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The Global Rate and Efficiency of Star Formation in Spiral Galaxies As a Function of Morphology and Environment Judith S

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The Global Rate and Efficiency of Star Formation in Spiral Galaxies As a Function of Morphology and Environment Judith S University of Massachusetts Amherst ScholarWorks@UMass Amherst Astronomy Department Faculty Publication Series Astronomy 1996 The global rate and efficiency of star formation in spiral galaxies as a function of morphology and environment Judith S. Young Lori Allen Jeffrey D.P. Kenney Amy Lesser Brooks Rownd Follow this and additional works at: https://scholarworks.umass.edu/astro_faculty_pubs Part of the Astrophysics and Astronomy Commons Recommended Citation Young, Judith S.; Allen, Lori; Kenney, Jeffrey D.P.; Lesser, Amy; and Rownd, Brooks, "The global rate and efficiency of star formation in spiral galaxies as a function of morphology and environment" (1996). Astronomical Journal. 859. Retrieved from https://scholarworks.umass.edu/astro_faculty_pubs/859 This Article is brought to you for free and open access by the Astronomy at ScholarWorks@UMass Amherst. It has been accepted for inclusion in Astronomy Department Faculty Publication Series by an authorized administrator of ScholarWorks@UMass Amherst. For more information, please contact [email protected]. THE ASTRONOMICAL JOURNAL VOLUME 112, NUMBER 5 NOVEMBER 1996 112.1903Y THE GLOBAL RATE AND EFFICIENCY OF STAR FORMATION IN SPIRAL GALAXIES AS A FUNCTION OF MORPHOLOGY AND ENVIRONMENT Judith S. Young,1 Lori Allen, Jeffrey D. P. Kenney,2 Amy Lesser,1 and Brooks Rownd 1996AJ Department of Physics and Astronomy and Five College Radio Astronomy Observatory, University of Massachusetts, Amherst, Massachusetts 01003 Electronic mail: [email protected] Received 1996 January 24; revised 1996 June 24 ABSTRACT CCD images of Ha: and R-band emission in 120 spiral galaxies were obtained using the now-retired No. 1-0.9 m telescope of Kitt Peak National Observatory. These images were used to derive the distribution and total flux of continuum-subtracted Ha: line emission, and therefore the Ha: surface brightnesses and high mass star formation rates in these galaxies. We find a small but significant variation in the mean Ha surface brightness for spiral galaxies along the Hubble sequence; the Sd-Ir galaxies exhibit a mean Ha surface brightness 1.4 times higher than the Sbc-Scd galaxies, and 2-3 times higher than the Sa-Sb galaxies. Estimates for the total formation rate for high mass stars have been compared with global molecular gas masses to determine the global efficiency of high mass star formation as a function of morphological type and environment. We find that the mean efficiency of high mass star formation in this sample of spiral galaxies shows little dependence on morphological type for galaxies of type Sa through Scd, although there is a wide range in star formation efficiencies within each type. Galaxies in disturbed environments (i.e., strongly interacting systems) are found to have a mean star formation efficiency ~4 times higher than in isolated spiral galaxies, uncorrected for extinction. This confirms previous findings (Young et al 1986a,b; Sanders et al 1986; Solomon & Sage 1988; Tinney et al 1990), based on the far-inffared luminosity rather than the Ha luminosity to trace the rate of high mass star formation, that the mean star formation efficiency among isolated galaxies is significantly lower than that among interacting systems. This result provides further confirmation that the rate of high mass star formation is reasonably well traced by both the Ha and the IR luminosity in spiral galaxies. © 1996 American Astronomical Society. 1. INTRODUCTION comparison of the far-inffared luminosity [L(IR)] with the molecular gas mass [^#(H2)]. For early- and late-type spi- If star formation in external galaxies takes place in mo- rals, a number of investigators have found no global varia- lecular clouds, as is observed in the Milky Way, then the tions in the mean L(IR)/^#(H2) with type (Rengarajan & evolution of those galaxies is largely driven by the rate at Yerma 1986; Young et al 1989; Thronson et al 1989; which molecular gas is converted into stars. Among spiral Wiklind & Henkel 1989; Devereux & Young 1991). Differ- galaxies, it is now well known that molecular gas is found in ences are found, however, in the mean L(IR)/L#(H2) for all spiral types (cf. Young & Knezek 1989), and it is of great interacting galaxies relative to isolated spirals (Young et al interest to determine whether the disks of galaxies of differ- 1986a, 1986b; Sanders et al 1986; Solomon & Sage 1988; ent Hubble types evolve at different rates. To this end, it is Tinney et al 1990), indicating that the yield of high mass important to determine the mean rate of high mass star for- stars per unit mass of molecular gas in interacting galaxies is mation per unit mass of molecular gas, which we also call ~5 times higher than in isolated galaxies. the