1985Apjs...58..533H the Astrophysical Journal Supplement Series, 58:533-560, 1985 August © 1985. the American Astronomical Soci
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The Astrophysical Journal Supplement Series, 58:533-560, 1985 August © 1985. The American Astronomical Society. All rights reserved. Printed in U.S.A. STAR-FORMING PROPERTIES AND HISTORIES OF DWARF IRREGULAR GALAXIES: 1985ApJS...58..533H DOWN BUT NOT OUT Deidre A. Hunter and John S. Gallagher III Kitt Peak National Observatory, National Optical Astronomy Observatories1 Received 1984 October 31; accepted 1985 January 28 ABSTRACT Star formation processes and their relationships to other galactic properties are investigated in a sample of low optical surface brightness dwarf irregular galaxies. New observations made for this study include B and R images taken with a CCD detector; flux-calibrated, Ha, narrow-band digital imagery; large-aperture spectropho- tometry; and spectrophotometry of individual H n regions. These data provide the basis for several measure- ments of star-forming characteristics of dwarf irregular galaxies: (1) Total current star formation rates are deduced from Ha luminosities. (2) Properties of typical star-forming centers are derived from the sizes, luminosities, and surface brightnesses of H ii regions. (3) Star formation histories are estimated from the ratio of Ha to blue luminosities and from the stellar content models. (4) The metallicity of the gas comes from oxygen emission lines. (5) Images provide information on the optical light distributions. 10 1 2 The dwarf irregulars in our sample have low stellar production rates of -10“ M0 yr“ pc“ and have formed stars at approximately constant rates over the last few Gyr. Because of their large H i gas contents, dwarf irregulars potentially can sustain constant star formation rates for very long times ( > 100 Gyr in some cases). The dwarf irregulars do make large star-forming regions, but on average they are smaller than those in the giant irregular galaxies. A comparison is made between properties of dwarf and giant irregular galaxies. Aside from characteristics directly associated with the levels of star-forming activity, the dwarf and giant irregulars are similar, which is indicative of the physical homogeneity of the normal irregular galactic structural class: (1) Despite local nonuniformities, irregulars have approximately exponential, disklike radial brightness profiles. (2) Population synthesis modeling yields equivalent stellar temperature mixes in samples of dwarf and giant irregulars. (3) Oxygen abundances in H n regions fall between those of the Small and Large Magellanic Clouds for most irregular galaxies. (4) In a mean sense, irregular galaxies form stars at nearly constant rates. These points are discussed in terms of physical mechanisms which might control evolutionary processes in galaxies. We suggest that the existence of constant star formation rates in galaxies covering a wide range in luminosity and mass points to local regulation of star formation processes, and we note the possible importance of a low-density cutoff for gas to effectively participate in star formation. Subject headings: galaxies: stellar content — stars: formation I. introduction fractional H i content, and modest gas oxygen abundances Dwarf irregular galaxies represent one extreme of the (see Hodge 1971; de Vaucouleurs and Freeman 1972; properties of gas-rich disk galaxies. Although they are the Feitzinger 1980; Gallagher and Hunter 1984, and references most numerous type of star-forming galaxy (Zwicky 1957; therein). The Irr galaxies thus belong in one structural family, de Vaucouleurs 1959h; van den Bergh 1966, 1977; Kraan- but cover a wide range in mass and luminosity. Korteweg and Tammann 1979), dwarf irregular (Irr) systems The characteristics which enable us to distinguish dwarf usually have uninspiring appearances on optical photographs Irr’s as a structural subclass stem largely from their low (cf. Sandage and Tammann 1981). They generally are less central surface densities of luminous matter coupled with their luminous, lower in optical surface brightness, and less massive small total masses, which result in optically tiny, low-surface than either spiral galaxies or their structural cousins, the brightness galaxies (Fisher and Tully 1975; Tully et al. 1978; high-surface brightness Magellanic-type irregulars, which we de Vaucouleurs, de Vaucouleurs, and Buta 1981, 1983). The will generically refer to as giant Irr’s. Despite differences in visibility of dwarf Irr galaxies is further reduced by the scale, many key properties of giant and dwarf Irr’s are similar; absence of numerous large star-forming complexes, which are e.g., chaotic optical morphologies with little or no spiral obvious and common features in giant Irr’s and late-type structure, low rotation velocities, blue optical colors, large spiral galaxies (compare images of giant Irr’s in Hunter 1982 h with those of dwarfs presented here and by Fisher and Tully 1979). This point leads to an interesting conundrum: