2016 STScI Symposium Posters
Click on each entry to access the abstract and view the poster. [slideshow]
Sudhanshu Barway, Staff/Faculty, SAAO
What physical processes shape S0 galaxies?
In near-by universe, S0 galaxies were originally conceived as a morphological transition class between ellipticals and early-type spirals on Hubble tuning fork diagram. S0 galaxies are often characterised by the presence of a central bulge and disc and the absence of spiral arms. Multi-wavelength observations over the decades have not led to any broad consensus about their origins, properties and evolution, although they are often thought to have formed most of their stars early in the history of the universe and to have evolved relatively passively since then. We present multi-wavelength study of the stellar mass dependent evolution for a sample of S0 galaxies using GALEX-SDSS-2MASS-WISE imaging as well as SDSS-SALT spectroscopy. We extracted galaxy parameters for bulge, disc and bar using GALFIT, a 2-D image decomposition technique. We estimated physical parameters like stellar mass, star formation rate, mass weighted stellar age, metalicity etc. by SED fitting to multi-wavelength photometry using publicly available code MAGPHYS. This helped us to create a catalog of these parameters along with those available from SDSS spectral database for our sample to S0 galaxies to study their origin and evolution. We find that S0 galaxies can be classified in two types based on their stellar mass. The star formation history of these two types of S0 are very different. High mass S0's have formed by major mergers at roughly the same epoch around z ∼ 2 after which star formation is rapidly quenched. On the other hand, low mass S0's are still seeing some star formation although the rate is decreasing, through various processes like ram pressure stripping for galaxies in clusters and bar driven instabilities in field galaxies.
Eric Bell, Staff/Faculty, University of Michigan
Supermassive black holes as the regulators of star formation in galaxies
(with Bryan Terrazas, University of Michigan)
Quiescence is one of the central observational features of many galaxies in the local and distant Universe. Yet, it is unclear what drives quiescence—is it feedback from AGN accretion, a change in cooling mode at high halo mass, a change in star formation efficiency in steep potentials, or something else entirely?
Connor Bottrell, Graduate Student, University of Victoria
Observational Properties of Simulated Galaxies: Computational Cosmology as Seen through a Telescope
The current generation of large-scale hydrodynamical cosmological simulations are valuable instruments for understanding the formation and evolution of galaxies. However, the crux in the interpretive power for such simulations is in whether they are fundamentally able to reproduce the observed properties of galaxies. Furthermore, it is crucial that any comparison between simulated and real galaxies is fair. That is, to facilitate a valid comparison, simulated galaxies must be anchored to the court of reality and adopt the observational biases that burden galaxies seen in the real Universe. We put galaxies from the Illustris simulation directly in the context of observational galaxy astronomy using an unprecedentedly rigorous suite of observational realism in the Sloan Digital Sky Survey (SDSS). Parametric photometry and structural analysis of simulated galaxies with observational realism is performed using the same pipeline that was used in the analysis of 1.12 million real galaxies in the SDSS — which collectively forms our comparison sample. In this talk, I will discuss promising similarities along with intriguing contrast between real and simulated galaxy populations. I will also highlight the broader applications of our methods — as such conjunctions between observational and computational astronomy stand to provide a wealth of feedback to each community alike.
Ryan Brennan, Graduate Student, Rutgers University
Studying the Relationship Between Star Formation and Morphology Using Semi-Analytic Models
We study how star formation and galaxy morphology are related both for a population of galaxies observed as part of the CANDELS survey as well a population generated using the Santa Cruz semi-analytic model (SAM). Specifically, we examine the relationship between the growth of a bulge component and the cessation of star formation. We first divide galaxies into broad types based on their star formation rates and morphologies to see whether our model can reproduce the buildup of these general subpopulations with its prescriptions for bulge growth and quenching due to mergers, disk instabilities and feedback due to active galactic nuclei (AGN). We then examine how morphological and structural properties of galaxies change as a continuous function of distance from the main sequence of star formation. We look to our model for insight into the physics of these transformations, and also discuss the ways in which our model fails to reproduce the observations, as well as how we might improve it.
Sebastien Carassou, Graduate Student, Institut d'Astrophysique de Paris
Inferring the spectro-morphometric evolution of galaxies from image simulations
After 20 years of large and small scale galaxy surveys (SDSS, CFHTLS, UDF etc.), the field of galaxy evolution is now well established in the era of Big Data. We can extract the spectrophotometric and morphometric properties of millions of galaxies, over a period that covers more than 9 billion years of cosmic history. But current constrains on models of galaxy evolution suffer from selection biases that, if not taken into account carefully, can lead to contradictory predictions (e.g. the size evolution of bulges from z~2).
To address this issue, we are developing a new approach combining sampling techniques (in a Bayesian framework) and empirical modeling with realistic image simulations that reproduce a large fraction of these selection effects. This will allow us to perform a direct comparison between observed and simulated images and therefore infer robust constraints on model parameters predicting the evolution of bulges and disks from z~2 to z~0.
