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In Pursuit of the Least Luminous Galaxies
Beth Willman
Advances in Astronomy , 2010, DOI: 10.1155/2010/285454
Abstract: The dwarf galaxy companions to the Milky Way are unique cosmological laboratories. With luminosities as low as , they inhabit the lowest mass dark matter halos known to host stars and are presently the most direct tracers of the distribution, mass spectrum, and clustering scale of dark matter. Their resolved stellar populations also facilitate detailed studies of their history and mass content. To fully exploit this potential requires a well-defined census of virtually invisible galaxies to the faintest possible limits and to the largest possible distances. I review the past and present impacts of survey astronomy on the census of Milky Way dwarf galaxy companions and discuss the future of finding ultra-faint dwarf galaxies around the Milky Way and beyond in wide-field survey data. 1. Introduction The least luminous known galaxies have historically been those closest to the Milky Way. Whether visually or with automated searches, resolved stars reveal the presence of nearby dwarf galaxies with surface brightnesses too low to be discovered by diffuse light alone. Even until recently, nearly all cataloged dwarfs fainter than resided within the Local Group of galaxies (LG) [1]. In 1999 the LG contained 36 known members, of which eleven are Milky Way (MW) satellites [2]. Four of these eleven MW dwarf galaxies are less luminous than , more than 10?000 times less luminous than the Milky Way itself. Although such low luminosity dwarfs almost certainly contribute a cosmologically insignificant amount to the luminosity budget of the Universe, all eight of the Milky Way's classical dwarf spheroidal companions ( , not including Sagittarius or the Magellanic Clouds) have been studied in extensive detail. (“Classical” will be used in the paper to refer to the Milky Way dwarf companions known prior to 2003.) There is now a new class of “ultra-faint” dwarf companions to the Milky Way known to have absolute magnitudes as low as ([3], see Section 3). The resolved stellar populations of these near-field cosmological laboratories have been used to derive their star formation and chemical evolution histories [4] and to model their dark mass content in detail (see article by Strigari in this volume and references therein). These complete histories of individual systems complement studies that rely on high redshift observations to stitch together an average view of the Universe's evolution with time. The need for an automated, “systematic, statistically complete, and homogeneous search” for LG dwarf galaxies has been known for some time [5]. A combination of theoretical
"Galaxy," Defined
Beth Willman,Jay Strader
Physics , 2012, DOI: 10.1088/0004-6256/144/3/76
Abstract: A growing number of low luminosity and low surface brightness astronomical objects challenge traditional notions of both galaxies and star clusters. To address this, we propose a definition of galaxy independent of the cold dark matter model of the universe: A galaxy is a gravitationally bound set of stars whose properties cannot be explained by a combination of baryons and Newton's laws of gravity. After exploring observational diagnostics of this definition, we examine the classification of ultra-faint dwarfs, globular clusters, ultra-compact dwarfs, and tidal dwarfs. While kinematic studies provide an effective galaxy diagnostic in many regimes, they can be less useful for compact or faint systems. To explore the use of [Fe/H] spread as a complementary diagnostic, we use published spectroscopic [Fe/H] measurements of 16 Milky Way dwarfs and 24 globular clusters to calculate their [Fe/H] spreads and uncertainties. Our principal results are: (i) no old star cluster less luminous then M_V = -10 has a significant (> 0.1 dex) spread in iron; (ii) known ultra-faint dwarfs can be classified with a combination of kinematics and [Fe/H]; (iii) the observed [Fe/H] spreads in massive (> 10^6 M_Sun) globular clusters do not necessarily imply they are the stripped nuclei of dwarfs, nor a need for dark matter; and (iv) if ultra-compact dwarfs reside in dark matter halos akin to those of ultra-faint dwarfs of the same half-light radii, then they will show no clear dynamical signature of dark matter. We suggest measurements that may assist future classification of massive globular clusters, ultra-compact dwarfs, and ultra-faint galaxies. Our galaxy definition is designed to be independent of the details of current observations and models, while our proposed diagnostics can be refined or replaced as our understanding of the universe evolves.
