The Enigmatic (Almost) Dark Galaxy Coma P: The Atomic Interstellar Medium

Ball, Catherine; Cannon, John; Adams, Elizabeth A. K.; Haynes, Martha P.; Józsa, G. I. G.; McQuinn, Kristen B. W.; Salzer, John J.; Brunker, Samantha W.; Giovanelli, Riccardo; Hallenbeck, Gregory; Janesh, William; Janowiecki, Steven; Jones, M. G. K.; Rhode, Katherine L.

doi:10.3847/1538-3881/aaa156

Cited by 23 publications

(32 citation statements)

References 37 publications

(76 reference statements)

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“…BST1047 shares many of the properties of field LSBs, but taken to extremes. For example, the most diffuse LSB in the "(Almost) Dark" sample is Coma P, with peak surface brightness µ B ≈ 26.6 mag arcsec −2 , B − V =0.13, and gas fraction f g = 0.97 (Janowiecki et al 2015;Ball et al 2018). BST1047 is similar in optical size and color to Coma P, but an order of magnitude lower in luminosity density.…”

Section: Discussionmentioning

confidence: 99%

BST1047+1156: An Extremely Diffuse and Gas-rich Object in the Leo I Group

Mihos

Carr

Watkins

et al. 2018

ApJL

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We report the detection of diffuse starlight in an extragalactic HI cloud in the nearby Leo I galaxy group. We detect the source, dubbed BST1047+1156, in both broadband optical and GALEX ultraviolet light. Spanning ∼ 2 kpc in radius, it has a peak surface brightness of µ B =28.8 mag arcsec −2 , making it the lowest surface brightness object ever detected via integrated light. Although the object is extremely gas-rich, with a gas fraction of f g = 0.99, its peak HI column density is well below levels where star formation is typically observed in galaxies. Nonetheless, BST1047+1156 shows evidence for young stellar populations: along with the detected UV emission, the object is extremely blue, with B − V = 0.14 ± 0.09. The object sports two tidal tails and is found embedded within diffuse gas connecting the spiral galaxy M96 to the group's extended HI Leo Ring. The nature of BST1047+1156 is unclear. It could be a disrupting tidal dwarf, recently spawned from star formation triggered in the Leo I Group's tidal debris. Alternatively, the object may have been a pre-existing galaxy -the most extreme example of a gas-rich field LSB known to date -which had a recent burst of star formation triggered by encounters in the group environment.

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Section: Discussionmentioning

confidence: 99%

BST1047+1156: An Extremely Diffuse and Gas-rich Object in the Leo I Group

Mihos

Carr

Watkins

et al. 2018

ApJL

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“…Our updated masses and luminosities are all dramatically lower than previously reported: Coma P is an extreme dwarf system. Although it is no longer such a severe outlier on galaxy scaling relations (e.g., the BTFR; Ball et al 2018), it is still an enigmatic system. For example, it has an H I mass to blue luminosity ratio M HI /L B of 28, much higher than the values found in more normal galaxies, and a hydrogen gas-to-star ratio M HI /M * of 81.…”

Section: Discussionmentioning

confidence: 99%

“…Furthermore, the larger distance increases the deviation of Coma P in galaxy scaling relations. For example, Coma P would lie a factor of ∼50 above the fiducial baryonic Tully−Fisher relation (BTFR; McGaugh 2012) if the 17.7 Mpc distance were adopted (see Figure 17 of Ball et al 2018). The current HST data do not make it possible to rule out the larger distance with certainty.…”

Section: Search For Additional Alternate Distancesmentioning

confidence: 99%

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The Enigmatic (Almost) Dark Galaxy Coma P: Distance Measurement and Stellar Populations from HST Imaging*

Brunker

McQuinn

Salzer

et al. 2019

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We present Hubble Space Telescope (HST) observations of the low surface brightness (SB) galaxy Coma P. This system was first discovered in the Arecibo Legacy Fast ALFA H I survey and was cataloged as an (almost) dark galaxy because it did not exhibit any obvious optical counterpart in the available survey data (e.g., Sloan Digital Sky Survey). Subsequent WIYN pODI imaging revealed an ultra-low SB stellar component located at the center of the H I detection. We use the HST images to produce a deep color-magnitude diagram (CMD) of the resolved stellar population present in Coma P. We clearly detect a red stellar sequence that we interpret to be a red giant branch, and use it to infer a tip of the red giant branch (TRGB) distance of 5.50 +0.28 −0.53 Mpc. The new distance is substantially lower than earlier estimates and shows that Coma P is an extreme dwarf galaxy. Our derived stellar mass is only 4.3 × 10 5 M , meaning that Coma P has an extreme H I-to-stellar mass ratio of 81. We present a detailed analysis of the galaxy environment within which Coma P resides. We hypothesize that Coma P formed within a local void and has spent most of its lifetime in a low-density environment. Over time, the gravitational attraction of the galaxies located in the void wall has moved it to the edge, where it had a recent "fly-by" interaction with M64. We investigate the possibility that Coma P is at a farther distance and conclude that the available data are best fit by a distance of 5.5 Mpc.

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“…This likely hampers a robust determination of their rotation curve and hence of their DM content. Stilp (2013) concluded that "rotation curves for the smallest dwarfs in our (VLA-ANGST) sample may be difficult to obtain due to their complex velocity fields" (see also Ball et al 2018 . This does not mean that there is no DM in the tiniest dIrrs, though it points out the need for a significantly more robust (3D) modeling of individual objects to disentangle effects due to star formation from those due to gravitation.…”

Section: What Are the Progenitors Of Dm-free Dsphs?mentioning

confidence: 97%

On the Absence of Dark Matter in Dwarf Galaxies Surrounding the Milky Way

Hammer

Yang

Wang

et al. 2019

ApJ

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This paper presents an alternative scenario to explain the observed properties of the Milky Way dwarf Spheroidals (MW dSphs). We show that instead of resulting from large amounts of dark matter (DM), the large velocity dispersions observed along their lines of sight (σ los ) can be entirely accounted for by dynamical heating of DM-free systems resulting from MW tidal shocks. Such a regime is expected if the progenitors of the MW dwarfs are infalling gas-dominated galaxies. In this case, gas lost through ram-pressure leads to a strong decrease of self-gravity, a phase during which stars can radially expand, while leaving a gas-free dSph in which tidal shocks can easily develop.The DM content of dSphs is widely derived from the measurement of the dSphs self-gravity acceleration projected along the line of sight. We show that the latter strongly anti-correlates with the dSph distance from the MW, and that it is matched in amplitude by the acceleration caused by MW tidal shocks on DM-free dSphs. If correct, this implies that the MW dSphs would have negligible DM content, putting in question, e.g., their use as targets for DM direct searches, or our understanding of the Local Group mass assembly history. Most of the progenitors of the MW dSphs are likely extremely tiny dIrrs, and deeper observations and more accurate modeling are necessary to infer their properties as well as to derive star formation histories of the faintest dSphs.

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The Enigmatic (Almost) Dark Galaxy Coma P: The Atomic Interstellar Medium

Cited by 23 publications

References 37 publications

BST1047+1156: An Extremely Diffuse and Gas-rich Object in the Leo I Group

BST1047+1156: An Extremely Diffuse and Gas-rich Object in the Leo I Group

The Enigmatic (Almost) Dark Galaxy Coma P: Distance Measurement and Stellar Populations from HST Imaging*

On the Absence of Dark Matter in Dwarf Galaxies Surrounding the Milky Way

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