Andres del Pino, Post-doc, Nicolaus Copernicus Astronomical Center (CAMK)
What shapes dwarf spheroidal galaxies? The case of Andromeda II and Fornax
How dwarf spheroidal galaxies (dSph) were formed and evolved is still an unsolved question. Several processes are thought to be affecting their evolution, shaping them into the systems we observe today. In this talk we will discuss the possible imprint of these mechanisms in two of the most conspicuous dwarf galaxies in the Local Group: the Fornax and Andromeda II dSph. Our results are based in a detailed observational study comprehending the deepest ground based Color-Magnitude Diagrams up to date, deep HST photometry and spectroscopy of ~3000 stars. By combining this data and using a new software tool, BEACON, specifically designed for finding motion streams in resolved stellar systems, we obtain surface density maps, radial density profiles, and rotation curves of the different stellar populations present in each galaxy. Our results demonstrate that both systems are complex, showing features which suggest a tumultuous past for them. For each galaxy, we study the mechanisms that might be implied in its evolution, and propose a merger scenario to explain its properties making use, in the case of Andromeda II, of N-Body simulations.
Gwendolyn Eadie, Graduate Student, McMaster University
Mass Estimates of the Milky Way using Kinematic Data
Obtaining an accurate mass estimate of the Milky Way is extremely important to cosmology and the study of dark matter. However, mass estimates for the Milky Way vary considerably, as evidenced by the range of results present in the literature. Many methods to estimate Galactic mass use the kinematic data of Galactic satellites. One issue with using kinematic data is that they are incomplete and subject to significant measurement uncertainty. It is possible that the variation in mass estimates is partly due to this issue. We have developed a method to estimate the Milky Way's mass which relies on a Bayesian analysis and a physical model. The advantage of using the Bayesian approach is twofold: we can incorporate both the incomplete kinematic data and the measurement errors using a hierarchical model. I will present our method and estimates for the Milky Way's cumulative mass profile when assuming two kinds of models: one in which the satellites and dark matter follow the same distribution, and one in which they do not.
Francesca Fragkoudi, Post-doc, Obervatoire de Paris
A close look at secular evolution: The effects of boxy/peanut bulges on gas inflow to the central regions
Non-axisymmetries such as bars and spirals are responsible for funnelling the gas found in disc galaxies to their central regions. These internal secular processes contribute to shaping the central regions of disc galaxies by creating discy (pseudo) bulges, triggering nuclear star formation and feedback, and have been suggested as a mechanism for fuelling AGN activity. Boxy/peanut (b/p) bulges are caused by vertical instabilities in bars and are therefore ubiquitous in barred galaxies. We show, using orbital structure analysis and idealised hydrodynamic simulations of isolated galaxies, that b/p bulges can reduce the gas inflow to the central regions by more than an order of magnitude. This has a significant effect on the amount of material available in the central kiloparsec. Our results thus add another layer of complexity to how secular evolution shapes galaxy properties.
Michael Fusco, Graduate Student, University of Arkansas
The Pitch Angle Distribution Function of Local Disk Galaxies and its Role in Understanding Galactic Structure
A recent study by Davis et al. (2014) has presented a pitch angle distribution function of local galaxies from the Carnegie-Irvine Survey (CGS), used to calculate the local super massive black hole mass function (BHMF) for those galaxies. For reasons of completeness that sample was limited both in luminosity and volume. We now present an extended analysis of the dimmer galaxies excluded from the previous sample. This subset consists of spiral galaxies with Absolute B-Band Magnitude greater than -19.12 and limiting luminosity distance (redshift independent) less than 25.4 Mpc (z=0.00572). These parameters yield a sample set of 74 spiral galaxies with 51 measurable pitch angles.
The incorporation of the new subset into the prior sample yields a sample solely limited only in Luminosity-Distance rather than both Absolute Magnitude and Luminosity-Distance. Not surprisingly, the more recent subset is morphologically distinct from the earlier magnitude limited sample, with more Sc and Sd classified galaxies, suggesting smaller central bulges. However, the pitch angle distribution is interestingly similar to that found by Davis et al for the brighter sample. The dispersion relation for spiral density waves, as discussed in Davis et al 2015, suggests that if the Sc and Sd galaxies actually have smaller bulges, they could still have broadly similar pitch angles if their disks had larger gas densities. A study of the link between these quantities is essential to constructing a complete BHMF, which would include the low mass end of the mass function.
