socrates

In terms of their energetics, interstellar cosmic rays are an insignificant by-product of star formation. However, due to their small mean free path, their coupling with interstellar gas is absolute in that they are the dominant source of momentum deposition on galactic scales. By defining an Eddington Limit in cosmic rays, we show that the maximum luminosity of bright starbursting galaxies is capped by the production and subsequent expulsion of cosmic rays. This simple provides an explanation for why galaxies are faint in comparison to quasars.

Post-starburst Galaxies and Fossil Galactic Winds: new constraints on feedback and the starburst-AGN connection

Christy Tremonti

Feedback from massive stars and accreting black holes has proven to be a key ingredient in successful models of galaxy evolution. Yet much about the feedback process is still poorly understood due to a lack of direct observational constraints. To help remedy this, we are studying post-starburst galaxies at z =0.5-1. These objects are the likely remnants of major mergers, observed a few hundred million years after the peak of their star formation and AGN activity. In 10/14 galaxies we detect absorption lines from the interstellar medium which are blueshifted with respect to the stars by 500 -- 2000 km/s. We hypothesize that the absorbing material represents a fossil galactic wind launched near the peak of the galaxy's activity. The outflow velocities we measure exceed typical values for starburst-driven winds, but overlap with the range of values measured in Broad Absorption Line (BAL) quasars. This suggests that feedback from an AGN may have played a role in powering the outflows and shutting down star formation. We consider the implications of these observations for the feedback-limited growth of galaxies and black holes, and for the pollution of the intergalactic medium.

Modeling the Clustering of Luminous Red Galaxies

Zeng Zeng

Luminous red galaxies (LRGs) constitute the bright end of the galaxy luminosity function. Their clustering properties encode information about their environment, formation, and evolution, which can provide a crucial test of theories of galaxy formation. I will first present the results of Halo Occupation Distribution (HOD) modeling of small-scale and intermediate-scale clustering of LRGs in the Sloan Digital Sky Survey. Most of these LRGs turn out to be central galaxies residing in massive halos of typical mass M~10^14 Msun/h, although a few percent of them are satellites within halos. I will also show the modeling result of the cross-correlation between LRGs and normal L* galaxies, which allows us to study how L_* galaxies are populated in massive halos. The HOD modeling result also leads to an intuitive understanding of the scale-dependent luminosity dependence seen in the cross-correlation measurements. Finally, I will present the HOD modeling of the evolution of LRG clustering from z~0.9 to z~0.5 measured in the NOAO Deep Wide Field Survey and study the merging and disruption of LRGs between these two epochs.