1,164 research outputs found
The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements
We present the first proper motion measurements for the galaxy M31. We
obtained new V-band imaging data with the HST ACS/WFC and WFC3/UVIS of a
spheroid field near the minor axis, an outer disk field along the major axis,
and a field on the Giant Southern Stream. The data provide 5-7 year time
baselines with respect to pre-existing deep first-epoch observations. We
measure the positions of thousands of M31 stars and hundreds of compact
background galaxies in each field. High accuracy and robustness is achieved by
building and fitting a unique template for each individual object. The average
proper motion for each field is obtained from the average motion of the M31
stars between the epochs with respect to the background galaxies. For the three
fields, the observed proper motions (mu_W,mu_N) are (-0.0458, -0.0376),
(-0.0533, -0.0104), and (-0.0179,-0.0357) mas/yr, respectively. The ability to
average over large numbers of objects and over the three fields yields a final
accuracy of 0.012 mas/yr. The robustness of the proper-motion measurements and
uncertainties are supported by the fact that data from different instruments,
taken at different times and with different telescope orientations, as well as
measurements of different fields, all yield statistically consistent results.
Papers II and III explore the implications for our understanding of the
history, future, and mass of the Local Group. (Abridged)Comment: 42 pages, 13 figures, to be published in ApJ. Version with high
resolution figures and N-body movies available at
http://www.stsci.edu/~marel/M31 . Press materials, graphics, and
visualizations available at
http://hubblesite.org/newscenter/archive/releases/2012/2
The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun
We study the future orbital evolution and merging of the MW-M31-M33 system,
using a combination of collisionless N-body simulations and semi-analytic orbit
integrations. Monte-Carlo simulations are used to explore the consequences of
varying the initial phase-space and mass parameters within their observational
uncertainties. The observed M31 transverse velocity implies that the MW and M31
will merge t = 5.86 (+1.61-0.72) Gyr from now, after a first pericenter at t =
3.87 (+0.42-0.32) Gyr. M31 may (probability p=41%) make a direct hit with the
MW (defined here as a first-pericenter distance less than 25 kpc). Most likely,
the MW and M31 will merge first, with M33 settling onto an orbit around them.
Alternatively, M33 may make a direct hit with the MW first (p=9%), or M33 may
get ejected from the Local Group (p=7%). The MW-M31 merger remnant will
resemble an elliptical galaxy. The Sun will most likely (p=85%) end up at
larger radius from the center of the MW-M31 merger remnant than its current
distance from the MW center, possibly further than 50 kpc (p=10%). The Sun may
(p=20%) at some time in the next 10 Gyr find itself moving through M33 (within
10 kpc), but while dynamically still bound to the MW-M31 merger remnant. The
arrival and possible collision of M31 (and possibly M33) with the MW is the
next major cosmic event affecting the environment of our Sun and solar system
that can be predicted with some certainty. (Abridged)Comment: 58 pages, 16 figures, to be published in ApJ. Version with high
resolution figures and N-body movies available at
http://www.stsci.edu/~marel/M31 . Press materials, graphics, and
visualizations available at
http://hubblesite.org/newscenter/archive/releases/2012/2
Connecting the Milky Way potential profile to the orbital timescales and spatial structure of the Sagittarius Stream
Recent maps of the halo using RR Lyrae from Pan-STARRS1 have clearly depicted
the spatial structure of the Sagittarius stream. These maps show the leading
and trailing stream apocenters differ in galactocentric radius by a factor of
two, and also resolve substructure in the stream at these apocenters. Here we
present dynamical models that reproduce these features of the stream in simple
Galactic potentials. We find that debris at the apocenters must be dynamically
young, in the sense of being stripped off in the last two pericentric passages,
while the Sagittarius dwarf is currently experiencing a third passage. The
ratio of apocenters is sensitive to both dynamical friction and the outer slope
of the Galactic rotation curve. These dependences can be understood with simple
regularities connecting the apocentric radii, circular velocities, and orbital
period of the progenitor. The effect of dynamical friction on the stream can be
constrained using substructure within the leading apocenter. Our ensembles of
models are not intended as statistically proper fits to the stream.
Nevertheless, out of the range of models we consider, we consistently find the
mass within 100 kpc to be , with a nearly
flat rotation curve between 50 and 100 kpc. This points to a more extended
Galactic halo than assumed in some current models. As in previous work, we find
prolate or triaxial halos ease agreement with the track of the leading stream.
