2,950 research outputs found
The baryonic halos of isolated elliptical galaxies
Without the interference of a number of events, galaxies may suffer in
crowded environments (e.g., stripping, harassment, strangulation); isolated
elliptical galaxies provide a control sample for the study of galaxy formation.
We present the study of a sample of isolated ellipticals using imaging from a
variety of telescopes, focusing on their globular cluster systems as tracers of
their stellar halos. Our main findings are: (a) GC color bimodality is common
even in the most isolated systems; (b) the specific frequency of GCs is fairly
constant with galaxy mass, without showing an increase towards high-mass
systems like in the case of cluster ellipticals; (c) on the other hand, the red
fraction of GCs follows the same inverted V shape trend with mass as seen in
cluster ellipticals; and (d) the stellar halos show low S\'ersic indices which
are consistent with a major merger origin.Comment: 5 pages. Published in the special issue of Galaxies, "On the Origin
(and Evolution) of Baryonic Galaxy Halos
Dark Matter Deprivation in Field Elliptical Galaxy NGC 7507
Previous studies have shown that the kinematics of the field elliptical
galaxy NGC 7507 do not necessarily require dark matter. This is troubling
because, in the context of LCDM cosmologies, all galaxies should have a large
dark matter component. We use penalised pixel fitting software to extract
velocities and velocity dispersions from GMOS slit mask spectra. Using Jeans
and MONDian modelling we produce best fit models to the velocity dispersion. We
find that NGC 7507 has a two component stellar halo, with the outer halo and
inner haloes counter rotating. The velocity dispersion profile exhibits an
increase at ~70" (~7.9 kpc), reminiscent of several other elliptical galaxies.
Our best fit models are those under mild anisotropy which include ~100 times
less dark matter than predicted by LCDM, although mildly anisotropic models
that are completely dark matter free fit almost equally well. Our MONDian
models, both isotropic and anisotropic, systematically fail to reproduce the
measured velocity dispersions at almost all radii. The counter rotating outer
halo implies a merger remnant, as does the increase in velocity dispersion at
~70". From simulations it seems plausible that the merger that caused the
increase in velocity dispersion was a spiral-spiral merger. Our Jeans models
are completely consistent with a no dark matter scenario, however, some dark
matter can be accommodated, although at much lower concentrations that
predicted by LCDM simulations. This indicates that NGC 7507 may be a dark
matter free elliptical galaxy. Whether NGC 7507 is completely dark matter free
or very dark matter poor, this is at odds with predictions from current LCDM
cosmological simulations. It may be possible that the observed velocity
dispersions could be reproduced if the galaxy is significantly flattened along
the line of sight (e.g. due to rotation), however, invoking this flattening is
problematic.Comment: 9 pages, 7 figures. Accepted for publication in A&
The Tidal Tails of 47 Tucanae
The Galactic globular cluster 47 Tucanae (47 Tuc) shows a rare increase in
its velocity dispersion profile at large radii, indicative of energetic, yet
bound, stars at large radii dominating the velocity dispersion and,
potentially, of ongoing evaporation. Escaping stars will form tidal tails, as
seen with several Galactic globular clusters, however, the tidal tails of 47
Tuc are yet to be uncovered. We model these tails of 47 Tuc using the most
accurate input data available, with the specific aim of determining their
locations, as well as the densities of the epicyclic overdensities within the
tails. The overdensities from our models show an increase of 3-4% above the
Galactic background and, therefore, should be easily detectable using matched
filtering techniques. We find that the most influential parameter with regard
to both the locations and densities of the epicyclic overdensities is the
Heliocentric distance to the cluster. Hence, uncovering these tidal features
observationally will contribute greatly to the ongoing problem of determining
the distance to 47 Tuc, tightly constraining the distance of the cluster
independent of other methods. Using our streakline method for determining the
locations of the tidal tails and their overdensities, we show how, in
principle, the shape and extent of the tidal tails of any Galactic globular
cluster can be determined without resorting to computationally expensive N-body
simulations.Comment: Original paper has 10 pages, 10 figures and 2 tables. Please note
that this now includes an erratum. Erratum has 6 pages, 8 figures and 2
tables. Ignore the exclamation marks in Section 2 of the erratum, these are
an artifact of the LaTeX class file used to produce the manuscrip
A Statement on the Appropriate Role for Research and Development in Climate Policy
This statement is issued by a group of economists and scientists which met at Stanford University on October 18, 2008 to discuss the role of research and development (R&D) in developing effective policies for addressing the adverse potential consequences of climate change. We believe that climate change is a serious issue that governments need to address. We also believe that research and development needs to be a central part of governments’ strategies for responding to this challenge. Solutions to manage long-term risks will require the development and global deployment of a range of technologies for energy supply and end-use, land-use, agriculture and adaptation that are not currently commercial. A key potential benefit of focused scientific and technological research and development investment is that it could dramatically reduce the cost of restricting greenhouse gas emissions by encouraging the development of more affordable, better performing technologies.
