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The Deepest Hubble Space Telescope Color-Magnitude Diagram of M32. Evidence for Intermediate-age Populations

Monachesi, Antonela and Trager, Scott C. and Lauer, Tod R. and Freedman, Wendy and Dressler, Alan and Grillmair, Carl and Mighell, Kenneth J. (2011) The Deepest Hubble Space Telescope Color-Magnitude Diagram of M32. Evidence for Intermediate-age Populations. Astrophysical Journal, 727 (1). Art. No. 55. ISSN 0004-637X.

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We present the deepest optical color-magnitude diagram (CMD) to date of the local elliptical galaxy M32. We have obtained F435W and F555W photometries based on Hubble Space Telescope (HST) Advanced Camera for Surveys/High-Resolution Channel images for a region 110" from the center of M32 (F1) and a background field (F2) about 320" away from M32 center. Due to the high resolution of our Nyquist-sampled images, the small photometric errors, and the depth of our data (the CMD of M32 goes as deep as F435W ~ 28.5 at 50% completeness level), we obtain the most detailed resolved photometric study of M32 yet. Deconvolution of HST images proves to be superior than other standard methods to derive stellar photometry on extremely crowded HST images, as its photometric errors are ~2× smaller than other methods tried. The location of the strong red clump in the CMD suggests a mean age between 8 and 10 Gyr for [Fe/H] = –0.2 dex in M32. We detect for the first time a red giant branch bump and an asymptotic giant branch (AGB) bump in M32 which, together with the red clump, allow us to constrain the age and metallicity of the dominant population in this region of M32. These features indicate that the mean age of M32's population at ~2' from its center is between 5 and 10 Gyr. We see evidence of an intermediate-age population in M32 mainly due to the presence of AGB stars rising to M _(F555W) ~ –2.0. Our detection of a blue component of stars (blue plume) may indicate for the first time the presence of a young stellar population, with ages of the order of 0.5 Gyr, in our M32 field. However, it is likely that the brighter stars of this blue plume belong to the disk of M31 rather than to M32. The fainter stars populating the blue plume indicate the presence of stars not younger than 1 Gyr and/or BSSs in M32. The CMD of M32 displays a wide color distribution of red giant branch stars indicating an intrinsic spread in metallicity with a peak at [Fe/H] ~ –0.2. There is not a noticeable presence of blue horizontal branch stars, suggesting that an ancient population with [Fe/H] < –1.3 does not significantly contribute to the light or mass of M32 in our observed fields. M32's dominant population of 8-10 Gyr implies a formation redshift of 1 ≲ z_f ≲ 2, precisely when observations of the specific star formation rates and models of "downsizing" imply galaxies of M32's mass ought to be forming their stars. Our CMD therefore provides a "ground truth" of downsizing scenarios at z = 0. Our background field data represent the deepest optical observations yet of the inner disk and bulge of M31. Its CMD exhibits a broad color spread of red giant stars indicative of its metallicity range with a peak at [Fe/H] ~ –0.4 dex, slightly more metal-poor than M32 in our fields. The observed blue plume consists of stars as young as 0.3 Gyr, in agreement with previous works on the disk of M31. The detection of bright AGB stars reveals the presence of intermediate-age population in M31, which is, however, less significant than that in M32 at our field's location.

Item Type:Article
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URLURL TypeDescription
Freedman, Wendy0000-0003-3431-9135
Grillmair, Carl0000-0003-4072-169X
Additional Information:© 2011 American Astronomical Society. Received 2010 July 5; accepted 2010 November 17; published 2011 January 3. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with GO proposal 10572. A.M. is grateful to Carme Gallart for very valuable discussions. Special thanks to Eline Tolstoy, Giuliana Fiorentino, Annette Ferguson, and Kathryn Johnston for useful comments and suggestions, and also to Eva Busekool for bringing to our attention to the BP in the archival WFPC2 observations. We are grateful to Kirsten Howley for providing us with the values of the contributions of M31 and M32 light to our fields before publication We thank Peter Stetson for providing his stand-alone DAOPHOT II software and guidance on its use. We thank the referee, Ivo Saviane, for his careful reading of the manuscript and comments that helped to improve this paper. This work has made use of the IAC-STAR synthetic CMD computation code. IAC-STAR is supported and maintained by the computer division of the Instituto de Astrofísica de Canarias. NOVA is acknowledged for financial support. Support for program GO- 10572 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Facility: HST (ACS)
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Instituto de Astrofisica de Canarias (IAC)UNSPECIFIED
Nederlandse Onderzoekschool voor de Astronomie (NOVA)UNSPECIFIED
NASANAS 5-26555
Subject Keywords:galaxies: elliptical and lenticular, cD – galaxies: evolution – galaxies: individual (M32, M31) – galaxies: photometry – galaxies: stellar content – Local Group
Issue or Number:1
Classification Code:PACS: 98.56.Ew, 98.56.Ne, 98.62.Lv, 97.20.Li, 98.62.Hr
Record Number:CaltechAUTHORS:20110323-111237005
Persistent URL:
Official Citation:Antonela Monachesi et al. 2011 ApJ 727 55 doi: 10.1088/0004-637X/727/1/55
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:23076
Deposited By: Benjamin Perez
Deposited On:23 Mar 2011 18:37
Last Modified:17 Nov 2019 06:03

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