139 research outputs found
Identification of the Microlens in Event MACHO-LMC-20
We report on the identification of the lens responsible for microlensing
event MACHO-LMC-20. As part of a \textit{Spitzer}/IRAC program conducting
mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields,
we discovered a significant flux excess at the position of the source star for
this event. These data, in combination with high resolution near-infrared
\textit{Magellan}/PANIC data has allowed us to classify the lens as an early M
dwarf in the thick disk of the Milky Way, at a distance of kpc. This
is only the second microlens to have been identified, the first also being a M
dwarf star in the disk. Together, these two events are still consistent with
the expected frequency of nearby stars in the Milky Way thin and thick disks
acting as lenses.Comment: 6 pages, 4 figures, submitted to ApJ Letter
Resolved Kinematics of Runaway and Field OB Stars in the Small Magellanic Cloud
We use GAIA DR2 proper motions of the RIOTS4 field OB stars in the Small
Magellanic Cloud (SMC) to study the kinematics of runaway stars. The data
reveal that the SMC Wing has a systemic peculiar motion relative to the SMC Bar
of (v_RA, v_Dec) = (62 +/-7, -18+/-5) km/s and relative radial velocity +4.5
+/- 5.0 km/s. This unambiguously demonstrates that these two regions are
kinematically distinct: the Wing is moving away from the Bar, and towards the
Large Magellanic Cloud with a 3-D velocity of 64 +/- 10 km/s. This is
consistent with models for a recent, direct collision between the Clouds. We
present transverse velocity distributions for our field OB stars, confirming
that unbound runaways comprise on the order of half our sample, possibly more.
Using eclipsing binaries and double-lined spectroscopic binaries as tracers of
dynamically ejected runaways, and high-mass X-ray binaries (HMXBs) as tracers
of runaways accelerated by supernova kicks, we find significant contributions
from both populations. The data suggest that HMXBs have lower velocity
dispersion relative to dynamically ejected binaries, consistent with the former
corresponding to less energetic supernova kicks that failed to unbind the
components. Evidence suggests that our fast runaways are dominated by
dynamical, rather than supernova, ejections.Comment: Accepted to ApJ Letters. 10 pages, 4 figure
GASS High Velocity Clouds in the Region of the Magellanic Leading Arm
We present a catalog of high-velocity clouds in the region of the Magellanic
Leading Arm. The catalog is based on neutral hydrogen (\HI) observations from
the Parkes Galactic All-Sky Survey (GASS). Excellent spectral resolution allows
clouds with narrow-line components to be resolved. The total number of detected
clouds is 419. We describe the method of cataloging and present the basic
parameters of the clouds. We discuss the general distribution of the
high-velocity clouds and classify the clouds based on their morphological type.
The presence of a significant number of head-tail clouds and their distribution
in the region is discussed in the context of Magellanic System simulations. We
suggest that ram-pressure stripping is a more important factor than tidal
forces for the morphology and formation of the Magellanic Leading Arm and that
different environmental conditions might explain the morphological difference
between the Magellanic Leading Arm and Magellanic Stream. We also discuss a
newly identified population of clouds that forms the LA IV and a new diffuse
bridge-like feature connecting the LA II and III complexes.Comment: 20 pages, 24 figures, accepted by ApJ on December 14, 201
A Widespread, Clumpy Starburst in the Isolated Ongoing Dwarf Galaxy Merger dm1647+21
Interactions between pairs of isolated dwarf galaxies provide a critical
window into low-mass hierarchical, gas-dominated galaxy assembly and the
buildup of stellar mass in low-metallicity systems. We present the first
VLT/MUSE optical IFU observations of the interacting dwarf pair dm1647+21,
selected from the TiNy Titans survey. The H emission is widespread and
corresponds to a total unobscured star formation rate (SFR) of 0.44 M
yr, 2.7 times higher than the SFR inferred from SDSS data. The implied
specific SFR (sSFR) for the system is elevated by more than an order of
magnitude above non-interacting dwarfs in the same mass range. This increase is
dominated by the lower-mass galaxy, which has a sSFR enhancement of 50.
Examining the spatially-resolved maps of classic optical line diagnostics, we
find the ISM excitation can be fully explained by star formation. The velocity
field of the ionized gas is not consistent with simple rotation. Dynamical
simulations indicate that the irregular velocity field and the stellar
structure is consistent with the identification of this system as an ongoing
interaction between two dwarf galaxies. The widespread, clumpy enhancements in
star formation in this system point to important differences in the effect of
mergers on dwarf galaxies, compared to massive galaxies: rather than the
funneling of gas to the nucleus and giving rise to a nuclear starburst,
starbursts in low-mass galaxy mergers may be triggered by large-scale ISM
compression, and thus be more distributed.Comment: Accepted for publication in ApJ. 11 pages, 5 figures, 1 table.
