.4300 .236. The Astrophysical Journal, 236:430-440, 1980 March 1 . © 1980. The American Astronomical Society. All rights reserved. Printed in U.S.A. 198OApJ. GALAXY SPECTRAL SYNTHESIS. IL M32 AND THE AGES OF GALAXIES Robert W. O’Connell Department of Astronomy, University of Virginia Received 1979 June 18; accepted 1979 September 6 ABSTRACT We perform a population synthesis on absolute spectrophotometry for the central 31" of M32. The main-sequence turnoff is at {B — V) ~ 0.5 or F8 equivalent spectral type. A variety of models for cooler turnoffs are considered and found to be excluded by the data. The metallicity of M32 is solar within ~0.1 dex. Taken together, these results imply that major star formation continued in M32 until ~ 5 Gyr ago, or 10 Gyr after the oldest globular clusters formed. Per- haps 50% of the giant light could arise in a significantly older population, and the peak star formation rate could have occurred much earlier. However, significant changes in the colors of low-mass elliptical galaxies are expected at lookback times of ~5 Gyr. The anomalously blue galaxies now being observed in distant clusters may be such objects. A tentative age dating for gE nuclei indicates an upper limit for the turnoff age of 6-8 Gyr. The synthesis models predict that the rate of mass return to the interstellar medium in M32 from evolving stars is ~ 8 x 10" 4 M© yr-1, which is in good agreement with counts of planetary nebulae. However, the upper limit for star formation during the past 1 Gyr is ~3 x 10“3 M© yr-1. Thus, the available optical observations do not exclude complete recycling of gas lost during stellar evolution into new generations of stars. Ultraviolet observations are required to demonstrate the need for galactic winds or other special gas removal mechanisms in elliptical galaxies. Subject headings: galaxies: individual — galaxies: nuclei — galaxies: stellar content — stars: evolution I. INTRODUCTION It is therefore of some interest to examine the age structure of the stellar population of bright E/S0 The evolution of galaxies now appears to be much galaxies. In principle, this information can be extrac- more complex than was envisioned only a few years ted from integrated light observations by spectral ago. Strong dynamical interaction between galaxies, synthesis, although most previous studies of early- ablation of the interstellar medium by intergalactic type galaxies have assumed that the stars were formed gas, and sweeping of gas from galaxies by supernovae- in a single burst. Here we perform such an analysis driven winds are all likely to be important at some on M32, and in a companion paper we examine the level in determining the history of star formation. case of NGC 4459, a blue-nucleated SO galaxy in the Major modifications in the stellar content of galaxies Virgo cluster which exhibits definite evidence of recent in large clusters may have occurred as recently as star formation. z ~ 0.4 (Butcher and Oemler 1978). M32 is an excellent candidate for such a study for This complexity is evident even among nearby three reasons, apart from the fact that it is the galaxies. Both NGC 205 (Hodge 1973) and NGC 1510 brightest elliptical galaxy. First, M32 has colors which (Disney and Pottash 1977; Kinman 1978) appear to are significantly bluer than a giant elliptical. While be normal elliptical galaxies which have probably the gross color differential is attributed not to recent suffered recent tidal interactions with nearby hydrogen- star formation but to lower metallicity than in gE’s rich spiral companions. The consequent transfer of (Faber 1973), nonetheless it provides a test of the fresh gas has produced bursts of star formation in the ability of spectral synthesis to discriminate between ellipticals. Other evidence is now accumulating that the effects of metallicity and young stellar popula- in the nearby dwarf spheroidals (Norris and Zinn tions. M32 is thus useful as a control for studies of 1975) and the bar of the Large Magellanic Cloud objects such as NGC 4459. (Butcher 1977) the “old” stellar population was Second, studies of the H i distribution and spiral- formed over a very long time interval, perhaps extend- arm pattern of M31 by Byrd (1977, 1978) suggest ing to only a few 109 years ago. This interpretation that M32 is in an orbit which passes through the disk is, of course, contrary to the classic analyses of our of M31. It is of some interest to know whether this own galaxy (Eggen, Lynden-Bell, and Sandage 1962; interaction has affected star formation in M32 over Eggen and Sandage 1969), which suggest that both the last ~ 109 years. the globular clusters and the bulk of the old disk Third, M32 is near enough that information on its population formed rapidly on essentially a collapse stellar content independent of integrated light observa- time scale. tions is already available, and more will be forth- 430 © American Astronomical Society • Provided by the NASA Astrophysics Data System .4300 .236. No. 2, 1980 M32 AND THE AGES OF GALAXIES 431 . coming in the near future. In particular, the planetary TABLE 1 nebula production rate, which determines the rate Corrected Absolute Energy Distributions of mass recycling to the interstellar medium, has been 198OApJ. estimated by Ford and Jenner (1975). AAB = -2.5 log [FV(A)/FV(A5050)] Four recent population syntheses of M32 have been published (Spinrad and Taylor 1971; Faber 1972; A (Â) M32 MeangE Williams 1976; and Pritchet 1977). All but Williams’s 3300.. 2.04 2.32 however, were based on the scanner data of Spinrad 3350 2.02 2.39 and Taylor, and none attempted to examine the age 3400 1.95 2.26 structure of the population. In this paper we apply the 3450.. 2.06 2.34 3570 2.03 2.36 spectral synthesis technique of O’Connell (1976a, 3620.. 1.81 2.00 hereafter Paper I) to new spectrophotometry of M32. 3784.. 1.45 1.82 The analysis is completely independent of earlier 3798.. ... 1.59 1.95 work and is more sensitive to possible young stellar 3815 1.61 1.98 populations because of the larger number of data 3835.. 1.91 2.29 points included below 4000 Â (14 compared with 4 in 3860 1.59 2.07 3889 1.54 1.71 Spinrad and Taylor). It provides the best limits on 3910.. ... 1.31 1.51 recent star formation in early type galaxies which are 3933 1.88 2.15 likely to be possible without far-ultraviolet observations 4015 0.82 0.99 from space telescopes. 4101 0.91 1.04 4200 0.77 0.96 It is worth noting at the outset that there is no 4270 0.73 0.83 evidence for a young stellar population in the outer 4305.. ... 0.93 1.03 parts of M32 (Baade 1944; van den Bergh 1975). 4340. 0.61 0.65 While it is not possible to study the resolution of the 4400.. 0.54 0.58 very crowded inner parts of the galaxy, there is also 4500 0.24 0.29 no evidence in the broad-band (de Vaucouleurs and 4785.. 0.08 0.11 4861. 0.27 0.17 de Vaucouleurs 1972) or narrow-band (Oke and 5050 0.00 0.00 Schwarzschild 1975) colors for a substantial massive 5174 0.13 0.22 star population. However, the relevant question to 5300 -0.20 -0.23 address here is whether meaningful quantitative limits 5820. -0.40 -0.50 5892 -0.25 -0.25 can be placed on star formation not only in the very 6100. -0.46 -0.55 recent past but over the last 109 years as well. 6180 -0.44 -0.52 We find, in fact, no evidence for recent star forma- 6370 -0.49 -0.60 tion and are able to place the relatively stringent upper 7050 -0.72 -0.84 limit of -0.2% of the galaxy’s mass involved in star 7100 -0.68 -0.76 formation during the last 109 yrs. However, the most 7400 -0.80 -0.98 8400 -0.97 -1.21 important result of this work is that major star forma- 9950 -1.25 -1.54 tion in M32 continued until only — 5Gyr ago, or 10400 -1.40 -1.68 10 Gyr after the formation of the oldest globular 10800 -1.47 -1.70 clusters in our Galaxy. (Sargent et al. 1977) was adopted. The largest correc- II. OBSERVATIONS AND DATA REDUCTION tion was to the Ca n A3933 feature and amounted to M32 was observed with the prime-focus scanner on —0.10 mag in ^4i?(A3933). Absorption-line indices de- the 91 cm Crossley reflector at Lick Observatory with rived from Table 1 are given in Table 2. Also given a 31?5 diameter entrance aperture (which projects to for comparison in Tables 1 and 2 is the well-defined 100 pc). Band widths were 16.4 Â for A < 5050, 26.2 Â mean s.e.d. for the nuclei of three normal giant ellip- for 5050 < A < 7400, and 130 Â at longer wave- tical galaxies in the Virgo cluster taken from Paper I. lengths. The near-infrared spectral features of Paper I were not included because of bad weather. The scans III. SPECTRAL SYNTHESIS TECHNIQUE were reduced to absolute fluxes as described in Paper I. The automatic linear programming technique de- Data from a number of nights were combined, and veloped in Paper I is applied here. A library of s.e.d.’s the standard error of the mean averaged over all for different population components is first estab- wavelengths is 0.015 mag.