86 research outputs found
The Stromlo Missing Satellites Survey
The Stromlo Missing Satellites (SMS) program is a critical endeavor to
investigate whether cold dark matter cosmology is flawed in its ability to
describe the matter distribution on galaxy scales or proves itself once again
as a powerful theory to make observational predictions. The project will
deliver unprecedented results on Milky Way satellite numbers, their
distribution and physical properties. It is the deepest, most extended survey
for optically elusive dwarf satellite galaxies to date, covering the entire
20,000 sq deg of the Southern hemisphere. 150TB of CCD images will be analysed
in six photometric bands, 0.5-1.0 mag fainter than SDSS produced by the ANU
SkyMapper telescope over the next five years. (For more details see:
http://msowww.anu.edu.au/~jerjen/SMS_Survey.html)Comment: 4 pages, 1 figure, in "Galaxies in the Local Volume" (Sydney, 8-13
July 2007), eds B. Koribalski and H. Jerjen, Springer Astrophysics and Space
Science Proceedings, p. 18
Formation and structure of halos in a warm dark matter cosmology
(Abridged) Using high-resolution cosmological N-body simulations, we study
how the density profiles of dark matter halos are affected by the filtering of
the density power spectrum below a given scale length and by the introduction
of a thermal velocity dispersion. In the warm dark matter (WDM) scenario, both
the free-streaming scale, R_f, and the velocity dispersion, v_w, are determined
by the mass m_w of the WDM particle. We find that v_w is too small to affect
the density profiles of WDM halos. Down to the resolution attained in our
simulations, there is not any significant difference in the density profiles
and concentrations of halos obtained in simulations with and without the
inclusion of v_w. The density profiles of halos with masses down to ~0.01 the
filtering mass M_f can be described by the NFW shape; significant soft cores
are not formed. Nevertheless, the concentrations of these halos are lower than
those of the CDM counterparts and are approximately independent of mass. The
lower concentrations of WDM halos with respect to their CDM counterparts can be
accounted for their late formation epoch. Overall, our results point to a
series of advantages of a WDM model over the CDM one. In addition to solving
the substructure problem, a WDM model with R_f~0.16 Mpc (m_w~0.75 kev; flat
cosmology with Omega_L=h=0.7) also predicts concentrations, a Tully-Fisher
relation, and formation epochs for small halos which seems to be in better
agreement with observations, relative to CDM predictions.Comment: Accepted for publication in ApJ. 34 pages, figs. 1a,1b, and 1c given
in separate files (high resolution versions available at
ftp://ftp.astroscu.unam.mx/pub/temporal/). Major modifications after referees
Report (more simulations and new figures are presented), but main conclusions
remain the sam
AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY
We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (Ό = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe
A Two Micron All-Sky Survey View of the Sagittarius Dwarf Galaxy: VI. s-Process and Titanium Abundance Variations Along the Sagittarius Stream
We present high-resolution spectroscopic measurements of the abundances of
titanium (Ti), yttrium (Y) and lanthanum (La) for M giant candidates of the
Sagittarius (Sgr) dwarf spheroidal (dSph) + tidal tail system pre-selected on
the basis of position and radial velocity. The majority of these stars show
peculiar abundance patterns compared to those of nominal Milky Way (MW) stars.
The overall [Ti/Fe], [Y/Fe], [La/Fe] and [La/Y] patterns with [Fe/H] of the Sgr
stream plus Sgr core do resemble those seen in the Large Magellanic Cloud (LMC)
and other dSphs, only shifted [Fe/H] by ~+0.4 from the LMC and by ~+1 dex from
the other dSphs; these relative shifts reflect the faster and/or more efficient
chemical evolution of Sgr compared to the other satellites, and show that Sgr
has had an enrichment history more like the LMC than the other dSphs. By
tracking the evolution of the abundance patterns along the Sgr stream we can
follow the time variation of the chemical make-up of dSph stars donated to the
MW halo by Sgr. This evolution demonstrates that while the bulk of the stars
currently in the Sgr dSph are quite unlike those of the MW halo, an increasing
number of stars farther along the Sgr stream have abundances like MW halo
stars, a trend that shows clearly how the MW halo could have been contributed
by present day satellite galaxies even if the present chemistry of those
satellites is now different from typical halo field stars. Finally, we analyze
the chemical abundances of a moving group of M giants among the Sgr leading arm
stars at the North Galactic Cap, but having radial velocities unlike the
infalling Sgr leading arm debris there. Through use of "chemical
fingerprinting", we conclude that these northern hemisphere M giants also are
Sgr stars, likely trailing arm debris overlapping the leading arm in the north.Comment: Accepted for publication in Ap
Toward an internally consistent astronomical distance scale
Accurate astronomical distance determination is crucial for all fields in
astrophysics, from Galactic to cosmological scales. Despite, or perhaps because
of, significant efforts to determine accurate distances, using a wide range of
methods, tracers, and techniques, an internally consistent astronomical
distance framework has not yet been established. We review current efforts to
homogenize the Local Group's distance framework, with particular emphasis on
the potential of RR Lyrae stars as distance indicators, and attempt to extend
this in an internally consistent manner to cosmological distances. Calibration
based on Type Ia supernovae and distance determinations based on gravitational
lensing represent particularly promising approaches. We provide a positive
outlook to improvements to the status quo expected from future surveys,
missions, and facilities. Astronomical distance determination has clearly
reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press
(chapter 8 of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201
Magellan/M2FS Spectroscopy of Tucana 2 and Grus 1
We present results from spectroscopic observations with the Michigan/Magellan
