4,233 research outputs found
Clues on the evolution of the Carina dwarf spheroidal galaxy from the color distribution of its red giant stars
The thin red giant branch (RGB) of the Carina dwarf spheroidal galaxy appears
at first sight quite puzzling and seemingly in contrast with the presence of
several distinct bursts of star formation. In this Letter, we provide a
measurement of the color spread of red giant stars in Carina based on new BVI
wide-field observations, and model the width of the RGB by means of synthetic
color-magnitude diagrams. The measured color spread, Sigma{V-I}=0.021 +/-
0.005, is quite naturally accounted for by the star-formation history of the
galaxy. The thin RGB appears to be essentially related to the limited age range
of its dominant stellar populations, with no need for a metallicity dispersion
at a given age. This result is relatively robust with respect to changes in the
assumed age-metallicity relation, as long as the mean metallicity over the
galaxy lifetime matches the observed value ([Fe/H] = -1.91 +/- 0.12 after
correction for the age effects). This analysis of photometric data also sets
some constraints on the chemical evolution of Carina by indicating that the
chemical abundance of the interstellar medium in Carina remained low throughout
each episode of star formation even though these episodes occurred over many
Gyr.Comment: 4 pages, 3 figures, accepted for publication in the Astrophysical
Journal Letter
Hierarchical galaxy formation and substructure in the Galaxy's stellar halo
We develop an explicit model for the formation of the stellar halo from
tidally disrupted, accreted dwarf satellites in the cold dark matter (CDM)
framework, focusing on predictions testable with the Sloan Digital Sky Survey
(SDSS) and other wide-field surveys. Subhalo accretion and orbital evolution
are calculated using a semi-analytic approach within the Press-Schechter
formalism. Motivated by our previous work, we assume that low-mass subhalos (v
< 30 km/s) can form significant populations of stars only if they accreted a
substantial fraction of their mass before the epoch of reionization. With this
assumption, the model reproduces the observed velocity function of galactic
satellites in the Local Group, solving the ``dwarf satellite problem'' without
modifying the popular LCDM cosmology. The disrupted satellites yield a stellar
distribution with a total mass and radial density profile consistent with those
observed for the Milky Way stellar halo. Most significantly, the model predicts
the presence of many large-scale, coherent substructures in the outer halo.
These substructures are remnants of individual, tidally disrupted dwarf
satellite galaxies. Substructure is more pronounced at large galactocentric
radii because of the smaller number density of tidal streams and the longer
orbital times. This model provides a natural explanation for the coherent
structures in the outer stellar halo found in the SDSS commissioning data, and
it predicts that many more such structures should be found as the survey covers
more of the sky. The detection (or non-detection) and characterization of such
structures could eventually test variants of the CDM scenario, especially those
that aim to solve the dwarf satellite problem by enhancing satellite
disruption.Comment: 12 pages, 8 figures, Submitted to Ap
Untangling the X-ray Emission From the Sa Galaxy NGC1291 With Chandra
We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa
galaxy NGC1291. The X-ray emission from the bulge resembles the X-ray emission
from a sub-class of elliptical and S0 galaxies with low L_X/L_B luminosity
ratios. The X-ray emission is composed of a central point-like nucleus, ~50
point sources that are most likely low mass X-ray binaries (LMXBs), and diffuse
gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global
temperature of 0.32^{+0.04}_{-0.03} keV and metallicity of 0.06 +/- 0.02 solar,
and both quantities marginally decrease with increasing radius. The hot gas
fills the hole in the HI distribution, and the softening of the spectrum of the
X-ray gas with radius might indicate a thermal coupling of the hot and cold
phases of the interstellar medium as previously suggested. The integrated X-ray
luminosity of the LMXBs, once normalized by the optical luminosity, is a factor
of 1.4 less than in the elliptical galaxy NGC4697 or S0 galaxy NGC1553. The
difference in L_{X,stellar}/L_B between the galaxies appears to be because of a
lack of very bright sources in NGC1291. No sources above 3 x 10^38 ergs/s were
found in NGC1291 when ~7 were expected from scaling from NGC4697 and NGC1553.
