3,475 research outputs found
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
The Physical Nature of Rest-UV Galaxy Morphology During the Peak Epoch of Galaxy Formation
Motivated by the irregular and little-understood morphologies of z ~ 2 - 3
galaxies, we use non-parametric coefficents to quantify the morphologies of 216
galaxies which have been spectroscopically confirmed to lie at redshifts z =
1.8 - 3.4 in the GOODS-N field. Using measurements of ultraviolet (UV) and
optical spectral lines, multi-band photometric data, and stellar population
models we statistically assess possible correlations between galaxy morphology
and physical observables such as stellar mass, star formation rate, and the
strength of galaxy-scale outflows. We find evidence that dustier galaxies have
more nebulous UV morphologies and that larger, more luminous galaxies may drive
stronger outflows, but otherwise conclude that UV morphology is either
statistically decoupled from the majority of physical observables or determined
by too complex a combination of physical processes to provide characterizations
with predictive power. Given the absence of strong correlations between UV
morphology and physical parameters such as star formation rates, we are
therefore unable to support the hypothesis that morphologically irregular
galaxies predominantly represent major galaxy mergers. Comparing galaxy
samples, we find that IR-selected BzK galaxies and radio-selected submillimeter
galaxies (SMGs) have UV morphologies similar to the optically selected sample,
while distant red galaxies (DRGs) are more nebulous.Comment: 26 pages. Accepted for publication in the ApJ. Version with full
resolution figures is available at
http://www.astro.caltech.edu/~drlaw/Papers/UVmorph.pd
A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: IV. Modeling the Sagittarius Tidal Tails
M giants recovered from the Two Micron All-Sky Survey (2MASS) have recently
been used to map the position and velocity distributions of tidal debris from
the Sagittarius (Sgr) dwarf spheroidal galaxy entirely around the Galaxy. We
compare this data set to both test particle orbits and N-body simulations of
satellite destruction run within a variety of rigid Milky Way potentials and
find that the mass of the Milky Way within 50 kpc of its center should be
3.8-5.6 x 10^11 Msun in order for any Sgr orbit to simultaneously fit the
velocity gradient in the Sgr trailing debris and the apocenter of the Sgr
leading debris. Orbital pole precession of young debris and leading debris
velocities in regions corresponding to older debris provide contradictory
evidence in favor of oblate/prolate Galactic halo potentials respectively,
leading us to conclude that the orbit of Sgr has evolved over the past few Gyr.
Based upon the velocity dispersion and width along the trailing tidal stream
we estimate the current bound mass of Sgr to be M_Sgr = 2 - 5 x 10^8 Msun
independant of the form of the Galactic potential; this corresponds to a range
of mass to light ratios (M/L)_Sgr = 14 - 36 (M/L)_Sun for the Sgr core. Models
with masses in this range best fit the apocenter of leading Sgr tidal debris
when they orbit with a radial period of roughly 0.85 Gyr and have periGalactica
and apoGalactica of about 15 kpc and 60 kpc respectively. These distances will
scale with the assumed distance to the Sgr dwarf and the assumed depth of the
Galactic potential. The density distribution of debris along the orbit in these
models is consistent with the M giant observations, and debris at all orbital
phases where M giants are obviously present is younger (i.e. was lost more
recently from the satellite) than the typical age of a Sgr M giant star.Comment: 42 pages, 13 figures; Accepted for publication by ApJ (October 08,
2004; originally submitted May 10, 2004). Fixed typos and added references.
PDF file with high resolution figures may be downloaded from
http://www.astro.caltech.edu/~drlaw/Papers/Sgr_paper4.pd
Measuring Fundamental Galactic Parameters with Stellar Tidal Streams and SIM PlanetQuest
Extended halo tidal streams from disrupting Milky Way satellites offer new
opportunities for gauging fundamental Galactic parameters without challenging
observations of the Galactic center. In the roughly spherical Galactic
potential tidal debris from a satellite system is largely confined to a single
plane containing the Galactic center, so accurate distances to stars in the
tidal stream can be used to gauge the Galactic center distance, R_0, given
reasonable projection of the stream orbital pole on the X_GC axis.
