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Where Are All the Gas-Bearing Local Dwarf Galaxies? Quantifying Possible Impacts of Reionization

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Where Are All the Gas-Bearing Local Dwarf Galaxies? Quantifying Possible Impacts of Reionization Draft version March 6, 2018 Preprint typeset using LATEX style AASTeX6 v. 1.0 WHERE ARE ALL THE GAS-BEARING LOCAL DWARF GALAXIES? QUANTIFYING POSSIBLE IMPACTS OF REIONIZATION Erik J. Tollerud1, J.E.G. Peek1, 2 1Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA 2Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA ABSTRACT We present an approach for comparing the detections and non-detections of Local Group (LG) dwarf galaxies in large H I surveys to the predictions of a suite of n-body simulations of the LG. This approach depends primarily on a set of empirical scaling relations to connect the simulations to the observations, rather than making strong theoretical assumptions. We then apply this methodology to the Galactic Arecibo L-band Feed Array H I (GALFA-HI) Compact Cloud Catalog (CCC), and compare it to the Exploring the Local Volume In Simulations (ELVIS) suite of simulations. This approach reveals a strong tension between the na¨ıve results of the model and the observations: while there are no LG dwarfs in the GALFA-HI CCC, the simulations predict ∼ 10. Applying a simple model of reionization can resolve this tension by preventing low-mass halos from forming gas. However, and if this effect operates as expected, the observations provide a constraint on the mass scale of dwarf galaxy that reionization impacts. Combined with the observed properties of Leo T, the halo virial 8:5 mass scale at which reionization impacts dwarf galaxy gas content is constrained to be ∼ 10 M , independent of any assumptions about star formation. Keywords: cosmology: dark ages, reionization, first stars | cosmology: dark matter | galaxies: dwarf | galaxies: Local Group | radio lines: galaxies 1. INTRODUCTION Band Feed Array (ALFA) at the William E. Gordon Dwarf galaxies of the Local Group (LG) provide 305 meter antenna at the Arecibo Observatory, the rate uniquely faint limits, yielding a similarly unique range of at which Arecibo can survey the 21-cm line of H I has constraints on galaxy formation and cosmology. These increased nearly 7-fold. Arecibo provides the sensitiv- constraints typically hinge on connecting simulations in ity of a single dish instrument unavailable to radio in- an assumed cosmology (usually the concordance ΛCDM terferometers with an angular resolution (∼ 1 kpc for cosmology) to a particular model of galaxy formation, objects 1 Mpc away) unavailable to smaller single-dish and comparing that in some manner to observations. observatories. Therefore, the surveys conducted with This general approach has led to a number of unexpected ALFA provide the best available data sets for search- findings (e.g. Klypin et al. 1999; Moore et al. 1999; ing for these gas-rich dwarf galaxies in the local group. Kravtsov 2010; Boylan-Kolchin et al. 2011; Brooks & These surveys, tuned to compact H Iclouds, have found Zolotov 2014). However, these approaches have tended interesting populations of Galactic disk clouds mixed in to focus on the stellar component of LG dwarfs and their with dwarf galaxy candidates. Some of these have later model equivalents. While this is a practical approach been confirmed to be galaxies beyond the local group given that most dwarf galaxies of the LG are passive (Saul et al. 2012; Tollerud et al. 2015; Sand et al. 2015). arXiv:1711.00485v2 [astro-ph.GA] 5 Mar 2018 and lack gas (Grcevich & Putman 2009; Spekkens et al. This suggests that such H I surveys may be useful for 2014), the dwarf galaxy Leo T demonstrates that even improving the understanding of nearby dwarf galaxies. ultra-faint dwarfs can have gas even to the present day A major uncertainty to any investigation of the gas (Ryan-Weber et al. 2008). Hence, it is important to con- (or stellar) content of low-mass galaxies is the impact of sider what additional constraints may be gained by in- reionization. Theory and simulations strongly suggest vestigating the presence or absence of gas-bearing galax- that dwarf galaxies with low enough mass dark matter ies. halos are significantly affected by reionization, dramat- Recent advancements have improved the prospects for ically altering how the galaxies form (e.g. Barkana & such investigations, especially in tracking the diffuse gas Loeb 1999; Okamoto et al. 2008; Ricotti 2009). This can traced by H I. With the installation of the Arecibo L- have major observational consequences, such as explain- ing the relatively small number of dwarf galaxies in the LG compared to the number of subhalos that are seen in ΛCDM simulations (e.g. Bullock et al. 2000). While [email protected] it is strongly suspected that these impacts are impor- 2 Tollerud & Peek tant, the