3,925 research outputs found
The star formation history of the Sculptor Dwarf Irregular Galaxy
[abridged] We study the resolved stellar populations and derive the SFH of
the SDIG, a gas-rich dwarf galaxy member of the NGC7793 subgroup in the
Sculptor group. We construct a CMD using archival HST observations and examine
its stellar content. We derive its SFH using a maximum-likelihood fit to the
CMD. The CMD shows that SDIG contains stars from 10Myr to several Gyr old, as
revealed from the MS, BL, luminous AGB, and RGB stars. The young stars with
ages less than ~250Myr show a spatial distribution confined to its central
regions, and additionally the young MS stars exhibit an off-center density
peak. The intermediate-age and older stars are more spatially extended. SDIG is
dominated by intermediate-age stars with an average age of 6.4Gyr. The average
metallicity inferred is [M/H]\approx -1.5dex. Its SFH is consistent with a
constant SFR, except for ages younger than ~200Myr. The lifetime average SFR is
1.3x10^{-3} Mo/yr. More recently than 100Myr, there has been a burst of SF at a
rate ~2-3 times higher than the average SFR. The inferred recent SFR from CMD
modelling is higher than inferred from the Ha flux of the galaxy; we interpret
this to mean that the upper end of the IMF is not being fully sampled due to
the low SFR. Additionally, an observed lack of bright blue stars in the CMD
could indicate a downturn in SFR on 10^7-yr timescales. A previous SF
enhancement appears to have occurred between 600-1100Myr ago, with amplitude
similar to the most recent 100Myr. Older bursts of similar peak SFR and
duration would not be resolvable with these data. The observed enhancements in
SF suggest that SDIG is able to sustain a complex SFH without the effect of
interactions with its nearest massive galaxy. Integrating the SFR over the
entire history of SDIG yields a total stellar mass 1.77x10^{7}Mo, and a current
V-band stellar mass-to-light ratio 3.2Mo/Lo.Comment: A&A accepted; 10 pages, 9 figure
Photometric metallicity map of the Large Magellanic Cloud
We have estimated a metallicity map of the Large Magellanic Cloud (LMC) using
the Magellanic Cloud Photometric Survey (MCPS) and Optical Gravitational
Lensing Experiment (OGLE III) photometric data. This is a first of its kind map
of metallicity up to a radius of 4 - 5 degrees, derived using photometric data
and calibrated using spectroscopic data of Red Giant Branch (RGB) stars. We
identify the RGB in the V, (VI) colour magnitude diagrams of small
subregions of varying sizes in both data sets. We use the slope of the RGB as
an indicator of the average metallicity of a subregion, and calibrate the RGB
slope to metallicity using spectroscopic data for field and cluster red giants
in selected subregions. The average metallicity of the LMC is found to be
[Fe/H] = 0.37 dex ([Fe/H] = 0.12) from MCPS data, and [Fe/H] =
0.39 dex ([Fe/H] = 0.10) from OGLE III data. The bar is found be the
most metal-rich region of the LMC. Both the data sets suggest a shallow radial
metallicity gradient up to a radius of 4 kpc (0.0490.002 dex kpc
to 0.0660.006 dex kpc). Subregions in which the mean metallicity
differs from the surrounding areas do not appear to correlate with previously
known features; spectroscopic studies are required in order to assess their
physical significance.Comment: 28 pages, 40 figures, 7 tables, Accepted for publication in MNRAS.
arXiv admin note: text overlap with arXiv:1101.1771, arXiv:1302.6211 by other
author
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The 2Mass Color-magnitude Diagram of the Center of the Sagittarius Dwarf Galaxy: Photometric Measurements of a Surprisingly High Mean Metallicity
We present the Two Micron All Sky Survey (2MASS) (J−K, K) color-magnitude diagram for the region within 1◦ of the center of the Sagittarius dwarf spheroidal galaxy. Using the slope of the red giant branch (RGB), we determine a mean metallicity for the main stellar population of [Fe/H] = −0.5 ±0.2. The Sagittarius RGB possesses a blue tail that overlaps with the foreground Milky Way giant branch, and suggests that ∼ 1/3 of the RGB is more metal-poor than [Fe/H] ~−1. Direct comparison to the Large Magellanic Cloud confirms the metal-rich nature of the bulk of the Sagittarius population. Our result is marginally consistent with the even higher metallicities determined from high-resolution spectroscopy
Ca II triplet spectroscopy of RGB stars in NGC 6822: kinematics and metallicities
We present a detailed analysis of the chemistry and kinematics of red giants
in the dwarf irregular galaxy NGC 6822. Spectroscopy at 8500 Angstroms was
acquired for 72 red giant stars across two fields using FORS2 at the VLT. Line
of sight extinction was individually estimated for each target star to
accommodate the variable reddening across NGC 6822. The mean radial velocity
was found to be v_helio = (52.8 +/- 2.2) km/s with dispersion rms = 24.1 km/s,
in agreement with other studies. Ca II triplet equivalent widths were converted
into [Fe/H] metallicities using a V magnitude proxy for surface gravity. The
average metallicity was [Fe/H] = (-0.84 +/- 0.04) with dispersion rms = 0.31
dex and interquartile range 0.48. Our assignment of individual reddening values
makes our analysis more sensitive to spatial variations in metallicity than
previous studies. We divide our sample into metal-rich and metal-poor stars;
the former are found to cluster towards small radii with the metal-poor stars
more evenly distributed across the galaxy. The velocity dispersion of the
metal-poor stars is higher than that of the metal-rich stars; combined with the
age-metallicity relation this indicates that older populations have either been
dynamically heated or were born in a less disclike distribution. The low ratio
(v_rot/v_rms) suggests that within the inner 10', NGC 6822's stars are
dynamically decoupled from the HI gas, possibly in a thick disc or spheroid.Comment: 15 pages, 12 figures, includes tabular dat
Comparing the Ancient Star Formation Histories of the Magellanic Clouds
We present preliminary results from a new HST archival program aimed at
tightly constraining the ancient (>4 Gyr ago) star formation histories (SFHs)
of the field populations of the SMC and LMC. We demonstrate the quality of the
archival data by constructing HST/WFPC2-based color-magnitude diagrams (CMDs;
M_{F555W} ~ +8) for 7 spatially diverse fields in the SMC and 8 fields in the
LMC. The HST-based CMDs are >2 magnitudes deeper than any from ground based
observations, and are particularly superior in high surface brightness regions,
e.g., the LMC bar, which contain a significant fraction of star formation and
are crowding limited from ground based observations. To minimize systematic
uncertainties, we derive the SFH of each field using an identical maximum
likelihood CMD fitting technique. We then compute an approximate mass weighted
average SFH for each galaxy. We find that both galaxies lack a dominant burst
of early star formation, which suggests either a suppression or an
under-fueling of early star formation. From 10-12 Gyr ago, the LMC experienced
a period of enhanced stellar mass growth relative to the SMC. Similar to some
previous studies, we find two notable peaks in the SFH of the SMC at ~4.5 and 9
Gyr ago, which could be due to repeated close passages with the LMC, implying
an interaction history that has persisted for at least 9 Gyr. We find little
evidence for strong periodic behavior in the lifetime SFHs of both MCs,
suggesting that repeated encounters with the Milky Way are unlikely. Beginning
~3.5 Gyr ago, both galaxies show increases in their SFHs, in agreement with
previous studies, and thereafter, track each other remarkably well. (abridged)Comment: 9 pages, 5 Figures, Accepted for Publication in MNRA
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