Rachel Ann Pildis

Gas-rich Dwarfs from the Second Palomar Sky Survey. I. Catalog and Characteristics

This project is a visual search for field dwarf galaxies using Second Palomar Sky Survey photographic plates. A morphologically selected sample of 310 objects yielded 145 detections of true dwarfs within a redshift search window of 0 to 10,000 km s-1. We confirm the low-mass, dwarf nature of the same by comparison of luminosity, isophotal size, H I mass and H I profile width distribution of other dwarf samples. The goal of this project is to use these newly discovered dwarf galaxies to map large-scale structure as a test of biased galaxy formation. Initial indicators are that the large-scale distribution of dwarf galaxies is identical to that of bright, high-mass galaxies, in contradiction to theory using biasing. The full analysis of the sample will be reported in the final paper of our series.

Gas-rich Dwarf Galaxies from the Second Palomar Sky Survey. II. Optical Properties

We describe the optical properties of a sample of 101 gas-rich field dwarf galaxies found on Second Palomar Sky Survey plates, most newly discovered as part of a survey to investigate the clustering properties of dwarf galaxies relative to giants. These galaxies have low surface brightnesses and are relatively distant, with recession velocities ranging up to 104 km s-1. They have bluer V-I colors (median value of 0.75) than either actively star-forming giant galaxies or low-metallicity globular clusters, implying that these dwarfs have both low metallicities and little past star formation. These galaxies are also extremely gas-rich, with a median H I mass-to-V luminosity ratio of approximately 2 in solar units. We divide the sample into two groups: true dwarfs with diameters (at 25 I mag arcsec-2) less than 7.5 kpc and Magellanic dwarfs with diameters greater than that value. The true dwarfs have greater H I mass-to-V luminosity ratios and slightly bluer V-I colors than the Magellanic dwarfs. Overall, the optical properties of our sample of dwarf galaxies point toward their being quiescent objects that have undergone little star formation over the age of the universe. They are not faded objects but instead may be going through one of their first periods of weak star formation.

Deep Optical Observations of Compact Groups of Galaxies

Compact groups of galaxies appear to be extremely dense, making them likely sites of intense galaxy interaction, while their small populations make them relatively simple to analyze. In order to search for optical interaction tracers such as diffuse light and galaxy tidal features in Hickson compact groups (HCGs), we carried out deep photometry in three filters on a sample of HCGs with

Properties of Simulated Compact Groups of Galaxies

ROSAT

Could a Local Group X-Ray Halo Affect the X-Ray and Microwave Backgrounds?

observations. Using a modeling procedure to subtract the light of bright early-type galaxies, we found shell systems and extended envelopes around many, but not all, of those galaxies. Only one group in our sample, HCG 94, has diffuse light in the group potential (with a luminosity of 7 L

The Environments of Dwarf Irregular Galaxies

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Deep X-Ray and Optical Observations of Hickson Compact Groups

); the other groups do not contain more than 1/3 L

A Test of Biased Galaxy Formation from PSS-II Dwarf Galaxies

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Optical and X-Ray Observations of Extraplanar Gas in the Edge-On Spiral Galaxy NGC 891

in diffuse light. With the exception of HCG 94 (which is the most X-ray--luminous HCG), we found no correlation between the presence of shells or other tidal features and the X-ray luminosity of a group. Better predictors of detectable group X-ray emission are a low spiral fraction and belonging to a larger galaxy condensation---neither of which are correlated with optical disturbances in the group galaxies. Two elliptical galaxies that are extremely optically luminous but X-ray--faint are found to have shells and very complex color structures. This is likely due to recent infall of gas-rich material into the galaxies, which would produce both the disruption of stellar orbits and a significant amount of star formation.

The Neutral Hydrogen Content of Dwarf Irregular Galaxies

We analyze compact groups of galaxies appearing in a galaxy formation simulation dominated by cold dark matter (0=1, baryon=0.1). The simulation uses an N-body code to model the behavior of the non-baryonic matter and smoothed particle hydrodynamics to model the baryons. One run includes gas dynamics alone, and the other incorporates star formation as well. Groups identified as physically compact at z=0 form originally along filaments and become compact in the final ∼20% of the simulation; they contain x-ray– luminous diffuse gas well before becoming compact. The component masses, baryon fractions, and gas-to-galaxy mass fractions of the simulated groups are roughly similar to those of Hickson compact groups (HCGs), although they tend to be somewhat more gas-rich and have x-ray temperatures a factor of 3 lower than those seen in HCGs. These discrepancies may be alleviated by adding the effects of energy input into the diffuse group gas by star formation and supernovae.