details of these effects, particularly when and (1991) catalog are categorized as HVCs, while those far at what mass scale the effects become important, are not from these clouds are called \Galaxy Candidates." This at all clear. In this work we investigate whether simply distinction was made under the assumption that small positing the presence of such an effect combined with H I clouds near larger high velocity clouds are much the empirically observed H I content of LG dwarf galax- more likely to be HVCs than to be galaxies. Practically, ies can provide independent constraints on the impacts this proximity is quantified using the parameter of reionization on dwarf galaxies and their dark matter r halos. D = Θ2 + f 2 (δv)2 ; (1) This paper is organized as follows: In x2, we de- scribe the Galactic Arecibo L-band Feed Array H I where Θ is the angular distance between two clouds, δv (GALFA-HI) survey and the compact high-velocity H I is the LSR velocity difference, and f is 0.5 degrees / km clouds it has identified as dwarf galaxy candidates; In x3 s−1 (Peek et al. 2009). For each cloud in Saul et al. we describe optical searches for dwarf galaxies near the (2012), this parameter is measured against all HVCs in H I clouds described in x2; In x4 we describe a method the Wakker & van Woerden(1991) catalog, and those for creating mock sets of GALFA-HI Compact Cloud clouds with minimum D > 25◦ are classified as Galaxy Catalog galaxies, using the Exploring the Local Volume Candidates. This procedure finds a total of 27 such In Simulations (ELVIS) suite of simulations; In x5 we de- candidates. scribe how the combination of all of the above provides This method of distinguishing between likely HVCs constraints on the scale at which reionization strips gas and possible dwarf galaxies is supported by work by from dwarf galaxies; and in x6 we conclude. To allow Donovan Meyer et al.(2015), who showed that includ- reproducibility, the analysis software used for this paper ing D < 25◦ clouds diluted an H I-based search for more is available at https://github.com/eteq/galvis. distant UV-bright galaxies. Our goal in this work is to identify which of these candidates are actually LG dwarf 2. GALFA-HI AND ITS COMPACT CLOUDS galaxies (x3), and create a mock version of this survey The GALFA-HI survey comprises observations of neu- using simulations in an appropriate cosmological context tral hydrogen with the 21-cm line taken with the 305 (x4). meter William E. Gordon telescope located in Arecibo, Puerto Rico. These observations began in 2004 with the 3. DWARF GALAXIES IN GALFA-HI installation of ALFA, which provided an almost 7-fold The set of Galaxy Candidates we describe in x2 re- increase in mapping speed over the L-band wide feed. quire follow-up to determine which, if any, have optical GALFA-HI observations have an angular resolution of galaxies within the GALFA-HI beam. It is precisely this 40, and a spectral resolution of 0.184 km s−1, over the that was the goal of the surveys described in Tollerud −1 velocity range -650 to +650 km s VLSR. This work et al.(2015) and Sand et al.(2015). These samples, as relies on the observations compiled into the GALFA-HI described in more detail in Bennet et al. (in prep), ob- Data Release 1 (DR1): 3046 hours of observations cov- served the GALFA-HI CCC Galaxy Candidate fields and ering 7520 deg2 of sky between Declination -1◦ and 38◦ resolve stars to a depth of ∼ 2 mags deeper than Sloan (Peek et al. 2011). Digital Sky Survey (SDSS). These surveys determined Saul et al.(2012) used GALFA-HI DR1 to generate a that 5 have optical counterparts that are likely nearby catalog of 1964 compact H I clouds using a template- galaxies, but they are at 2 to 10 Mpc (Tollerud et al. matching technique, the GALFA-HI Compact Cloud 2016), placing them in the Local Volume (LV) rather Catalog (CCC). The search was designed to be sen- than the LG. sitive to clouds smaller than 200 and with linewidths While the description above focuses on new unknown between 2.5 and 35 km=s. The sensitivity of the search galaxies, the GALFA-HI footprint also contains nearby was measured empirically through a signal injection ap- dwarf galaxies that are not in the CCC simply because proach (section 3.3 of Saul et al.(2012)). Simulated they were previously-known. Specifically, Sextans B, clouds were injected with a range of positions, veloc- GR 8, and KKH 86 are in the GALFA-HI footprint. ities, linewidths, sizes, aspect ratios, and brightnesses However, as with the confirmed galaxy candidates de- and the detection algorithm run. It was found that a scribed above, and as discussed in more detail in Mc- rather simple function of these parameters was able to Connachie(2012, Figure 5), all three of these galaxies reliably predict whether the search algorithm could find are clearly in the Hubble Flow (both by distance and a given cloud.
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