5,395 research outputs found
Extragalactic archeology with the GHOSTS Survey I. - Age-resolved disk structure of nearby low-mass galaxies
We study the individual evolution histories of three nearby low-mass edge-on
galaxies (IC 5052, NGC4244, and NGC5023). Using resolved stellar populations,
we constructed star count density maps for populations of different ages and
analyzed the change of structural parameters with stellar age within each
galaxy.
We do not detect a separate thick disk in any of the three galaxies, even
though our observations cover a wider range in equivalent surface brightness
than any integrated light study. While scale heights increase with age, each
population can be well described by a single disk. Two of the galaxies contain
a very weak additional component, which we identify as the faint halo. The mass
of these faint halos is lower than 1% of the mass of the disk. The three
galaxies show low vertical heating rates, which are much lower than the heating
rate of the Milky Way. This indicates that heating agents, such as giant
molecular clouds and spiral structure, are weak in low-mass galaxies. All
populations in the three galaxies exhibit no or only little flaring. While this
finding is consistent with previous integrated light studies, it poses strong
constraints on galaxy simulations, where strong flaring is often found as a
result of interactions or radial migration.Comment: Accepted for publication in A&
Testing galaxy formation models with the GHOSTS survey: The color profile of M81's stellar halo
We study the properties of the stellar populations in M81's outermost part,
which hereafter we will term the stellar halo, using HST ACS/WFC observations
of 19 fields from the GHOSTS survey. The observed fields probe the stellar halo
out to a projected distance of ~ 50 kpc from the galactic center. Each field
was observed in both F606W and F814W filters. The 50% completeness levels of
the color magnitude diagrams (CMDs) are typically at 2 mag below the tip of the
red giant branch (TRGB). Fields at distances closer than 15 kpc show evidence
of disk-dominated populations whereas fields at larger distances are mostly
populated by halo stars. The RGB of the M81's halo CMDs is well matched with
isochrones of ~ 10 Gyr and metallicities [Fe/H] ~ -1.2 dex, suggesting that the
dominant stellar population of M81's halo has a similar age and metallicity.
The halo of M81 is characterized by a color distribution of width ~ 0.4 mag and
an approximately constant median value of (F606W - F814W) ~ 1 mag measured
using stars within the magnitude range 23.7 < F814W < 25.5. When considering
only fields located at galactocentric radius R > 15 kpc, we detect no color
gradient in the stellar halo of M81. We place a limit of 0.03+/-0.11 mag
difference between the median color of RGB M81 halo stars at ~ 15 and at 50
kpc, corresponding to a metallicity difference of 0.08+/-0.35 dex over that
radial range for an assumed constant age of 10 Gyr. We compare these results
with model predictions for the colors of stellar halos formed purely via
accretion of satellite galaxies. When we analyze the cosmologically motivated
models in the same way as the HST data, we find that they predict no color
gradient for the stellar halos, in good agreement with the observations.Comment: 16 pages, 7 figures. Accepted to Ap
Diverse Stellar Haloes in Nearby Milky Way-Mass Disc Galaxies
We have examined the resolved stellar populations at large galactocentric
distances along the minor axis (from 10 kpc up to between 40 and 75 kpc), with
limited major axis coverage, of six nearby highly-inclined Milky Way-mass disc
galaxies using HST data from the GHOSTS survey. We select red giant branch
stars to derive stellar halo density profiles. The projected minor axis density
profiles can be approximated by power laws with projected slopes of between
and and a diversity of stellar halo masses of , or of the total galaxy stellar masses. The typical
intrinsic scatter around a smooth power law fit is dex owing to
substructure. By comparing the minor and major axis profiles, we infer
projected axis ratios at kpc between . The GHOSTS
stellar haloes are diverse, lying between the extremes charted out by the
(rather atypical) haloes of the Milky Way and M31. We find a strong correlation
between the stellar halo metallicities and the stellar halo masses. We compare
our results with cosmological models, finding good agreement between our
observations and accretion-only models where the stellar haloes are formed by
the disruption of dwarf satellites. In particular, the strong observed
correlation between stellar halo metallicity and mass is naturally reproduced.
