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1. Introduction THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 122:109È150, 1999 May ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. GALAXY STRUCTURAL PARAMETERS: STAR FORMATION RATE AND EVOLUTION WITH REDSHIFT M. TAKAMIYA1,2 Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637; and Gemini 8 m Telescopes Project, 670 North Aohoku Place, Hilo, HI 96720 Received 1998 August 4; accepted 1998 December 21 ABSTRACT The evolution of the structure of galaxies as a function of redshift is investigated using two param- eters: the metric radius of the galaxy(Rg) and the power at high spatial frequencies in the disk of the galaxy (s). A direct comparison is made between nearby (z D 0) and distant(0.2 [ z [ 1) galaxies by following a Ðxed range in rest frame wavelengths. The data of the nearby galaxies comprise 136 broad- band images at D4500A observed with the 0.9 m telescope at Kitt Peak National Observatory (23 galaxies) and selected from the catalog of digital images of Frei et al. (113 galaxies). The high-redshift sample comprises 94 galaxies selected from the Hubble Deep Field (HDF) observations with the Hubble Space Telescope using the Wide Field Planetary Camera 2 in four broad bands that range between D3000 and D9000A (Williams et al.). The radius is measured from the intensity proÐle of the galaxy using the formulation of Petrosian, and it is argued to be a metric radius that should not depend very strongly on the angular resolution and limiting surface brightness level of the imaging data. It is found that the metric radii of nearby and distant galaxies are comparable to each other. The median value of the radius of the local sample is SR T D 5 ^ 1 kpc, and the median radius of the HDF sample isSR T D 6 ^ 2 kpc for q \ 0.5, H \ 65 g ~1 ~1 \ \ g 0 0 km s Mpc ; however, forq0 0.1, SRgT D 7 kpc and forq0 1, SRgT D 5 kpc. In the HDF, galaxies with redshifts larger than z [ 0.6 have ÑatterRg distributions than galaxies with redshifts smaller than z ¹ 0.6. However, the medianRg values of high- and low-redshift galaxies are consistent with each other. This result is consistent with the simulations of galaxy images at redshifts z \ 0.35, z \ 0.5, and z \ 0.9, which show that the metric sizes can be recovered within ^2 kpc. The Ñocculency or power at high spatial frequencies is quantiÐed using a simple method that is based on surface photometry in one band and that depends on the size of the star-forming regions and on the intensity proÐle of the galaxy. In nearby galaxies, the Ñocculency is found to trace the star formation rate as s is correlated with optical colors (B[V ) and the strength of the hydrogen recombination lines (Ha). In the HDF, galaxies at redshifts smaller than z D 1 and with Ñuxes brighter than B \ 25 have values of s similar to what is measured in nearby galaxies and to what is expected from simulations of distant galaxy images. Among the HDF galaxies, I Ðnd that at most 4% can be identiÐed as dwarf galaxies with rates of star formation similar to NGC 4449 and NGC 1569. Most HDF galaxies are giants with star formation rates similar to those in nearby giant galaxies. In summary, in this study I have introduced a method to measure the metric sizes and Ñocculency of the two-dimensional light distribution of galaxies. As a result, I Ðnd that the high spatial frequency power is related to the star formation rate. Further, I Ðnd that the sizes and power at high spatial fre- quencies of HDF galaxies remain largely unchanged between the present epoch and redshifts lower than z D 1. Subject headings: galaxies: evolution È galaxies: fundamental parameters È galaxies: photometry È galaxies: structure 1. INTRODUCTION for the faint blue excess have apparent sizes typically of 1A,a detail study of their morphologies had to await the delivery Studies of the origin and nature of galaxies to constrain by the Hubble Space Telescope (HST ) of subarcsecond their evolution with cosmological time have focused pri- resolution images. marily on deep multiwavelength photometric surveys, spec- In the context of galaxy counts, galaxy morphology has troscopic follow up, and on extensive modeling of the stellar provided new insight to the problem of galaxy evolution content of galaxies (see reviews of Koo & Kron 1992; Ellis (Glazebrook et al. 1995; Naim et al. 1995a; Abraham et al. 1997, and references therein). The results show a large 1996a; van den Bergh et al. 1996; Odewahn et al. 1996; population of blue galaxies that exceeds the no-evolution Driver et al. 1998). By 1995, the morphologically segregated prediction by a factor of 2 at B D 22 mag and by a factor of galaxy counts out toB [ 25 mag were interpreted as most up to 10 at B D 25 mag. Given that the galaxies responsible of the evolution occurring in irregular galaxies, particularly in dwarf galaxies undergoing high rates of star formation 1 Submitted in partial fulÐllment of the requirements for the degree of (Glazebrook et al. 1995; Babul & Ferguson 1996). By 1998, Doctor of Philosophy, The University of Chicago. 