* Astronomy

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NGC 221

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RE: Messier 32

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Title: An Ancient Metal-Poor Population in M32, and Halo Satellite Accretion in M31, Identified by RR Lyrae Stars
Authors: Ata Sarajedini, Soung-Chul Yang, Antonela Monachesi, Tod R. Lauer, Scott C. Trager

We present time-series photometry of two fields near M32 using archival observations from ACS/WFC onboard HST. One field is centred about 2 arcmin from M32 while the other is located 15 arcmin to the southeast of M31. We identify a total of 1139 RR Lyrae variables of which 821 are ab-type and 318 are c-type. In the field near M32, we find a radial gradient in the density of RR Lyraes relative to the center of M32. This gradient is consistent with the surface brightness profile of M32 suggesting that a significant number of the RR Lyraes in this region belong to M32. This provides further confirmation that M32 contains an ancient stellar population formed around the same time as the oldest population in M31 and the Milky Way. The RR Lyrae stars in M32 exhibit a mean metal abundance of [Fe/H] ~ -1.42 ±0.02, which is ~15 times lower than the metal abundance of the overall M32 stellar population. Moreover, the abundance of RR Lyrae stars normalised to the luminosity of M32 in the field analysed further indicates that the ancient metal-poor population in M32 represents only a very minor component of this galaxy, consistent with the 1% to 4.5% in mass inferred from the CMD analysis of Monachesi et al. In the other field, we find unprecedented evidence for two populations of RR Lyraes in M31 as shown by two distinct sequences among the ab-type variables in the Bailey Diagram. When interpreted in terms of metal abundance, one population exhibits a peak at [Fe/H] ~ -1.3 and the other is at [Fe/H] ~ -1.9. One possible interpretation of this result is that the more metal-rich population represents the dominant M31 halo, while the metal-poorer group could be a disrupted dwarf satellite galaxy orbiting M31. If true, this represents a further indication that the formation of the M31 spheroid has been significantly influenced by the merger and accretion of dwarf galaxy satellites.

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Title: The ancient stellar population of M32: RR Lyr Variable stars confirmed
Authors: G. Fiorentino, R. Contreras Ramos, E. Tolstoy, G. Clementini, A. Saha

Using archival multi--epoch ACS/WFC images in the F606W and F814W filters of a resolved stellar field in Local Group dwarf elliptical galaxy M32 we have made an accurate Colour-Magnitude Diagram and a careful search for RR Lyr variable stars. We identified 416 bona fide RR Lyr stars over our field of view, and their spatial distribution shows a rising number density towards the centre of M32. These new observations clearly confirm the tentative result of Fiorentino et al. (2010), on a much smaller field of view, associating an ancient population of RR Lyr variables to M32. We associate at least 83 RR Lyr stars in our field to M32. In addition the detection of 4 Anomalous Cepheids with masses in the range 1.2-1.9 Mo indicates the presence of relatively young, 1-4 Gyr old, stars in this field. They are most likely associated to the presence of the blue plume in the Colour-Magnitude Diagram. However these young stars are unlikely to be associated with M32 because the radial distribution of the blue plume does not follow the M32 density profile, and thus they are more likely to belong to the underlying M31 stellar population. Finally the detection of 3 Population II Cepheids in this field gives an independent measurement of the distance modulus in good agreement with that obtained from the RRLyr, mu0=24.33 ± 0.21 mag.

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Title: The Star Formation History of M32
Authors: Antonela Monachesi (1,2), Scott C. Trager (1), Tod R. Lauer (3), Sebastián L. Hidalgo (4), Wendy Freedman (5), Alan Dressler (5), Carl Grillmair (6), Kenneth J. Mighell (3) ((1) Kapteyn Astronomical Institute, University of Groningen, (2) Department of Astronomy, University of Michigan, (3) NOAO - National Optical Astronomy Observatory, (4) IAC - Instituto de Astrofísica de Canarias, (5) OCIW - Observatories of the Carnegie Institution of Washington, (6) Spitzer Science Centre)

We use deep HST ACS/HRC observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5Gyr old stars contribute ~40% ±17% of M32's mass, while 55% ±21% of M32's mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are <Age>=6.8 ±1.5 Gyr and <[M/H]>=-0.01 ±0.08 dex. The SFH additionally indicates the presence of young (<2 Gyr old), metal-poor ([M/H] ~-0.7) stars, suggesting that blue straggler stars contribute ~2% of the mass at F1; the remaining ~3% of the mass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single-stellar-population-equivalent parameters of 2.9 ±0.2 Gyr and [M/H]=0.02 ±0.01 dex at F1 (~2.7 re). This contradicts spectroscopic studies that show a steep age gradient from M32's center to 1re. The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with ~95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; ~30% ±7.5% of its mass is in a 5-8 Gyr old population, and ~65% ±9% of the mass is in a 8-14 Gyr old population. The mass-weighted mean age and metallicity of F2 are <Age>=9.2 ±1.2 Gyr and <[M/H]>=-0.10 ±0.10 dex, respectively. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31's disk at F2 (~1 disk scale length) to be 5-9 Gyr, our results agree with an inside-out disk formation scenario for M31's disk.