The cumulative L_{X,stellar}/L_B value including only sources below 1.0 x 10^38
ergs/s is remarkably similar between NGC1291 and NGC4697, if a recent surface
brightness fluctuation-determined distance is assumed for NGC4697. If this is a
common feature of the LMXB population in early-type systems, it might be used
as a distance indicator. Finally, a bright, variable (1.6-3.1 x 10^39 ergs/s)
source was detected at the optical center of the galaxy. Its spectrum shows
excess soft emission superimposed on a highly absorbed power law component,
similar to what has been found in several other low luminosity AGN (ABRIDGED).Comment: 13 pages in emulateapj5 style with 11 embedded Postscript figures;
minor revisions since last version; accepted by Ap
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Measurement of the quality factor of a new low-frequency differential accelerometer for testing the equivalence principle
A cryogenic differential accelerometer has been developed to test the weak equivalence principle to a few parts in 1015 within the framework of the general relativity accuracy test in an Einstein elevator experiment. The prototype sensor was designed to identify, address, and solve the major issues associated with various aspects of the experiment. This paper illustrates the measurements conducted on this prototype sensor to attain a high quality factor (Q ∼ 105) at low frequencies (<20 Hz). Such a value is necessary for reducing the Brownian noise to match the target acceleration noise of 10−14 g/√Hz, hence providing the desired experimental accuracy.AstronomyPhysic
A New Method for Searching for Free Fractional Charge Particles in Bulk Matter
We present a new experimental method for searching for free fractional charge
in bulk matter; this new method derives from the traditional Millikan liquid
drop method, but allows the use of much larger drops, 20 to 100 mm in diameter,
compared to the traditional method that uses drops less than 15 mm in diameter.
These larger drops provide the substantial advantage that it is then much
easier to consistently generate drops containing liquid suspensions of powdered
meteorites and other special minerals. These materials are of great importance
in bulk searches for fractional charge particles that may have been produced in
the early universe.Comment: 17 pages, 5 figures in a singl PDF file (created from WORD Doc.).
Submitted to Review of Scientific Instrument
Public Benefits of Undeveloped Lands on Urban Outskirts: Non-Market Valuation Studies and their Role in Land Use Plans
Over the past three decades, the economics profession has developed methods for estimating the public benefits of green spaces, providing an opportunity to incorporate such information into land-use planning. While federal regulations routinely require such estimates for major regulations, the extent to which they are used in local land use plans is not clear. This paper reviews the literature on public values for lands on urban outskirts, not just to survey their methods or empirical findings, but to evaluate the role they have played--or have the potential to play-- in actual land use plans. Based on interviews with authors and representatives of funding agencies and local land trusts, it appears that academic work has had a mixed reception in the policy world. Reasons for this include a lack of interest in making academic work accessible to policy makers, emphasizing revealed preference methods which are inconsistent with policy priorities related to nonuse values, and emphasis on benefit-cost analyses. Nevertheless, there are examples of success stories that illustrate how such information can play a vital role in the design of conservation policies. Working Paper 07-2
Milky Way Demographics with the VVV Survey II. Color Transformations and Near-Infrared Photometry for 136 Million Stars in the Southern Galactic Disk
The new multi-epoch near-infrared VVV survey (VISTA Variables in the Via
Lactea) is sampling 562 sq. deg of the Galactic bulge and adjacent regions of
the disk. Accurate astrometry established for the region surveyed allows the
VVV data to be merged with overlapping surveys (e.g., GLIMPSE, WISE, 2MASS,
etc.), thereby enabling the construction of longer baseline spectral energy
distributions for astronomical targets. However, in order to maximize use of
the VVV data, a set of transformation equations are required to place the VVV
JHKs photometry onto the 2MASS system. The impetus for this work is to develop
those transformations via a comparison of 2MASS targets in 152 VVV fields
sampling the Galactic disk. The transformation coefficients derived exhibit a
reliance on variables such as extinction. The transformed data were
subsequently employed to establish a mean reddening law of
E_{J-H}/E_{H-Ks}=2.13 +/- 0.04, which is the most precise determination to date
and merely emphasizes the pertinence of the VVV data for determining such
important parameters.Comment: 24 pages, 12 figures, published in A&
A deep WISE search for very late type objects and the discovery of two halo/thick-disc T dwarfs : WISE 0013+0634 and WISE 0833+0052
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reservedA method is defined for identifying late-T and Y dwarfs in WISE down to low values of signal-to-noise. This requires a WISE detection only in the W2-band and uses the statistical properties of the WISE multiframe measurements and profile fit photometry to reject contamination resulting from non-point-like objects, variables and moving sources. To trace our desired parameter space, we use a control sample of isolated non-moving non-variable point sources from the Sloan Digital Sky Survey (SDSS), and identify a sample of 158 WISE W2-only candidates down to a signal-to-noise limit of eight. For signal-to-noise ranges >10 and 8-10, respectively, similar to 45 and similar to 90 per cent of our sample fall outside the selection criteria published by the WISE team, mainly due to the type of constraints placed on the number of individual W2 detections. We present follow-up of eight candidates and identify WISE 0013+0634 and WISE 0833+0052, T8 and T9 dwarfs with high proper motion (similar to 1.3 and similar to 1.8 arcsec yr(-1)). Both objects show a mid-infrared/near-infrared excess of similar to 1-1.5 mag and are K band suppressed. Distance estimates lead to space motion constraints that suggest halo (or at least thick disc) kinematics. We then assess the reduced proper motion diagram of WISE ultracool dwarfs, which suggests that late-T and Y dwarfs may have a higher thick-disc/halo population fraction than earlier objects.Peer reviewe
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