Charting Unexplored Dwarf Galaxy Territory With RR Lyrae
Mariah Baker,Beth Willman
Physics , 2015, DOI: 10.1088/0004-6256/150/5/160
Abstract: Observational bias against finding Milky Way (MW) dwarf galaxies at low Galactic latitudes (b < 20 deg) and at low surface brightnesses (fainter than 29 mag arcsec^-2, in the V-band) currently limits our understanding of the faintest limits of the galaxy luminosity function. This paper is a proof-of-concept that groups of two or more RR Lyrae stars reveal MW dwarf galaxies at d > 50 kpc in these unmined regions of parameter space, with only modest contamination from interloper groups when large halo structures are excluded. For example, a friends-of-friends (FOF) search with a linking length of 500 pc could reveal dwarf galaxies more luminous than M_V = -3.2 mag and with surface brightnesses as faint as 31 mag arcsec^-2 (or even fainter, depending on RR Lyrae specific frequency). Although existing public RR Lyrae catalogs are highly incomplete at d > 50 kpc and/or include <1% of the MW halo's volume, a FOF search reveals two known dwarfs (Bootes I and Sextans) and two dwarf candidate groups possibly worthy of follow-up. PanSTARRS 1 (PS1) may catalog RR Lyrae to 100 kpc which would include ~15% of predicted MW dwarf galaxies. Groups of PS1 RR Lyrae should therefore reveal very low surface brightness and low Galactic latitude dwarfs within its footprint, if they exist. With sensitivity to RR Lyrae to d >600 kpc, LSST is the only planned survey that will be both wide-field and deep enough to use RR Lyrae to definitively measure the Milky Way's dwarf galaxy census to extremely low surface brightnesses, and through the Galactic plane.
Turning the Tides on the Ultra-Faint Dwarf Spheroidal Galaxies: Coma Berenices and Ursa Major II
Ricardo R. Munoz,Marla Geha,Beth Willman
Physics , 2009, DOI: 10.1088/0004-6256/140/1/138
Abstract: We present deep CFHT/MegaCam photometry of the ultra-faint Milky Way satellite galaxies Coma Berenices (ComBer) and Ursa Major II (UMa II). These data extend to r~25, corresponding to three magnitudes below the main sequence turn-offs in these galaxies. We robustly calculate a total luminosity of M_V=-3.8 +/- 0.6 for ComBer and M_V=-3.9 +/- 0.5 for UMa II, in agreement with previous results. ComBer shows a fairly regular morphology with no signs of active tidal stripping down to a surface brightness limit of 32.4 magarcsec^-2. Using a maximum likelihood analysis, we calculate the half-light radius of ComBer to be r_half=74 +/- 4 pc (5.8 +/- 0.3 arcmin) and its ellipticity e=0.36 +/- 0.04. In contrast, UMa II shows signs of on-going disruption. We map its morphology down to mu_V=32.6 mag arcsec^-2 and found that UMa II is larger than previously determined, extending at least ~700 pc (1.2 deg on the sky) and it is also quite elongated with an ellipticity of e=0.50 +/- 0.2. However, our estimate for the half-light radius, 123 +/- 3 pc (14.1 +/- 0.3 arcmin) is similar to previous results. We discuss the implications of these findings in the context of potential indirect dark matter detections and galaxy formation. We conclude that while ComBer appears to be a stable dwarf galaxy, UMa II shows signs of on-going tidal interaction.
