127 research outputs found
Modeling the gravitational potential of a cosmological dark matter halo with stellar streams
Stellar streams result from the tidal disruption of satellites and star
clusters as they orbit a host galaxy, and can be very sensitive probes of the
gravitational potential of the host system. We select and study narrow stellar
streams formed in a Milky-Way-like dark matter halo of the Aquarius suite of
cosmological simulations, to determine if these streams can be used to
constrain the present day characteristic parameters of the halo's gravitational
potential. We find that orbits integrated in static spherical and triaxial NFW
potentials both reproduce the locations and kinematics of the various streams
reasonably well. To quantify this further, we determine the best-fit potential
parameters by maximizing the amount of clustering of the stream stars in the
space of their actions. We show that using our set of Aquarius streams, we
recover a mass profile that is consistent with the spherically-averaged dark
matter profile of the host halo, although we ignored both triaxiality and time
evolution in the fit. This gives us confidence that such methods can be applied
to the many streams that will be discovered by the Gaia mission to determine
the gravitational potential of our Galaxy.Comment: ApJ sub
Action-space clustering of tidal streams to infer the Galactic potential
We present a new method for constraining the Milky Way halo gravitational
potential by simultaneously fitting multiple tidal streams. This method
requires full three-dimensional positions and velocities for all stars to be
fit, but does not require identification of any specific stream or
determination of stream membership for any star. We exploit the principle that
the action distribution of stream stars is most clustered when the potential
used to calculate the actions is closest to the true potential. Clustering is
quantified with the Kullback-Leibler Divergence (KLD), which also provides
conditional uncertainties for our parameter estimates. We show, for toy
Gaia-like data in a spherical isochrone potential, that maximizing the KLD of
the action distribution relative to a smoother distribution recovers the true
values of the potential parameters. The precision depends on the observational
errors and the number of streams in the sample; using KIII giants as tracers,
we measure the enclosed mass at the average radius of the sample stars accurate
to 3% and precise to 20-40%. Recovery of the scale radius is precise to 25%,
and is biased 50% high by the small galactocentric distance range of stars in
our mock sample (1-25 kpc, or about three scale radii, with mean 6.5 kpc).
About 15 streams, with at least 100 stars per stream, are needed to obtain
upper and lower bounds on the enclosed mass and scale radius when observational
errors are taken into account; 20-25 streams are required to stabilize the size
of the confidence interval. If radial velocities are provided for stars out to
100 kpc (10 scale radii), all parameters can be determined with 10% accuracy
and 20% precision (1.3% accuracy in the case of the enclosed mass), underlining
the need for ground-based spectroscopic follow-up to complete the radial
velocity catalog for faint halo stars observed by Gaia.Comment: Accepted versio
Building an Acceleration Ladder with Tidal Streams and Pulsar Timing
We analyze stellar streams in action-angle coordinates combined with recent
local direct acceleration measurements to provide joint constraints on the
potential of our Galaxy. Our stream analysis uses the Kullback-Leibler
divergence with a likelihood analysis based on the two-point correlation
function. We provide joint constraints from pulsar accelerations and stellar
streams for local and global parameters that describe the potential of the
Milky Way (MW). Our goal is to build an ``acceleration ladder", where direct
acceleration measurements that are currently limited in dynamic range are
combined with indirect techniques that can access a much larger volume of the
MW. To constrain the MW potential with stellar streams, we consider the Palomar
5, Orphan, Nyx, Helmi and GD1 streams. Of the potential models that we have
considered here, the preferred potential for the streams is a two-component
Staeckel potential. We also compare the vertical accelerations from stellar
streams and pulsar timing, defining a function , where is the MW potential determined
from stellar streams, and is the vertical acceleration
determined from pulsar timing observations. Our analysis indicates that the
Oort limit determined from streams is consistently (regardless of the choice of
potential) lower than that determined from pulsar timing observations. The
calibration we have derived here may be used to correct the estimate of the
acceleration from stellar streams.Comment: 8 pages, 4 figures, 1 table. Submitted to ApJ Letter
Constraining the Tilt of the Milky Way's Dark Matter Halo with the Sagittarius Stream
Recent studies have suggested that the Milky Way (MW)'s Dark Matter (DM) halo
may be significantly tilted with respect to its central stellar disk, a feature
that might be linked to its formation history. In this work, we demonstrate a
method of constraining the orientation of the minor axis of the DM halo using
the angle and frequency variables. This method is complementary to other
traditional techniques, such as orbit fitting. We first test the method using a
simulated tidal stream evolving in a realistic environment inside an MW-mass
host from the FIRE cosmological simulation, showing that the theoretical
description of a stream in the action-angle-frequency formalism still holds for
a realistic dwarf galaxy stream in a cosmological potential. Utilizing the
