Dwarf spheroidals in the M 81 group – Metallicity distribution functions and population gradients

Lianou, S.; Grebel, Eva K.; Koch, Andreas

doi:10.1051/0004-6361/200913364

Cited by 32 publications

(58 citation statements)

References 67 publications

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“…For the five studied Galactic dSphs, we derive their photometric metallicities using linear interpolation between Dartmouth isochrones with a fixed age of 12.5 Gyr, with a range in metallicities from −2.5 dex to −0.3 dex and with a step size of 0.05 dex (e.g., Lianou et al 2010). In practice, for each star under consideration, we interpolate between the two closest isochrones that bracket its color in order to find its metallicity.…”

Section: Photometric Metallicitiesmentioning

confidence: 99%

“…The latter two techniques serve to either bracket the range of the metal abundances or to interpolate between them in order to derive the metallicity distribution function. Examples using the mean color of the RGB can be found in Mould et al (1983), Caldwell et al (1998), Grebel & Guhathakurta (1999), Caldwell (2006); using GC fiducials or analytic fits of GC fiducial loci in Harris et al (1999), Sarajedini et al (2002); using interpolation between theoretical tracks or isochrones in Harris & Harris (2000), Mouhcine et al (2005), Crnojevic et al (2010), Bird et al (2010), Lianou et al (2010). In the case of GCs, dwarf spheroidals (dSphs), and the stellar haloes of galaxies, the assumption under which these techniques are used is that the red giants represent populations of an old age (≥10 Gyr).…”

Section: Introductionmentioning

confidence: 99%

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Spectroscopic versus photometric metallicities: Milky Way dwarf spheroidal companions as a test case

Lianou

Grebel

Koch

2011

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Aims. The method of deriving photometric metallicities using red giant branch stars is applied to resolved stellar populations under the common assumption that they mainly consist of single-age old stellar populations. We explore the effect of the presence of mixed-age stellar populations on deriving photometric metallicities. Methods. We use photometric data sets for the five Galactic dwarf spheroidals Sculptor, Sextans, Carina, Fornax, and Leo II in order to derive their photometric metallicity distribution functions from their resolved red giant branches using isochrones of the Dartmouth Stellar Evolutionary Database. We compare the photometric metallicities with published spectroscopic metallicities based on the analysis of the near-infrared Ca triplet (Ca T), both on the metallicity scale of Carretta & Gratton and on the scale defined by the Dartmouth isochrones. In addition, we compare the photometric metallicities with published spectroscopic metallicities based on spectral synthesis and medium-resolution spectroscopy, and on high resolution spectra where available. Results. The mean properties of the spectroscopic and photometric metallicity samples are comparable within the intrinsic scatter of each method although the mean metallicities of dSphs with pronounced intermediate-age population fractions may be underestimated by the photometric method by up to a few tenths of dex in [Fe/H]. The star-by-star differences of the spectroscopic minus the photometric metallicities show a wide range of values along the fiducial spectroscopic metallicity range, with the tendency to have systematically lower photometric metallicities for those dwarf spheroidals with a higher fraction of intermediate-age populations. Such discrepancies persist even in the case of the purely old Sculptor dSph, where one would naïvely expect a very good match when comparing with medium or low resolution metallicity measurements. Overall, the agreement between Ca T metallicities and photometric metallicities is very good in the metallicity range from ∼−2 dex to ∼−1.5 dex. We find that the photometric method is reliable in galaxies that contain small (less than 15%) intermediate-age stellar fractions. Therefore, in the presence of mixed-age stellar populations, one needs to quantify the fraction of the intermediate-age stars in order to assess their effect on determining metallicities from photometry alone. Finally, we note that the comparison of spectroscopic metallicities of the same stars obtained with different methods reveals similarly large discrepancies as the comparison with photometric metallicities.

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Section: Photometric Metallicitiesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Spectroscopic versus photometric metallicities: Milky Way dwarf spheroidal companions as a test case

Lianou

Grebel

Koch

2011

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“…In particular, dwarf galaxies in nearby groups have been predominantly found to have similar properties to those of our close neighbors (e.g. Karachentsev et al 2002aKarachentsev et al ,b, 2003aKarachentsev et al ,b, 2004Karachentsev et al , 2007Trentham & Tully 2002;Karachentsev 2005;Sharina et al 2008;Bouchard et al 2009;Weisz et al 2008;Côté et al 2009;Dalcanton et al 2009;Koleva et al 2009;McQuinn et al 2009;McQuinn et al 2010;Crnojević et al 2010Crnojević et al , 2011bLianou et al 2010;Makarova et al 2010). A recent Hubble Space Telescope (HST) survey of dwarf galaxies (ANGST, Weisz et al 2011), comprising about 60 objects located within ∼4 Mpc, has led to a large compilation of color-magnitude diagrams (CMDs) of their resolved stellar populations.…”

