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1982Apjs...49...53H the Astrophysical Journal Supplement The Astrophysical Journal Supplement Series, 49:53-88, 1982 May © 1982. The American Astronomical Society. All rights reserved. Printed in U.S.A. 1982ApJS...49...53H GLOBAL PROPERTIES OF IRREGULAR GALAXIES D. A. Hunter1 and J. S. Gallagher Department of Astronomy, University of Illinois at Urbana-Champaign AND D. Rautenkranz Steward Observatory, University of Arizona Received 1981 June 8; accepted 1981 October 12 ABSTRACT We present optical and radio observations of global properties for a sample of noninteracting irregular galaxies and a few comparison objects. Program galaxies were chosen primarily on the basis of their blue colors and thus may be expected to represent systems with high rates of star formation. The data consist of 40" aperture photometry through intermediate-band filters, 25" aperture spectrophotometry in the region 3500-5900 À, spectrophotometry of individual H n regions primarily from 4500-7500 Á, and 21 cm H i observations. From the filter photometry we determine a Mg index and a reddening-free color parameter Q. An iterative population synthesis technique is applied to the large aperture spectra in order to separate Hß emission from absorption, and a star formation rate (SFR) is estimated from the number of ionizing photons coupled with a Miller and Scalo initial mass function. Line fluxes are used to determine the abundance ratios O/H and N+/S+, and the radio data give information on gas masses and kinematics. None of the program galaxies have spiral arms, and many have low rotational velocities; nevertheless, there is a large range in SFR, and it can be very high (up to 103 times the local galactic mean). No correlation is found between the SFR and global gas or abundance parameters, implying that local processes dominate. The fractional involvement of the galaxy in the current star-forming activity decreases with increasing SFR and with decreasing time scale for exhausting the gas at the current rate. This correlation, coupled with the low metallicities, suggests that the star-forming activity moves around the galaxy and that the overall SFR is likely to vary substantially with time. The nitrogen-to-sulfur ratio remains fairly constant, rising steeply only for high oxygen abundance. This fact may show that these galaxies are less evolved than typical spirals or that mixing from a reservoir of unprocessed material is taking place. We briefly compare these properties with predic- tions from three global models of galaxian evolution: the closed system model, the stochastic star-induced star formation model, and the gas infall model. Subject headings: galaxies: photometry — galaxies: stellar content — nebulae: H n regions — radio sources: 21 cm radiation — stars: formation I. INTRODUCTION still are not understood. Many of these galaxies have The star-forming histories of noninteracting irregular low rotation velocities, and none have spiral arms, so the galaxies that show evidence for unusual stellar popula- galactic shock density wave model of star formation is tions are not understood. These galaxies include ones not dominant here. Nevertheless, there are irregular with anomalous UBV colors, disturbed optical morphol- systems in which the star formation is seen to be highly ogies, emission lines, and regions of high surface efficient, at least at the current epoch; therefore, one brightness, implying a recent epoch of enhanced star would like to uncover the large-scale processes govern- formation. Larson and Tinsley (1978) have shown that ing the star formation rates and the relationship of these many of the galaxies with unusual colors are interacting systems to the less active low luminosity dwarfs. The systems, but those isolated systems with unusual colors stochastic processes in which star formation continues by means of the energy dumped back into the interstel- Visiting Student, Kitt Peak National Observatory, which is lar medium by the evolving massive stars through H n operated by the Association of Universities for Research in region expansion, stellar winds, and supemovae may be Astronomy, Inc., under contract with the National Science Foun- most relevant. However, this is an inherently local pro- dation; and the National Radio Astronomy Observatory, which is operated by Associated Universities, Inc., under contract with the cess, and systems with globally extended star formation National Science Foundation. present a further puzzle. We would like to know what © American Astronomical Society • Provided by the NASA Astrophysics Data System 54 HUNTER, GALLAGHER, AND RAUTENKRANZ parameters control the star formation, whether the sto- camera images (Hunter 1981) through b, y, /, and Ha chastic processes can account for the observed star filters show that the program galaxies form a surpris- formation rates (SFR), efficiency, distribution, abun- ingly homogeneous morphological class. They are irregu- dances, and stellar content, and what the nature of the lar in structure, but with the exceptions noted