arXiv subjects

Stacy McGaugh

Publications and source records attributed to Stacy McGaugh.

93 records · Page 6Linked to original sources

arXiv1993

A Possible Local Counterpart to the Excess Population of Faint Blue Galaxies

OBSERVATIONS of galaxies to very faint magnitudes have revealed a population of blue galaxies at intermediate redshift$^{1-5}$. These galaxies represent a significant excess over the expectation of standard cosmological models for reasonable amounts of evolution of the locally observed galaxy population. However, the surveys which define the local galaxy population are strongly biased against objects of low surface brightness$^{6-9}$. Low surface brightness galaxies have properties very similar to those of the excess blue population$^{10,11}$, and recent work suggests that they are comparable in abundance to the more readily detected normal galaxies$^{9,12}$. I show that the very deep surveys which reveal the excess population can easily detect low surface brightness galaxies to large redshifts, but that local surveys will miss them because they are not comparably sensitive. This suggests that the excess faint galaxies {\it are\/} low surface brightness galaxies. No alteration of standard cosmology is required, but it is necessary to reconsider the way in which the galaxy distribution function is specified.

astro-ph↗

arXiv1993

Oxygen Abundances in Low Surface Brightness Disk Galaxies

The oxygen abundances in the \HII regions of a sample of low surface brightness (LSB) disk galaxies are presented. In general, LSB galaxies are found to be metal poor ($Z < {1 \over 3} Z\solar$). Indeed, some LSB galaxies rival the lowest abundance extragalactic objects known. These low metallicities indicate that LSB galaxies evolve slowly, forming relatively few stars over a Hubble time. The low metallicities of LSB galaxies occur even though many are comparable in size and mass to the prominent spirals which define the Hubble sequence. As well as being low in surface brightness, these galaxies tend to be isolated. This suggests that surface mass density and environment are more relevant to galaxy evolution than gross size. Despite the low surface brightness of the disks, massive ($M > 60 M\solar$) stars are inferred to be present and no abnormality of the IMF is indicated. Many low excitation \HII regions exist at low metallicity in LSB galaxies, and the ionization parameter is not tightly correlated with metallicity. However, there does seem to be a significant envelope of maximum ionization at a given metallicity.

astro-ph↗

arXiv1993

Structural Characteristics and Stellar Composition of Low Surface Brightness Disk Galaxies

We present UBVI surface photometry of a sample of low surface brightness (LSB) disk galaxies. LSB disk galaxies are fairly well described as exponential disks with no preferred value for either scale length, central surface brightness, or rotational velocity. Indeed, the distribution of scale lengths is indistinguishable from that of high surface brightness spirals, indicating that dynamically similar galaxies (e.g., those with comparable Rv^2) exist over a large range in surface density. These LSB galaxies are strikingly blue. The complete lack of correlation between central surface brightness and color rules out any fading scenario. Similarly, the oxygen abundances inferred from HII region spectra are uncorrelated with color so the low metallicities are not the primary cause of the blue colors. While these are difficult to interpret in the absence of significant star formation, the most plausible scenario is a stellar population with a young mean age stemming from late formation and subsequent slow evolution. These properties suggest that LSB disks formed from low initial overdensities with correspondingly late collapse times.

astro-ph↗