Gregory David Bothun

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.

Galaxy collisions and mergers - the genesis of very powerful radio sources

Results are reported from optical, long-slit interferometry, narrow-band imaging and VLA radiotelescope scans of 43 radio galaxies. The strongest radio galaxies (log P at 408 MHz exceeding 25.5 W/Hz for a Hubble parameter of 75 km/sec per MPC) displayed the most distinct optical morphologies, e.g., bridges, fans, dust features, etc. The sources also emitted continuum lines and other line features which were morphologically distinct from the source regions, conditions which suggest that collisions between galaxies occurred. Other characteristics of the strong radio galaxies are discussed, including lowered optical luminosities, reduced galactic densities, and an absence of edge-darkened radio morphologies. 101 references.

The Luminosity of SN 1999by in NGC 2841 and the Nature of “Peculiar” Type Ia Supernovae

We present UBVRIJHK photometry and optical spectroscopy of the so-called peculiar Type Ia supernova 1999by in NGC 2841. The observations began 1 week before visual maximum light, which is well defined by daily observations. The light curves and spectra are similar to those of the prototypical subluminous event SN 1991bg. We find that maximum light in B occurred on 1999 May 10.3 UT (JD 2,451,308.8 ± 0.3) with B = 13.66 ± 0.02 and a color of Bmax - Vmax = 0.51 ± 0.03. The late-time color implies minimal dust extinction from the host galaxy. Our photometry, when combined with the recent Cepheid distance to NGC 2841, gives a peak absolute magnitude of MB = -17.15 ± 0.23, making SN 1999by one of the least luminous Type Ia events ever observed. We estimate a decline rate parameter of Δm15(B) = 1.90 mag, versus 1.93 for SN 1991bg, for which 1.10 is typical for so-called normal events. We compare SN 1999by with other subluminous events and find that the Bmax - Vmax color correlates strongly with the decline rate and may be a more sensitive indicator of luminosity than the fading rate for these objects. We find a good correlation between luminosity and the depth of the spectral feature at 580 nm, which had been attributed solely to Si II. We show that in cooler photospheres the 580 nm feature is dominated by Ti II, which provides a simple physical explanation for the correlation. Using only subluminous Type Ia supernovae, we derive a Hubble parameter of H0 = 75 km s-1 Mpc-1, consistent with values found from brighter events.

Low surface brightness galaxies in the local universe .1. The catalog

Data are presented for 693 galaxies identified in a large new survey for low surface brightness galaxies in the nearby universe (z less than or similar to 0.1). The survey covers 786 square degrees centered on the equator, and it extends significantly the surface brightness range of galaxy surveys in which there are a substantial number of galaxies with redshifts. The data are derived from the Automated Plate Measuring machine scans of survey plates from the UK Schmidt Telescope and from follow-up observations at radio and optical wavelengths. Accurate positions, total B magnitudes, surface brightness parameters, and angular sizes are tabulated for each galaxy. Radial velocities, optical luminosities, and neutral hydrogen masses are listed for a subset of the sample. Finding charts are also presented for those objects having a large enough angular size that the scans from survey plates provide some morphological information. The selection function and the luminosity function that can be derived from the survey are discussed in two companion papers.

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.

Discovery of a huge low-surface-brightness galaxy - a protodisk galaxy at low redshift

The accidental discovery of an extremely large, extremely H I-rich low-surface-brightness galaxy located at a redshift of z = 0.083 is reported. Its nuclear spectrum exhibits broad, low-level emission lines. Surface photometry at V indicates the presence of a bulge component and a very extended disk, with scale length of about 45 arcsec and with central surface brightness of V(0) of about 25.5 mag/sq arcsec. The total amount of H I is at least 1.0 x 10 to the 11th solar masses. This amount of H I is at least 5 times more H I than any spiral galaxy previously observed. If disk formation is a quiescent process, then it is likely that a disk was caught in the process of formation. The properties of this disk are likely to be similar to the suspected sources that produce the observed damped Ly-alpha absorption profiles that are so conspicuous at z of about 2. 31 references.

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.

Modeling the spatial distribution of star formation in interacting disk galaxies

Models of star-forming interacting galaxies are combined with observational data to investigate the nature of star formation in interacting and merging disk galaxies. Detailed models of specific interacting systems are created using the observed morphology and kinematics of the system to constrain model parameters describing the interaction. The models employ an N-body code to calculate the gravitational dynamics, a discrete cloud model to govern the ISM dynamics, and a modified Schmidt law to describe star formation

Observations with the parking lot camera. I. Surface photometry and color distribution of the Magellanic Clouds

An optical system with a 6 x 10 deg field of view was used to obtain B, V, and R CCD images of the SMC and LMC. SMC is found to have a coherent connected pattern of very recent star formation and the overall properties of star-forming dwarf ellipticals in the Virgo Cluster. While the LMC shows a largely random pattern of star formation, some hierarchical clustering of different age regions is noted. Neither galaxy displays significant evidence for a globally large star-formation rate at the present epoch. It is suggested that if the mass distribution of the LMC was once exponential in form, beyond the 1/2 mass radius, approximately 30 percent of this mass has been lost. 24 references.

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.