Eridanus II: A Fossil from Reionization with an Off-Center Star Cluster
Joshua D. Simon, Thomas M. Brown, Alex Drlica-Wagner, Ting S. Li, Roberto J. Avila, Keith Bechtol, Gisella Clementini, Denija Crnojevic, Alessia Garofalo, Marla… 展开作者
Joshua D. Simon, Thomas M. Brown, Alex Drlica-Wagner, Ting S. Li, Roberto J. Avila, Keith Bechtol, Gisella Clementini, Denija Crnojevic, Alessia Garofalo, Marla Geha, David J. Sand, Jay Strader, Beth Willman
We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf galaxy Eridanus II (Eri II). Eri II, which has an absolute magnitude of M_V = -7.1, is located at a distance of 339 kpc, just beyond the virial radius of the Milky Way. We determine the star formation history of Eri II and measure the structure of the galaxy and its star cluster. We find that a star formation history consisting of two bursts, constrained to match the spectroscopic metallicity distribution of the galaxy, accurately describes the Eri II stellar population. The best-fit model implies a rapid truncation of star formation at early times, with >80% of the stellar mass in place before z~6. A small fraction of the stars could be as young as 8 Gyr, but this population is not statistically significant; Monte Carlo simulations recover a component younger than 9 Gyr only 15% of the time, where they represent an average of 7 +/- 4% of the population. These results are consistent with theoretical expectations for quenching by reionization. The HST depth and angular resolution enable us to show that Eri II's cluster is offset from the center of the galaxy by a projected distance of 23 +/- 3 pc. This offset could be an indication of a small (~50-75 pc) dark matter core in Eri II. Moreover, we demonstrate that the cluster has a high ellipticity of 0.31 +0.05/-0.06 and is aligned with the orientation of Eri II within 3 +/- 6 degrees, likely due to tides. The stellar population of the cluster is indistinguishable from that of Eri II itself.
We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (10$^4$ $<$ M$_{\star}$ $<$ 10$^9$ M$_{\odot}$). The SFHs are derived from color-magnitude diagrams using archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five quenched galaxies (And V, And VI, And XIII, Leo IV, Hercules) are consistent with forming the bulk of their stars before reionization, when full uncertainties are considered. Observations of 13 of the predicted `true fossils' identified by Bovill & Ricotti show that only two (Hercules and Leo IV) in…
We present the star formation history (SFH) of the Sextans dwarf spheroidal galaxy based on deep archive $B$,$I$ photometry taken with Suprime-Cam at Subaru telescope focusing our analysis on the inner region of the galaxy, fully located within the core radius. Within the errors of our SFH we have not detected any metallicity gradient along the considered radial distance interval. As a main result of this work we can state that the Sextans dwarf spheroidal stopped forming stars less than $\sim1.3$ Gyr after Big Bang in correspondance to the end of the reionization epoch. We have been able to c…
In cold dark matter cosmologies, small mass halos outnumber larger mass halos at any redshift. However, the lower bound for the mass of a galaxy is unknown, as are the typical luminosity of the smallest galaxies and their numbers in the universe. The answers depend on the extent to which star formation in the first population of small mass halos may be suppressed by radiative feedback loops operating over cosmological distance scales. If early populations of dwarf galaxies did form in significant number, their relics should be found today in the Local Group. These relics have been termed "foss…
Eridanus II (EriII) is an ultra-faint dwarf (UFD) galaxy (M_V=-7.1) located at a distance close to the Milky Way virial radius. Early shallow color-magnitude diagrams (CMD) indicated that it possibly hosted an intermediate-age or even young stellar population, which is unusual for a galaxy of this mass. In this paper, we present new ACS/HST CMDs reaching the oldest main sequence turnoff with excellent photometric precision, and derive a precise star formation history (SFH) for this galaxy through CMD-fitting. This SFH shows that the bulk of the stellar mass in Eri II formed in an extremely sho…
Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies (UFDs; ${\rm M}_{\rm V}$ $\sim -2$ or ${\rm M}_{\star}$ $\sim 10^{2}$ at $z=0$) had ultra-violet (UV) luminosities of ${\rm M}_{\rm UV}$ $\sim -3$ to $-6$ during reionization ($z\sim6-10$). The existence of such faint galaxies has substantial implications for early epochs of galaxy formation and reionization. If the faint-end slopes of the UV luminosity functions (UVLFs) during reionization are steep ($α\lesssim-2$) to ${\rm M}_{\rm UV}$ …