Section: Introductionmentioning

confidence: 99%

A close look at the Centaurus A group of galaxies

Crnojević

Grebel

Cole

2012

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Aims. We study a sample of five dwarf irregular galaxies in the CenA/M 83 group, which are companions to the giant elliptical CenA. We aim at deriving their physical properties over their lifetime and compare them to those of dwarfs located in different environments. Methods. We use archival HST/ACS data and apply synthetic color-magnitude diagram fitting in order to reconstruct the past star formation activity of the target galaxies. Results. The average star formation rate for the studied galaxies ranges from 10 −3 M yr −1 up to ∼7 × 10 −2 M yr −1 , and their mean metallicities correlate with their luminosities (from [Fe/H] ∼ −1.4 up to ∼−1.0). The form of the star formation histories varies across the sample, with quiescent periods alternating with intermittent enhancements in the star formation (from a few up to several times the average lifetime value). The dwarfs in this sample formed ∼35% to 60% of their stellar content prior to ∼5 Gyr ago. Conclusions. The resulting star formation histories for the CenA companions are similar to those found for comparable Local Group and M 81 group dwarfs. We consider this sample of dwarfs together with five previously studied M 83 dwarf irregular companions. We find no trend of the average star formation rate with tidal index or distance from the main galaxy of the group. However, dwarfs with higher baryonic masses do show higher average star formation rates, underlining the importance of intrinsic properties in governing the evolution of these galaxies. On the other hand, there is also a clear trend when looking at the recent (∼0.5−1 Gyr) level of activity. Namely, dwarfs within a denser region of the group appear to have had their star formation quenched while dwarfs located in the group outskirts show a wide range of possible star formation rates, thus indicating that external processes play a fundamental role, complementary to mass, in shaping the star formation histories of dwarf galaxies.

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“…(7) and (8) of Table 2. We note here that we use Dartmouth isochrones (Dotter et al 2007(Dotter et al , 2008 to define the luminous AGB stars in order to remain consistent with our previous investigations (Lianou et al 2010(Lianou et al , 2011.…”

Section: Transition-type Dwarfsmentioning

confidence: 84%

“…We use for our analysis Dartmouth isochrones (Dotter et al 2007(Dotter et al , 2008, in order to remain consistent with our previous investigations on photometric metallicities (Lianou et al 2010(Lianou et al , 2011. Using Padova isochrones instead to derive the RGB photometric metallicities introduces an offset in metallicity on the order of ∼0.3 dex, making the stars appear more metalrich than when using Dartmouth isochrones (see Appendix B).…”

Section: Photometric Metallicity Distribution Functionsmentioning

confidence: 95%

Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group

Lianou¹,

Grebel²,

Costa³

et al. 2013

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Aims. We focus on the resolved stellar populations of one early-type and four transition-type dwarf galaxies in the Sculptor group, with the aim to examine the potential presence of population gradients and place constraints on their mean metallicities. Methods. We use deep Hubble Space Telescope images to construct color-magnitude diagrams, from which we select stellar populations that trace different evolutionary phases in order to constrain their range of ages and metallicities, as well as to examine their spatial distribution. In addition, we use the resolved stars in the red giant branch in order to derive photometric metallicities. Results. All studied dwarfs contain intermediate-age stars with ages of ∼1 Gyr and older as traced by the luminous asymptotic giant branch and red clump stars, while the transition-type dwarfs contain also stars younger than ∼1 Gyr as traced by a young main sequence and vertical red clump stars. Moreover, the spatial distribution of the stars that trace different evolutionary phases shows a population gradient in all transition-type dwarfs. The derived error-weighted mean metallicities, assuming purely old stellar populations, range from −1.5 dex for ESO294-G010 to −1.9 dex for Scl-dE1, and should be considered as lower limits to their true metallicities. Assuming intermediate-age stellar populations to dominate the dwarfs, we derive upper limits for the metallicities that are 0.3 to 0.2 dex higher than the metallicities derived assuming purely old populations. We discuss how photometric metallicity gradients are affected by the age-metallicity degeneracy, which prevents strong conclusions regarding their actual presence. Finally, the transition-type dwarfs lie beyond the virial radius of their closest bright galaxy, as also observed for the Local Group transition-type dwarfs. Scl-dE1 is the only dwarf spheroidal in our sample and is an outlier in a potential morphology-distance relation, similar as the two isolated dwarf spheroidals of the Local Group, Tucana, and Cetus.

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Dwarf spheroidals in the M 81 group – Metallicity distribution functions and population gradients

Cited by 32 publications

References 67 publications

Spectroscopic versus photometric metallicities: Milky Way dwarf spheroidal companions as a test case

Spectroscopic versus photometric metallicities: Milky Way dwarf spheroidal companions as a test case

A close look at the Centaurus A group of galaxies

Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group

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