Caryl Gronwall, Staff/Faculty, Penn State University
Metallicities, Star Formation Rates and Dust Attenuation of z~2 Star-Forming Galaxies
We have been investigating the physical and chemical properties of emission-line galaxies in the z ~ 2 universe using galaxies in the GOODS and COSMOS fields identified via their strong [O III]-emitting emission on WFC3 G141 grism frames. We use our sample of ~250 galaxies to explore issues such as the (non)-Fundamental Metallicity Relation, the systematics of star-formation rate indicators, and the behavior of dust attenuation laws versus stellar mass. We find that the distribution of z ˜ 2 galaxies in stellar mass-SFR-metallicity space is clearly different from that derived for our sample of local galaxy analogs, and this offset cannot be explained by simple systematic offsets in the derived quantities. We also show that at z ~ 2, the H-beta star formation rate (SFR) is a factor of ~1.8 higher than what would be expected from the systems' rest-frame UV flux density, suggesting a shift in the standard conversion between these quantities and SFR. Finally, our analysis demonstrates that the average reddening law of z ~ 2 starburst galaxies has a shallower far-UV slope that the locally derived Calzetti (2001) law, and that the slope steepens with stellar mass.
William Janesh, Graduate Student, Indiana University
The New Local Group Member AGC 198606 (Friend of Leo T) and the Search for Optical Counterparts to ALFALFA-Detected UCHVCs
The ALFALFA survey is a blind neutral hydrogen (HI) survey aimed at constructing a statistically-complete census of HI gas in the local universe. ALFALFA has detected a sample of compact, isolated neutral gas clouds called ultra-compact high-velocity HI clouds (UCHVCs) with properties that make them good candidates for low-mass galaxies in the Local Volume. The UCHVCs have no obvious optical counterpart in existing catalogs, and their HI properties are similar to those of the Local Group star-forming dwarf galaxy Leo T. At a distance of 1 Mpc, UCHVCs would have HI masses of ~10^5-10^6 M_Sun, HI diameters of ~2-3 kpc, and dynamical masses of ~10^7-10^8 M_Sun. Deep imaging by our group of a UCHVC has led to the discovery of Leo P, a dwarf galaxy at 1.6 Mpc with an extremely low metallicity (~2% solar; Giovanelli et al. 2013, Rhode et al. 2013, Skillman et al. 2013; McQuinn et al. 2015). Leo P is gas-rich and is currently forming stars; its location in a relatively benign, low-density environment has likely been a main contributor to its particular evolutionary status.
We have initiated a campaign to obtain deep optical imaging of the other UCHVCs using the WIYN 3.5-m telescope and wide-field ODI camera, with the goal of identifying resolved stellar counterparts to these HI clouds. The overall motivation is to understand the origin and evolution of these objects in the context of star formation and baryon retention in low-mass dark matter halos. We have developed observational and numerical techniques for finding stellar counterparts to the UCHVCs in our optical images and quantifying the significance of any counterparts we detect. The campaign has so far identified a probable stellar counterpart to AGC 198606, an HI cloud that is close to Leo T in both spatial position and velocity (Friend of Leo T; Janesh et al. 2015). We estimate a distance of ~380 kpc for the optical counterpart, placing it ~40 kpc from Leo T and implying that the two objects could have had a close encounter in the past. Here we present details about our observing campaign and detection methods, the detection of the counterpart to AGC 198606, and the latest results from the project.
Sandor Kruk, Graduate Student, University of Oxford
Studying stellar populations of barred galaxies using structural decomposition of multiband galaxy images
Considering that bars are seen in at least one third of the disc galaxies, understanding their role on the evolution of the host galaxies is crucial in our interpretation of the present day Universe. We study the stellar populations in disc galaxy structures (discs, bulges and bars), using morphological classifications from the Galaxy Zoo citizen science project. Using data from SDSS we have created a representative sample of ~3,000 local barred galaxies (z < 0.06), which have been visually inspected for the presence of a bar. On these galaxies, we have performed three-component (disc/bulge/bar) multiwavelength parametric decomposition in five SDSS bands, ugriz, using GALFITM developed by the MegaMorph project. With detailed structural analysis we estimate physical quantities such as the bar and bulge-to-total luminosity ratios, Sersic indices and colour distribution of the components. By comparing with a similar sample of unbarred late-type galaxies, we examine the connection between the presence of a large-scale galactic bar and the properties of discs and bulges. Using Galaxy Zoo morphologies we are able to perform 2D disc/bulge/bar decomposition on the largest sample of barred disc galaxies to date and study the secular evolution of their detailed stellar structures.
Peter Kurczynski, Staff/Faculty, Rutgers University
Evolution of Intrinsic Scatter in the SFR-Stellar Mass Correlation at 0.5<z<3
We present estimates of intrinsic scatter in the Star Formation Rate (SFR) - Stellar Mass (M*) correlation in the redshift range 0.5 < z < 3.0 and in the mass range 10^7 < M* < 10^11 Msun. We utilize photometry in the Hubble Ultradeep Field from UDF12 and Ultraviolet Ultra Deep Field (UVUDF) campaigns and CANDELS/GOODS-S and estimate SFR, M* from broadband Spectral Energy Distributions (SEDs) and the best available redshifts. The maximum depth of the UDF photometry (F160W 29.9 AB, 5 sigma depth) probes the SFR-M* correlation down to M* ~ 10^7 Msun, a factor of 10-100X lower in M* than previous studies, and comparable to dwarf galaxies in the local universe. We find the slope of the SFR-M* relationship to be near unity at all redshifts and the normalization to decrease with cosmic time. We find a moderate increase in intrinsic scatter with cosmic time from 0.2 to 0.4 dex across the epoch of peak star formation. None of our redshift bins show a statistically significant increase in intrinsic scatter at low mass. However, it remains possible that intrinsic scatter increases at low mass on timescales shorter than then ~100 Myr star formation rates probed by our SED fitting. These results are consistent with a picture of gradual and self-similar assembly of galaxies across more than three orders of magnitude in stellar mass to as low as 10^7 Msun. This work has been submitted to ApJ Letters.
