The nature of Markarian galaxies and studies of star formation in blue galaxies

Citation

Huchra, John Peter (1977) The nature of Markarian galaxies and studies of star formation in blue galaxies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/F34C-J170. https://resolver.caltech.edu/CaltechETD:etd-09242008-110419

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.

Integrated UBV photometry is given for 197 non-Seyfert Markarian galaxies and is compared with existing photometry of field galaxies. The distributions of galaxy colors overlap by almost seventy-five percent, and the color-color relations are the same. The color-aperture relations derived from multi-aperture observations show that the Markarian galaxies generally get bluer towards their centers, unlike all but the latest types of field galaxies.

Thirteen binary galaxies with good radial velocity data are used to derive the mean minimum mass-to-light ratio for Markarian galaxies. It is the same as the mean minimum mass-to-light ratio for field binary galaxies.

All Markarian galaxies with m[subscript pg][...] 14.0 are morphologically classified in the modified Hubble sequence. The distibution of morphological types for Markarian galaxies is generally similar to that of field galaxies. The only significant difference is that the Markarian sample shows a factor of three excess of very late type galaxies. The Markarian galaxies are generally blue for their morphological type but fall within the distributions of colors for their field counterparts. The bluest types - Magellanic spirals and irregulars - extend 0.3 magnitude bluer in U-B than the bluest field galaxies.

The Markarian galaxy luminosity function is derived from the available data for all seven lists of objects. Markarian galaxies represent approximately one tenth of all galaxies with absolute magnitudes fainter than M[subscript pg] = -22 (H[subscript o] = 50 km sec[superscript -1] Mpc[superscript -1]). A crude estimate is made of the space density as a function of color for both Markarian and field galaxies. Markarian appears to be finding all galaxies with U-B < -0.3 plus a decreasing fraction of the redder galaxies.

For the Markarian galaxies with available data, there are strong correlations of the emission line strength, measured by Hβ equivalent width, with color and absolute magnitude. There are also weaker correlations of the excitation parameter I([O III]λ5007)/I(Hβ) with color.

The optical and radio data indicate that the non-Seyfert Markarian galaxies (85% of all Markarian objects) are not a new class of object but rather a subset of normal galaxies. Markarian selects the bluest galaxies and those galaxies which are blue for their morphological type.

The Markarian galaxy photometry and photometry of galaxies in the lists of Haro and Zwicky extend the observed galaxy two color distribution significantly bluewards. Existing evolutionary models of galaxy colors (Tinsley 1968, 1972; Searle, Sargent and Bagnuolo 1973) do not match the observed colors of these blue galaxies. Models of galaxies are constructed with a variety of star formation histories and initial mass functions. The models include the effects of emission by gas ionized by the hot stars in the galaxy.

Three general classes of models were studied. 'Old' galaxies are systems that are more than 10[superscript 10] years old. 'Young' galaxies are systems that are less than one tenth that age. 'Composite' galaxies are 'old' galaxies, with properties similar to ordinary spirals and ellipticals, plus a burst of recent star formation. Powerlaw initial mass functions were used.

In order to fit the observed colors, 'old' galaxies must have high mass star enriched initial mass functions and nearly constant rates of star formation. 'Young' galaxies must have large amounts of internal reddening. If their initial mass function is the Salpeter function, they must be less than 10[superscript 8] years old. 'Composite' galaxies must have bursts of star formation that contribute approximately twenty to fifty percent of the V band light.

Comparison of model predictions with the observed metal abundance, color-magnitude distribution, and internal reddening lead us to conclude that 'composite' models best fit the observed properties. There is no compelling evidence for the existence of 'young' galaxies - objects only recently condensed out of the intergalactic medium.

In addition, comparison of model predictions with the observed Hβ emission as a function of color in blue galaxies leads to the conclusion that some of these galaxies are forming stars with an initial mass function flatter than the Salpeter function.

Thesis Files