The Astrophysical Journal Supplement Series, 86:5-93, 1993 May ....5K © 1993. The American Astronomical Society. All rights reserved. Printed in U.S.A. 86 ... AN ATLAS OF ULTRAVIOLET SPECTRA OF STAR-FORMIJN G GALAXIES 1993ApJS A. L. Kinney, R. C. Bohlin, and D. Calzetti Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 N. Pan agía1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; and Istituto di Astronomía, Université di Catania AND Rosemary F. G. Wyse2 Physics and Astronomy Department, Johns Hopkins University, Homewood Campus, Baltimore, MD 21218 Received 1992 April 13; accepted 1992 September 21 ABSTRACT An atlas of ultraviolet spectra of the central regions of 143 spiral, irregular, blue compact, Seyfert 2, and starburst galaxies is produced by combining 387 low-resolution spectra from the IUE data archives. The spectra have been extracted with an optimal algorithm and co-added to produce spectra with the best possible signal-to- noise ratio. Our data support the picture proposed earlier on the basis of UV spectra from the Orbiting Astronomical Observatory and from the Astronomical Netherlands Satellite that spiral galaxies of later Hubble class have higher flux at the shortest UV wavelengths than do spiral galaxies of earlier Hubble class. A comparison of the UV spectra of low-ionization nuclear emission-line region (LINER) galaxies with the spectra of normal spiral galaxies shows that the UV continuum of LINERs is dominated by the galaxy continuum. In contrast, Seyfert 2 galaxies show a featureless blue continuum plus emission lines, with the UV spectrum of the host galaxy apparent only at the longest U V wavelengths. The starbursting, blue compact, and blue compact dwarf galaxies have UV continua a26±014 that are flat or increasing toward short wavelengths, with a wide range in spectral index, going from Fx cc x 1 85 06 toFx oc X- - ^- . The spectral signature of dust with a wavelength dependence of the extinction such as is seen in the Milky Way is not apparent in these galaxies. The lack of an apparent dust feature (e.g., the 2200 Á bump) imphes either that the extinction law is diflerent or that any dust present in the galaxies is in the form of clumps and does not contribute to the flux, owing to the very high optical depth. Subject headings: atlases — galaxies: ISM — galaxies: Seyfert — galaxies: spiral — galaxies: starburst — ultraviolet: galaxies 1. INTRODUCTION tra, and the ultraviolet ionizing continuum that often drives the activity observed in the optical. The IUE archives do not We present a systematic study of the ultraviolet spectra of star-forming galaxies of diflerent morphological type and activ- contain a complete, well-selected sample of galaxies, but rather ity class using a sample drawn from a uniformly reduced IUE contain a collection of the individual galaxies in which the data set. The spectra for a wide variety of galaxies, including scientific community has had the most interest. In spite of the normal spiral, low-ionization nuclear emission-hne region fact that the sample is not particularly complete, the IUE ar- (LINER), starburst, blue compact, blue compact dwarf, and chives contain a large number of galaxies, so that conclusions Seyfert 2 galaxies, are presented, both in the form of spectral about UV spectral properties as a function of galaxy type can energy distributions, to demonstrate the overall characteristics be made. However, acquisition of a well-defined sample of according to morphology and activity class and in the form of normal disk galaxies should be a high priority in the later years absolute flux distributions, to better show the absorption and of IUE. emission features of individual objects. Our data support the The object selection, data compilation, and reduction are picture based on UV spectra of the Orbiting Astronomical Ob- described in § 2; digital distribution of the atlas is described in servatory (OAO) (Pence 1976) and of Xht Astronomical Neth- § 3; the spectral signatures of various contributors to the UV, erlands Satellite {ANS) (Coleman, Wu, & Weedman 1980) such as the interstellar medium (ISM), early O and B stars, that spiral galaxies of later Hubble class have more flux at the Wolf-Rayet stars, and later type stars, is discussed in § 4.1 ; the shortest UV wavelengths than do spiral galaxies of earlier Hub- characteristics of the various types of galaxies and the impact ble class. With this atlas we aim to clarify the connection be- of dust upon their spectra are discussed in § 4.2; and the indi- tween morphological type, activity class based on optical spec- vidual objects and their UV spectra are discussed in § 4.3. The ultraviolet spectra in this atlas will be analyzed in the 1 Affiliated with the