Janice Lee, Staff/Faculty, STScI
Connections between Galaxy Properties and the Luminosity Function Slope of Star Forming Regions
Authors: David Cook (University of Wyoming / CalTech); Lee, J.; Dale, D.; Thilker, D.; Calzetti, D.; Kennicutt, R.
We present the first study of far-UV luminosity functions of individual star forming regions within a sample of nearby galaxies spanning a large range in total stellar mass and star formation properties. Our analysis is based on the Local Volume Legacy Survey, which provides multi-wavelength imaging from 1500A to 24 microns for 258 galaxies within ~11 Mpc. We investigate correlations between the far-UV luminosity function slope and global galaxy properties such as Hubble type, metallicity, and SFR surface density. We find weak to moderate correlations, the strongest of which are with SFR and SFR surface density, in the sense that more active star-forming galaxies have flatter luminosity function slopes (a relative larger number of bright star forming regions). Simulations do not show evidence that the results are due to the effects of blending or number statistics. We speculate on the connection to systematic variations in the underlying star formation efficiency.
Sandra Liss, Graduate Student, University of Virginia
Hα Imaging of Interacting Dwarf Galaxies
We present Hα imaging of the interacting dwarf galaxies from the TiNy Titans (TNT) survey, providing an unparalleled view of the ongoing star formation in the first systematic study of these low mass interacting systems. Galaxy interactions are known to be of fundamental importance to the evolution of massive galaxies — they have been observed to impact morphology, star formation rates, and ISM composition. Although the majority of mergers at all redshifts are expected to take place between low mass galaxies, until now there have not been comparable systematic studies of dwarf galaxy interactions, leaving open the question of whether interactions between low mass galaxies can strongly affect their own evolution.
Early TNT results include the finding that Hα fluxes measured from the 3'' Sloan Digital Sky Survey spectroscopic fibers suggest an enhancement in the star formation rates of paired versus unpaired dwarf galaxies. To develop a more complete view of the star formation in these interacting dwarf galaxies, we present narrowband Hα observations obtained at the 6.5m MMT, 6.5m Magellan Baade telescope, and 8.1m Gemini North telescope. These compelling images reveal the clumpy morphology and asymmetric distribution of the ongoing star formation, typically oriented in the direction of the interaction. Additionally, we calculate the total star formation rates in each dwarf galaxy and compare these to the fiber-derived values, providing a valuable check of methodology.
Mireia Montes, Post-doc, Yale University
The buildup of the outskirts of massive galaxies
Massive early-type galaxies are believed to have been assembled hierarchically. According to this picture, the innermost regions have formed the majority of their stars at high redshift and on short time-scales whereas their outer parts are a consequence of multiple major and minor merging. To probe this scenario, radial metallicity profiles are a useful tool as different formation histories predict different chemical enrichments, and, therefore, different gradients.
In my contribution, I will discuss the evidence for supporting the hierarchical scenario. In particular, I will focus on the outskirts of M87 and how the properties of this galaxy fits with this picture. I will describe the connection of this galaxy with its likely progenitors and how its ongoing assembly is manifested today on the formation of the intra-cluster light.
Preethi Nair, Staff/Faculty, University of Alabama - Tuscaloosa
Determining the dominant mechanisms of bulge and disk growth since z~1
Recent simulations suggest that bulge+disk galaxy growth proceeds via multiple paths. Bulge+disk galaxies can evolve (a) secularly, (b) through disk instabilities with shorter time scales (clump fragmentation) or by merger driven processes with subsequent disk regrowth. We compare the detailed morphologies of galaxies in field (COSMOS), groups and clusters (Frontier Fields) to characterize the growth of different mass components and the evolution of SFR in disks from z~1 till the present day. We present example galaxies in the local Universe which suggest disk regrowth as a possible, though not dominant, mechanism in the local Universe but which may be more important at z>1.
Charles Nelson, Staff/Faculty, Drake University
What Shapes Narrow LIne Regions of Active Galaxies? Bulges Do.
