120 research outputs found
Spatially Resolved Spectroscopic Star Formation Histories of Nearby Disks: Hints of Stellar Migration
We use the Mitchell Spectrograph (formerly VIRUS-P) to observe 12 nearby disk
galaxies. We successfully measure ages in the outer disk in six systems. In
three cases (NGC 2684, NGC 6155, and NGC 7437), we find that a downward break
in the disk surface brightness profile corresponds with a change in the
dominant stellar population with the interior being dominated by active star
formation and the exterior having older stellar populations that are best-fit
with star formation histories that decline with time. The observed increase in
average stellar ages beyond a profile break is similar to theoretical models
that predict surface brightness breaks are caused by stellar migration, with
the outer disk being populated from scattered old interior stars. In three more
cases (IC 1132, NGC 4904, and NGC 6691), we find no significant change in the
stellar population as one crosses the break radius. In these galaxies, both the
inner and outer disks are dominated by active star formation and younger
stellar populations. While radial migration can contribute to the stellar
populations beyond the break, it appears more than one mechanism is required to
explain all of our observed stellar profile breaks.Comment: 18 pages, 21 figures. Accepted for publication in Ap
A Panoply of Cepheid Light Curve Templates
We have generated accurate V and I template light curves using a combination
of Fourier decomposition and principal component analysis for a large sample of
Cepheid light curves. Unlike previous studies, we include short period Cepheids
and stars pulsating in the first overtone mode in our analysis. Extensive Monte
Carlo simulations show that our templates can be used to precisely measure
Cepheid magnitudes and periods, even in cases where there are few observational
epochs. These templates are ideal for characterizing serendipitously discovered
Cepheids and can be used in conjunction with surveys such as Pan-Starrs and
LSST where the observational sampling may not be optimized for Cepheids.Comment: 12 pages, 14 figures. Accepted for publication in AJ fixed
embarrassing typo
SDSS IV MaNGA - Rotation Velocity Lags in the Extraplanar Ionized Gas from MaNGA Observations of Edge-on Galaxies
We present a study of the kinematics of the extraplanar ionized gas around
several dozen galaxies observed by the Mapping of Nearby Galaxies at the Apache
Point Observatory (MaNGA) survey. We considered a sample of 67 edge-on galaxies
out of more than 1400 extragalactic targets observed by MaNGA, in which we
found 25 galaxies (or 37%) with regular lagging of the rotation curve at large
distances from the galactic midplane. We model the observed emission
velocity fields in the galaxies, taking projection effects and a simple model
for the dust extinction into the account. We show that the vertical lag of the
rotation curve is necessary in the modeling, and estimate the lag amplitude in
the galaxies. We find no correlation between the lag and the star formation
rate in the galaxies. At the same time, we report a correlation between the lag
and the galactic stellar mass, central stellar velocity dispersion, and axial
ratio of the light distribution. These correlations suggest a possible higher
ratio of infalling-to-local gas in early-type disk galaxies or a connection
between lags and the possible presence of hot gaseous halos, which may be more
prevalent in more massive galaxies. These results again demonstrate that
observations of extraplanar gas can serve as a potential probe for accretion of
gas.Comment: 13 pages, 11 figures, accepted for publication in Ap
The ACS Nearby Galaxy Survey Treasury III: Cepheids in the Outer Disk of M81
The ACS Nearby Galaxy Survey Treasury (ANGST) has acquired deep ACS imaging
of a field in the outer disk of the large spiral galaxy M81. These data were
obtained over a total of 20 HST orbits, providing a baseline long enough to
reliably identify Cepheid variable stars in the field. Fundamental mode and
first overtone types have been distinguished through comparative fits with
corresponding Cepheid light curve templates derived from principal component
analysis of confirmed Cepheids in the LMC, SMC, and Milky Way. A distance
modulus of 27.78 pm 0.05_random pm 0.14_systematic with a corresponding
distance of 3.60 pm 0.23 Mpc has been calculated from a sample of 11
fundamental mode and 2 first overtone Cepheids (assuming an LMC distance
modulus of mu_LMC=18.41 pm 0.10_r pm 0.13_s).Comment: 10 pages, 9 figures. Accepted for publication in AJ Fixed typo
Lick Indices in the Thin and Thick Disks of Edge-On Disk Galaxies
We have measured Lick index equivalent widths to derive luminosity weighted
stellar ages and metallicities for thin and thick disk dominated regions of 9
edge-on disk galaxies with the ARC 3.5 meter telescope at Apache Point
Observatory. In all cases, the thick disks are confirmed to be old stellar
populations, with typical ages between 4 and 10 Gyr. The thin disks are
uniformly younger than the thick disks, and show strong radial age gradients,
with the outer regions of the disks being younger than 1 Gyr. We do not detect
any significant metallicity differences or alpha-element enhancement in the
thick disk stars compared to the thin disk, due to the insensitivity of the
Lick indices to these differences at low metallicity. We compare these results
to thick disks measured in other systems and to predictions from thick disk
formation models.Comment: 14 pages, 10 figures, accepted for publication in Ap
The Kinematics of Thick Disks in External Galaxies
We present kinematic measurements of the thick and thin disks in two edge-on
galaxies. We have derived stellar rotation curves at and above the galaxies'
midplanes using Ca II triplet features measured with the GMOS spectrograph on
Gemini North. In one galaxy, FGC 1415, the kinematics above the plane show
clear rotation that lags that of the midplane by ~20-50%, similar to the
behavior seen in the Milky Way. However, the kinematics of the second galaxy,