Astrophysics Division, Space Sciences Department context of a larger data set. Ground-based optical spectra are of ESA. currently being obtained in an aperture matched to the size of 2 Alfred P. Sloan Foundation Fellow. the IUE aperture, so that wavelength coverage for the atlas 5 © American Astronomical Society • Provided by the NASA Astrophysics Data System 6 KINNEY ET AL. ....5K 86 objects will eventually go from 1200 to 7800 Â. The UV spec- with no normalization between the cameras. The flux levels are ... tra will be used to form template spectra of star-forming galax- generally well matched between the cameras, which is evi- ies according to Hubble class and activity class. Template spec- dence that the center of the galaxy was contained in the large tra will be used to predict the contribution to the diffuse UV IUE aperture for all spectra co-added for a given object. We background from star-forming galaxies. The template spectra have compared our fluxes with those of Longo, Capaccioli, & 1993ApJS will also be used to quantify UV colors of galaxies and to pre- Ceriello 1991, who report IUE fluxes for normal galaxies, and dict the optical colors of star-forming galaxies at high redshift find that they agree to about 5%. (z « 1-2). A number of galaxies in the atlas are redshifted so The fist of atlas galaxies is given in Table 1 with the number that the region of the Lya emission Une can be observed. The of short-wavelength spectra (SW), the number of long-wave- observations of emission and absorption at Lya will be ana- length (LW) spectra, the galaxy name, the morphological type lyzed in an attempt to understand the implications of our low- of the galaxy (where we used the simpler classification of those redshift star-forming galaxies for high-redshift star-forming gal- from the Revised Shapley-Ames Catalog [Sandage & Tam- axies. mann 1987, hereafter RSA] and the NASA Extragalactic Da- tabase [NED] ), the activity class of the galaxy, the heliocentric 2. THE DATA velocity, the photographic magnitude, the absolute magnitude (assuming q = \ and H = 50), the Galactic reddening (Bur- The ultraviolet data are from the IUE archives and include Q 0 stein & Heiles 1982, 1984), the Galactic latitude and longi- spectra from the short-wavelength (SWP) and long-wave- tude, the right ascension and declination, and, finally, other length (LWR and LWP) cameras taken in the large (10" X names. For cross-reference, Table 2 lists the Messier objects 20") aperture. The resolution is approximately 5 Â for the that are included in the atlas, along with their positions and SWP and 8 Â for the LWR and LWP (see Tumrose & Thomp- other names; and Table 3 lists Markarian objects and positions son 1984 for a complete description of IUE data). and other names. 2.1. Object Selection 2.2. Extraction and Reduction First a list of the positions of all spiral, irregular, and com- In order to minimize the noise in the atlas spectra, the opti- pact galaxies in the Catalogue of Principal Galaxies (Paturel et mal extraction technique of Kinney, Bohlin, & Neill ( 1991 c ) is al. 1989, hereafter CPG) was produced. The CPG contains used to extract the spectra from the IUE line-by-Une data files. 73,000 galaxies and includes, among other catalogs, the Sec- This sht-weighted extraction can choose slit heights of either 9 ond Reference Catalogue of Bright Galaxies (de Vancouleurs, or 15 fines, where the 9 or 15 is understood to be 18 or 30 fines de Vaucouleurs, & Corwin 1976) and the Markarian lists of in the more modem line-by-line files that have twice the num- galaxies. Then a list of objects common to both the IUE ar- ber of fines in the file. Despite the optimal nature of our spec- chives and the CPG was produced, from which the Seyfert 1 tral extraction technique, the added noise in 15 fines seriously galaxies present in the Véron-Cetty & Véron catalog ( 1986) degrades the signal-to-noise ratio in the resultant spectra in were excluded, except for those low-luminosity objects found many cases. Therefore, all spectra presented here are from 9 to have broad emission lines (called dwarf Seyfert 1 galaxies by fine optimal extractions. The loss of fight in a 9 fine slit is not Filippenko & Sargent 1985). Our initial list of galaxies was large, as evidenced by the following facts: (a) An IUE slit of 9 cross-referenced with the IUE archives object classes 80 and 82 fines corresponds to 19 "4, or 91 % of the 21 "4 high IUE slit. ( b ) (spiral and irregular galaxies) and cross-referenced with lists of In the case of trailed spectra, 87% of the fight is in the central 9 starburst galaxies (Balzano 1983; Mazzarella & Balzano fines, (c) For one of the more extended galaxies, NGC 7590, 1986 ), so that missing objects were added.