Recent investigations of AGN feedback, have indicated that interaction between the host galaxy and the nucleus is significant only in high luminosity active galaxies (Kormendy & Ho 2013). The upshot for Seyfert galaxies and other low luminosity AGN is that the Narrow Line Region (NLR) must reach a state of near pressure equilibrium with the host galaxy ISM. Thus, the NLR emerges into, and is constrained by the properties of the bulge. Kormendy & Kennicut (2004) review the notion that many bulges are not akin to classical ellipticals but are constructs of secular evolution in disks. Canonically, Seyferts are found in early type spirals but many, if not most of them, must be pseudo-bulges. Also much of the NLR kinematics are dominated by virial motions (Ho 2009, Nelson & Whittle 1996), suggesting that the gas most likely originates as mass loss from bulge stars or molecular clouds that become ionized by the AGN, simply because they are in the cone''. Seyferts with pseudo-bulges, therefore should be largely supported by rotation, while in the classical bulges random motions should dominate.
We report preliminary results on a project to assess the NLR-bulge relationship, based on a sample of Seyferts with broad-band HST F606W images, narrow-band HST [OIII] images, measured central velocity dispersions and nuclear gas kinematics from emission line widths. We will present detailed analyses of the bulge morphologies using GALFIT as a first step in searching for new links between the state of the NLR and the bulge gravitational potential.
Viraj Pandya, Graduate Student, Princeton University
Ionized Gas in the Most MASSIVE Local Elliptical Galaxies
We present a study of the distribution and kinematics of ionized gas in the most MASSIVE elliptical galaxies in the local Universe. Classically, local giant elliptical galaxies were thought to be devoid of gas, dust, and young stars. However, there have been hints since the 1980s that these ancient stellar systems are far more complex, with many such galaxies hosting large reservoirs of ionized and/or molecular gas, serving as the sites of residual star formation, and even exhibiting varying degrees of rotation among their stellar populations. Previous studies such as ATLAS3D were restricted to nearby galaxies and thus did not include a substantial number of the most massive elliptical galaxies in the local Universe. The MASSIVE survey is the first systematic study of the 100 most massive ellipticals out to the distance of the Coma cluster with integral field spectroscopy as its backbone. In this talk, I will describe our results on the existence and origin of warm ionized gas in these systems. In particular, I will show some striking examples of large reservoirs of rotating or patchy ionized gas, and examine whether that gas is of internal origin (e.g., stellar mass loss) or external origin (e.g., accretion). I will conclude by speculating on how observations at X-ray and radio wavelengths can offer clues as to the fate of the gas.
Michael Petersen, Graduate Student, University of Massachusetts at Amherst
Dark Matter Trapping by Stellar Bars: The Shadow Bar
I present an investigation into the complex interactions between the baryonic (stellar) disc and the dark-matter halo during bar formation and evolution using N-body simulations, revealing that the forming stellar bar traps dark matter in the vicinity of the stellar bar into bar-supporting orbits—the shadow bar. The shadow bar modifies both the location and magnitude of the angular momentum transfer between the disc and dark matter halo and adds 10% to the mass of the stellar bar over 4 Gyr. The (potentially) observable consequences of the shadow bar, its density and velocity signature in spheroid stars, are presented. Numerical tests demonstrate that the shadow bar can diminish the rate of angular momentum transport from the bar to the dark matter halo by more than a factor of three over the naive dynamical friction prediction, and thus provides a possible physical explanation for the observed prevalence of fast bars in nature.
Nor Pirzkal, Staff/Faculty, STScI
Slitless Spectroscopy and Star Formation in 0.3 < z < 2 Galaxies
The Faint Infrared Galaxy Survey (FIGS) provides us with a unique opportunity to identify emission line galaxies. Emission lines such as [OII], [OIII], Hα and Lyα lines can be identified in the FIGS slitless spectroscopic observations down to faint line fluxes of a few times 10^-17 erg/s/cm2. Crucially, the use of multiple observations, taken at different position angles on the sky allows us to accurately determine the location of these star forming regions within individual galaxies using the Emission Line 2D (EM2D) method. Our ability to detect high equivalent width lines independently of any host galaxies allows us to search for naked emission line objects. Combining this method with the wavelength coverage of the G102 grism, we are able to identify emission line objects using [OII] and [OIII], and Hα over 0.2 < z < 2 and using Lyman alpha from 6 < z < 8. Here, we present the first results on star forming galaxies selected using this method and demonstrate the wealth of data to be expected from the FIGS project.
Sedona Price, Graduate Student, University of California - Berkeley
Kinematics of z~2 star-forming galaxies with MOSDEF, CANDELS, and MassiveFIRE
The internal kinematics of star-forming galaxies are an ideal probe of the structural evolution of disks over time. Detailed observations of local galaxies constrain the late-time nature of disks, but observations at earlier times are necessary to determine the physical mechanisms behind the structural growth of star-forming galaxies. Recent near-infrared multi-object spectrographs allow us to probe the physical structure of large samples of galaxies at z~2, when galaxies were most rapidly forming their stellar mass. I present gas kinematics of star-forming galaxies at z~2 from the MOSFIRE Deep Evolution Field (MOSDEF) survey combined with structural measurements from HST/CANDELS imaging. We use forward modeling of gas kinematics observed with fixed-angle multi-object spectrographs to measure the kinematics and dynamical masses for galaxies both with and without detected rotation. By comparing the dynamical and baryonic (stellar + gas) masses, we constrain the physical structure (i.e., V/σ) of galaxies for which we do not resolve rotation. Finally, I present mock observations constructed from the MassiveFIRE (Feedback in Realistic Environments) simulation, which allow us to test how well we recover kinematic properties of star-forming galaxies with MOSDEF and from future observations with JWST.