above, no 1982ApJS...49...53H underlying galaxy, the history of the SFR, including one galaxy stands out as being distinct from the rest. possible bursts, and the form and parameter dependence The program galaxies are listed in Table 1, and the of the initial mass function (IMF) are. columns are as follows: column (1)—galaxy identifica- In order to develop a picture of these galaxies in tion; column (2)—alternate identifications; column (3) terms of the history of star formation and the relation of —morphological type from de Vaucouleurs and the star formation to other systemic properties, we have Freeman (1972) or de Vaucouleurs, de Vaucouleurs, and explored the stellar and gaseous contents and metallicity Corwin (1976; hereafter RC2). In columns (4)-(7) a on global and local scales through various optical and “Y” indicates that data were obtained for that galaxy: radio observations. The data presented here concern the Prairie Observatory (PO) photometry; large aperture global properties of these systems and consist of inter- Reticon (1RS) spectrophotometry; 21 cm H i, or mediate-band filter photometry, large aperture spectro- Steward Observatory 90 inch (2.3 m) Reticon spectro- photometry, small aperture spectrophotometry of photometry of H ii regions. Columns (8)-(9)—total individual H n regions, and H i 21 cm observations. reddening-corrected UBV colors from de Vaucouleurs, Limits to telescope time and/or weather, however, have de Vaucouleurs, and Corwin (1981), the RC2, or Huchra meant that not all galaxies are present in all data sets. (1977û); column (10)—color excesses used to correct Nevertheless, all data are presented here with the hope UBV colors for galactic reddening, determined from that gaps will be filled in the future. The data are given Burstein and Heiles (1978); column (11)—distance to in § II, analysis of the spectrophotometric data is out- galaxy in Mpc. The Hubble constant is chosen to be lined in § III, the global properties are presented and 50 km s-1 Mpc-1. Distances are taken from Kraan- discussed in § IV, a comparison with various models is Korteweg and Tammann (1979), whenever possible, or made in § V, and § VI summarizes our results. The from the recession velocities of Rood (1980) or our data paper presenting local properties of these irregulars (Table 5). Column (12)—the major axis in arc minutes (Hunter 1981) is currently being prepared. on Holmberg’s (1958) system. If an original Holmberg diameter was not available, one was constructed from the blue major axis from Nilson (1973; hereafter the II. OBSERVATIONS AND DATA REDUCTION UGC) or the isophotal diameter D25 from the RC2 using The galaxies observed in our optical and radio pro- the relationships given by Fisher and Tully (1980). We gram were selected from a pool of noninteracting, have calculated their factor for correcting the optical intermediate luminosity irregulars chosen for showing dimension for galactic extinction, but except for NGC evidence of unusual evolutionary histories, as indicated 1569 and IC 334, for which this factor is 2 and 1.12, primarily by anomalous UBV colors. Most of the galax- respectively, there is no measurable correction. Column ies are quite blue (£/—i?<—0.3; see Table 1) and, (13)—the ratio of minor to major Holmberg axis; col- hence, are selected to be those in a highly active star umn (14)—comments on the nature of each irregular formation phase of their evolution. We have, however, galaxy which may help clarify the question of why it was avoided the low luminosity, extremely blue, intergalactic chosen for this sample. “Blue Irr” identifies those that H ii regions which are most likely the result of episodic form the primary thrust of this investigation. Those star formation events in gas-dominated galaxies (Searle, labeled “? colors” were chosen on the basis of morphol- Sargent, and Bagnuolo 1973). Most of the galaxies in ogy alone, including irregular structure, high surface our sample have no close neighbors, and their star brightness, bright knots, etc. formation rates are therefore unlikely to have been influenced by recent encounters. As the investigation a) Photometry progressed, some systems with unknown colors and Intermediate-band filter photometry was obtained some with more normal spiral-like colors but disturbed with the University of Illinois Prairie Observatory 1 m morphologies and emission lines were added for com- telescope at Oakland, Illinois, using a single-channel parison (see Fig. 1 for a color-color plot). Also included photometer with a thermoelectrically cooled RCA for comparison were the less active irregulars DDO 168, C31034A photomultiplier. The standard uby filters were NGC 5474, NGC 7292, the dusty spiral-like peculiars used, but the v filter was shifted to a central wavelength NGC 972, NGC 2146, and NGC 7625, and the peculiar of 4200 À with a width of 150 À in order to avoid HÔ. low luminosity spiral NGC 3310 which currently has a The 38, 52, and 74 filters on Wood’s (1969) photometric high star formation rate (see Balick and Heckman 1981).
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