24 research outputs found
Stellar Populations in the Outskirts of the Small Magellanic Cloud: No Outer Edge Yet
We report the detection of intermediate-age and old stars belonging to the
SMC at 6.5 kpc from the SMC center in the southern direction. We show, from the
analysis of three high quality 34\arcmin 33\arcmin CMDs, that the age
composition of the stellar population is similar at galactocentric distances of
4.7 kpc, 5.6 kpc, and 6.5 kpc. The surface
brightness profile of the SMC follows an exponential law, with no evidence of
truncation, all the way out to 6.5 kpc. These results, taken together, suggest
that the SMC `disk' population is dominating over a possible old Milky Way-like
stellar halo, and that the SMC may be significantly larger than previously
thought.Comment: Accepted for publication in ApJ Letters. High resolution figures are
available at ftp://ftp.iac.es/out/noe
The Chemical Enrichment History of the Small Magellanic Cloud and Its Gradients
We present stellar metallicities derived from Ca II triplet spectroscopy in
over 350 red giant branch stars in 13 fields distributed in different positions
in the SMC, ranging from 1\arcdeg\@ to 4\arcdeg\@ from its center.
In the innermost fields the average metallicity is [Fe/H] . This value
decreases when we move away towards outermost regions. This is the first
detection of a metallicity gradient in this galaxy. We show that the
metallicity gradient is related to an age gradient, in the sense that more
metal-rich stars, which are also younger, are concentrated in the central
regions of the galaxy.Comment: 30 pages, 13 figures, accepted for publication in Astronomical
Journa
EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies
Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to
the Milky Way and other massive spiral galaxies. As such, their projected
stellar ellipticity and extended light distributions are often thought to owe
to tidal forces. In this paper, we study the projected stellar ellipticities
and faint stellar outskirts of tidally isolated ultra-faints drawn from the
'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) cosmological simulation
suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range
of projected ellipticities (), with many possessing
anisotropic extended stellar haloes that mimic tidal tails, but owe instead to
late-time accretion of lower mass companions. Furthermore, we find a strong
causal relationship between ellipticity and formation time of an UFD, which is
robust to a wide variation in the feedback model. We show that the distribution
of projected ellipticities in our suite of simulated EDGE dwarfs matches well
with that of 21 Local Group dwarf galaxies. Given the ellipticity in EDGE
arises from an ex-situ accretion origin, the agreement in shape indicates the
ellipticities of some observed dwarfs may also originate from a similar
non-tidal scenario. The orbital parameters of these observed dwarfs further
support that they are not currently tidally disrupting. If the baryonic content
in these galaxies is still tidally intact, then the same may be true for their
dark matter content, making these galaxies in our Local Group pristine
laboratories for testing dark matter and galaxy formation models.Comment: 10 pages, 4 figures; submitted to MNRA
Old Main Sequence Turnoff Photometry in the Small Magellanic Cloud. II. Star Formation History and Its Spatial Gradients
We present a quantitative analysis of the SFH of 12 fields in the SMC. We
find that there are four main periods of enhancement of star formation: a young
one peaked at around 0.2-0.5 Gyr old, only present in the eastern and in the
central-most fields; two at intermediate ages present in all fields (a
conspicuous one peaked at 4-5 Gyr old, and a less significant one peaked at
1.5-2.5); and an old one, peaked at 10 Gyr in all fields but the western ones.
In the western fields, this old enhancement splits into two, one peaked at
around 8 Gyr old and another at around 12 Gyr old. This "two-enhancement" zone
seems to be a robust feature since it is unaffected by our choice of stellar
evolutionary library but more data covering other fields of the SMC are
necessary in order to ascertain its significancy.
Some correlation could exist with encounters taken from the orbit
determination of Kallivayalil et al. (2006). But our results would be also fit
in a first pericenter passage scenario like the one claimed by Besla et al.
