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Register a new userStellar Populations in the Andromeda V Dwarf Spheroidal Galaxy
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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.
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(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.
To understand the history and formation mechanisms of galaxies it is crucial to determine their current multidimensional structure. Here we focus on stellar population properties, such as metallicity and [$alpha$/Fe] enhancement. We devise a new technique to recover the distribution of these parameters using spatially resolved, line-of-sight averaged data. Our chemodynamical method is based on the made-to-measure (M2M) framework and results in an $N$-body model for the abundance distribution. We test our method on a mock data set and find that the radial and azimuthal profiles are well-recovered, however only the overall shape of the vertical profile matches the true profile. We apply our procedure to spatially resolved maps of mean [Z/H] and [$alpha$/Fe] for the Andromeda Galaxy, using an earlier barred dynamical model of M31. We find that the metallicity is enhanced along the bar, with possible maxima at the ansae. In the edge-on view the [Z/H] distribution has an X shape due to the boxy/peanut bulge; the average vertical metallicity gradient is equal to $-0.133pm0.006$ dex/kpc. We identify a metallicity-enhanced ring around the bar, which also has relatively lower [$alpha$/Fe]. The highest [$alpha$/Fe] is found in the centre, due to the classical bulge. Away from the centre, the $alpha$-overabundance in the bar region increases with height, which could be an indication of a thick disc. We argue that the galaxy assembly resulted in a sharp peak of metallicity in the central few hundred parsecs and a more gentle negative gradient in the remaining disc, but no [$alpha$/Fe] gradient. The formation of the bar lead to the re-arrangement of the [Z/H] distribution, causing a flat gradient along the bar. Subsequent star formation close to the bar ends may have produced the metallicity enhancements at the ansae and the [Z/H] enhanced lower-$alpha$ ring.
We present B,V time-series photometry of Andromeda XIX (And XIX), the most extended (half-light radius of 6.2) of Andromedas dwarf spheroidal companions, that we observed with the Large Binocular Cameras at the Large Binocular Telescope. We surveyed a 23x 23 area centered on And XIX and present the deepest color magnitude diagram (CMD) ever obtained for this galaxy, reaching, at V~26.3 mag, about one magnitude below the horizontal branch (HB). The CMD shows a prominent and slightly widened red giant branch, along with a predominantly red HB, which, however, extends to the blue to significantly populate the classical instability strip. We have identified 39 pulsating variable stars, of which 31 are of RR Lyrae type and 8 are Anomalous Cepheids (ACs). Twelve of the RR Lyrae variables and 3 of the ACs are located within And XIXs half light radius. The average period of the fundamental mode RR Lyrae stars (<Pab> = 0.62 d, sigma= 0.03 d) and the period-amplitude diagram qualify And XIX as an Oosterhoff-Intermediate system. From the average luminosity of the RR Lyrae stars (<V (RR)> = 25.34 mag, sigma= 0.10 mag) we determine a distance modulus of (m-M)$_0$=$24.52pm0.23$ mag in a scale where the distance to the Large Magellanic Cloud (LMC) is $18.5pm0.1$ mag. The ACs follow a well defined Period-Wesenheit (PW) relation that appears to be in very good agreement with the PW relationship defined by the ACs in the LMC.
We present a Bayesian method to identify multiple (chemodynamic) stellar populations in dwarf spheroidal galaxies (dSphs) using velocity, metallicity, and positional stellar data without the assumption of spherical symmetry. We apply this method to a new Keck/DEIMOS spectroscopic survey of the Ursa Minor (UMi) dSph. We identify 892 likely members, making this the largest UMi sample with line-of-sight velocity and metallicity measurements. Our Bayesian method detects two distinct chemodynamic populations with high significance ($ln{B}sim33$). The metal-rich ($[{rm Fe/H}]=-2.05pm0.03$) population is kinematically colder (radial velocity dispersion of $sigma_v=4.9pm0.8 , {rm km , s^{-1}}$) and more centrally concentrated than the metal-poor ($[{rm Fe/H}]=-2.29pm0.05$) and kinematically hotter population ($sigma_v =11.5pm0.9, {rm km , s^{-1}}$). Furthermore, we apply the same analysis to an independent MMT/Hectochelle data set and confirm the existence of two chemodynamic populations in UMi. In both data sets, the metal-rich population is significantly flattened ($epsilon=0.75pm0.03$) and the metal-poor population is closer to spherical ($epsilon=0.33_{-0.09}^{+0.12}$). Despite the presence of two populations, we are unable to robustly estimate the slope of the dynamical mass profile. We found hints for prolate rotation of order $sim 2 , {rm km , s^{-1}}$ in the MMT data set, but further observations are required to verify this. The flattened metal-rich population invalidates assumptions built into simple dynamical mass estimators, so we computed new astrophysical dark matter annihilation (J) and decay profiles based on the rounder, hotter metal-poor population and inferred $log_{10}{(J(0.5^{circ})/{rm GeV^{2} , cm^{-5}})}approx19.1$ for the Keck data set. Our results paint a more complex picture of the evolution of Ursa Minor than previously discussed.
We report on the discovery of a faint (M_V ~ -10.6 +/- 0.2) dwarf spheroidal galaxy on deep F606W and F814W Hubble Space Telescope images of a Virgo intracluster field. The galaxy is easily resolved in our images, as our color magnitude diagram (CMD) extends > 1 magnitude beyond the tip of the red giant branch (RGB). Thus, it is the deepest CMD for a small dwarf galaxy inside a cluster environment. Using the colors of the RGB stars, we derive a metal abundance for the dwarf of [M/H]= -2.3 +/- 0.3, and show that the metallicity dispersion is less than 0.6 dex at 95% confidence. We also use the galaxys lack of AGB stars and the absence of objects brighter than M_bol ~ -4.1 +/- 0.2 to show that the system is old (t >~10 Gyr). Finally, we derive the objects structural parameters, and show that the galaxy displays no obvious evidence of tidal threshing. Since the tip of the red giant branch distance ((m-M)_0 = 31.23 +/- 0.17 or D = 17.6 +/- 1.4 Mpc) puts the galaxy near the core of the Virgo cluster, one might expect the object to have undergone some tidal processing. Yet the chemical and morphological similarity between the dwarf and the dSph galaxies of the Local and M81 Group demonstrates that the object is indeed pristine, and not the shredded remains of a much larger galaxy. We discuss the possible origins of this galaxy, and suggest that it is just now falling into Virgo for the first time.
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