star formation efficiency for disk galaxies Using the luminosity in Ha as opposed to that in the along the Hubble sequence. This approach to the study of far-inffared, we have undertaken a program to determine the spiral galaxies is independent of bulge luminosity—which present rate of high mass star formation in disk galaxies, as changes as a function of morphological type (Boroson 1981; well as the star formation rate per unit mass of molecular Kent 1985)—since the molecular clouds and young stars are both located in the disk. gas. The primary incentive for this program was originally to Previous studies of galaxies have addressed both the mor- determine the distribution of high mass star formation within phological type dependence and the environmental depen- galaxies, since the IRAS resolution is insufficient to resolve dence of the high mass star formation efficiency through a most of the galaxies studied here. Because the flux in Ha is proportional to the number of ionizing photons from OB stars, in the absence of absorption by dust, it directly traces Visiting JCMT Fellow, Institue for Astronomy, University of Hawaii, Honolulu, HI 96822. the presence of recently formed high mass stars. Thus, the department of Astronomy, Yale University, New Haven, CT 06511. ratio of the Ha luminosity to the mass of molecular gas is an 1903 Astron. J. 112 (5), November 1996 0004-6256/96/112(5)/1903/25/$ 10.00 © 1996 Am. Astron. Soc. 1903 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1904 YOUNG ET AL. : STAR FORMATION IN SPIRAL GALAXIES 1904 112.1903Y independent indicator of the yield of high mass stars per unit 2.2 Imaging Observations mass of molecular gas, or the star formation efficiency. In this paper, we present initial results from our CCD Measurements of the Ha+[N n] emission were made for imaging program on the global high mass star formation rate 120 galaxies using the now-retired No. 1-0.9 m telescope of 1996AJ and efficiency in a sample of 120 spiral galaxies. We include KPNO (/7.5) with either the T12, T13, RCA3, or TEK1 a comparison of interacting and isolated galaxies, as well as CCD direct imaging system on photometric nights between an examination of the high mass star formation efficiency in 1986 October and 1989 April. The CCDs were operated to spiral galaxies of different morphological types. In a later produce a scale of either 0.77" or 0.86" per pixel, depending paper, we will examine the radial variations in the rate of star on the particular CCD. The area imaged was either 6' by 6' formation and the yield of high mass stars per unit mass of or 7' by 4', also depending on the particular CCD. A sum- molecular gas within these galaxies. mary of the observations is given in Table 2, including the CCD used to image each galaxy, and the characteristics of each CCD. The Ha+[N n] emission line fluxes were obtained by ob- serving each galaxy through two filters: a narrow (AX 2. THE Ha DATA =76-86 A) interference filter centered on Ha at the velocity 2.1 Galaxy Sample of the target galaxy, and either a broad 7?-band filter (X =6470 A, AX=1110 A) or a narrow R filter (X=7024 A, We have obtained Ha and red continuum CCD images for AX=380 A) to determine the nearby continuum level. Inte- 120 of the 300 galaxies observed as part of the Five College gration times were typically 5-10 min in the R band and 1 h Radio Astronomy Observatory (FCRAO) Extragalactic CO in Ha. Airmass corrections were made using the standard Survey (Young et al 1995). The galaxies included in the extinction curve for Kitt Peak given in the KPNO 1RS Stan- Extragalactic CO Survey cover a wide range in properties, dard Star Manual (Barnes & Hayes 1984). The airmass of including lenticulars, spirals, and irregulars; galaxies with the observations, the specific Ha interference filters used, the optical diameters between 4 and 100 kpc; environments from characteristics of these filters, and the Ha and R-band inte- pair to cluster, and from isolated to merging systems. The gration times are given in Table 2. larger sample of galaxies is not complete in a flux-limited Because of the range in redshifts for the galaxies in our sense, nor it is volume limited, but rather the galaxies ob- sample, six different Ha interference filters were needed for served were selected to span a wide range of parameter the imaging observations. For each filter, the transmission space. The majority of the galaxies observed as part of the characteristics as a function of wavelength were made avail- FCRAO Extragalactic CO Survey were selected from the able to us by the staff at KPNO. Most of the Ha interference RC2 (de Vaucouleurs et al. 1976) or the IRAS database filters were characterized by transmission which decreased based on their optical or infrared properties, such as the total by less than 5% for wavelengths within ±20 A of the peak, blue magnitude, the global 60 pm flux density, S60, or so that no transmission corrections were needed for most of the global 100 pm flux density, Sjoq.
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