Figures slightly degraded to meet arXiv size restrictions. For more
information about TiNy Titans see https://lavinia.as.arizona.edu/~tinytitans
Properties and Origin of the High-Velocity Gas Toward the Large Magellanic Cloud
In the spectra of 139 early-type Large Magellanic Cloud (LMC) stars observed
with FUSE and with deep radio Parkes HI 21-cm observations along those stars,
we search for and analyze the absorption and emission from high-velocity gas at
+90<v<+175 km/s. The HI column density of the high-velocity clouds (HVCs) along
these sightlines ranges from <10^18.4 to 10^19.2 cm^-2. The incidence of the
HVC metal absorption is 70%, significantly higher than the HI emission
occurrence of 32%. We find that the mean metallicity of the HVC is [OI/HI] =
-0.51 (+0.12,-0.16). There is no strong evidence for a large variation in the
HVC metallicity, implying that thes e HVCs have a similar origin and are part
of the same complex. The mean and scatter of the HVC metallicities are more
consistent with the present-day LMC oxygen abundance than that of the Small
Magellanic Cloud or the Milky Way. We find that on average [SiII/OI] = +0.48
(+0.15,- 0.25) and [FeII/OI] = +0.33 (+0.14,-0.21), implying that the HVC
complex is dominantly ionized. The HVC complex has a multiphase structure with
a neutral (OI, FeII), weakly ionized (FeII, NII), and highly ionized (OVI)
components, and has evidence of dust but no molecules. All the observed
properties of the HVC can be explained by an energetic outflow from the LMC.
This is the first example of a large (>10^6 M_sun) HVC complex that is linked
to stellar feedback occurring in a dwarf spiral galaxy.Comment: Accepted for publication in the Ap
Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Orbits, Milky Way Mass, Tidal Influences, Planar Alignments, and Group Infall
A proper understanding of the Milky Way (MW) dwarf galaxies in a cosmological
context requires knowledge of their 3D velocities and orbits. However, proper
motion (PM) measurements have generally been of limited accuracy and available
only for more massive dwarfs. We therefore present a new study of the
kinematics of the MW dwarf galaxies. We use the Gaia DR2 for those dwarfs that
have been spectroscopically observed in the literature. We derive systemic PMs
for 39 galaxies and galaxy candidates out to 420 kpc, and generally find good
consistency for the subset with measurements available from other studies. We
derive the implied Galactocentric velocities, and calculate orbits in canonical
MW halo potentials of "low" () and "high" mass
(). Comparison of the distributions of orbital
apocenters and 3D velocities to the halo virial radius and escape velocity,
respectively, suggests that the satellite kinematics are best explained in the
high-mass halo. Tuc III, Crater II, and additional candidates have orbital
pericenters small enough to imply significant tidal influences. Relevant to the
missing satellite problem, the fact that fewer galaxies are observed to be near
apocenter than near pericenter implies that there must be a population of
distant dwarf galaxies yet to be discovered. Of the 39 dwarfs: 12 have orbital
poles that do not align with the MW plane of satellites (given reasonable
assumptions about its intrinsic thickness); 10 have insufficient PM accuracy to
establish whether they align; and 17 satellites align, of which 11 are
co-orbiting and (somewhat surprisingly, in view of prior knowledge) 6 are
counter-orbiting. Group infall might have contributed to this, but no
definitive association is found for the members of the Crater-Leo group.Comment: 18 pages, 7 figures, 4 tables, accepted for publication in A&
AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY
We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (μ = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe
Measurement of a Magnetic Field in a Leading Arm High Velocity Cloud
Using a recent catalogue of extragalactic Faraday rotation derived from the
NRAO VLA Sky Survey we have found an agreement between Faraday rotation
structure and the HI emission structure of a High Velocity Cloud (HVC)
associated with the Leading Arm of the Magellanic System. We suggest that this
morphological agreement is indicative of Faraday rotation through the HVC.
Under this assumption we have used 48 rotation measures through the HVC,
together with estimates of the electron column density from H-\alpha\
measurements and QSO absorption lines to estimate a strength for the
line-of-sight component of the coherent magnetic field in the HVC of >
6 {\rm \mu G}B_{r}/B_{||} < 0.8$, which suggests that the random field does not dominate
over the coherent field as it does in the Magellanic Clouds from which this HVC
likely originates.Comment: 17 pages, 3 figure
X-Ray Searches for Emission from the WHIM in the Galactic Halo and the Intergalactic Medium
At least 50% of the baryons in the local universe are undetected and
predicted to be in a hot dilute phase (1E5-1E7 K) in low and moderate
overdensity environments. We searched for the predicted diffuse faint emission
through shadowing observations whereby cool foreground gas absorbs more distant
diffuse emission. Observations were obtained with Chandra and XMM-Newton. Using
the cold gas in two galaxies, NGC 891 and NGC 5907, shadows were not detected
and a newer observation of NGC 891 fails to confirm a previously reported X-ray
shadow. Our upper limits lie above model predictions. For Local Group studies,
we used a cloud in the Magellanic Stream and a compact high velocity cloud to
search for a shadow. Instead of a shadow, the X-ray emission was brighter
towards the Magellanic Stream cloud and there is a less significant brightness
enhancement toward the other cloud also. The brightness enhancement toward the
Magellanic Stream cloud is probably due to an interaction with a hot ambient
medium that surrounds the Milky Way. We suggest that this interaction drives a
shock into the cloud, heating the gas to X-ray emitting temperatures.Comment: 10 ApJ pages with 10 figure
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