Star-Galaxy Classification in Multi-Band Optical Imaging
Ross Fadely,David W. Hogg,Beth Willman
Physics , 2012, DOI: 10.1088/0004-637X/760/1/15
Abstract: Ground-based optical surveys such as PanSTARRS, DES, and LSST, will produce large catalogs to limiting magnitudes of r > 24. Star-galaxy separation poses a major challenge to such surveys because galaxies---even very compact galaxies---outnumber halo stars at these depths. We investigate photometric classification techniques on stars and galaxies with intrinsic FWHM < 0.2 arcsec. We consider unsupervised spectral energy distribution template fitting and supervised, data-driven Support Vector Machines (SVM). For template fitting, we use a Maximum Likelihood (ML) method and a new Hierarchical Bayesian (HB) method, which learns the prior distribution of template probabilities from the data. SVM requires training data to classify unknown sources; ML and HB don't. We consider i.) a best-case scenario (SVM_best) where the training data is (unrealistically) a random sampling of the data in both signal-to-noise and demographics, and ii.) a more realistic scenario where training is done on higher signal-to-noise data (SVM_real) at brighter apparent magnitudes. Testing with COSMOS ugriz data we find that HB outperforms ML, delivering ~80% completeness, with purity of ~60-90% for both stars and galaxies, respectively. We find no algorithm delivers perfect performance, and that studies of metal-poor main-sequence turnoff stars may be challenged by poor star-galaxy separation. Using the Receiver Operating Characteristic curve, we find a best-to-worst ranking of SVM_best, HB, ML, and SVM_real. We conclude, therefore, that a well trained SVM will outperform template-fitting methods. However, a normally trained SVM performs worse. Thus, Hierarchical Bayesian template fitting may prove to be the optimal classification method in future surveys.
Are the Ultra-Faint Dwarf Galaxies Just Cusps?
Adi Zolotov,David W. Hogg,Beth Willman
Physics , 2010, DOI: 10.1088/2041-8205/727/1/L14
Abstract: We develop a technique to investigate the possibility that some of the recently discovered ultra-faint dwarf satellites of the Milky Way might be cusp caustics rather than gravitationally self-bound systems. Such cusps can form when a stream of stars folds, creating a region where the projected 2-D surface density is enhanced. In this work, we construct a Poisson maximum likelihood test to compare the cusp and exponential models of any substructure on an equal footing. We apply the test to the Hercules dwarf (d ~ 113 kpc, M_V ~ -6.2, e ~ 0.67). The flattened exponential model is strongly favored over the cusp model in the case of Hercules, ruling out at high confidence that Hercules is a cusp catastrophe. This test can be applied to any of the Milky Way dwarfs, and more generally to the entire stellar halo population, to search for the cusp catastrophes that might be expected in an accreted stellar halo.
High-Resolution Spectroscopy of Extremely Metal-Poor Stars in the Least Evolved Galaxies: Ursa Major II and Coma Berenices
Anna Frebel,Joshua D. Simon,Marla Geha,Beth Willman
Physics , 2009, DOI: 10.1088/0004-637X/708/1/560
Abstract: We present Keck/HIRES observations of six metal-poor stars in two of the ultra-faint dwarf galaxies orbiting the Milky Way, Ursa Major II and Coma Berenices. These observations include the first high-resolution spectroscopic observations of extremely metal-poor stars ([Fe/H]<-3.0) stars not belonging to the Milky Way (MW) halo field star population. We obtain abundance measurements and upper limits for 26 elements between carbon and europium. The entire sample of stars spans a range of -3.2<[Fe/H]<-2.3, and we confirm that each galaxy contains a large intrinsic spread of Fe abundances. A comparison with MW halo stars of similar metallicities reveals substantial agreement between the abundance patterns of the ultra-faint dwarf galaxies and the MW halo for the light, alpha and iron-peak elements (C to Zn). This agreement contrasts with the results of earlier studies of more metal-rich stars (-2.5<[Fe/H]<-1.0) in more luminous dwarf spheroidal galaxies (dSphs), which found significant abundance discrepancies with respect to the MW halo data. The abundances of neutron-capture elements (Sr to Eu) in the ultra-faint dwarf galaxies are extremely low, consistent with the most metal-poor halo stars, but not with the typical halo abundance pattern at [Fe/H]>-3.0. Our results are broadly consistent with a galaxy formation model that predicts that massive dwarf galaxies are the source of the metal-rich component ([Fe/H]>-2.5) of the MW halo, but we also suggest that the faintest known dwarfs may be the primary contributors to the metal-poor end of the MW halo metallicity distribution.
