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Register a new userOrigin of Two Distinct Populations in Dwarf Spheroidal Galaxies
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Authors
Daisuke Kawata
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We study the chemical and kinematic properties of the first galaxies which formed at a high redshift, using high resolution cosmological numerical simulations, and compared them with the recent observational results for the Sculptor dwarf spheroidal galaxy by Tolstoy et al., who found two distinct stellar populations: the lower metallicity stars are more spatially extended and possess a higher velocity dispersion than the higher metallicity stars. Our calculations reproduce these observations as the result of a steep metallicity gradient, within a single populations, induced by dissipative collapse of the gas component. We also predict strong [N/O] enhancements in the lowest metallicity stars in dwarf spheroidals, due to the preferential retention of ejected gas from intermediate mass stars, compared to Type II supernovae.
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We present new spectroscopic data for twenty six stars in the recently-discovered Canes Venatici I (CVnI) dwarf spheroidal galaxy. We use these data to investigate the recent claim of the presence of two dynamically inconsistent stellar populations in this system (Ibata et al., 2006). We do not find evidence for kinematically distinct populations in our sample and we are able to obtain a mass estimate for CVnI that is consistent with all available data, including previously published data. We discuss possible differences between our sample and the earlier data set and study the general detectability of sub-populations in small kinematic samples. We conclude that in the absence of supporting observational evidence (for example, metallicity gradients), sub-populations in small kinematic samples (typically fewer than 100 stars) should be treated with extreme caution, as their detection depends on multiple parameters and rarely produces a signal at the 3sigma confidence level. It is therefore essential to determine explicitly the statistical significance of any suggested sub-population.
Dwarf spheroidal galaxies are the most dark matter dominated systems in the nearby Universe and their origin is one of the outstanding puzzles of how galaxies form. Dwarf spheroidals are poor in gas and stars, making them unusually faint, and those known as ultra-faint dwarfs have by far the lowest measured stellar content of any galaxy. Previous theories require that dwarf spheroidals orbit near giant galaxies like the Milky Way, but some dwarfs have been observed in the outskirts of the Local Group. Here we report simulations of encounters between dwarf disk galaxies and somewhat larger objects. We find that the encounters excite a process, which we term ``resonant stripping, that can transform them into dwarf spheroidals. This effect is distinct from other mechanisms proposed to form dwarf spheroidals, including mergers, galaxy-galaxy harassment, or tidal and ram pressure stripping, because it is driven by gravitational resonances. It may account for the observed properties of dwarf spheroidals in the Local Group, including their morphologies and kinematics. Resonant stripping predicts that dwarf spheroidals should form through encounters, leaving detectable long stellar streams and tails.
We provide manganese abundances (corrected for the effect of the hyperfine structure) for a large number of stars in the dwarf spheroidal galaxies Sculptor and Fornax, and for a smaller number in the Carina and Sextans dSph galaxies. Abundances had already been determined for a number of other elements in these galaxies, including alpha and iron-peak ones, which allowed us to build [Mn/Fe] and [Mn/alpha] versus [Fe/H] diagrams. The Mn abundances imply sub-solar [Mn/Fe] ratios for the stars in all four galaxies examined. In Sculptor, [Mn/Fe] stays roughly constant between [Fe/H]sim -1.8 and -1.4 and decreases at higher iron abundance. In Fornax, [Mn/Fe] does not vary in any significant way with [Fe/H]. The relation between [Mn/alpha] and [Fe/H] for the dSph galaxies is clearly systematically offset from that for the Milky Way, which reflects the different star formation histories of the respective galaxies. The [Mn/alpha] behavior can be interpreted as a result of the metal-dependent Mn yields of type II and type Ia supernovae. We also computed chemical evolution models for star formation histories matching those determined empirically for Sculptor, Fornax, and Carina, and for the Mn yields of SNe Ia, which were assumed to be either constant or variable with metallicity. The observed [Mn/Fe] versus [Fe/H] relation in Sculptor, Fornax, and Carina can be reproduced only by the chemical evolution models that include a metallicity-dependent Mn yield from the SNe Ia.
Using archival imaging from the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope, we investigate the stellar populations of the Local Group dwarf spheroidal Andromeda V - a companion satellite galaxy of M31. The color-magnitude diagram (CMD) extends from above the first ascent red giant branch (RGB) tip to approximately one magnitude below the horizontal branch (HB). The steep well-defined RGB is indicative of a metal-poor system while the HB is populated predominantly redward of the RR Lyrae instability strip. Utilizing Galactic globular cluster fiducial sequences as a reference, we calculate a mean metallicity of [Fe/H] = -2.20 +/- 0.15 and a distance of (m-M)0 = 24.57 +/- 0.04 after adopting a reddening of E(B-V) = 0.16. This metal abundance places And V squarely in the absolute magnitude - metallicity diagram for dwarf spheroidal galaxies. In addition, if we attribute the entire error-corrected color spread of the RGB stars to an abundance spread, we estimate a range of ~0.5 dex in the metallicities of And V stars. Our analysis of the variable star population of And V reveals the presence of 28 potential variables. Of these, at least 10 are almost certainly RR Lyrae stars based on their time sequence photometry.
(abridged) We present B,V,I CCD photometry of about 40000 stars in four regions of the Fornax dSph. Using the resultant CMDs, many already known age-tracers are investigated, and new CMD features are also detected: we show that blue HB stars may be present in the outer regions, and measure the luminosity of the AGB bump. We measure a corrected distance modulus (m-M)o = 20.70+/-0.12, based on the RGB tip luminosity, which is in good agreement with that obtained from the mean magnitude of old HB stars [(m-M)o=20.76+/-0.04] and with previous results. The (B-I) distribution of the RG stars can be approximately described as the superposition of two populations. The dominant component, comprising ~70% of the red giant stars, consists of relatively metal-enriched intermediate-age stars, whose mean metallicity is [Fe/H]=-1.39+/-0.15 (the age-corrected metallicity would be [Fe/H] ~ -1.0+/-0.15). The dominant intermediate-age component has an intrinsic color dispersion sigma(B-I)=0.06+/-0.01mag, corresponding to a relatively low abundance dispersion, sigma[Fe/H]=0.12+/-0.02dex. In addition, there is a small population of giants on the blue side of the RGB, whose spatial distribution is consistent with that of old horizontal branch stars, and completely different from that of the younger population. This unambiguously qualifies them as old and metal-poor. The exceptional color width of the Fornax RGB is therefore due to the presence of two main populations, yielding a large abundance range (-2.0<[Fe/H]<-0.7). This evidence suggests a scenario in which this galaxy started forming a stellar halo and its surrounding clusters together about 10-13 Gyr ago, then a major SF episode occurred after several Gyr.
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