The distribution of [/Fe] in the Milky Way disc
Using a sample of red giant stars from the Apache Point Observatory Galactic
Evolution Experiment (APOGEE) Data Release 16, we infer the conditional
distribution in the Milky Way disk
for the -elements Mg, O, Si, S, and Ca. In each bin of [Fe/H] and
Galactocentric radius , we model as a sum of two
Gaussians, representing "low-" and "high-" populations with
scale heights and , respectively.
By accounting for age-dependent and -dependent selection effects in APOGEE,
we infer the [/Fe] distributions that would be found for a fair sample
of long-lived stars covering all . Near the Solar circle, this distribution
is bimodal at sub-solar [Fe/H], with the low- and high- peaks
clearly separated by a minimum at intermediate [/Fe]. In agreement with
previous results, we find that the high- population is more prominent
at smaller , lower [Fe/H], and larger , and that the sequence
separation is smaller for Si and Ca than for Mg, O, and S. We find significant
intrinsic scatter in [/Fe] at fixed [Fe/H] for both the low-
and high- populations, typically -dex. The means,
dispersions, and relative amplitudes of this two-Gaussian description, and the
dependence of these parameters on , [Fe/H], and -element, provide a
quantitative target for chemical evolution models and a test for hydrodynamic
simulations of disk galaxy formation. We argue that explaining the observed
bimodality will probably require one or more sharp transitions in the disk's
gas accretion, star formation, or outflow history in addition to radial mixing
of stellar populations.Comment: Accepted for publication in MNRA
Are Milky-Way-like galaxies like the Milky Way? A view from SDSS-IV/MaNGA
In this paper, we place the Milky Way (MW) in the context of similar-looking
galaxies in terms of their star-formation and chemical evolution histories. We
select a sample of 138 Milky-Way analogues (MWAs) from the SDSS-IV/MaNGA survey
based on their masses, Hubble types, and bulge-to-total ratios. To compare
their chemical properties to the detailed spatially-resolved information
available for the MW, we use a semi-analytic spectral fitting approach, which
fits a self-consistent chemical-evolution and star-formation model directly to
the MaNGA spectra. We model the galaxies' inner and outer regions assuming that
some of the material lost in stellar winds falls inwards. We also incorporate
chemical enrichment from type II and Ia supernovae to follow the alpha-element
abundance at different metallicities and locations. We find some MWAs where the
stellar properties closely reproduce the distribution of age, metallicity, and
alpha enhancement at both small and large radii in the MW. In these systems,
the match is driven by the longer timescale for star formation in the outer
parts, and the inflow of enriched material to the central parts. However, other
MWAs have very different histories. These divide into two categories:
self-similar galaxies where the inner and outer parts evolve identically; and
centrally-quenched galaxies where there is very little evidence of late-time
central star formation driven by material accreted from the outer regions. We
find that, although selected to be comparable, there are subtle morphological
differences between galaxies in these different classes, and that the
centrally-quenched galaxies formed their stars systematically earlier.Comment: 16 pages, 9 figures, MNRAS accepted versio
Halo globular clusters observed with AAOmega: dark matter content, metallicity and tidal heating
Globular clusters have proven to be essential to our understanding of many
important astrophysical phenomena. Here we analyse spectroscopic observations
of ten Halo globular clusters to determine their dark matter content, their
tidal heating by the Galactic disc and halo, describe their metallicities and
the likelihood that Newtonian dynamics explain their kinematics. We analyse a
large number of members in all clusters, allowing us to address all these
issues together, and we have included NGC 288 and M30 to overlap with previous
studies. We find that any flattening of the velocity dispersion profiles in the
outer regions of our clusters can be explained by tidal heating. We also find
that all our GCs have M/L_V < 5, therefore, we infer the observed dynamics do
not require dark matter, or a modification of gravity. We suggest that the lack
of tidal heating signatures in distant clusters indicates the Halo is not
triaxial. The isothermal rotations of each cluster are measured, with M4 and
NGC 288 exhibiting rotation at a level of 0.9 +/- 0.1 km/s and 0.25 +/- 0.15
km/s, respectively. We also indirectly measure the tidal radius of NGC 6752,
determining a more realistic figure for this cluster than current literature
values. Lastly, an unresolved and intriguing puzzle is uncovered with regard to
the cooling of the outer regions of all ten clusters.Comment: 12 pages, 8 figures, 1 table. Accepted for publication in MNRAS
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