The ALFALFA Almost-Dark Galaxy AGC~229101: A Two Billion Solar Mass HI Cloud with a Very Low Surface Brightness Optical Counterpart

Abstract

We present results from deep HI and optical imaging of AGC 229101, an unusual HI source detected at v${}_{\mathrm{helio}}$ = 7116 km/s in the ALFALFA survey. Initially classified as a candidate "dark" source because it lacks a clear optical counterpart in SDSS or DSS2 imaging, AGC 229101 has $10^{9.31\pm 0.05}$ solar masses of HI, but an HI line width of only 43$\pm$9 km/s. Low resolution WSRT imaging and higher resolution VLA B-array imaging show that the source is significantly elongated, stretching over a projected length of ~80 kpc. The HI imaging resolves the source into two parts of roughly equal mass. WIYN pODI optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the HI. The peak surface brightness of the optical source is only ${\mu }_g$ = 26.6 mag arcsec${}^{-2}$, well below the typical cutoff that defines the isophotal edge of a galaxy, and its estimated stellar mass is only $10^{7.32\pm 0.33}$ solar masses, yielding an overall neutral gas-to-stellar mass ratio of M${}_{\mathrm{HI}}$/M${}_{\ast }=$~98$_{+111}\atop^{-52}$. We demonstrate the extreme nature of this object by comparing its properties to those of other HI-rich sources in ALFALFA and the literature. We also explore potential scenarios that might explain the existence of AGC~229101, including a tidal encounter with neighboring objects and a merger of two dark HI clouds.

arXiv comment18 pages, 8 figures, accepted by AJ

References in corpus (54)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• The physical properties of star forming galaxies in the low redshift universe
• The Optical and Near-Infrared Properties of Galaxies: I. Luminosity and Stellar Mass Functions
• SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way Galaxy, and Extra-Solar Planetary Systems
• UV Star Formation Rates in the Local Universe
• The Arecibo Legacy Fast ALFA Survey: I. Science Goals, Survey Design and Strategy
• The Arecibo Legacy Fast ALFA Survey: The alpha.40 HI Source Catalog, its Characteristics and their Impact on the Derivation of the HI Mass Function
• The SDSS View of the Palomar-Green Bright Quasar Survey
• Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster
• Multi-Scale CLEAN deconvolution of radio synthesis images
• The Arecibo Legacy Fast ALFA Survey: The ALFALFA Extragalactic HI Source Catalog
• The GALEX Arecibo SDSS Survey. I. Gas Fraction Scaling Relations of Massive Galaxies and First Data Release
• The Absolute Magnitude of the Sun in Several Filters
• The Atlas3D project -- XIII. Mass and morphology of HI in early-type galaxies as a function of environment
• The Arecibo Legacy Fast ALFA Survey: The Galaxy Population Detected by ALFALFA
• NIHAO XI: Formation of Ultra-Diffuse Galaxies by outflows
• Ultra-diffuse galaxies: the high-spin tail of the abundant dwarf galaxy population
• (Almost) Dark Galaxies in the ALFALFA Survey: Isolated HI Bearing Ultra Diffuse Galaxies
• Ultra diffuse galaxies outside clusters: clues to their formation and evolution
• The EAGLE simulations: atomic hydrogen associated with galaxies
• From Tidal Dwarf Galaxies to Satellite Galaxies
• Off the baryonic Tully-Fisher relation: a population of baryon-dominated ultra-diffuse galaxies
• Uncovering Additional Clues to Galaxy Evolution. I. Dwarf Irregulars in the Field
• Arecibo HI Mapping of a Large Sample of Dwarf Irregular Galaxies
• Robust HI kinematics of gas-rich ultra-diffuse galaxies: hints of a weak-feedback formation scenario
• A Study in Blue: The Baryon Content of Isolated Low Mass Galaxies
• The Bluedisks project, a study of unusually HI-rich galaxies: I. HI Sizes and Morphology
• A Catalog of Ultra-compact High Velocity Clouds from the ALFALFA Survey: Local Group Galaxy Candidates?
• The ALFALFA "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-to-Light Ratio Systems
• (Almost) Dark HI Sources in the ALFALFA Survey: The Intriguing Case of HI1232+20
• The evolution of ultra-diffuse galaxies in nearby galaxy clusters from the Kapteyn IAC WEAVE INT Clusters Survey ($\text{KIWICS}$)
• The abundance of dark galaxies
• On Star Formation and the Non-Existence of Dark Galaxies
• Neutral hydrogen in dwarf galaxies I. The spatial distribution of HI
• Are Newly Discovered HI High Velocity Clouds Minihalos in the Local Group?
• Mass-to-Light versus Color Relations for Dwarf Irregular Galaxies
• A Study of Two Diffuse Dwarf Galaxies in the Field
• The environment of HI-bearing ultra diffuse galaxies in the ALFALFA survey
• Self-consistent Color-Mass-to-Light-Ratio relations for Low Surface Brightness Galaxies
• ALFALFA and WSRT Imaging of Extended HI Features in the Leo Cloud of Galaxies
• The frequency and properties of young tidal dwarf galaxies in nearby gas-rich groups
• An extremely optically dim tidal feature in the gas-rich interacting galaxy group NGC 871/NGC 876/NGC 877
• The Enigmatic (Almost) Dark Galaxy Coma P: Distance Measurement and Stellar Populations from HST Imaging
• Dark influences III. Structural characterization of minor mergers of dwarf galaxies with dark satellites
• The Enigmatic (Almost) Dark Galaxy Coma P: The Atomic Interstellar Medium
• Five Gas-rich Ultra-faint Dwarf Galaxy Candidates Discovered in WIYN Imaging of ALFALFA Sources
• Dark influences II: gas and star formation in minor mergers of dwarf galaxies with dark satellites
• Robust automatic photometry of local galaxies from SDSS Dissecting the color magnitude relation with color profiles
• Relationship between Hubble type and spectroscopic class in local galaxies
• Updated Analysis of a "Dark" Galaxy and its Blue Companion in the Virgo Cloud HI 1225+01
• Identifying OH Imposters in the ALFALFA Neutral Hydrogen Survey
• The Distance to the Galaxy Coma P
• VLA Imaging of HI-bearing Ultra-Diffuse Galaxies from the ALFALFA Survey

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• Not so-dark: High resolution HI imaging of J0139+4328 and identification of an optical counterpart
• Unveiling the nature and fate of the almost-dark cloud AGC 226178 through HI mapping
• HI-bearing dark galaxies predictions from constrained Local Group simulations: how many and where to find them
• Pearls on a string: Dark and bright galaxies on a strikingly straight and narrow filament
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