Hydrogen Envelopes Around Low Luminosity Galaxies

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Session 108 -- Dwarf and Irregular Galaxies
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[108.06] Hydrogen Envelopes Around Low Luminosity Galaxies

L.van Zee, M.P.Haynes, R.Giovanelli (Cornell Univ.)

Prototypical ``young'' galaxies such as IZw18 and 1225+01 are distinguished not only by their low metal abundances but also by the large hydrogen envelopes surrounding the compact optical galaxy. IZw18 and 1225+01 are further distinguished by extremely high M$_H$/L$_B$ ratios. The existence of objects such as 1225+01 suggests that some low mass systems may only be optically detectable for short intervals of time, when they are undergoing bursts of star formation. Since objects still in the stage of collapse might retain their nascent HI envelopes, we have undertaken a survey to identify potentially young objects via HI mapping observations of a sample of compact emission-line galaxies and galaxies with high M$_H$/L. HI mapping observations were undertaken between July 1992 and July 1993 with the Arecibo 305m telescope. Of an initial sample of 78 candidate galaxies, 49 were found to have extended HI envelopes (integrated HI flux one beamwidth from center $>$ 5\% of integrated flux from the optical center). Most of the emission-line galaxies are compact in both optical light and in HI. Only 16 of the 41 were detected one beamwidth (3.9 arcmin) from the optical center. In contrast, 29 of the 33 galaxies with an M$_H$/L $>$ 10 had some evidence of extended hydrogen. The sample of galaxies with extended HI envelopes will be studied further to determine if these galaxies are truly ``young'' galaxies with collapse times on the order of the Hubble time. Further studies will include broad band imaging to determine the age of the stellar populations and optical spectroscopy to determine metal abundances of the HII regions.

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