Population Gradients in Local Group Dwarf Spheroidal Galaxies

By Daniel Harbeck, E. Grebel, J. Holtzman, P. Guhathakurta, W. Brandner, D. Geisler, A. Sarajedini, A. Dolphin, D. Hurley-Keller, M. Mateo. 2001

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We present a systematic and homogeneous analysis of population gradients for the Local Group dwarf spheroidal galaxies (dSph's) Carina, Sculptor, Sextans, Tucana, and Andromeda I–III, V, and VI. For all of the Milky Way companions studied here, we find significant population gradients. The same is true for the remote dSph Tucana located at the outskirts of the Local Group. Among the M31 dSph companions, only Andromeda I and VI show obvious gradients. In all cases where a horizontal-branch (HB) morphology gradient is visible, the red HB stars are more centrally concentrated. The occurrence of a HB morphological gradient shows a correlation with a morphology gradient in the red giant branch. It seems likely that metallicity is the driver of the gradients in Sextans, Sculptor, Tucana, and Andromeda VI, while age is an important factor in Carina. We find no evidence that the vicinity of a nearby massive spiral galaxy influences the formation of the population gradients.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/d50870b9eebcc94ce27e40dd212a0c5137d4df3d

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A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: searching for the least massive dwarf galaxies (2007, background): “…is growing evidence that many dwarf galaxies harbor kinematically and structurally distinct populations or at least population gradients (see e.g. Harbeck et al. 2001; Tolstoy et al. 2004 for Sculptor; Battaglia et al. 2006 for Fornax), CVnI is to date the most extreme case of such a behaviour.”
“Sculptor-ing” the Galaxy? The Chemical Compositions of Red Giants in the Sculptor Dwarf Spheroidal Galaxy (2004, background): “Hurley-Keller et al. (1999), Majewski et al. (1999), and Harbeck et al. (2001) found a gradient in the morphology of the horizontal branch (HB), with a much higher percentage of red HB stars in the inner 1 The estimation ofmetallicities bymeasurement of the Ca ii triplet has proven to be very…”
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H i IN LOCAL GROUP DWARF GALAXIES AND STRIPPING BY THE GALACTIC HALO (2009, background): “Nondetection of HI towards And III was confirmed by Robishaw et al. (2000), and the velocity measured by Harbeck et al. (2001) of Andromeda V indicates that the detection in BR00 is false.”
The DART Imaging And CaT Survey of the Fornax Dwarf Spheroidal Galaxy (2006, background): “Population gradients have been detected in several other dwarf spheroidal galaxies in the Local Group from imaging (e.g., Harbeck et al. 2001), and spectroscopy (e.g., Tolstoy et al. 2004).”
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Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. II. Early-Type Dwarfs with Central Star Formation (2006, background): “In all of these cases, the younger populations are more centrally concentrated than the old ones (Harbeck et al. 2001).”
Redefining the Missing Satellites Problem (2007, background): “Some dSphs show evidence for multiple, dynamically distinct stellar populations, with each population described by its own surface density and velocity dispersion (Harbeck et al. 2001; Tolstoy et al. 2004; Westfall et al. 2006; McConnachie et al. 2006; Ibata et al. 2006).”
A Comprehensive Model for the Monoceros Tidal Stream (2004, background): “…with the highest metallicity, as star formation nd metallicity enrichment happen more intensely in the center of dwarf galaxies (Pagel & Edmunds 1981, Harbeck et al. 2001) and 2) those processes generating or enhancing any metallicity gradient have stopped, or are very slow, in the last 3 Gyr5.”
The Observed Properties of Dark Matter on Small Spatial Scales (2007, background): “It can be shown, however, that younger and/ormoremetal-rich populations aremore centrally concentrated (e.g., Harbeck et al. 2001) and that these populations also tend to exhibit lower velocity dispersions (Tolstoy et al. 2004).”
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The Progenitors of Dwarf Spheroidal Galaxies (2002, background): “…episodes (Shetrone et al. 2001, 2003; Ikuta & Arimoto 2002; Carigi, Hernandez & Gilmore 2002) as well as radial gradients in old populations (Harbeck et al. 2001) indicate that dSphs were not only able to hold on to a significant amount of the metals that they produced, but also experienced…”
The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2 (2018, result): “The difference in metallicities is only marginal, however it is not unexpected, as similar trends with centrally concentrated more metal-rich stellar populations have been observed in many (especially classical) dwarf galaxies (Harbeck et al. 2001; Koch et al. 2006).”
The Adequacy of Stellar Evolution Models for the Interpretation of the Color-Magnitude Diagrams of Resolved Stellar Populations (2005, background): “…indications have been found, many of the “structural” second parameter candidates (e.g., high mass loss due to high concentration), are less likely to be effective in dSphs, and differences in HB morphology are easier to explain in terms of age and metallicity gradients (Harbeck et al. 2001).”
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Formation Histories of Dwarf Galaxies in the Local Group (2004, methodology): “…for a specific range of metallicities and ages using the so-called “horizontal branch index” (G. Gilmore, private communication; also see Zinn 1993; Harbeck et al. 2001; Makey & Gilmore 2004): younger galaxies have redder horizontal branch for the same metallicity, although, how precise the…”
New calibrations for abundance determinations in H ii regions (2016, background): “…population and may show abundance gradients, which in turn hold clues about galaxy evolution (e.g., Searle 1971; Janes 1979; Maciel & Quireza 1999; Harbeck et al. 2001; Andrievsky et al. 2002; Chen et al. 2003; Mehlert et al. 2003; Cioni 2009; Haschke et al. 2012; Boeche et al. 2013, 2014;…”