Marc Rafelski, Post-doc, NASA Goddard
The Star Formation Rate Efficiency of Atomic-dominated Hydrogen Gas from z~1 to z~3
Current observational evidence suggests that the star formation rate (SFR) efficiency of neutral atomic hydrogen gas measured in Damped Lyman-alpha System (DLAs) at z~3 is a factor of 10 lower than predicted by the Kennicutt-Schmidt relation. To understand the origin of this deficit, we measure the SFR efficiency of atomic gas at z~1, z~2, and z~3 in the outskirts of star forming galaxies to investigate possible coevolution with galactic properties. We use new robust photometric redshifts to create galaxy stacks in these three redshift bins, and measure the SFR efficiency by combining DLA absorber statistics with the observed rest-frame UV emission in the galaxies' outskirts. Contrary to simulations and models that predict a reduced SFR efficiency with decreasing metallicity and thus with increasing redshift, we find no significant evolution in the SFR efficiency. We conclude that the reduced SFR efficiency is driven by the low molecular content of this atomic-dominated phase, with metallicity perhaps playing a second order effect in regulating the conversion between atomic and molecular gas. This interpretation is supported by the similarity between the observed SFR efficiency and that observed in local atomic-dominated gas, such as in the outskirts of local spiral galaxies and local dwarf galaxies.
Ruben Sanchez-Janssen, Canada, NRC Herzberg
The intrinsic shapes of dSph galaxies in the Virgo cluster, and a comparison with the Local Group
I will present an investigation on the intrinsic shapes of low-luminosity galaxies in the core of the Virgo cluster. We use very deep imaging obtained as part of the Next Generation Virgo Cluster Survey (NGVS) to build a sample of nearly 300 red-sequence cluster members in the yet unexplored -14 < M_g < -8 magnitude range. We fit the distribution of apparent axis ratios with models for triaxial ellipsoids, and find that faint Virgo galaxies are best described as a family of thick, nearly oblate spheroids with mean intrinsic axis ratios 1 : 0.94 : 0.57. I will also show that Local Group dSphs have slightly flatter and more triaxial mean shapes, 1 : 0.76 : 0.49. Considering literature results for faint dIrr galaxies, we find that the intrinsic flattening in this low-luminosity regime is almost independent of the environment in which the galaxy resides—but there is a hint that objects may be moderately rounder in denser environments. The comparable flattening distributions of low-luminosity galaxies that have experienced very different degrees of environmental effects suggests that internal processes are the main drivers of galaxy structure at low masses—with external mechanisms playing a secondary role.
Tabatha Sauvaget, Graduate Student, Observatoire de Paris - GEPI
Bulges formation in spiral galaxies via gas-rich major mergers
The mechanism of bulge formation in spiral galaxies is still debated. While classical bulges have been associated to the result of major mergers, pseudo-bulges have been rather associated to secular evolution. Over the past decade, increasing observations have suggested that most present-day spiral galaxies could have formed via gas-rich major mergers in the past 10 Gyr. Gas-rich mergers can change dramatically the properties of the merger remnant because gas can damp the violent relaxation occuring in the stellar phase. Here, using N-body hydrodynamic simulations, we have investigated whether pseudo-bulges can form as a result of such gas-rich major mergers. We have simulated and analyzed 12 simulated gas-rich major mergers that represent typical merger orbit and spin parameters in the LCDM context. We have analyzed the luminosity distribution of the merger remnants and decomposed it into central bulge plus disk components. Based on the Sersic index of the bulge component, we have classified the remnant bulges into classical bulges and pseudo-bulges. We find that pseudo-bulges can be created via gas-rich major mergers in about 75% of the cases, which appears to be consistent with observations.
Evan Schneider, Graduate Student, University of Arizona
Zooming in on Galactic Winds
The supply and removal of gas is a key process in galactic evolution. By the present day, almost all galaxies lack significant quantities of cold gas, as and a result, the majority of star formation in the universe has ceased. While the lack of cold gas in the z = 0 universe is obvious, the processes by which this fuel is removed remain controversial. Whether galaxies manage to consume all of the fuel available to them, or expel significant quantities into the circumgalactic medium remains unclear. Cosmological simulations have incorporated the effect of gas removal via prescriptive galactic winds, but due to resolution limits are unable to shed light on the driving mechanisms and physical processes acting in galactic gas removal. In an effort to better understand the galactic wind mechanism, we perform high-resolution fixed-grid simulations with the new, extremely efficient, GPU-parallelized hydro code Cholla. We focus on the interplay between cool gas entrained from the ISM, and hot winds driven by supernovae in rapidly star-forming regions. We examine the results of our simulations with a focus on the velocity and phase structure of the cool gas leaving starburst regions, and compare the resulting photoionization structure to the observed wind in M82. A suite of simulations allows us to study the effects of the density structure of the cool gas, contrasting the results of constant density and turbulent clouds. Our results have implications both for the interpretation of observations as well as for the mass-loading of hot winds that allow mass to fully escape galaxies.
