Optical Observations of Candidate Intergalactic HI Clouds

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Session 51 -- Dwarf Galaxies
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

[51.16] Optical Observations of Candidate Intergalactic HI Clouds

C.L. Taylor, E. Brinks (NRAO), E.D. Skillman (U. Minnesota)

We present new R-Band CCD observations of candidate extragalactic HI clouds discovered in a search for HI companions of HII galaxies (Taylor et al. 1994: ApJS submitted). These new data extend our optical detection threshold down to $\sim$ 25 mag~/~$\sq~\arcsec$. For one HI cloud there is a tentative optical counterpart, but no optical emission is detected from the other five clouds. Our detection thresholds are low enough such that if the HI clouds had stellar components comparable to the Local Group dwarf spheroidals, we would have detected them. We use our detection threshold to estimate upper limits in luminosity for the undetected objects and calculate lower limit mass-to-light ratios. Under very conservative assumptions we arrive at lower limits on M$_{HI}$/L$_{B}$ in the range from 10 to 34 and lower limits on M$_{tot}$/L$_{B}$ in the range from 34 to 370. These estimated lower limits on total mass-to-light ratios are comparable to the observed mass-to-light ratios for the Local Group dwarf spheroidals, which have the highest known mass-to-light ratios of any stellar systems. The optical surface brightness limits presented here imply that we may have discovered true intergalactic HI clouds, devoid of any stellar component. At the very least they certainly represent a new class of extragalactic objects.

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