The analysis of the early star formation history (SFH) of nearby galaxies, obtained from their resolved stellar populations is relevant as a test for cosmological models. However, the early time resolution of observationally derived SFHs is limited by several factors. Thus, direct comparison of observationally derived SFHs with those derived from theoretical models of galaxy formation is potentially biased. Here we investigate and quantify this effect. For this purpose, we analyze the duration of the early star formation activity in a sample of four Local Group dwarf galaxies and test whether …
Observations of the Milky Way (MW), M31, and their vicinity, known as the Local Group (LG), can provide clues about the sources of reionization. We present a suite of radiative transfer simulations based on initial conditions provided by the Constrained Local UniversE Simulations (CLUES) project that are designed to recreate the Local Universe, including a realistic MW-M31 pair and a nearby Virgo. Our box size (91 Mpc) is large enough to incorporate the relevant sources of ionizing photons for the LG. We employ a range of source models, mimicking the potential effects of radiative feedback for…
EDGE-INFERNO: The chemical signatures of assembly history in reionization fossils
American Museum of Natural History（美国自然历史博物馆）；University of Bath（巴斯大学）；University of Chicago（芝加哥大学）；Kavli Institute for Cosmological Physics, University of Chicago（芝加哥大学卡弗里宇宙学物理研究所）；NSF-Simons AI Institute for the Sky (SkAI)（NSF-Simons 天空人工智能研究所 (SkAI)）；University of Surrey（萨里大学）；Lund University（隆德大学）；Carnegie Observatories（卡内基观测站）；Carnegie Theoretical Astrophysics Center, Carnegie Observatories（卡内基观测站卡内基理论天体物理学中心）；Columbia University（哥伦比亚大学）；Center for Astrophysics | Harvard & Smithsonian（哈佛-史密森尼天体物理中心）
The large-scale overdensities of galaxies at z~2-7 known as protoclusters are believed to be the sites of cluster formation, and deep, wide survey projects such as the Large Synoptic Survey Telescope (LSST) and the Wide Field Infrared Survey Telescope (WFIRST) will deliver significant numbers of these interesting structures. Spectroscopic confirmation and interpretation of these targets, however, is still challenging, and will require wide-field multi-plexed spectroscopy on >20 m-class telescopes in the optical and near-infrared. In the coming decade, detailed studies of protoclusters will ena…
Rise of the forsaken relics: connecting present-day stellar streams and phase-mixed galaxies to the Epoch of Reionization
University of Pennsylvania（宾夕法尼亚大学）；Yale University（耶鲁大学）；University of California, Davis（加州大学戴维斯分校）；Universität Zürich（苏黎世大学）；The Observatories of the Carnegie Institution for Science（卡内基科学研究所天文台）；California Institute of Technology（加州理工学院）
Ghostly stellar haloes are extended haloes of stars composed solely of debris of pre-reionization fossil galaxies and should exist in dwarf galaxies with total masses $<10^{10}$ M$_\odot$. Fossil galaxies are even smaller mass dwarf galaxies that stopped forming stars after the epoch of reionization and have been identified in the Local Group as the ultra-faint dwarf satellites. Using cosmological N-body simulations we present an empirical model for the shapes and masses of ghostly stellar haloes. We compare the model to available observations of stellar haloes in six isolated dwarf galaxies i…
We present the star formation history (SFH) of the Sculptor dwarf spheroidal galaxy based on deep g,r photometry taken with DECam at the Blanco telescope, focusing our analysis on the central region of the galaxy extended up to $\sim 3$ core radii. We have investigated how the SFH changes radially, subdividing the sampled area into four regions, and have detected a clear trend of star formation. All the SFHs show a single episode of star formation, with the innermost region presenting a longer period of star formation of $\sim 1.5$ Gyr and for the outermost region the main period of star forma…
Dwarf spheroidal satellite galaxies have emerged a powerful probe of small-scale dark matter clustering and of cosmic reionization. They exhibit structural and chemical continuity with dwarf irregular galaxies in the field and with spheroidal galaxies in high-density environments. By combining empirical constraints derived for star formation at low gas column densities and metallicities in the local universe with a model for dark matter and baryonic mass assembly, we provide an analytical description of how the dwarf spheroidals acquired their stellar content. Their progenitors formed stars un…
We review numerical work carried out over the last decade on the role of environmental mechanisms in shaping the nature of the faintest galaxies known, dwarf spheroidals (dSphs). In particular we discuss a model in which dSphs originated from gas dominated, disky dwarfs that were accreted by massive galaxies several billions of years ago. Dwarfs accreting at $z > 1$, when the cosmic ultraviolet ionizing flux was much higher than today and was thus able to keep the gas in the dwarfs warm and diffuse, were rapidly stripped of their baryons via ram pressure and tidal forces, producing very dark m…
