Gas, Stars, and Star Formation in Alfalfa Dwarf Galaxies

Abstract

We examine the global properties of the stellar and HI components of 229 low HI mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with HI masses < 10 7.7 M ⊙ and HI line widths < 80 km s −1 . SDSS data are combined with photometric properties derived from GALEX to derive stellar masses (M * ) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs) and estimates of their SFRs and M * obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M * 10 8 M ⊙ is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo cluster. The imposition of an upper HI mass limit yields the selection of a sample with lower gas fractions for their M * than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have HI depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation laws is consistent with the general assumptions that gas-rich galaxies have lower star formation efficiencies than do optically selected populations and that HI disks are more extended than stellar ones.1 Based on observations made with the Arecibo Observatory and the NASA Galaxy Evolution Explorer (GALEX). The Arecibo Observatory is The 11HUGS sample is complete in HI mass only above 2 × 10 8 M ⊙ , becoming rapidly incomplete at smaller HI masses. To develop further the current understanding of how the gas supply regulates star formation in the lowest mass systems, a larger sample of extreme dwarf galaxies is needed. Making use of the Arecibo L-band Feed Array (ALFA), the on-going Arecibo Legacy Fast ALFA (ALFALFA) extragalactic HI line survey is specially designed to identify low mass, gas rich objects in the local universe . Because of its combination of wide areal coverage, sensitivity, and velocity resolution, ALFALFA has already detected more than 400 galaxies with HI masses M HI < 10 8 M ⊙ (Haynes et al. 2011). While star formation is more directly linked to the molecular interstellar component, the detection of CO in low-metallicity dwarfs is difficult (Leroy et al. 2005, and references therein), suggesting further that CO no longer traces H 2 well. Furthermore, in many gas-rich dwarf galaxies, the HI component dominates both the gas as well as the baryonic mass (e.g. Leroy et al. 2007). The combination of HI parameters from ALFALFA with complementary multi-wavelength data contributed by SDSS and GALEX provides an ideal dataset to investigate the abundance and distribution of gas-rich dwarfs and t...

Dark influences

“…The stellar masses of the systems follow generally (extrapolated) stellar mass -halo mass relations available in the literature (Behroozi et al 2013;Moster et al 2013;Garrison-Kimmel et al 2014;Sawala et al 2015). The gas fraction, f g = M gas /(M + M gas ), ranges from f g = 0.3 to f g = 0.9 in agreement with observational estimates (Huang et al 2012;McQuinn et al 2015). This means that the baryonic mass can be quite high, and that in the most gas-rich systems M gas = 9 M .…”

“…Although the simulated systems have slowly declining star formation rates in general (as a result of the lack of external gas infall), they agree quite well with observations, also in terms of gas content (Hunter & Elmegreen 2006;Weisz et al 2012;Ott et al 2012;Warren et al 2012;Huang et al 2012;McQuinn et al 2015). However, the lower mass disks tend to be more extended, slightly thinner, and have lower surface brightness compared to observations.…”

“…The intrinsic axis ratios are computed by fitting an ellipsoid to the stellar particle distribution (normalized by the ellipsoidal distance of the particles within this ellipsoid, Allgood et al 2006). The ellipsoids are centered around the center of mass defined by the stellar component and the axis ratios computed for particles within major axis length of a ∼ 1.4 kpc, but we find similar Weisz et al (2012) 185 galaxies from M , M B , SFR the Spitzer LVL survey (e.g., Dale et al 2009) and 11HUGS (Kennicutt et al 2008;Lee et al 2011) Huang et al (2012 229 low HI mass galaxies from M , M HI , SFR the ALFALFA survey (Giovanelli et al 2005) Ott et al (2012 VLA-ANGST survey: 35 galaxies from M B , SFR the ANGST survey (Dalcanton et al 2009) Warren et al (2012) 31 nearby low-mass galaxies taken from M B , SFR THINGS (Walter et al 2008) and the VLA-ANGST survey (Ott et al 2012) McQuinn et al (2015 12 galaxies from the SHIELD survey (Cannon et al 2011) Karachentsev et al (2004) all-sky catalog of basic optical and HI properties of Huang et al (2012), andMcQuinn et al (2015). Since the stellar mass grows during the simulations and that we do not include fresh gas inflow, the gas fraction of our simulated systems necessarily decreases.…”

“…However, sampled at an arbitrary point in time, the match is quite remarkable. Huang et al (2012) and McQuinn et al (2015) for the left panels, and in addition for the right panel from Ott et al (2012), Warren et al (2012), and Hunter & Elmegreen (2006). Note that the stellar mass grows during the simulations.…”

“…14 shows the SFR versus stellar mass M * for the systems discussed in this paper compared to observational values reported in Weisz et al (2012), Huang et al (2012), andMcQuinn et al (2015). For each simulation the evolution of the SFR and M * are plotted at four epochs in steps of 1 Gyr, from 1 Gyr after the start of the simulation.…”