slopes of the line in angle and frequency space, we show that the correct
rotation frame yields a minimal slope difference, allowing us to put a
constraint on the minor axis location. Finally, we apply this method to the
Sagittarius stream's leading arm. We report that the MW's DM halo is oblate
with the flattening parameter in the potential and the minor
axis pointing toward . Our constraint on the minor
axis location is weak and disagrees with the estimates from other works; we
argue that the inconsistency can be attributed in part to the observational
uncertainties and in part to the influence of the Large Magellanic Cloud.Comment: 16 pages, 12 figure
Modeling the orbital histories of satellites of Milky Way-mass galaxies: testing static host potentials against cosmological simulations
Understanding the evolution of satellite galaxies of the Milky Way (MW) and
M31 requires modeling their orbital histories across cosmic time. Many works
that model satellite orbits incorrectly assume or approximate that the host
halo gravitational potential is fixed in time and is spherically symmetric or
axisymmetric. We rigorously benchmark the accuracy of such models against the
FIRE-2 cosmological baryonic simulations of MW/M31-mass halos. When a typical
surviving satellite fell in ( Gyr ago), the host halo mass and radius
were typically per cent of their values today, respectively. Most of
this mass growth of the host occurred at small distances, kpc,
opposite to dark-matter-only simulations, which experience almost no growth at
small radii. We fit a near-exact axisymmetric gravitational potential to each
host at and backward integrate the orbits of satellites in this static
potential, comparing against the true orbit histories in the simulations.
Orbital energy and angular momentum are not well conserved throughout an
orbital history, varying by 25 per cent from their current values already
Gyr ago. Most orbital properties are minimally biased,
per cent, when averaged across the satellite population as a whole. However,
for a single satellite, the uncertainties are large: recent orbital properties,
like the most recent pericentre distance, typically are per cent
uncertain, while earlier events, like the minimum pericentre or the infall
time, are per cent uncertain. Furthermore, these biases and
uncertainties are lower limits, given that we use near-exact host mass profiles
at .Comment: 24 pages, 12 figures, 3 appendices, 2 appendix figures. Accepted for
publication in MNRA
Reconciling observed and simulated stellar halo masses
We use cosmological hydrodynamical simulations of Milky-Way-mass galaxies
from the FIRE project to evaluate various strategies for estimating the mass of
a galaxy's stellar halo from deep, integrated-light images. We find good
agreement with integrated-light observations if we mimic observational methods
to measure the mass of the stellar halo by selecting regions of an image via
projected radius relative to the disk scale length or by their surface density
in stellar mass . However, these observational methods systematically
underestimate the accreted stellar component, defined in our (and most)
simulations as the mass of stars formed outside of the host galaxy, by up to a
factor of ten, since the accreted component is centrally concentrated and
therefore substantially obscured by the galactic disk. Furthermore, these
observational methods introduce spurious dependencies of the estimated accreted
stellar component on the stellar mass and size of galaxies that can obscure the
trends in accreted stellar mass predicted by cosmological simulations, since we
find that in our simulations the size and shape of the central galaxy is not
strongly correlated with the assembly history of the accreted stellar halo.
This effect persists whether galaxies are viewed edge-on or face-on. We show
that metallicity or color information may provide a way to more cleanly
delineate in observations the regions dominated by accreted stars. Absent
additional data, we caution that estimates of the mass of the accreted stellar
component from single-band images alone should be taken as lower limits.Comment: Version accepted by Ap
On the stability of tidal streams in action space
In the Gaia era it is increasingly apparent that traditional static,
parameterized models are insufficient to describe the mass distribution of our
complex, dynamically evolving Milky Way (MW). In this work, we compare
different time-evolving and time-independent representations of the
gravitational potentials of simulated MW-mass galaxies from the FIRE-2 suite of
cosmological baryonic simulations. Using these potentials, we calculate actions
for star particles in tidal streams around three galaxies with varying merger
histories at each snapshot from 7 Gyr ago to the present day. We determine the
action-space coherence preserved by each model using the Kullback-Leibler
Divergence to gauge the degree of clustering in actions and the relative
stability of the clusters over time. We find that all models produce a
clustered action space for simulations with no significant mergers. However, a
massive (mass ratio prior to infall more similar than 1:8) interacting galaxy
not present in the model will result in mischaracterized orbits for stars most
affected by the interaction. The locations of the action space clusters (i.e.
the orbits of the stream stars) are only preserved by the time-evolving model,
while the time-independent models can lose significant amounts of information
as soon as 0.5--1 Gyr ago, even if the system does not undergo a significant
merger. Our results imply that reverse-integration of stream orbits in the MW
using a fixed potential is likely to give incorrect results if integrated
longer than 0.5 Gyr into the past
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