FGC 227, are quite different. The rotation above the plane is extremely slow,
showing <25% of the rotation speed of the stars at the midplane. We decompose
the observed rotation curves into a superposition of thick and thin disk
kinematics, using 2-dimensional fits to the galaxy images to determine the
fraction of thick disk stars at each position. We find that the thick disk of
FGC 1415 rotates at 30-40% of the rotation speed of the thin disk. In contrast,
the thick disk of FGC 227 is very likely counter-rotating, if it is rotating at
all. These observations are consistent with the velocity dispersion profiles we
measure for each galaxy. The detection of counter-rotating thick disks
conclusively rules out models where the thick disk forms either during
monolithic collapse or from vertical heating of a previous thin disk. Instead,
the data strongly support models where the thick disk forms from direct
accretion of stars from infalling satellites.Comment: 13 pages, 10 figures. Accepted for publication in Ap
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
Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies
We analyze the global structure of 34 late-type, edge-on, undisturbed, disk
galaxies spanning a wide range of mass. We measure structural parameters for
the galaxies using two-dimensional least-squares fitting to our -band
photometry. The fits require both a thick and a thin disk to adequately fit the
data. The thick disks have larger scale heights and longer scale lengths than
the embedded thin disks, by factors of ~2 and ~1.25, respectively. The observed
structural parameters agree well with the properties of thick and thin disks
derived from star counts in the Milky Way and from resolved stellar populations
in nearby galaxies. We find that massive galaxies' luminosities are dominated
by the thin disk. However, in low mass galaxies (Vc < 120 km/s), thick disk
stars contribute nearly half of the luminosity and dominate the stellar mass.
Thus, although low mass dwarf galaxies appear blue, the majority of their stars
are probably quite old.
Our data are most easily explained by a formation scenario where the thick
disk is assembled through direct accretion of stellar material from merging
satellites while the thin disk is formed from accreted gas. The baryonic
fraction in the thin disk therefore constrains the gas-richness of the merging
pre-galactic fragments. If we include the mass in HI as part of the thin disk,
the thick disk contains <10% of the baryons in high mass galaxies, and ~25-30%
of the baryons in low-mass galaxies. We discuss how our trends can be explained
by supernova-driven outflow at early times as well as the possibilities for
predicting abundance trends in thick disks, and for removing discrepancies
between semi-analytic galaxy formation models and the observed colors of low
mass galaxies. (abstract abridged)Comment: 25 pages, 24 figures, accepted for publication in A
A Radial Age Gradient in the Geometrically Thick Disk of the Milky Way
In the Milky Way, the thick disk can be defined using individual stellar abundances, kinematics, or age, or geometrically, as stars high above the midplane. In nearby galaxies, where only a geometric definition can be used, thick disks appear to have large radial scale lengths, and their red colors suggest that they are uniformly old. The Milky Way's geometrically thick disk is also radially extended, but it is far from chemically uniform: α-enhanced stars are confined within the inner Galaxy. In simulated galaxies, where old stars are centrally concentrated, geometrically thick disks are radially extended, too. Younger stellar populations flare in the simulated disks' outer regions, bringing those stars high above the midplane. The resulting geometrically thick disks therefore show a radial age gradient, from old in their central regions to younger in their outskirts. Based on our age estimates for a large sample of giant stars in the APOGEE survey, we can now test this scenario for the Milky Way. We find that the geometrically defined thick disk in the Milky Way has indeed a strong radial age gradient: the median age for red clump stars goes from ~9 Gyr in the inner disk to 5 Gyr in the outer disk. We propose that at least some nearby galaxies could also have thick disks that are not uniformly old, and that geometrically thick disks might be complex structures resulting from different formation mechanisms in their inner and outer parts
Rubin LSST observing strategies to maximize volume and uniformity coverage of Star Forming Regions in the Galactic Plane
A complete map of the youngest stellar populations of the Milky Way in the
era of all-sky surveys, is one of the most challenging goals in modern
astrophysics. The characterisation of the youngest stellar component is crucial
not only for a global overview of the Milky Way structure, of the Galactic thin
disk, and its spiral arms, but also for local studies. In fact, the
identification of the star forming regions (SFRs) and the comparison with the
environment in which they form are also fundamental to put them in the context
of the surrounding giant molecular clouds and to understand still unknown
physical mechanisms related to the star and planet formation processes. In 10
yrs of observations, Vera C. Rubin Legacy Survey of Space and Time (Rubin-LSST)
will achieve an exquisite photometric depth that will allow us to significantly
extend the volume within which we will be able to discover new SFRs and to
enlarge the domain of a detailed knowledge of our own Galaxy. We describe here
a metrics that estimates the total number of young stars with ages t < 10 Myr
and masses >0.3M that will be detected with the Rubin LSST observations
in the gri bands at a 5 {\sigma} magnitude significance. We examine the results
of our metrics adopting the most recent simulated Rubin-LSST survey strategies
in order to evaluate the impact that different observing strategies might have
on our science case.Comment: 11 pages, 5 figures, 1 table; accepted for publication in The
Astrophysical Journal Supplement Serie
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