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1. ALFALFA Harvest: HI Imaging of Candidate Local Group Dwarf Galaxies

   Paine, S.; Bralts-Kelly, L.; Adams, E.; Cannon, J.; Giovanelli, R.; Haynes, M.; Janesh, W.; Janowiecki, S.; Oosterloo, T.; Rhode, K.; et al (January 2020, American Astronomical Society meeting)

   The ALFALFA blind extragalactic survey has populated the faint end of the neutral hydrogen (HI) mass function with statistical confidence for the first time. Of particular interest is a subset of the ALFALFA detections, termed "ultra-compact high-velocity clouds" (UCHVCs). These systems, if located within ~1 Mpc, would populate the lowest-mass end of the HI mass function. Subsequent optical imaging has revealed that some of these UCHVCs harbor associated (though sparse) stellar populations, revealing that they may be some of the most extreme galaxies known in the Local Volume, with optical properties akin to ultra-faint dwarf galaxies but with significant neutral gas reservoirs. In this campaign, we investigate the neutral hydrogen properties of six UCHVC candidate galaxies using deep VLA HI spectral line imaging. A companion poster (Bralts-Kelly et al.) presents 3D kinematic modeling of selected sources. Here, we show the imaging products and discuss the morphological and kinematic properties of the six chosen sources: AGC 198606, AGC 215417, AGC219656, AGC 249525, AGC 258237, and AGC 268069. 

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2. ALFALFA Harvest: 3D Modeling of HI-Rich Candidate Local Group Dwarf Galaxies

   Bralts-Kelly, L.; Paine, S.; Adams, E. A.; Cannon, J. M.; Giovanelli, R.; Haynes, M. P.; Janesh, W.; Janowiecki, S.; Oosterloo, T.; Rhode, K. L.; et al (January 2020, American Astronomical Society meeting)

   The ALFALFA blind extragalactic survey has populated the faint end of the neutral hydrogen (HI) mass function with statistical confidence for the first time. Of particular interest is a subset of the ALFALFA detections, termed "ultra-compact high-velocity clouds" (UCHVCs). These systems, if located within ~1 Mpc, would populate the lowest-mass end of the HI mass function. Subsequent optical imaging has revealed that some of these UCHVCs harbor associated (though sparse) stellar populations, revealing that they may be some of the most extreme galaxies known in the Local Volume, with optical properties akin to ultra-faint dwarf galaxies but with significant neutral gas reservoirs. In this campaign, we investigate the neutral hydrogen properties of six UCHVC candidate galaxies using deep VLA HI spectral line imaging. A companion poster (Paine et al.) presents details on the data reduction, imaging, and resulting products. Here, we examine the morphological and kinematic properties of selected sources. We apply the modeling software 3D-Barolo to our deep HI images in order to derive the rotation curve and constrain the inclination angle for each source. Successful modeling allows us to determine the dynamical masses of these objects and thus to consider them in the context of various fundamental scaling relations defined by more massive galaxies. 

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3. Catching Tidal Dwarf Galaxies at a Later Evolutionary Stage with ALFALFA

   https://doi.org/10.3847/1538-3881/acc4c6  

   Gray, Laurin M.; Rhode, Katherine L.; Leisman, Lukas; Mancera Piña, Pavel E.; Cannon, John M.; Salzer, John J.; Gault, Lexi; Fuson, Jackson; Józsa, Gyula I.; Adams, Elizabeth A.; et al (April 2023, The Astronomical Journal)

   Abstract We present deep optical imaging and photometry of four objects classified as “Almost-Dark” galaxies in the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have H i masses of 10 7 –10 9 M ⊙ and distances of ∼9–100 Mpc. Observations with the WIYN 3.5 m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of ∼24–25 mag arcsec − 2 in the g band. We also present the results of H i synthesis observations with the Westerbork Synthesis Radio Telescope. These Almost-Dark galaxies have been identified as possible tidal dwarf galaxies (TDGs) based on their proximity to one or more massive galaxies. We demonstrate that AGC 229398 and AGC 333576 likely have the low dark matter content and large effective radii representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. AGC 219369 is likely dark matter dominated, while AGC 123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDG candidates such as a traditional major merger scenario and gas ejection from a high-velocity flyby. Blind H i surveys like ALFALFA enable the detection of gas-rich, optically faint TDGs that can be overlooked in other surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle. 

