The Dearth of Neutral Hydrogen in Galactic Dwarf Spheroidal Galaxies

Abstract

We present new upper limits on the neutral hydrogen (H I) content within the stellar half-light ellipses of 15 Galactic dwarf spheroidal galaxies (dSphs), derived from pointed observations with the Green Bank Telescope (GBT) as well as Arecibo L-band Fast ALFA (ALFALFA) survey and Galactic All-Sky Survey (GASS) data. All of the limits M lim HI are more stringent than previously reported values, and those from the GBT improve upon contraints in the literature by a median factor of 23.−5 , irrespective of location in the Galactic halo. Comparing these relative H I contents to those of the Local Group and nearby neighbor dwarfs compiled by McConnachie, we find that the Galactic dSphs are extremely gas-poor. Our H I upper limits therefore provide the clearest picture yet of the environmental dependence of the H I content in Local Volume dwarfs. If ram pressure stripping explains the dearth of H I in these systems, then orbits in a relatively massive Milky Way are favored for the outer halo dSph Leo I, while Leo II and Canes Venatici I have had a pericentric passage in the past. For Draco and Ursa Minor, the interstellar medium mass that should accumulate through stellar mass loss in between pericentric passages exceeds M lim HI by a factor of ∼ 30. In Ursa Minor, this implies that either this material is not in the atomic phase, or that another mechanism clears the recycled gas on shorter timescales.

A survey of HI gas toward the Andromeda galaxy

“…Figure 8 shows the EBHIS HI column density distribution toward And XIX and And XX. As reported by Grcevich & Putman (2009) and Spekkens et al (2014), there is a striking nondetection of HI gas toward Milky Way dwarf galaxies but also towardAndromeda's satellite galaxies. Using the EBHIS data we can confirm the non-detection and then consistently lower their limits partly by a factor of two to ten (Kerp et al, in prep.).…”

Neutral Gas Accretion onto Nearby Galaxies

“…For the dwarf galaxies currently known around the Milky Way, it amounts to M HI ≈ 3 × 10 7 M , an value almost identical to the estimated total H I mass of HVCs in each of the HVC systems of the Milky Way, M31, and M33 (Putman et al, 2012;Westmeier et al, 2008;Keenan et al, 2016). It is tempting to dismiss this as a coincidence, especially because the inferred mass of the "stripped" H I for the Milky Way is dominated by the most massive galaxy, the Sagittarius dSph (Spekkens et al, 2014). But we do not have a good understanding of the inflow rate and location of the stripped material, which likely eventually make its way to the disk.…”

“…Because dwarf galaxies > 400 kpc from the Milky Way seem to have retained their gas, the deficit for nearer dwarfs presumably reflects their stripping by the CGM (Blitz & Robishaw, 2000;Grcevich & Putman, 2009). Distant dwarfs have M HI /L * ≈ 1 (Spekkens et al, 2014), so we can estimate the H I mass that has been deposited in the CGM through this stripping. For the dwarf galaxies currently known around the Milky Way, it amounts to M HI ≈ 3 × 10 7 M , an value almost identical to the estimated total H I mass of HVCs in each of the HVC systems of the Milky Way, M31, and M33 (Putman et al, 2012;Westmeier et al, 2008;Keenan et al, 2016).…”

Under pressure: quenching star formation in low-mass satellite galaxies via stripping

“…fHI 0.136, Spekkens et al 2014); that is, ram pressure only quenches 15% of our low-mass satellite population, such that fHI < 0.136. Ultimately, to reproduce the observed HI gas fractions for satellites in the Local Group and thus the inferred satellite quenching timescales at low stellar masses, ram-pressure stripping would need to be substantially more efficient than our predictions (i.e.…”