77 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
Revisiting the Role of M31 in the Dynamical History of the Magellanic Clouds
We study the dynamics of the Magellanic Clouds in a model for the Local Group
whose mass is constrained using the timing argument/two-body limit of the
action principle. The goal is to evaluate the role of M31 in generating the
high angular momentum orbit of the Clouds, a puzzle that has only been
exacerbated by the latest proper motion measurements. We study the
effects of varying the total Local Group mass, the relative mass of the Milky
Way and M31, the proper motion of M31, and the proper motion of the LMC on this
problem. Over a large part of this parameter-space we find that tides from M31
are insignificant. For a range of LMC proper motions approximately
higher than the mean and total Local Group mass ,
M31 can provide a significant torque to the LMC orbit. However, if the LMC is
bound to the MW, then M31 is found to have negligible effect on its motion and
the origin of the high angular momentum of the system remains a puzzle.
Finally, we use the timing argument to calculate the total mass of the MW-LMC
system based on the assumption that they are encountering each other for the
first time, their previous perigalacticon being a Hubble time ago, obtaining
.Comment: 10 pages, 5 figures, accepted to Ap
The Impact of Patchy Reionization on Ultra-faint Dwarf Galaxies
We investigate how patchy reionization affects the star formation history
(SFH) and stellar metallicity of ultra-faint dwarf galaxies (UFDs). Patchy
reionization refers to varying ultraviolet (UV) background strengths depending
on a galaxy's environment. Recent observations highlight the significance of
this effect on UFDs, as UFDs can have different SFHs depending on their
relative position with respect to their host halo during the period of
reionization. However, most cosmological hydrodynamic simulations do not
consider environmental factors such as patchy reionization, and the effect of
reionization is typically applied homogeneously. Using a novel approach to
implement patchy reionization, we show how SFHs of simulated UFDs can change.
Our cosmological hydrodynamic zoom-in simulations focus on UFD analogs with
M_vir~10^9solar mass, M_star < 10^5 solar mass at . We find that patchy
reionization can weaken the effect of reionization by two orders of magnitude
up to , enabling late star formation in half of the simulated UFDs, with
quenching times 460 Myr later than those with homogeneous reionization.
We also show that halo merger and mass assembly can affect the SFHs of
simulated UFDs, in addition to patchy reionization. The average stellar
iron-to-hydrogen ratio, [Fe/H], of the simulated UFDs with patchy reionization
increases by 0.22-0.42 dex. Finally, our findings suggest that patchy
reionization could be responsible for the extended SFHs of Magellanic UFDs
compared to non-Magellanic UFDs.Comment: 27 pages, 16 figures, Accepted for publication in Astrophysical
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