Low-resolution hydrodynamical models have unrealistically high stellar halo
masses. Current high-resolution hydrodynamical models appear to predict stellar
halo masses somewhat higher than observed but with reasonable metallicities,
metallicity gradients and density profiles.Comment: 26 pages, 17 figures. Accepted for publication in MNRA
Constraining the age of the NGC 4565 HI Disk Warp: Determining the Origin of Gas Warps
We have mapped the distribution of young and old stars in the gaseous HI warp
of NGC 4565. We find a clear correlation of young stars (<600 Myr) with the
warp, but no coincident old stars (>1 Gyr), which places an upper limit on the
age of the structure. The formation rate of the young stars, which increased
~300 Myr ago relative to the surrounding regions, is (6.3 +2.5/-1.5) x 10^-5
M_sol/yr/kpc^2. This implies a ~60+/-20 Gyr depletion time of the HI warp,
similar to the timescales calculated for the outer HI disks of nearby spiral
galaxies. While some stars associated with the warp fall into the asymptotic
giant branch (AGB) region of the color magnitude diagram, where stars could be
as old as 1 Gyr, further investigation suggests that they may be interlopers
rather than real AGB stars. We discuss the implications of these age
constraints for the formation of HI warps, and the gas fueling of disk
galaxies.Comment: 12 pages, 9 figures. Accepted for publication in Ap
GHOSTS I: A New Faint very Isolated Dwarf Galaxy at D = 12 +/- 2 Mpc
We report the discovery of a new faint dwarf galaxy, GHOSTS I, using HST/ACS
data from one of our GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick
disk, and Star clusters) fields. Its detected individual stars populate an
approximately one magnitude range of its luminosity function (LF). Using
synthetic color-magnitude diagrams (CMDs) to compare with the galaxy's CMD, we
find that the colors and magnitudes of GHOSTS I's individual stars are most
consistent with being young helium-burning and asymptotic giant branch stars at
a distance of 12 +/- 2 Mpc. Morphologically, GHOSTS I appears to be actively
forming stars, so we tentatively classify it as a dwarf irregular (dIrr)
galaxy, although future HST observations deep enough to resolve a larger
magnitude range in its LF are required to make a more secure classification.
GHOSTS I's absolute magnitude is , making it one
of the least luminous dIrr galaxies known, and its metallicity is lower than
[Fe/H] =-1.5 dex. The half-light radius of GHOSTS I is 226 +/- 38 pc and its
ellipticity is 0.47 +/- 0.07, similar to Milky Way and M31 dwarf satellites at
comparable luminosity. There are no luminous massive galaxies or galaxy
clusters within ~ 4 Mpc from GHOSTS I that could be considered as its host,
making it a very isolated dwarf galaxy in the Local Universe.Comment: 8 pages, 7 figures. Accepted for publication in Ap
Testing galaxy formation models with the GHOSTS survey : the color profile of M81\u27s stellar halo.
We study the properties of the stellar populations in M81’s outermost part, which hereafter we will call the stellar halo, using Hubble Space Telescope (HST) Advanced Camera for Surveys observations of 19 fields from the GHOSTS survey. The observed fields probe the stellar halo out to a projected distance of ∼50 kpc from the galactic center. Each field was observed in both F606W and F814W filters. The 50% completeness levels of the color–magnitude diagrams (CMDs) are typically at 2 mag below the tip of the red giant branch (TRGB). Fields at distances closer than 15 kpc show evidence of disk-dominated populations whereas fields at larger distances are mostly populated by halo stars. The red giant branch (RGB) of the M81’s halo CMDs is well matched with isochrones of ∼10 Gyr and metallicities [Fe/H] ∼ −1.2 dex, suggesting that the dominant stellar population of M81’s halo has a similar age and metallicity. The halo of M81 is characterized by a color distribution of width ∼0.4 mag and an approximately constant median value of (F606W − F814W) ∼ 1 mag measured using stars within the magnitude range 23.7 F814W 25.5. When considering only fields located at galactocentric radius R \u3e 15 kpc, we detect no color gradient in the stellar halo of M81. We place a limit of 0.03 ± 0.11 mag difference between the median color of RGB M81 halo stars at ∼15 and at 50 kpc, corresponding to a metallicity difference of 0.08 ± 0.35 dex over that radial range for an assumed constant age of 10 Gyr. We compare these results with model predictions for the colors of stellar halos formed purely via accretion of satellite galaxies. When we analyze the cosmologically motivated models in the same way as the HST data, we find that they predict no color gradient for the stellar halos, in good agreement with the observations
Outer-Disk Populations in NGC 7793: Evidence for Stellar Radial Migration
We analyzed the radial surface brightness profile of the spiral galaxy NGC
7793 using HST/ACS images from the GHOSTS survey and a new HST/WFC3 image
across the disk break. We used the photometry of resolved stars to select
distinct populations covering a wide range of stellar ages. We found breaks in
the radial profiles of all stellar populations at 280" (~5.1 kpc). Beyond this
disk break, the profiles become steeper for younger populations. This same
trend is seen in numerical simulations where the outer disk is formed almost
entirely by radial migration. We also found that the older stars of NGC 7793
extend significantly farther than the underlying HI disk. They are thus
unlikely to have formed entirely at their current radii, unless the gas disk
was substantially larger in the past. These observations thus provide evidence
for substantial stellar radial migration in late-type disks.Comment: 8 pages, 6 figure. Accepted for publication in Ap
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