3,959 research outputs found
A Natural Formalism for Microlensing
If the standard microlensing geometry is inverted so that the Einstein ring
is projected onto the observer plane rather than the source plane, then the
relations between the observables (\theta_E,\tilde r_E) and the underlying
physical quantities (M,\pi_rel) become immediately obvious. Here \theta_E and
\tilde r_E are the angular and projected Einstein radii, M is the mass of the
lens, and \pi_rel is the lens-source relative parallax. I recast the basic
formalism of microlensing in light of this more natural geometry and in terms
of observables. I then find that the relations between observable and physical
quantities assume an exceptionally simple form. In an appendix, I propose a set
of notational conventions for microlensing.Comment: 8 pages, 1 figure tells all. Interested parties are requested to vote
on a proposed standard for microlensing notation given in the appendix.
Submitted to Ap
The importance of preventive feedback: inference from observations of the stellar masses and metallicities of Milky Way dwarf galaxies
Dwarf galaxies are known to have remarkably low star formation efficiency due
to strong feedback. Adopting the dwarf galaxies of the Milky Way as a
laboratory, we explore a flexible semi-analytic galaxy formation model to
understand how the feedback processes shape the satellite galaxies of the Milky
Way. Using Markov-Chain Monte-Carlo, we exhaustively search a large parameter
space of the model and rigorously show that the general wisdom of strong
outflows as the primary feedback mechanism cannot simultaneously explain the
stellar mass function and the mass--metallicity relation of the Milky Way
satellites. An extended model that assumes that a fraction of baryons is
prevented from collapsing into low-mass halos in the first place can be
accurately constrained to simultaneously reproduce those observations. The
inference suggests that two different physical mechanisms are needed to explain
the two different data sets. In particular, moderate outflows with weak halo
mass dependence are needed to explain the mass--metallicity relation, and
prevention of baryons falling into shallow gravitational potentials of low-mass
halos (e.g. "pre-heating") is needed to explain the low stellar mass fraction
for a given subhalo mass.Comment: 14 pages, 4 figures, accepted for publication in Ap
Brightest galaxies as halo centre tracers in SDSS DR7
Determining the positions of halo centres in large-scale structure surveys is
crucial for many cosmological studies. A common assumption is that halo centres
correspond to the location of their brightest member galaxies. In this paper,
we study the dynamics of brightest galaxies with respect to other halo members
in the Sloan Digital Sky Survey DR7. Specifically, we look at the line-of-sight
velocity and spatial offsets between brightest galaxies and their neighbours.
We compare those to detailed mock catalogues, constructed from high-resolution,
dark-matter-only -body simulations, in which it is assumed that satellite
galaxies trace dark matter subhaloes. This allows us to place constraints on
the fraction of haloes in which the brightest galaxy is not the
central. Compared to previous studies we explicitly take into account the
unrelaxed state of the host haloes, velocity offsets of halo cores and
correlations between and the satellite occupation. We find that
strongly decreases with the luminosity of the brightest galaxy
and increases with the mass of the host halo. Overall, in the halo mass range
we find , in good
agreement with a previous study by Skibba et al. We discuss the implications of
these findings for studies inferring the galaxy--halo connection from satellite
kinematics, models of the conditional luminosity function and galaxy formation
in general.Comment: 24 pages, 15 figures. Accepted for publication in MNRA
Modeling the Impact of Baryons on Subhalo Populations with Machine Learning
We identify subhalos in dark matter-only (DMO) zoom-in simulations that are
likely to be disrupted due to baryonic effects by using a random forest
classifier trained on two hydrodynamic simulations of Milky Way (MW)-mass host
halos from the Latte suite of the Feedback in Realistic Environments (FIRE)
project. We train our classifier using five properties of each disrupted and
surviving subhalo: pericentric distance and scale factor at first pericentric
passage after accretion, and scale factor, virial mass, and maximum circular
velocity at accretion. Our five-property classifier identifies disrupted
subhalos in the FIRE simulations with an out-of-bag classification
score. We predict surviving subhalo populations in DMO simulations of the FIRE
host halos, finding excellent agreement with the hydrodynamic results; in
particular, our classifier outperforms DMO zoom-in simulations that include the
gravitational potential of the central galactic disk in each hydrodynamic
simulation, indicating that it captures both the dynamical effects of a central
disk and additional baryonic physics. We also predict surviving subhalo
populations for a suite of DMO zoom-in simulations of MW-mass host halos,
finding that baryons impact each system consistently and that the predicted
amount of subhalo disruption is larger than the host-to-host scatter among the
subhalo populations. Although the small size and specific baryonic physics
prescription of our training set limits the generality of our results, our work
suggests that machine-learning classification algorithms trained on
hydrodynamic zoom-in simulations can efficiently predict realistic subhalo
populations.Comment: 20 pages, 14 figures. Updated to published version. Code available at
https://github.com/ollienad/subhalo_randomfores
The connection between the host halo and the satellite galaxies of the Milky Way
Many properties of the Milky Way's dark matter halo, including its mass
assembly history, concentration, and subhalo population, remain poorly
constrained. We explore the connection between these properties of the Milky
Way and its satellite galaxy population, especially the implication of the
presence of the Magellanic Clouds for the properties of the Milky Way halo.