While we 1 Operated by the Association of Universities for Research in Astron- were led in our studies of giant Irr galaxies to ask how blatant omy, Inc., under contract with the National Science Foundation. levels of OB star formation could be sustained in such simple © American Astronomical Society • Provided by the NASA Astrophysics Data System 534 HUNTER AND GALLAGHER Vol. 58 galaxies (Hunter, Gallagher, and Rautenkranz 1982, hereafter b) Images HGR; Gallagher, Hunter, and Tutukov 1984), in examining the structurally similar dwarf Irr’s, we instead ask why they i) Videocamera are so unspectacular in their production of OB stars. The Kitt Peak National Observatory (KPNO) videocamera Understanding the underlying physical mechanisms which system was used on the 2.1 m telescope in 1982 November. 1985ApJS...58..533H differentiate even closely related types of galaxies like dwarf Digital images were obtained through filters centered on Ha and giant Irr’s holds considerable promise for providing in- (central wavelength 6577 A and FWHM of 54 A; or central sight into the factors which influence global star formation wavelength 6612 Á and FWHM of 72 À) and the nearby rates in simple galaxies. For exsimple, do giant and dwarf continuum (centered at 7096 À and FWHM of 83 À; or systems show systematic evidence for differences in star for- centered at 6204 À and FWHM of 149 À). Integration times mation histories, or are the dwarfs merely scaled-down ver- were typically 14 minutes. The resulting images are 256 X 256 sions of the giants? There is some support, both observational pixels at 0755 per pixel for a field of view of ~ 2' X 2'. (Searle, Sargent, and Bagnuolo 1973; Huebra 1911b; Stewart Small- and large-scale pixel-to-pixel gain variations were etal. 1982) and theoretical (Gerola, Seiden, and Schulman corrected with observations of a quartz lamp and of blank sky 1980; Seiden, Schulman, and Feitzinger 1982; Comins 1983, in twilight. Geometrical distortions introduced by the image 1984), for the view that dwarf systems are dominated by tube were removed using a standard reduction program at short-lived periods of enhanced star formation, or “bursts.” Kitt Peak. An Ha emission picture was produced by aligning We also are interested in exploring variations in the properties the Ha-on and Ha-off images, scaling one to the other, and of star-forming regions as functions of galaxy size and star subtracting to remove the stellar continuum. The scaling fac- formation rates. tor was determined from standard star fields. The Ha flux In order to further explore the issues associated with star calibration was found through observations of NGC 595, formation processes in dwarf Irr galaxies and their evolution- NGC 604, and NGC 2363, for which Ha measurements were ary relationships with the more actively star-forming giant available (Kennicutt, Balick, and Heckman 1980; Hunter, Irr’s, we have undertaken an observational program which unpublished). The Ha-on filters also passed [N n] \6584, and parallels that of our earlier work on giant Irr galaxies corrections to the calibration were made to account for this, (HGR; Hunter 1982 a, b) and yields results which are directly although no correction has been made for [N n] in the object comparable with those studies. The data base consists of large- galaxies themselves. In addition, a correction has been made and small aperture spectrophotometry and broad- and nar- for the displacement of Ha within the filter bandpasses due to row-band digital imagery, which we analyze to provide em- redshift differences between the calibrating and program pirical probes of gas chemical abundances, star formation galaxies. Mean atmospheric coefficients for Kitt Peak were rates and stellar populations, characteristics of star-forming used for extinction corrections. Some objects were observed centers, and spatial distributions of the stars. The observations through cirrus, and these are flagged in subsequent tables. The are reviewed in the next section, derived properties are pre- videocamera images are shown in Figure 1 (Plates 19-25). sented in § III, a comparison is made with the giant Irr’s in § IV, and additional discussion is in § V. ii) CCD In 1982 November the RCA CCD direct imaging camera was used at f/7.5 on the KPNO No. 1 0.9 m telescope. The II. THE DATA resulting digital array has a scale of 0786 per pixel and a field of view of 7'3 (E-W)X4'6 (N-S). The galaxies were observed a) The Objects through two filters: one which is similar to the Johnson B The program galaxies are primarily cataloged in van den filter but avoids the major emission lines at 3727 À and 4861 Bergh’s (1959) survey of dwarfs (designated by his DDO Á, and a quasi-R filter (6204 Á, FWHM 149 Á) which also number) and were selected form Fisher and Tully’s H i survey avoids emission lines. Integration times were usually 30-60 (1975) on the basis of their morphological classification as minutes. The nights were clear, and standard stars for B and Irr’s. Preference was given for systems closer than 10 Mpc, R were observed.