James M. Schombert

The Baryonic Tully-Fisher Relation

We explore the Tully-Fisher relation over five decades in stellar mass in galaxies with circular velocities ranging over 30 less, similarVc less, similar300 km s-1. We find a clear break in the optical Tully-Fisher relation: field galaxies with Vc less, similar90 km s-1 fall below the relation defined by brighter galaxies. These faint galaxies, however, are very rich in gas; adding in the gas mass and plotting the baryonic disk mass Md=M*+Mgas in place of luminosity restores the single linear relation. The Tully-Fisher relation thus appears fundamentally to be a relation between rotation velocity and total baryonic mass of the form Md~V4c.

THE BARYON CONTENT OF COSMIC STRUCTURES

We make an inventory of the baryonic and gravitating mass in structures ranging from the smallest galaxies to rich clusters of galaxies. We find that the fraction of baryons converted to stars reaches a maximum between M 500 = 1012 and 1013 M ☉, suggesting that star formation is most efficient in bright galaxies in groups. The fraction of baryons detected in all forms deviates monotonically from the cosmic baryon fraction as a function of mass. On the largest scales of clusters, most of the expected baryons are detected, while in the smallest dwarf galaxies, fewer than 1% are detected. Where these missing baryons reside is unclear.

Color Evolution from z=0 to z=1

Rest frame Stroemgren photometry (3500 A, 4100 A, 4750 A, and 5500 A) is presented for 509 galaxies in 17 rich clusters between z = 0 and z = 1 as a test of color evolution. Our observations confirm a strong, rest frame, Butcher-Oemler effect where the fraction of blue galaxies increases from 20% at z = 0.4 to 80% at z = 0.9. We also find that a majority of these blue cluster galaxies are composed of normal disk or post-starburst systems based on color criteria. When comparing our colors to the morphological results from Hubble Space Telescope HST imaging, we propose that the blue cluster galaxies are a population of late-type, low surface brightness objects which fade and are then destroyed by the cluster tidal field. After isolating the red objects from Butcher-Oemler objects, we have compared the mean color of these old, non-star-forming objects with spectral energy distribution models in the literature as a test for passive galaxy evolution in ellipticals. We find good agreement with single-burst models which predict a mean epoch of galaxy formation at z = 5. Tracing the red envelope for ellipticals places the earliest epoch of galaxy formation at z = 10.

A CATALOG OF LOW SURFACE BRIGHTNESS GALAXIES - LIST-II

A list of galaxies characterized by low surface brightness (LSB) is presented which facilitates the recognition of galaxies with brightnesses close to that of the sky. A total of 198 objects and 140 objects are listed in the primary and secondary catalogs respectively, and LSB galaxies are examined by means of H I redshift distributions. LSB disk galaxies are shown to have similar sizes and masses as the high-surface-brightness counterparts, and ellipticals and SOs are rarely encountered. Many LSB spirals have stellarlike nuclei, and most of the galaxies in the present catalog are late-type galaxies in the Sc, Sm, and Im classes. The LSB region of observational parameter space is shown to encompass a spectrum of types as full as that of the Hubble sequence. It is suggested that studies of LSB galaxies can provide important data regarding the formation and star-formation history of all galaxies.

Radial Acceleration Relation in Rotationally Supported Galaxies

We report a correlation between the radial acceleration traced by rotation curves and that predicted by the observed distribution of baryons. The same relation is followed by 2693 points in 153 galaxies with very different morphologies, masses, sizes, and gas fractions. The correlation persists even when dark matter dominates. Consequently, the dark matter contribution is fully specified by that of the baryons. The observed scatter is small and largely dominated by observational uncertainties. This radial acceleration relation is tantamount to a natural law for rotating galaxies.

The structure of brightest cluster members. III: cD envelopes

Single-band photographic and CCD surface photometry is used to determine luminosity and structural properties of the extended, faint envelopes around 27 cD galaxies. The distribution of envelope light follows a similar slope as other cluster luminous material. Several correlations between envelope luminosity and cluster properties substantiate arguments for the dynamical formation of envelopes. A correlation between underlying galaxy luminosity and envelope luminosity implies a parallel but separate process between the growth of the first-ranked elliptical by mergers and cD envelope formation. Strong color gradients, in particular the blue disk colors as predicted by other studies, were not found upon examination of multicolor profiles for three cD envelopes. 50 references.

The structure of brightest cluster members. II - Mergers

Surface photometry of 342 bright elliptical galaxies in 103 clusters is analyzed for evidence of mergers. Structural differences between brightest cluster members (BCMs) and normal ellipticals can be summarized as having enlarged characteristic radii and shallow profile slopes (beta greater than -1.7). Profile morphology criteria for the elliptical types gE, D, and cD are outlined. Comparison of observations with numerical simulations of mergers strongly suggests a past history of dynamical growth for BCMs. Weak correlations of global cluster properties to BCMs supports the hypothesis proposed by Merritt (1984) that mergers are important in early subgroups before virialization and formation of a cluster identity. 65 references.

The Rotation Velocity Attributable to Dark Matter at Intermediate Radii in Disk Galaxies

We examine the amplitude of the rotation velocity that can be attributed to the dark matter halos of disk galaxies, focusing on well-measured intermediate radii. The data for 60 galaxies spanning a large range of mass and Hubble types, taken together, are consistent with a dark halo velocity log Vh = + log R with = 1.47 and ≈ over the range of 1 1.6. This problem is not specific to a particular type of galaxy or to the innermost region of the halo (cusp or core); the velocity attributable to dark matter is too low at all radii.

The Morphology of Low Surface Brightness Disk Galaxies

Images of a sample of low surface brightness (LSB) disk galaxies are presented. These galaxies are generally late types; however, they are not dwarfs, being intrinsically large and luminous. The morphology of LSB galaxies is discussed in terms of the physical interpretation of the Hubble sequence, the stages of which are found to be nonlinear in the sense that smaller physical differences separate mid to early type spirals than late types.

Weighing Galaxy Disks With the Baryonic Tully?Fisher Relation

We estimate the stellar masses of disk galaxies with two independent methods: a photometrically self-consistent color