Brooke Simmons, Post-doc, UC San Diego
The Evolution of Disk Galaxies to z ~ 2 from Galaxy Zoo CANDELS
Visual galaxy morphologies have a long tradition of driving discovery in Astrophysics. Galaxy Zoo has established that quantitative visual morphologies provided via consensus classifications of typically 40 or more human classifiers are a powerful tool for analysis of galaxy evolution surveys, alongside significant advances in computational morphological analyses.
Galaxy Zoo and CANDELS are publicly releasing visual morphologies of approximately 50,000 galaxies having z <~ 3 and observed in 3 HST legacy fields by CANDELS. Detailed morphological information is available on galaxy features such as clumpiness, bar instabilities, spiral structure, and merger and tidal signatures. We apply a consensus-based classifier weighting method that preserves classifier independence while effectively down-weighting significantly errant classifications. We demonstrate how the classifications of bar features may be used to trace the settling of disks: the bar fraction declines steadily from 0 < z < 1, then is consistent with no evolution to z ~ 2. Comparing the Galaxy Zoo classifications to other human and machine classifications of the same galaxies shows very good agreement; in some cases the high number of independent classifications provided by Galaxy Zoo provides an advantage in selecting galaxies with a particular morphological profile, while in others the combination of Galaxy Zoo with other classifications is a more promising approach than using any one method alone. As an example, we discuss how visual classifications, in combination with parametric morphologies, may be used to reveal a population of featureless disk galaxies at 1 < z < 2, which may represent a dynamically warmer progenitor population to the settled disk galaxies seen at later epochs.
Rebecca Smethurst, Graduate Student, University of Oxford
Galaxy Zoo: Evidence for diverse morphologically dependent quenching histories across the green valley and the role of AGN feedback
Two open issues in modern astrophysics are: (i) how do galaxies fully quench their star formation and (ii) how is this affected - or not - by AGN feedback? We present the results of a new Bayesian-MCMC analysis of the star formation histories of over 126,000 galaxies across the colour magnitude diagram showing that diverse, morphologically dependent quenching mechanisms are instrumental in the formation of the present day red sequence. To do this we utilise the classifications from Galaxy Zoo in a novel way, weighting by the vote fraction to include results from all galaxies rather than using a threshold to split our sample into late- and early-type morphology. We show that the rate at which quenching can occur is morphologically dependent in each of the blue cloud, green valley & red sequence, and that the green valley is a transition population for all morphological types, not a “red herring. We discuss the nature of the possible quenching mechanisms which give rise to the distribution of quenching parameters, considering the influence of secular evolution, galaxy interactions and mergers, both with and without black hole activity. We focus particularly on the relationship between these quenched star formation histories and the presence of an AGN, showing a population of type 2 AGN host galaxies have recently (within 2 Gyr) undergone a rapid (τ 〈 1 Gyr) drop in their star formation rate. The diversity of this new method also highlights that such rapid quenching histories cannot account fully for all the quenching across the current AGN host population. We demonstrate that slower (τ 〉 2 Gyr) quenching rates dominate for high stellar mass (log10[M*/M☉] 〉 10.75) hosts of AGN with both early- and late-type morphology. We discuss how these results show that both merger-driven and non-merger processes are contributing to the co-evolution of galaxies and supermassive black holes across the entirety of the colour magnitude diagram.
Aleksandra Sokolowska, Graduate Student, University of Zurich
Specific angular momentum evolution of late-type galaxies in high-resolution zoom-in cosmological hydro simulations
In the standard theory of disk galaxy formation the sizes of galaxies can be easily explained if baryons retain their specific angular momentum, initially resulting from tidal torques, as they collapse into the halo centers. We use high-resolution zoom-in cosmological simulations of late-type galaxies in order to understand the evolutionary tracks of gas and stars on the j/M* diagram, as well as how closely the angular momentum of baryons is coupled with that of dark matter. With an inventory of five simulations that yield normal disk galaxies in the halos of the Milky Way mass, we investigate how several differences in the sub-grid physics modify their evolution on that diagram. Among the main differences are: the strength of supernova feedback, the presence or not of AGN thermal feedback, the radiative cooling rates, as well as quiet vs. violent merger histories. Furthermore, owing to the resolution high enough to address single galaxy components at sub-kpc scales, we decompose each galaxy into a spheroid and a disk, using several photometric and kinematic methods, and study their angular momentum evolution separately.