(2007). There is a strong dichotomy between East/Southeast and West in the
current irregular shape of the SMC. We find that this dichotomy is produced by
the youngest population and began about 1 Gyr ago or later. We do not find yet
a region dominated by an old halo at 4.5 kpc from the SMC center, indicating
either that this old stellar halo does not exist in the SMC or that its
contribution to the stellar populations, at the galactocentric distances of our
outermost field, is negligible. We derive the age-metallicity relation and find
that the metallicity increased continuously from early epochs until now.Comment: Accepted for publication in AJ, 39 pages, 13 Postscript figures. High
resolution available at:
http://www.iac.es/galeria/noelia/PaperII_Figures/index.html or via email to:
[email protected]
EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies
Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. In this paper, we study the projected stellar ellipticities and faint stellar outskirts of tidally isolated ultra-faints drawn from the ‘Engineering Dwarfs at Galaxy Formation’s Edge’ (EDGE) cosmological simulation suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range of projected ellipticities (0.03 < ε < 0.85), with many possessing anisotropic extended stellar haloes that mimic tidal tails, but owe instead to late-time accretion of lower mass companions. Furthermore, we find a strong causal relationship between ellipticity and formation time of a UFD, which is robust to a wide variation in the feedback model. We show that the distribution of projected ellipticities in our suite of simulated EDGE dwarfs matches well with a sample of 19 Local Group dwarf galaxies and a sample of 11 isolated dwarf galaxies. Given ellipticity in EDGE arises from an ex-situ accretion origin, the agreement in shape indicates the ellipticities of some observed dwarfs may also originate from a non-tidal scenario. The orbital parameters of these observed dwarfs further support that they are not currently tidally disrupting. If the baryonic content in these galaxies is still tidally intact, then the same may be true for their dark matter content, making these galaxies in our Local Group pristine laboratories for testing dark matter and galaxy formation models
Slicing The Monoceros Overdensity with Suprime-Cam
We derive distance, density and metallicity distribution of the stellar
Monoceros Overdensity (MO) in the outer Milky Way, based on deep imaging with
the Subaru Telescope. We applied CMD fitting techniques in three stripes at
galactic longitudes: l=130 deg, 150 deg, 170 deg; and galactic latitudes: +15 <
b [deg] < +25 . The MO appears as a wall of stars at a heliocentric distance of
~ 10.1\pm0.5 kpc across the observed longitude range with no distance change.
The MO stars are more metal rich ([Fe/H] ~ -1.0) than the nearby stars at the
same latitude. These data are used to test three different models for the
origin of the MO: a perturbed disc model, which predicts a significant drop in
density adjacent to the MO that is not seen; a basic flared disc model, which
can give a good match to the density profile but the MO metallicity implies the
disc is too metal rich to source the MO stars; and a tidal stream model, which
bracket the distances and densities we derive for the MO, suggesting that a
model can be found that would fully fit the MO data. Further data and modeling
will be required to confirm or rule out the MO feature as a stream or as a
flaring of the disc.Comment: 15 pages, 12 figures, accepted for publication in Ap
The intrinsic reddening of the Magellanic Clouds as traced by background galaxies -- II. The Small Magellanic Cloud
We present a map of the total intrinsic reddening across ~34 deg of the
Small Magellanic Cloud (SMC) derived using optical () and near-infrared
(IR; ) spectral energy distributions (SEDs) of background
galaxies. The reddening map is created using a subsample of 29,274 galaxies
with low levels of intrinsic reddening based on the LePhare
minimisation SED-fitting routine. We find statistically significant enhanced
levels of reddening associated with the main body of the SMC compared with
regions in the outskirts [ mag]. A comparison with
literature reddening maps of the SMC shows that, after correcting for
differences in the volume of the SMC sampled, there is good agreement between
our results and maps created using young stars. In contrast, we find
significant discrepancies between our results and maps created using old stars
or based on longer wavelength far-IR dust emission that could stem from biased
samples in the former and uncertainties in the far-IR emissivity and the
optical properties of the dust grains in the latter. This study represents one
of the first large-scale categorisations of extragalactic sources behind the
SMC and as such we provide the LePhare outputs for our full sample of ~500,000
sources.Comment: Accepted for publication in MNRAS, 13 pages, 6 figures and 4 tables.
Tables 1, 3 and 4 will be available in full as Supporting Information in the
online version of the articl