An Ultra-Faint, Chemically Primitive Galaxy Forming in the Reionization Era
Kanazawa University（金泽大学）；National Astronomical Observatory of Japan（日本国立天文台）；The University of Tokyo（东京大学）；SOKENDAI (The Graduate University for Advanced Studies)（综合研究大学院大学）；Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo（东京大学宇宙物质量子研究所（WPI））；Istituto Nazionale di Astrofisica (INAF)（意大利国家天体物理研究所）；University of Cambridge（剑桥大学）
We present early results from a Hubble Space Telescope survey of the ultra-faint dwarf galaxies. These Milky Way satellites were discovered in the Sloan Digital Sky Survey, and appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in the efforts to understand the missing satellite problem. Because they are the least luminous, most dark matter dominated, and least chemically evolved galaxies known, the ultra-faint dwarfs are the best candidate fossils from the early universe. The primary goal of the survey is to measure the star-formation histori…
We present new constraints on the star formation histories of the ultra-faint dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space Telescope (HST). A galaxy class recently discovered in the Sloan Digital Sky Survey, the UFDs appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in efforts to understand the missing satellite problem. They are the least luminous, most dark-matter dominated, and least chemically-evolved galaxies known. Our HST survey of six UFDs seeks to determine if these galaxies are true fossils from the ea…
We present results from multifrequency radiative hydrodynamical chemistry simulations addressing primordial star formation and related stellar feedback from various populations of stars, stellar energy distributions (SEDs) and initial mass functions. Spectra for massive stars, intermediate-mass stars and regular solar-like stars are adopted over a grid of 150 frequency bins and consistently coupled with hydrodynamics, heavy-element pollution and non-equilibrium species calculations. Powerful massive population III stars are found to be able to largely ionize H and, subsequently, He and He$^+$,…
We present a semi-analytic model of isolated dwarf galaxy evolution and use it to study the build-up of observed correlations between dwarf galaxy properties. We analyse the evolution using models with averaged and individual halo mass assembly histories in order to determine the importance of stochasticity on the present-day properties of dwarf galaxies. The model has a few free parameters, but when these are calibrated using the halo mass - stellar mass and stellar mass-metallicity relations, the results agree with other observed dwarf galaxy properties remarkably well. Redshift evolution sh…
Pop III stars are typically massive stars of primordial composition forming at the centers of the first collapsed dark matter structures. Here we estimate the optimal X-ray emission in the early universe for promoting the formation of Pop III stars. This is important in determining the number of dwarf galaxies formed before reionization and their fossils in the local universe, as well as the number of intermediate-mass seed black holes. A mean X-ray emission per source above the optimal level reduces the number of Pop III stars because of the increased Jeans mass of the intergalactic medium (I…
We present the first comparison between the lifetime star formation histories (SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we obtained deep optical imaging of Andromeda II (M$_{V} = -$12.0; log(M$_{\star}$/M$_{\odot}$) $\sim$ 6.7) and Andromeda XVI (M$_{V} = -$7.5; log(M$_{\star}$/M$_{\odot}$) $\sim$ 4.9) yielding color-magnitude diagrams that extend at least 1 magnitude below the oldest main sequence turnoff, and are similar in quality to those available for the MW companions. And II and And XVI show strikingly similar S…
EDGE-INFERNO: How chemical enrichment assumptions impact the individual stars of a simulated ultra-faint dwarf galaxy
American Museum of Natural History（美国自然历史博物馆）；University of Bath（巴斯大学）；University of Hertfordshire（赫特福德大学）；University of Surrey（萨里大学）；Lund University（隆德大学）；University of Chicago（芝加哥大学）；Columbia University（哥伦比亚大学）；Center for Astrophysics | Harvard & Smithsonian（哈佛-史密森尼天体物理中心）
A dense, metal-rich absorber driven by a heavily dust-obscured galaxy: The direct evidence of CGM enrichment at $z\sim7$
University of Toronto（多伦多大学）；Dunlap Institute for Astronomy and Astrophysics（邓普天文学与天体物理研究所）；University of Copenhagen（哥本哈根大学）；Niels Bohr Institute（尼尔斯·玻尔研究所）；Steward Observatory, University of Arizona（亚利桑那大学斯图尔德天文台）；MIT Kavli Institute for Astrophysics and Space Research（麻省理工学院卡弗里天体物理学与空间研究学院）；Massachusetts Institute of Technology（麻省理工学院）
