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A. Jangren (PSU), M.A. Bershady (UW-Madison, PSU), D.P. Schneider (PSU)
We present the results of our analysis of a sample of luminous, compact, H\alpha-selected emission-line galaxies from the Palomar Transit Grism Survey (PTGS; Schneider, Schmidt, & Gunn 1995). As part of an ongoing study where the aim is to characterize the evolution of compact emission-line galaxies (ELGs) from z ~1 to the present, we derive their local space density, and construct a bivariate size-luminosity function. We also infer the contribution of luminous, compact ELGs to the comoving star-formation rate at low z, and compare our results to those of the UCM and CFRS surveys (Gallego et al. 1995; Tresse & Maddox 1998).
The galaxies in the PTGS sample were selected to have small physical sizes and comparatively high luminosities; they are thus similar to the sample of Compact Narrow Emission-Line Galaxies (CNELGs) that has been studied extensively. However, the PTGS galaxies lie at low redshift (0.05 < z <0.12), while the CNELGs lie at intermediate redshift (0.1 < z < 0.7). Based on existing long-slit spectra (resolution ~ 25 Å) we were able to exclude AGNs from the sample. Diagnostic line-ratio diagrams suggest that the majority of the PTGS sample galaxies are starburst or H II galaxies, characteristics that they share with the CNELGs.
High-resolution WIYN images of a subset of our sample allow us to explore the morphology of the galaxies. Several are interacting or merging pairs, others are disk-like objects; however, a large fraction have compact, near-stellar image structure. Compact galaxies like these have been observed out to z ~4 (Smail et al. 1998). Recent evidence indicates that the comoving number of luminous, small galaxies increases at intermediate redshifts, and persists (perhaps even increases) at high redshift. Ironically, we know little about the low-z comoving number. Our study helps address the space density at low redshift, thus allowing us to get a more definitive measure of the evolution in number of these systems.
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The author(s) of this abstract have provided an email address for comments about the abstract: jangren@astro.psu.edu