828 research outputs found
Simulations of the Magellanic Stream in a First Infall Scenario
Recent high precision proper motions from the Hubble Space Telescope (HST)
suggest that the Large and Small Magellanic Clouds (LMC and SMC, respectively)
are either on their first passage or on an eccentric long period (>6 Gyr) orbit
about the Milky Way (MW). This differs markedly from the canonical picture in
which the Clouds travel on a quasi-periodic orbit about the MW (period of ~2
Gyr). Without a short period orbit about the MW, the origin of the Magellanic
Stream, a young (1-2 Gyr old) coherent stream of HI gas that trails the Clouds
~150 degrees across the sky, can no longer be attributed to stripping by MW
tides and/or ram pressure stripping by MW halo gas. We propose an alternative
formation mechanism in which material is removed by LMC tides acting on the SMC
before the system is accreted by the MW. We demonstrate the feasibility and
generality of this scenario using an N-body/SPH simulation with cosmologically
motivated initial conditions constrained by the observations. Under these
conditions we demonstrate that it is possible to explain the origin of the
Magellanic Stream in a first infall scenario. This picture is generically
applicable to any gas-rich dwarf galaxy pair infalling towards a massive host
or interacting in isolation.Comment: 9 pages, 2 figures,1 table, submitted to apj
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
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
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