Details

Autor(en) / Beteiligte

• Leisman, Lukas Ross

Titel

Almost Dark Galaxies in the ALFALFA Survey

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2017

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• Star formation laws in galaxies appear to be tied to their available reservoirs of neutral hydrogen (HI). However, gas rich sources that are unable to convert their HI into stars would be difficult to detect in optical surveys, potentially dipping below detectable surface brightness limits in the most extreme cases. Theoretical predictions have been mixed as to whether these potential "(almost) dark," HI rich sources should exist. Thus, one of the main scientific drivers behind blind HI surveys is the search for optically dark, or almost dark, but gas-bearing dark matter halos. The Arecibo Legacy Fast ALFA blind HI survey (ALFALFA) has detected over 30,000 clearly extragalactic sources, ∼99% of which can be readily identified with probable stellar counterparts in public SDSS or DSS2 images. Here we present the results of follow up efforts to understand the other ∼300 "dark" systems in ALFALFA, in a search for potential "almost dark" galaxies. We expect most optically "dark" ALFALFA sources to be HI clouds stripped from galaxies in tidal interactions: tidal interactions are well known to play an important role in galactic evolution in group environments. Indeed, we confirm that over half of the "dark" systems in ALFALFA have a nearby neighbor (projected within 100 kpc and at a similar redshift) or are a clear tidal appendage. However, in at least two cases we found that candidate "dark" sources far from their nearest neighbor, were in fact part of large HI plumes with extents exceeding 1.4 deg (∼600 kpc). These "dark" HI plumes constitute up to 40% of the total HI mass in their host groups, potentially implying significant undetected HI in other groups, and greater prevalence of group scale interactions than previously thought. These observations demonstrate the importance of sensitive, wide field HI surveys in understanding the role of groups in galaxy evolution. After removing tidal plumes, searching for (almost) dark galaxies requires extensive confirmation observations to remove spurious sources and OH Megamasers (the 18cm line from rare molecular masers in luminous infrared galaxies redshifted into the ALFALFA bandpass). Further, since Arecibo does not resolve sources with its ∼3.5' beam, determining their nature requires detailed optical and HI follow up observations. Through a series of HI observations with the Jansky Very Large Array (VLA) and the Westerbork Synthesis Radio Telescope (WSRT), and optical observations with the WIYN 3.5m Telescope at Kitt Peak National Observatory, we have imaged 25 of the best ALFALFA (almost) dark candidates. One early result of this imaging was the discovery of AGC 229385, a bright HI source (log (MHI/Msun) = 8.7 at D=25 Mpc), but dark at the surface brightness limit of SDSS. Deep imaging revealed a very blue optical counterpart, with a peak surface brightness of μg = 26.4 mag/arcsec2, an order of magnitude fainter than low surface brightness (LSB) galaxies detected in SDSS, placing it far off HI-stellar mass scaling relations. AGC 229385 has the largest accurately measured HI mass to light ratio of any known non-tidal, optically detected galaxy, MHI/LB = 38 Msun/ Lsun and appears to lie far off the Tully-Fisher relation. The HI and optical follow up observations also reveal that several of the "almost dark" sources are undetectable in current optical surveys not because they do not have stars, but because the stars are too spread out. These sources appear to be "ultra diffuse" galaxies (UDGs) - extreme galaxies with stellar masses of dwarf galaxies, but radii similar to galaxies as large as the Milky Way. Most previously detected UDGs are in clusters, without detected HI. We broaden our definition of "almost dark" to search for more UDGs in ALFALFA, and present a sample of ∼100 isolated ALFALFA galaxies that have similar stellar surface brightnesses, radii, and absolute magnitudes to optically selected UDGs in denser environments. Though these ALFALFA `"HI-bearing ultra-diffuse" galaxies have similarly large radii for their stellar mass, they differ from cluster UDGs in that they are richer in atomic gas, bluer, and have more irregular morphologies. We present follow up HI and optical imaging of three of these HI-bearing UDGs, and show significant diffuse optical emission, large HI disks extending well beyond the stellar emission to radii of nearly 20 kpc, and evidence of ordered rotation. Indeed, comparison of this rotation to halo models may suggest that these sources have halo masses more consistent with dwarf galaxies, and may suggest they formed in high angular momentum halos.