232 research outputs found
The Magellanic Puzzle: origin of the periphery
In this paper, we analyse the metallicity structure of the Magellanic Clouds
using parameters derived from the Gaia DR3 low-resolution XP spectra,
astrometry and photometry. We find that the qualitative behavior of the radial
metallicity gradients in the LMC and SMC are quite similar, with both of them
having a metallicity plateau at intermediate radii and a second at larger
radii. The LMC has a first metallicity plateau at [Fe/H]-0.8 for
37\degr, while the SMC has one at [Fe/H]-1.1 at 35\degr. The
outer LMC periphery has a fairly constant metallicity of [Fe/H]-1.0
(1018\degr), while the outer SMC periphery has a value of
[Fe/H]-1.3 (610\degr). The sharp drop in metallicity in the LMC at
8\dgr and the marked difference in age distributions in these two regions
suggests that there were two important evolutionary phases in the LMC. In
addition, we find that the Magellanic periphery substructures, likely
Magellanic debris, are mostly dominated by LMC material stripped off in old
interactions with the SMC. This presents a new picture in contrast with the
popular belief that the debris around the Clouds had been mostly stripped off
from the SMC due to having a lower mass. We perform a detailed analysis for
each known substructure and identify its potential origin based on
metallicities and motions with respect to each galaxy.Comment: 11 pages, 7 figures. Submitted to MNRAS. Comments welcome
The HI Chronicles of LITTLE THINGS BCDs II: The Origin of IC 10's HI Structure
In this paper we analyze Very Large Array (VLA) telescope and Green Bank
Telescope (GBT) atomic hydrogen (HI) data for the LITTLE THINGS(1) blue compact
dwarf galaxy IC 10. The VLA data allow us to study the detailed HI kinematics
and morphology of IC 10 at high resolution while the GBT data allow us to
search the surrounding area at high sensitivity for tenuous HI. IC 10's HI
appears highly disturbed in both the VLA and GBT HI maps with a kinematically
distinct northern HI extension, a kinematically distinct southern plume, and
several spurs in the VLA data that do not follow the general kinematics of the
main disk. We discuss three possible origins of its HI structure and kinematics
in detail: a current interaction with a nearby companion, an advanced merger,
and accretion of intergalactic medium. We find that IC 10 is most likely an
advanced merger or a galaxy undergoing accretion.
1:Local Irregulars That Trace Luminosity Extremes, The HI Nearby Galaxy
Survey; https://science.nrao.edu/science/surveys/littlethingsComment: 36 pages, 17 figures, accepted for publication in The Astronomical
Journa
The shapes of Milky Way satellites: looking for signatures of tidal stirring
We study the shapes of Milky Way satellites in the context of the tidal
stirring scenario for the formation of dwarf spheroidal galaxies. The standard
procedures used to measure shapes involve smoothing and binning of data and
thus may not be sufficient to detect structural properties like bars, which are
usually subtle in low surface brightness systems. Taking advantage of the fact
that in nearby dwarfs photometry of individual stars is available we introduce
discrete measures of shape based on the two-dimensional inertia tensor and the
Fourier bar mode. We apply these measures of shape first to a variety of
simulated dwarf galaxies formed via tidal stirring of disks embedded in dark
matter halos and orbiting the Milky Way. In addition to strong mass loss and
randomization of stellar orbits, the disks undergo morphological transformation
that typically involves the formation of a triaxial bar after the first
pericenter passage. These tidally induced bars persist for a few Gyr before
being shortened towards a more spherical shape if the tidal force is strong
enough. We test this prediction by measuring in a similar way the shape of
nearby dwarf galaxies, satellites of the Milky Way. We detect inner bars in
Ursa Minor, Sagittarius, LMC and possibly Carina. In addition, six out of
eleven studied dwarfs show elongated stellar distributions in the outer parts
that may signify transition to tidal tails. We thus find the shapes of Milky
Way satellites to be consistent with the predictions of the tidal stirring
model.Comment: 14 pages, 11 figures, accepted for publication in Ap
Determining Ages of APOGEE Giants with Known Distances
We present a sample of local red giant stars observed using the New Mexico
State University 1 m telescope with the APOGEE spectrograph, for which we
estimate stellar ages and the age distribution from the high-resolution
spectroscopic stellar parameters and accurate distance measurements from
Hipparcos. The high-resolution (R ~ 23,000), near infrared (H-band, 1.5-1.7
micron) APOGEE spectra provide measurements of the stellar atmospheric
parameters (temperature, surface gravity, [M/H], and [alpha/M]). Due to the
smaller uncertainties in surface gravity possible with high-resolution spectra
and accurate Hipparcos distance measurements, we are able to calculate the
stellar masses to within 40%. For red giants, the relatively rapid evolution of
stars up the red giant branch allows the age to be constrained based on the
mass. We examine methods of estimating age using both the mass-age relation
directly and a Bayesian isochrone matching of measured parameters, assuming a
constant star formation history (SFH). To improve the prior on the SFH, we use
a hierarchical modeling approach to constrain the parameters of a model SFH
from the age probability distribution functions of the data. The results of an
alpha dependent Gaussian SFH model shows a clear relation between age and
[alpha/M] at all ages. Using this SFH model as the prior for an empirical
Bayesian analysis, we construct a full age probability distribution function
and determine ages for individual stars. The age-metallicity relation is flat,
with a slight decrease in [M/H] at the oldest ages and a ~ 0.5 dex spread in
metallicity. For stars with ages < 1 Gyr we find a smaller spread, consistent
with radial migration having a smaller effect on these young stars than on the
older stars.Comment: 14 page, 18 figures, accepted to ApJ with minor revisions, full
electronic table of data available upon publicatio
Exploring Halo Substructure with Giant Stars: The Dynamics and Metallicity of the Dwarf Spheroidal in Bootes
We report the results of a spectroscopic study of the Bootes (Boo) dwarf
spheroidal (dSph) galaxy carried out with the WIYN telescope and the Hydra
multifiber spectrograph. Radial velocities have been measured for 58 Boo
candidate stars selected to have magnitudes and colors consistent with its red
and asymptotic giant branches. Within the 13' half-light radius, seven members
of Boo yield a systemic velocity of V_r=95.6+-3.4 km/s and a velocity
dispersion of 6.6+-2.3 km/s. This implies a mass on the order of 1 x 10^7
M_sun, similar to the inferred masses of other Galactic dSphs. Adopting a total
Boo luminosity of L=1.8 x 10^4 L_sun to 8.6 x 10^4 L_sun implies M/L ~ 610 to
130, making Boo, the most distorted known Milky Way dwarf galaxy, potentially
also the darkest. From the spectra of Boo member stars we estimate its
metallicity to be [Fe/H] ~ -2.5, which would make it the most metal poor dSph
known to date.Comment: Accepted for publication in ApJ Letter
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