Using a suite of high-resolution -body simulations of Milky Way-mass halos
with a fixed final Mvir ~ 10^{12.1}Msun, we find that the presence of
Magellanic Cloud-like satellites strongly correlates with the assembly history,
concentration, and subhalo population of the host halo, such that Milky
Way-mass systems with Magellanic Clouds have lower concentration, more rapid
recent accretion, and more massive subhalos than typical halos of the same
mass. Using a flexible semi-analytic galaxy formation model that is tuned to
reproduce the stellar mass function of the classical dwarf galaxies of the
Milky Way with Markov-Chain Monte-Carlo, we show that adopting host halos with
different mass-assembly histories and concentrations can lead to different
best-fit models for galaxy-formation physics, especially for the strength of
feedback. These biases arise because the presence of the Magellanic Clouds
boosts the overall population of high-mass subhalos, thus requiring a different
stellar-mass-to-halo-mass ratio to match the data. These biases also lead to
significant differences in the mass--metallicity relation, the kinematics of
low-mass satellites, the number counts of small satellites associated with the
Magellanic Clouds, and the stellar mass of Milky Way itself. Observations of
these galaxy properties can thus provide useful constraints on the properties
of the Milky Way halo.Comment: 20 pages, 12 figures, accepted for publication in ApJ. A new section
on the effect of host halo mass-assembly history on the central galaxy
stellar mass is adde
Einstein Radii from Binary Lensing Events
We show that the Einstein ring radius and transverse speed of a lens
projected on the source plane, and , can be
determined from the light curve of a binary-source event, followed by the
spectroscopic determination of the orbital elements of the source stars. The
determination makes use of the same principle that allows one to measure the
Einstein ring radii from finite-source effects. For the case when the orbital
period of the source stars is much longer than the Einstein time scale, , there exists a single two-fold degeneracy in determining
. However, when the degeneracy can
often be broken by making use of the binary-source system's orbital motion.
%Once , and thus are determined, one can
%distinguish self-lensing events in the Large Magellanic Cloud %from Galactic
halo events. For an identifiable 8\% of all lensing events seen toward the
Large Magellanic Cloud (LMC), one can unambiguously determine whether the
lenses are Galactic, or whether they lie in the LMC itself. The required
observations can be made after the event is over and could be carried out for
the events seen by Alcock et al.\ and Aubourg et al.. In addition, we
propose to include eclipsing binaries as sources for gravitational lensing
experiments.Comment: 18 pages, revised version, submitted to Ap
Statistical properties of Faraday rotation measure from large-scale magnetic fields in intervening disc galaxies
To constrain the large-scale magnetic field strengths in cosmologically
distant galax- ies, we derive the probability distribution function of Faraday
rotation measure (RM) when random lines of sight pass through a sample of disc
galaxies, with axisymmetric large-scale magnetic fields. We find that the width
of the RM distribution of the galaxy sample is directly related to the mean
large-scale field strength of the galaxy population, provided the dispersion
within the sample is lower than the mean value. In the absence of additional
constraints on parameters describing the magneto-ionic medium of the
intervening galaxies, and in the situation where RMs produced in the
intervening galaxies have already been statistically isolated from other RM
contributions along the lines of sight, our simple model of the magneto-ionic
medium in disc galaxies suggests that the mean large-scale magnetic field of
the population can be measured to within ~ 50% accuracy.Comment: 4 pages, Proceedings of FM8 "New Insights in Extragalactic Magnetic
Fields", XXXth General Assembly of the IAU, Vienna, August 20-31, 201
How to Optimally Constrain Galaxy Assembly Bias: Supplement Projected Correlation Functions with Count-in-cells Statistics
Most models for the connection between galaxies and their haloes ignore the
possibility that galaxy properties may be correlated with halo properties other
than mass, a phenomenon known as galaxy assembly bias. Yet, it is known that
such correlations can lead to systematic errors in the interpretation of survey
data. At present, the degree to which galaxy assembly bias may be present in
the real Universe, and the best strategies for constraining it remain
uncertain. We study the ability of several observables to constrain galaxy
assembly bias from redshift survey data using the decorated halo occupation
distribution (dHOD), an empirical model of the galaxy--halo connection that
incorporates assembly bias. We cover an expansive set of observables, including
the projected two-point correlation function ,
the galaxy--galaxy lensing signal , the void
probability function , the distributions of
counts-in-cylinders , and counts-in-annuli
, and the distribution of the ratio of counts in cylinders
of different sizes . We find that despite the frequent use of the
combination in
interpreting galaxy data, the count statistics, and
, are generally more efficient in constraining galaxy
assembly bias when combined with . Constraints
based upon and
share common degeneracy directions in the parameter space, while combinations
of with the count statistics are more
complementary. Therefore, we strongly suggest that count statistics should be
used to complement the canonical observables in future studies of the
galaxy--halo connection.Comment: Figures 3 and 4 show the main results. Published in Monthly Notices
of the Royal Astronomical Societ
- …