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4. The ALFALFA Almost Dark Galaxy AGC 229101: A 2 Billion Solar Mass H i Cloud with a Very Low Surface Brightness Optical Counterpart

   https://doi.org/10.3847/1538-3881/ac2a38  

   Leisman, Lukas; Rhode, Katherine L.; Ball, Catherine; Pagel, Hannah J.; Cannon, John M.; Salzer, John J.; Janowiecki, Steven; Janesh, William F.; Józsa, Gyula I.; Giovanelli, Riccardo; et al (December 2021, The Astronomical Journal)

   Abstract We present results from deep H i and optical imaging of AGC 229101, an unusual H i source detected at v helio =7116 km s −1 in the Arecibo Legacy Fast ALFA (ALFALFA) blind H i survey. Initially classified as a candidate “dark” source because it lacks a clear optical counterpart in Sloan Digital Sky Survey (SDSS) or Digitized Sky Survey 2 (DSS2) imaging, AGC 229101 has 10 9.31±0.05 M ⊙ of H i , but an H i line width of only 43 ± 9 km s −1 . Low-resolution Westerbork Synthesis Radio Telescope (WSRT) imaging and higher-resolution Very Large Array (VLA) B-array imaging show that the source is significantly elongated, stretching over a projected length of ∼80 kpc. The H i imaging resolves the source into two parts of roughly equal mass. WIYN partially populated One Degree Imager (pODI) optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the H i . The peak surface brightness of the optical source is only μ 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±0.33 M ⊙ , yielding an overall neutral gas-to-stellar mass ratio of M / M * = 98 − 52 + 111 . We demonstrate the extreme nature of this object by comparing its properties with those of other H i -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 H i clouds. 

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5. Hubble Space Telescope Imaging of Isolated Local Volume Dwarfs GALFA Dw3 and Dw4

   https://doi.org/10.3847/1538-4357/ac356c  

   Bennet, P.; Sand, D. J.; Crnojević, D.; Weisz, D. R.; Caldwell, N.; Guhathakurta, P.; Hargis, J. R.; Karunakaran, A.; Mutlu-Pakdil, B.; Olszewski, E.; et al (January 2022, The Astrophysical Journal)

   Abstract We present observations of the dwarf galaxies GALFA Dw3 and GALFA Dw4 with the Advanced Camera for Surveys on the Hubble Space Telescope. These galaxies were initially discovered as optical counterparts to compact H i clouds in the GALFA survey. Both objects resolve into stellar populations which display old red giant branch (RGB), younger helium-burning, and massive main sequence stars. We use the tip of the RGB method to determine the distance to each galaxy, finding distances of 7.61 − 0.29 + 0.28 Mpc and 3.10 − 0.17 + 0.16 Mpc, respectively. With these distances we show that both galaxies are extremely isolated, with no other confirmed objects within ∼1.5 Mpc of either dwarf. GALFA Dw4 is also found to be unusually compact for a galaxy of its luminosity. GALFA Dw3 and Dw4 contain H ii regions with young star clusters and an overall irregular morphology; they show evidence of ongoing star formation through both ultraviolet and H α observations and are therefore classified as dwarf irregulars (dIrrs). The star formation histories of these two dwarfs show distinct differences: Dw3 shows signs of a recently ceased episode of active star formation across the entire dwarf, while Dw4 shows some evidence for current star formation in spatially limited H ii regions. Compact H i sources offer a promising method for identifying isolated field dwarfs in the Local Volume, including GALFA Dw3 and Dw4, with the potential to shed light on the driving mechanisms of dwarf galaxy formation and evolution. 

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