Too Many, Too Few, or Just Right? The Predicted Number and Distribution of Milky Way Dwarf Galaxies
Jonathan R. Hargis,Beth Willman,Annika H. G. Peter
Physics , 2014, DOI: 10.1088/2041-8205/795/1/L13
Abstract: We predict the spatial distribution and number of Milky Way dwarf galaxies to be discovered in the DES and LSST surveys, by completeness correcting the observed SDSS dwarf population. We apply most massive in the past, earliest forming, and earliest infall toy models to a set of dark matter-only simulated Milky Way/M31 halo pairs from Exploring the Local Volume In Simulations (ELVIS). The observed spatial distribution of Milky Way dwarfs in the LSST-era will discriminate between the earliest infall and other simplified models for how dwarf galaxies populate dark matter subhalos. Inclusive of all toy models and simulations, at 90% confidence we predict a total of 37-114 L $\gtrsim 10^3$L$_{\odot}$ dwarfs and 131-782 L $\lesssim 10^3$L$_{\odot}$ dwarfs within 300 kpc. These numbers of L $\gtrsim 10^3$L$_{\odot}$ dwarfs are dramatically lower than previous predictions, owing primarily to our use of updated detection limits and the decreasing number of SDSS dwarfs discovered per sky area. For an effective $r_{\rm limit}$ of 25.8 mag, we predict: 3-13 L $\gtrsim 10^3$L$_{\odot}$ and 9-99 L $\lesssim 10^3$L$_{\odot}$ dwarfs for DES, and 18-53 L $\gtrsim 10^3$L$_{\odot}$ and 53-307 L $\lesssim 10^3$L$_{\odot}$ dwarfs for LSST. These enormous predicted ranges ensure a coming decade of near-field excitement with these next generation surveys.
An Orphan No Longer? Detection of the Southern Orphan Stream and a Candidate Progenitor
Carl J. Grillmair,Lauren Hetherington,Raymond G. Carlberg,Beth Willman
Physics , 2015, DOI: 10.1088/2041-8205/812/2/L26
Abstract: Using a shallow, two-color survey carried out with the Dark Energy Camera, we detect the southern, possibly trailing arm of the Orphan Stream. The stream is reliably detected to a declination of $-38^\circ$, bringing the total known length of the Orphan stream to $108^\circ$. We find a slight offset or "S" shape in the stream at $\delta \simeq -14^\circ$ that would be consistent with the transition from leading to trailing arms. This coincides with a moderate concentration of $137 \pm 25$ stars (to $g = 21.6$) that we consider a possible remnant of the Orphan progenitor. The position of this feature is in agreement with previous predictions.
The Dual Origin of Stellar Halos II: Chemical Abundances as Tracers of Formation History
Adi Zolotov,Beth Willman,Alyson Brooks,Fabio Governato,David W. Hogg,Sijing Shen,James Wadsley
Physics , 2010, DOI: 10.1088/0004-637X/721/1/738
Abstract: Fully cosmological, high resolution N-Body + SPH simulations are used to investigate the chemical abundance trends of stars in simulated stellar halos as a function of their origin. These simulations employ a physically motivated supernova feedback recipe, as well as metal enrichment, metal cooling and metal diffusion. As presented in an earlier paper, the simulated galaxies in this study are surrounded by stellar halos whose inner regions contain both stars accreted from satellite galaxies and stars formed in situ in the central regions of the main galaxies and later displaced by mergers into their inner halos. The abundance patterns ([Fe/H] and [O/Fe]) of halo stars located within 10 kpc of a solar-like observer are analyzed. We find that for galaxies which have not experienced a recent major merger, in situ stars at the high [Fe/H] end of the metallicity distribution function are more [alpha/Fe]-rich than accreted stars at similar [Fe/H]. This dichotomy in the [O/Fe] of halo stars at a given [Fe/H] results from the different potential wells within which in situ and accreted halo stars form. These results qualitatively match recent observations of local Milky Way halo stars. It may thus be possible for observers to uncover the relative contribution of different physical processes to the formation of stellar halos by observing such trends in the halo populations of the Milky Way, and other local L* galaxies.
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