2 Visiting Astronomer, Kitt Peak National Observatory. KPNO is the morphologically split galaxy counts out toB [ 29 mag operated by AURA, Inc. under contract to the National Science Founda- suggest that the evolution of galaxies is due to a population tion. of high-redshift spiral galaxies and that very few true irregu- 109 110 TAKAMIYA Vol. 122 lars are responsible for the faint blue galaxy excess (Driver that had typically magnitudes B ¹ 17 mag, and apparent et al. 1998; Ferguson & Babul 1998). [email protected] º D º 2@ were classiÐed by six experts. In a Although galaxy morphological classiÐcation is com- slightly di†erent experiment, Abraham et al. (1996a, 1996b) monly used, we do not yet have a uniÐed physical interpre- had two experts classify a sample of 507 galaxies imaged tation for the various shapes of galaxies and we cannot yet with the HST with I \ 22 mag,0A.2\D\4@@, redshifts in unambiguously classify the morphologies of galaxies (which the range 0.1 \ z \ 0.6. In both studies there is agreement depend on observed wavelength, inclination angle, and on average within roughly 2È3 Hubble types among the number of resolution elements across the image). Much as di†erent classiÐers. Most of the disagreement is found the classiÐcation of stellar spectra reÑects primarily a among images of either edge-on galaxies (inclination angles sequence in temperature, if the properties chosen to deÐne a i Z 60¡) or among galaxies of irregular and peculiar types classiÐcation system are relevant, the underlying physical (T [ 7). The limitations of the Hubble system in classifying processes controlling the morphology of galaxies may be irregular galaxies are especially evident when applied to revealed. This paper develops a technique designed to distant galaxies. At redshifts above z D 0.4 and I [ 21 most measure an image structural parameter, namely, the power of the population of galaxies has been classiÐed as irregu- at high spatial frequencies (hereafter referred to as s) and lars, interacting, or merger systems including new classes attempts to give a simple physical interpretation to it. such as the ““ blue nucleated galaxies ÏÏ (Schade et al. 1996) To this date many galaxy classiÐcation systems have been and ““ chain galaxies ÏÏ (Cowie, Hu, & Songaila 1995). As a proposed (see Sandage 1975 for a review). The systems in result, the population responsible for the excess blue counts use today are all in some way related to the Hubble classi- down to those limits has been identiÐed as galaxies under- Ðcation whose criteria for spirals are (1) ““ the relative size of going enhanced star formation (Dressler et al. 1994; Gla- the unresolved nuclear region,ÏÏ (2) ““ the extent to which the zebrook et al. 1995; Abraham et al. 1996a). At even fainter arms are unwound, ÏÏ and (3) ““ the degree of resolution in the Ñux limits I \ 26 and larger redshifts z D 1.5, the excess arm ÏÏ (Hubble 1926). Conventional classiÐcation is gener- population has been identiÐed with starbursting dwarfs and ally done by visually assessing each one of the above the precursors of present-day spirals (Driver et al. 1998). It properties in the photometric B band. is, then, unfortunate that visual classiÐcation of faint gal- A number of considerations must be taken into account axies into irregular types has proven to be particularly sub- when assigning morphological types and interpreting the jective (Abraham et al. 1996b) and that ArtiÐcial Neural results. First, the optical appearance of a galaxy is a strong Networks classiÐcation is prone to failure (Naim et al. function of wavelength. Galaxies that look regular at 1995a, 1995b). Perhaps part of the answer to the problem of D4500A where the morphological classiÐcation systems interpreting distant galaxy images lies in the very fact that have been established can appear abnormal when observed the classiÐcation of galaxies at high redshifts is not the same in the rest frame UV (Giavalisco et al. 1996; OÏConnell & task as the classiÐcation at low redshifts. Marcum 1997), which is often the rest frame wavelengths Despite the shortcomings of galaxy classiÐcation men- where distant galaxies are observed. Unless we know and tioned above, Hubble morphological types are correlated, account for the redshifts when classifying distant galaxies although with much scatter, with a number of global we will be comparing morphologies at di†erent rest frames properties.
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