Alternatively, a tidal stream with orbital pole near the Y_GC axis, like the
Sagittarius stream, can be used to derive the speed of the Local Standard of
Rest (\Theta_LSR). Modest improvements in current astrometric catalogues might
allow this measurement to be made, but NASA's Space Interferometry Mission (SIM
PlanetQuest) can definitively obtain both R_0 and \Theta_LSR using tidal
streams.Comment: 8 pages, 4 figures, accepted for publication in ApJ Letters (minor
text revisions). Version with high resolution figures available at
http://www.astro.caltech.edu/~drlaw/Papers/GalaxyParameters.pd
Morphologies of Galaxies in and around a Protocluster at z=2.300
We present results from the first robust investigation of galaxy morphology
as a function of environment at z>1.5. Our study is motivated by the fact that
star-forming galaxies contained within a protocluster at z=2.3 in the HS1700+64
field have significantly older ages and larger stellar masses on average than
those at similar redshifts but more typical environmental densities. In the
analysis of HST/ACS images, we apply non-parametric statistics to characterize
the rest-frame UV morphologies of a sample of 85 UV-selected star-forming
galaxies at z=1.7-2.9, 22 of which are contained in the protocluster. The
remaining 63 control-sample galaxies are not in the protocluster but have a
similar mean redshift of ~2.3. We find no environmental dependence for the
distributions of morphological properties. Combining the measured morphologies
with the results of population synthesis modeling, we find only weak
correlations, if any, between morphological and stellar population properties
such as stellar mass, age, extinction and star-formation rate. Given the
incomplete census of the protocluster galaxy population, and the lack of
correlation between rest-frame UV morphology and star-formation history at z~2
within our sample, the absence of environmental trends in the distribution of
morphological properties is not surprising. Additionally, using a larger sample
of photometric candidates, we compare morphological distributions for 282
UV-selected and 43 near-IR-selected galaxies. While the difference in the
degree of nebulosity between the two samples appears to be a byproduct of the
fainter average rest-frame UV surface brightness of the near-IR-selected
galaxies, we find that, among the lowest surface brightness galaxies, the
near-IR-selected objects have significantly smaller angular sizes (abridged).Comment: 25 pages including 16 figures. Accepted for publication in ApJ.
Version with full resolution figures available at
http://www.astro.princeton.edu/~apeter/LBG/papers/peter2007_fullres.ps.g
Exploring Halo Substructure with Giant Stars: Spectroscopy of Stars in the Galactic Anticenter Stellar Structure
To determine the nature of the recently discovered, ring-like stellar
structure at the Galactic anticenter, we have collected spectra of a set of
presumed constituent M giants selected from the 2MASS point source catalog.
Radial velocities have been obtained for stars spanning ~100 degrees,
exhibiting a trend in velocity with Galactic longitude and an estimated
dispersion of 20 +/- 4 km/sec. A mean metallicity [Fe/H] = -0.4 +/- 0.3
measured for these stars combines with previous evidence from the literature to
suggest a population with a significant metallicity spread. In addition, a
curious alignment of at least four globular clusters of lower mean metallicity
is noted to be spatially and kinematically consistent with this stellar
distribution. We interpret the M giant sample position and velocity variation
with Galactic longitude as suggestive of a satellite galaxy currently
undergoing tidal disruption in a non-circular, prograde orbit about the Milky
Way.Comment: (1) University of Virginia, 4 pages, 3 figures, accepted for
publication in The Astrophysical Journal Letter
The infrared imaging spectrograph (IRIS) for TMT: the science case
The InfraRed Imaging Spectrograph (IRIS) is a first-light instrument being
designed for the Thirty Meter Telescope (TMT). IRIS is a combination of an
imager that will cover a 16.4" field of view at the diffraction limit of TMT (4
mas sampling), and an integral field unit spectrograph that will sample objects
at 4-50 mas scales. IRIS will open up new areas of observational parameter
space, allowing major progress in diverse fields of astronomy. We present the
science case and resulting requirements for the performance of IRIS.
Ultimately, the spectrograph will enable very well-resolved and sensitive
studies of the kinematics and internal chemical abundances of high-redshift
galaxies, shedding light on many scenarios for the evolution of galaxies at
early times. With unprecedented imaging and spectroscopy of exoplanets, IRIS
will allow detailed exploration of a range of planetary systems that are
inaccessible with current technology. By revealing details about resolved
stellar populations in nearby galaxies, it will directly probe the formation of
systems like our own Milky Way. Because it will be possible to directly
characterize the stellar initial mass function in many environments and in
galaxies outside of the the Milky Way, IRIS will enable a greater understanding
of whether stars form differently in diverse conditions. IRIS will reveal
detailed kinematics in the centers of low-mass galaxies, allowing a test of
black hole formation scenarios. Finally, it will revolutionize the
characterization of reionization and the first galaxies to form in the
universe.Comment: to appear in Proc. SPIE 773
Towards practice-based studies of HRM: an actor-network and communities of practice informed approach
HRM may have become co-terminus with the new managerialism in the rhetorical orthodoxies of the HRM textbooks and other platforms for its professional claims. However, we have detailed case-study data showing that HR practices can be much more complicated, nuanced and indeed resistive toward management within organizational settings.
Our study is based on ethnographic research, informed by actor-network theory and community of practice theory conducted by one of the authors over an 18-month period. Using actor-network theory in a descriptive and critical way, we analyse practices of managerial resistance, enrolment and counter-enrolment through which an unofficial network of managers used a formal HRM practice to successfully counteract the official strategy of the firm, which was to close parts of a production site. As a consequence, this network of middle managers effectively changed top management strategy and did so through official HRM practices, coupled with other actor-network building processes, arguably for the ultimate benefit of the organization, though against the initial views of the top management.
The research reported here, may be characterized as a situated study of HRM-in-practice and we draw conclusions which problematize the concept of HRM in contemporary management literature
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