Fiber System (M2FS) of stellar targets along the line of sight to the
newly-discovered `ultrafaint' stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru
1). Based on simultaneous estimates of line-of-sight velocity and
stellar-atmospheric parameters, we identify 8 and 7 stars as probable members
of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to
resolve an internal velocity dispersion of km s
about a mean of km s (solar rest frame), and to
estimate a mean metallicity of [Fe/H]= . These results
place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies,
suggesting a dominant dark matter component with dynamical mass
enclosed within the central
pc, and dynamical mass-to-light ratio
. For Gru 1 we estimate a mean velocity of
km s and a mean metallicity of
[Fe/H]=, but our sample does not resolve Gru 1's
velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic
phase space suggest that their orbits are among the most energetic within
distance kpc. Moreover, their proximity to each other in this space
arises naturally if both objects are trailing the Large Magellanic Cloud.Comment: replaced with ApJ-accepted version, all spectra and data products
(including samples from posterior PDFs) are available at
http://www.andrew.cmu.edu/user/mgwalker/tuc2gru1_dataproducts.tar.g
Kinematics and chemistry of recently discovered Reticulum 2 and Horologium 1 dwarf galaxies
Photometry alone is not sufficient to unambiguously distinguish between
ultra-faint star clusters and dwarf galaxies because of their overlap in
morphological properties. Here we report on VLT/GIRAFFE spectra of candidate
member stars in two recently discovered ultra-faint satellites Reticulum 2 and
Horologium 1, obtained as part of the ongoing Gaia-ESO Survey. We identify 18
members in Reticulum 2 and 5 in Horologium 1. We find Reticulum 2 to have a
velocity dispersion of ~3.22 km/s, implying a M/L ratio of ~ 500. We have
inferred stellar parameters for all candidates and we find Reticulum 2 to have
a mean metallicity of [Fe/H] = -2.46+/-0.1, with an intrinsic dispersion of ~
0.29, and is alpha-enhanced to the level of [alpha/Fe]~0.4. We conclude that
Reticulum 2 is a dwarf galaxy. We also report on the serendipitous discovery of
four stars in a previously unknown stellar substructure near Reticulum 2 with
[Fe/H] ~ -2 and V_hel ~ 220 km/s, far from the systemic velocity of Reticulum
2. For Horologium 1 we infer a velocity dispersion of 4.9^{+2.8}_{-0.9} km/s
and a consequent M/L ratio of ~ 600, leading us to conclude that Horologium 1
is also a dwarf galaxy. Horologium 1 is slightly more metal-poor than Reticulum
2 [Fe/H] = -2.76 +/- 0.1 and is similarly alpha-enhanced: [alpha/Fe] ~ 0.3.
Despite a large error-bar, we also measure a significant spread of
metallicities of 0.17 dex which strengthen the evidence that Horologium 1 is
indeed a dwarf galaxy. The line-of-sight velocity of Reticulum 2 is offset by
some 100 km/s from the prediction of the orbital velocity of the LMC, thus
making its association with the Cloud uncertain. However, at the location of
Horologium 1, both the backward integrated LMC's orbit and the LMC's halo are
predicted to have radial velocities similar to that of the dwarf. Therefore, it
is likely that Horologium 1 is or once was a member of the Magellanic Family.Comment: submitted to ApJ, abstract abridge
BEASTS OF THE SOUTHERN WILD: DISCOVERY OF NINE ULTRA FAINT SATELLITES IN THE VICINITY OF THE MAGELLANIC CLOUDS
We have used the publicly released Dark Energy Survey data to hunt for new
satellites of the Milky Way in the Southern hemisphere. Our search yielded a
large number of promising candidates. In this paper, we announce the discovery
of 9 new unambiguous ultra-faint objects, whose authenticity can be established
with the DES data alone. Based on the morphological properties, three of the
new satellites are dwarf galaxies, one of which is located at the very
outskirts of the Milky Way, at a distance of 380 kpc. The remaining 6 objects
have sizes and luminosities comparable to the Segue~1 satellite and can not be
classified straightforwardly without follow-up spectroscopic observations. The
satellites we have discovered cluster around the LMC and the SMC. We show that
such spatial distribution is unlikely under the assumption of isotropy, and,
therefore, conclude that at least some of the new satellites must have been
associated with the Magellanic Clouds in the past.Comment: Accepted to ApJ; See also 1503.02584; Changes w.r.t. v2: the
discussion section updated slightly; small changes in the table with
satellite parameter
Large-Scale Spatio-Temporal Patterns of Mediterranean Cephalopod Diversity
Species diversity is widely recognized as an important trait of ecosystemsâ functioning and resilience. Understanding the causes of diversity patterns and their interaction with the environmental conditions is essential in order to effectively assess and preserve existing diversity. While diversity patterns of most recurrent groups such as fish are commonly studied, other important taxa such as cephalopods have received less attention. In this work we present spatio-temporal trends of cephalopod diversity across the entire Mediterranean Sea during the last 19 years, analysing data from the annual bottom trawl survey MEDITS conducted by 5 different Mediterranean countries using standardized gears and sampling protocols. The influence of local and regional environmental variability in different Mediterranean regions is analysed applying generalized additive models, using species richness and the Shannon Wiener index as diversity descriptors. While the western basin showed a high diversity, our analyses do not support a steady eastward decrease of diversity as proposed in some previous studies. Instead, high Shannon diversity was also found in the Adriatic and Aegean Seas, and high species richness in the eastern Ionian Sea. Overall diversity did not show any consistent trend over the last two decades. Except in the Adriatic Sea, diversity showed a hump-shaped trend with depth in all regions, being highest between 200â400 m depth. Our results indicate that high Chlorophyll a concentrations and warmer temperatures seem to enhance species diversity, and the influence of these parameters is stronger for richness than for Shannon diversityVersiĂłn del editor4,411
- âŠ