Daniel G. Fabricant

Spectrophotometry of Nearby Field Galaxies: The Data

We have obtained integrated and nuclear spectra as well as U, B, R surface photometry for a representative sample of 196 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (MB = -14 to -22). Here we present the spectrophotometry for these galaxies. The selection of the sample and the U, B, R surface photometry is described in a companion paper. Our goals for the project include measuring the current star formation rates and metallicities of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high redshift. We describe the observing, data reduction, and calibration techniques and demonstrate that our spectrophotometry agrees well with that of Kennicutt. The spectra span the range 3550-7250 A at a resolution (FWHM) of ~6 A and have an overall relative spectrophotometric accuracy of ~±6%. We present a spectrophotometric atlas of integrated and nuclear rest-frame spectra as well as tables of equivalent widths and synthetic colors. The atlas and tables of measurements will be made available electronically. We study the correlations of galaxy properties determined from the spectra and images. Our findings include: (1) galaxies of a given morphological class display a wide range of continuum shapes and emission-line strengths if a broad range of luminosities are considered, (2) emission-line strengths tend to increase and continua tend to get bluer as the luminosity decreases, and (3) the scatter on the general correlation between nuclear and integrated Hα emission-line strengths is large.

Surface photometry of nearby field galaxies: The data

We have obtained integrated spectra and multifilter photometry for a representative sample of similar to 200 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (M(B) = -14 to -22) and colors (B-R = 0.4-1.8). Here we describe the sample selection criteria and the U, B, R surface photometry for these galaxies. The spectrophotometric results will be presented in a companion paper. Our goals for the project include measuring the current star formation rates and metallicity of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high red-shift. We discuss the observing, data reduction, and calibration techniques and show that our photometry agrees well with previous work in those cases in which earlier data are available. We present an atlas of images, radial surface brightness profiles, and color profiles as well as tables of derived parameters. The atlas and tables of measurements will be made available electronically. We study the correlations of galaxy properties determined from the galaxy images. Our findings include the following: (1) colors determined within the effective radius correlate better with morphological type than with M(B) and (2) 50% of the low-luminosity galaxies are bluest in their centers.

PHYSICAL SOURCES OF SCATTER IN THE TULLY-FISHER RELATION

We analyze residuals from the Tully-Fisher relation for the emission-line galaxies in the Nearby Field Gal- axy Survey, a broadly representative survey designed to fairly sample the variety of galaxy morphologies and environments in the local universe for luminosities from MB ¼� 15 to � 23. For a subsample consisting of the spiral galaxies brighter than M i R ¼� 18, we find strong correlations between Tully-Fisher residuals and both BR color and EW(H� ). The extremes of the correlations are populated by Sa galaxies, which show consistently red colors, and spiral galaxies with morphological peculiarities, which are often blue. If we apply an EW(H� )-dependent or BR color-dependent correction term to the Tully-Fisher relation, the scatter in the relation no longer increases from R to B to U but instead drops to a nearly constant level in all bands, close to the scatter we expect from measurement errors. We argue that these results probably reflect corre- lated offsets in luminosity and color as a function of star formation history. Broadening the sample in mor- phology and luminosity, we find that most nonspiral galaxies brighter than M i ¼� 18 follow the same correlations between Tully-Fisher residuals and BR color and EW(H� ) as do spirals, albeit with greater scatter. However, the color and EW(H� ) correlations do not apply to galaxies fainter than M i R ¼� 18 or to emission-line S0 galaxies with anomalous gas kinematics. For the dwarf galaxy population, the parameters controlling Tully-Fisher residuals are instead related to the degree of recent evolutionary disturbance: overlu- minous dwarfs have higher rotation curve asymmetries, brighter U-band effective surface brightnesses, and shorter gas consumption timescales than their underluminous counterparts. As a result, sample selection strongly affects the measured faint-end slope of the Tully-Fisher relation, and a sample limited to include only passively evolving, rotationally supported galaxies displays a break toward steeper slope at low luminosities.

A broad search for counterrotating gas and stars: Evidence for mergers and accretion

We measure the frequency of bulk gas-stellar counterrotation in a sample of 67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative survey of the local galaxy population down to MB ~ -15. We detect four counterrotators among 17 E/S0s with extended gas emission (24 %). In contrast, we find no clear examples of bulk counterrotation among 38 Sa–Sbc spirals, although one Sa does show peculiar gas kinematics. This result implies that, at 95% confidence, no more than 8% of Sa–Sbc spirals are bulk counterrotators. Among types Sc and later, we identify only one possible counterrotator, a Magellanic irregular. We use these results together with the physical properties of the counterrotators to constrain possible origins for this phenomenon.