Published in ApJS, 132: 129-198, 2001


AN ULTRAVIOLET/OPTICAL ATLAS OF BRIGHT GALAXIES

Pamela M. Marcum1, Robert W. O'Connell2, Michael N. Fanelli 3,4, Robert H. Cornett4, William H. Waller4,5, Ralph C. Bohlin6, Susan G. Neff7, Morton S. Roberts8, Andrew M. Smith7, K.-P. Cheng9, Nicholas R. Collins4, Gregory S. Hennessy2,10, Jesse K. Hill4, Robert S. Hill4, Paul Hintzen11, Wayne B. Landsman4, Raymond G. Ohl2, Ronald A. Parise12, Eric P. Smith7, Wendy L. Freedman13, Leslie E. Kuchinski14, Barry Madore14, Ronald Angione15, Christopher Palma2, Freddie Talbert15, and Theodore P. Stecher7


Abstract. We present wide-field imagery and photometry of 43 selected nearby galaxies of all morphological types at ultraviolet and optical wavelengths. The ultraviolet (UV) images, in two broad bands at 1500 and 2500 Å, were obtained using the Ultraviolet Imaging Telescope (UIT) during the Astro-1 Spacelab mission. The UV images have ~ 3" resolution, and the comparison sets of ground-based CCD images (in one or more of B, V, R, and Halpha) have pixel scales and fields of view closely matching the UV frames. The atlas consists of multiband images and plots of UV/optical surface brightness and color profiles. Other associated parameters, such as integrated photometry and half-light radii, are tabulated. In an appendix, we discuss the sensitivity of different wavebands to a galaxy's star formation history in the form of "history weighting functions" and emphasize the importance of UV observations as probes of evolution during the past 10-1000 Myr. We find that UV galaxy morphologies are usually significantly different from visible band morphologies as a consequence of spatially inhomogeneous stellar populations. Differences are quite pronounced for systems in the middle range of Hubble types, Sa through Sc, but less so for ellipticals or late-type disks. Normal ellipticals and large spiral bulges are fainter and more compact in the UV. However, they typically exhibit smooth UV profiles with far-UV/optical color gradients which are larger than any at optical/IR wavelengths. The far-UV light in these cases is probably produced by extreme horizontal branch stars and their descendants in the dominant, low-mass, metal-rich population. The cool stars in the large bulges of Sa and Sb spirals fade in the UV while hot OB stars in their disks brighten, such that their Hubble classifications become significantly later. In the far-UV, early-type spirals often appear as peculiar, ringlike systems. In some spiral disks, UV-bright structures closely outline the spiral pattern; in others, the disks can be much more fragmented and chaotic than at optical wavelengths. Contributions by bright active galactic nuclei (AGNs) to the integrated UV light in our sample range from less than 10% to nearly 100%. A number of systems have unusual UV-bright structures in their inner disks, including rings, compact knots, and starburst nuclei, which could easily dominate the UV light in high-redshift analogs. A significant but variable fraction of the far-UV light in spiral disks is diffuse rather than closely concentrated to star-forming regions. Dust in normal spiral disks does not control UV morphologies, even in some highly inclined disk systems. The heaviest extinction is apparently confined to thin layers and the immediate vicinity of young H II complexes; the UV light emerges from thicker star distributions, regions evacuated of dust by photodestruction or winds, or by virtue of strong dust clumpiness. Only in cases where the dust layers are disturbed does dust appear to be a major factor in UV morphology. The UV-bright plume of M82 indicates that dust scattering of UV photons can be important in some cases. In a companion paper, we discuss far-UV data from the Astro-2 mission and optical comparisons for another 35 galaxies, emphasizing face-on spirals.

Key words: galaxies: evolution-galaxies: fundamental parameters-galaxies: photometry-galaxies: structure-ultraviolet: galaxies


Table of Contents

INTRODUCTION

BACKGROUND AND MOTIVATION FOR ULTRAVIOLET IMAGING OF GALAXIES
Ultraviolet Probes of Galaxy Astrophysics
Ages and Metal Abundances of Stellar Populations
Star Formation Histories for t < 1 Gyr
Cold Interstellar Material
Hot Interstellar Material
Nuclear Structures
Low Surface Brightness Systems
Applications to High-Redshift Galaxies
UV Imaging of Galaxies

OBSERVATIONS AND DATA REDUCTION
UIT Imaging
Ground-based Optical Imaging

DATA ANALYSIS AND ATLAS PRESENTATION
Sample Selection
Treatment of Images
Flux Calibration and Error Analysis
Image Atlas
Photometry
Length Scales and Light Concentration

COMMENTS ON INDIVIDUAL OBJECTS
Primary Objects
Secondary Objects

DISCUSSION

CONCLUSION

APPENDIX A: STAR FORMATION HISTORY WEIGHTING FUNCTIONS

APPENDIX B: GROWTH CURVE PHOTOMETRY

REFERENCES

For a postscript version of the article, click here.



1 Physics and Astronomy Department, Texas Christian University, Box 298840, Fort Worth, TX 76129; p.marcum@tcu.edu.
2 Astronomy Department, University of Virginia, P. O. Box 3818, Charlottesville, VA 22903-0818; rwo@virginia.edu.
3 Physics Department, University of North Texas, Denton, TX 76203-5370; fanelli@unt.edu.
4 Raytheon ITSS, Code 681, NASA/GSFC, Greenbelt, MD 20771.
5 Department of Physics and Astronomy, Tufts University, Medford, MA 02155.
6 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218.
7 Laboratory for Astronomy and Solar Physics, Code 681, NASA/GSFC, Greenbelt, MD 20771.
8 National Radio Astronomy Observatory, Charlottesville, VA 22903.
9 Physics Department, California State University, Fullerton, CA 92634.
10 Visiting Astronomer, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory of the National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation.
11 US Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392-5420.
12 Physics and Astronomy Department, California State University, Long Beach CA 90840.
13 Computer Sciences Corporation, NASA/GSFC, Code 684.9, Greenbelt, MD 20771.
14 Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101-1292.
15 Caltech, IPAC 100-22, Pasadena, CA 91125.
16 Astronomy Department, San Diego State University, San Diego, CA 92182.

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