David Stark, Post-doc, Kavli IPMU
Multi-Scale Environmental Influences on Galaxy Gas Content
To explore whether galaxy gas content is governed by more than just dark matter halos and their internal properties, we examine the dependence of global galaxy gas fractions on local and large scale environment. For this purpose, we combine the new 21cm census for the RESOLVE survey — a volume-limited census of ~1500 galaxies spanning diverse environments and probing baryonic masses down to ~10^9 Msun — with metrics that parametrize environment from group scales (group dark matter halo mass) up to large-scale structure (relative density of the cosmic web and classification into filaments and walls). First, we show evidence for systematic gas deficiency in satellites in group halos down to at least 10^12 Msun, suggesting that group processes that deplete gas content are active well below the large group/cluster scale. In addition, at fixed halo mass both centrals and satellites in large-scale walls have systematically lower gas fractions than galaxies in large-scale filaments, and this trend cannot be fully explained by differing stellar mass distributions within these large-scale environments. Lastly, we show that the abundance of gas-deficient centrals increases with large-scale structure density, and this correlation is most pronounced in large-scale walls. The population of gas-poor but low halo-mass (<10^11.4 Msun) centrals tends to reside in closer proximity to larger groups than do more gas-rich but otherwise analogous low halo-mass centrals, suggesting that the gas-poor centrals have lost their gas due to the influence of nearby groups. We discuss how the observed trends may be shaped by a number of physical processes such as gas stripping, starvation, and halo assembly bias.
Jonelle Walsh, Post-doc, Texas A&M University
The Black Holes in Compact, High-dispersion Galaxies
Correlations between black hole (BH) masses and galaxy bulge properties have been clearly established over the past 15 years; however major questions remain, particularly concerning the sparsely populated upper end of the BH mass distribution. Through a large survey with the Hobby Eberly Telescope (HET), we have identified a sample of dense, rapidly rotating, early-type galaxies, with small sizes and large stellar velocity dispersions, that appear similar to the z~2 quiescent galaxies. The HET spectra hint that these nearby compact, high-dispersion galaxies could host some of the most massive BHs known, and that the BHs could weigh a surprisingly large fraction of its host galaxy's mass. If the compact galaxies are relics of the z~2 quiescent galaxies and indeed contain over-massive black holes, this could suggest that BH growth precedes that of its host galaxy. I will present observations of the nuclear stellar kinematics for six compact, high-dispersion galaxies made using the integral field units (IFUs) on the Gemini and Keck telescopes assisted by adaptive optics. By combining with large-scale IFU kinematics and Hubble Space Telescope imaging observations, we construct orbit-based stellar dynamical models. I will discuss the dynamical modeling results, including constraints on the BH masses, measurements of the stellar mass-to-light ratios, and the location of the six compact, high-dispersion galaxies on the BH mass - host galaxy relationships.
Xin Wang, Graduate Student, University of California - Los Angeles
Measuring gas-phase metallicity gradients on galaxies at z=1~3 in the deepest field probed by HST spectroscopy
How baryons and metals cycle in and out of galaxies has remained to be a crucial question in galaxy formation and evolution. A variety of physical processes, including gas inflows/outflows, galactic feedback, and major/minor mergers, manifest themselves as a complex interplay which regulates baryonic mass assembly and star formation. A key diagnostic of this complex system is the radial gradient of gas-phase oxygen abundance or metallicity. With the ultra deep HST near-infared grism spectroscopy from the supernova Refsdal follow-up project (Kelly et al. 2016) and the Grism Lens-Amplified Survey from Space (Treu et al. 2015), we are able to observe the cycle of baryons to the lowest mass galaxies, at the peak epoch of cosmic star formation. I will present the first results on the metallicity gradient measurements with high spatial sampling (sub-kpc scales on the source plane), and unprecedentedly small stellar masses (at the level of 10 mission solar masses), in the field of the Hubble Frontier Field cluster MACS1149.6+2223. The synergy of extra deep diffraction-limited exposure and lensing magnification offers a great gain in the spatially resolved spectroscopy of high-z galaxies, in terms of source plane angular resolution and emission line limiting flux, which makes this study unique in all the metallicity gradient analyses so far.
Tao Wang, Post-doc, CEA - Saclay
Galaxy transformation in the most distant galaxy cluster at z = 2.505
We have recently discovered the most distant X-ray galaxy cluster (currently known) at z = 2.505 (11 spectroscopic members). Unlike any clusters discovered so far, the 80-kpc core of this cluster exhibits outrageous star formation activity with a combined star formation rate 3400 Msun yr^-1 (11 massive galaxies in the 80-kpc core, 5 detected with ALMA) and a gas depletion time of only 200 Myr. This cluster bridges the gap between high-redshift protoclusters(extended, not yet collapsed) and low- redshift galaxy clusters (collapsed, little star formation), unveiling a key rapid formation phase of massive galaxy clusters. With unprecedented multiwavelength data (HST, Herschel, IRAM-NOEMA, ALMA and JVLA), I will present how the unique properties of this cluster help to constrain some of the most fundamental questions on the (trans)formation of massive galaxies in a cluster-like environment.