We display the behavior of our models in various observational spaces and
characterize the substructure expected within the stream. In particular, the
young trailing stream visible near trailing apocenter should exhibit a tight
trend of velocity with distance separate from the older debris, and we suggest
that this will serve as an especially useful probe of the outer Galactic
potential.Comment: Submitted to MNRA
Systemic Proper Motions of Milky Way Satellites from Stellar Redshifts: the Carina, Fornax, Sculptor and Sextans Dwarf Spheroidals
The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute
line-of-sight components that increase with angular distance from the dSph
centers, inducing detectable gradients in stellar redshift. In the absence of
an intrinsic velocity gradient (e.g., due to rotation or streaming), an
observed gradient in the heliocentric rest frame (HRF) relates simply to a
dSph's systemic proper motion (PM). Kinematic samples for the Milky Way's
brightest dSph satellites are now sufficiently large that we can use stellar
redshifts to constrain systemic PMs independently of astrometric data. Data
from our Michigan/MIKE Fiber System (MMFS) Survey reveal significant HRF
velocity gradients in Carina, Fornax and Sculptor, and no significant gradient
in Sextans. Assuming there are no intrinsic gradients, the data provide a
relatively tight constraint on the PM of Fornax,
(mu_{alpha}^{HRF},mu_{delta}^{HRF})=(+48 +/- 15,-25 +/- 14) mas/century, that
agrees with published HST astrometric measurements. Smaller data sets yield
weaker constraints in the remaining galaxies, but our Carina measurement,
(mu_{alpha}^{HRF},mu_{delta}^{HRF})=(+25 +/- 36,+16 +/- 43) mas/century, agrees
with the published astrometric value. The disagreement of our Sculptor
measurement, (mu_{alpha}^{HRF},mu_{delta}^{HRF})= (-40 +/- 29, -69 +/- 47)
mas/century, with astrometric measurements is expected if Sculptor has a
rotational component as reported by Battaglia et al. (2008). For Sextans, which
at present lacks an astrometric measurement, we measure
(mu_{alpha}^{HRF},mu_{delta}^{HRF})=(-26 +/- 41, +10 +/- 44) mas/century.Comment: Accepted for Publication by ApJ Letter
Jakobshavn Glacier, west Greenland: 30 years of spaceborne observations
This is the published version, also available here: http://dx.doi.org/10.1029/98GL01973.Early 1960's reconnaissance satellite images are compared to more recent image and map data in an interannual and seasonal study of West Central Greenland margin fluctuations. From 1962 to 1992, ice sheet margins to the north and south of Jakobshavn Glacier retreated despite a decline in average summer temperatures. The retreat may be reversing along the southern flank of the ice stream where regional mass balance estimates are positive. From 1950 to 1996, the terminus of Jakobshavn Glacier seasonally fluctuated βΌ2.5 km around its annual mean position. The total calving flux during the summer is more than six times that during winter. We identified that summer melting and the break-up of sea ice and icebergs in the fjord are important in controlling the rate of iceberg production. If correct, calving rates may be expected to increase should climate become warmer in the near future
Evidence for an Intermediate-mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
We estimate the mass of the Milky Way (MW) within 21.1 kpc using the
kinematics of halo globular clusters (GCs) determined by Gaia. The second Gaia
data release (DR2) contained a catalogue of absolute proper motions (PMs) for a
set of Galactic GCs and satellite galaxies measured using Gaia DR2 data. We
select from the catalogue only halo GCs, identifying a total of 34 GCs spanning
kpc, and use their 3D kinematics to estimate the anisotropy
over this range to be , in good agreement, though
slightly lower than, a recent estimate for a sample of halo GCs using HST PM
measurements further out in the halo. We then use the Gaia kinematics to
estimate the mass of the MW inside the outermost GC to be , which
corresponds to a circular velocity of km/s. The implied virial mass is . The error bars encompass the
uncertainties on the anisotropy and on the density profile of the MW dark halo,
and the scatter inherent in the mass estimator we use. We get improved
estimates when we combine the Gaia and HST samples to provide kinematics for 46
GCs out to 39.5 kpc: , , and . We show that these results are
robust to potential substructure in the halo GC distribution. While a wide
range of MW virial masses have been advocated in the literature, from below
to above , these
new data imply that an intermediate mass is most likely
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