4,152 research outputs found
The Stellar Populations of Pixels and Frames
Derived from first physical principles, a few simple rules are presented that
can help in both the planning and interpretation of CCD and IR-array camera
observations of resolvable stellar populations. These rules concern the overall
size of the population sampled by a frame as measured by its total luminosity,
and allow to estimate the number of stars (in all evolutionary stages) that are
included in the frame. The total luminosity sampled by each pixel (or
resolution element) allows instead to estimate to which depth meaningful
stellar photometry can be safely attempted, and below which crowding makes it
impossible. Simple relations give also the number of pixels (resolution
elements) in the frame that will contain an unresolved blend of two stars of
any kind. It is shown that the number of such blends increases quadratically
with both the surface brightness of the target, as well as with the angular
size of the pixel (or resolution element). A series of examples are presented
illustrating how the rules are practically used in concrete observational
situations. Application of these tools to existing photometric data for the
inner parts of the bulge of M31, M32 and NGC 147 indicates that no solid
evidence has yet emerged for the presence of a significant intermediate age
population in these objects.Comment: 28 pages, LaTeX file using aasms4.sty, 2 postscript figures To appear
on: The Astronomical Journa
NASA rotor system research aircraft flight-test data report: Helicopter and compound configuration
The flight test activities of the Rotor System Research Aircraft (RSRA), NASA 740, from June 30, 1981 to August 5, 1982 are reported. Tests were conducted in both the helicopter and compound configurations. Compound tests reconfirmed the Sikorsky flight envelope except that main rotor blade bending loads reached endurance at a speed about 10 knots lower than previously. Wing incidence changes were made from 0 to 10 deg
Star-Formation in Low Radio Luminosity AGN from the Sloan Digital Sky Survey
We investigate faint radio emission from low- to high-luminosity Active
Galactic Nuclei (AGN) selected from the Sloan Digital Sky Survey (SDSS). Their
radio properties are inferred by co-adding large ensembles of radio image
cut-outs from the FIRST survey, as almost all of the sources are individually
undetected. We correlate the median radio flux densities against a range of
other sample properties, including median values for redshift, [OIII]
luminosity, emission line ratios, and the strength of the 4000A break. We
detect a strong trend for sources that are actively undergoing star-formation
to have excess radio emission beyond the ~10^28 ergs/s/Hz level found for
sources without any discernible star-formation. Furthermore, this additional
radio emission correlates well with the strength of the 4000A break in the
optical spectrum, and may be used to assess the age of the star-forming
component. We examine two subsamples, one containing the systems with emission
line ratios most like star-forming systems, and one with the sources that have
characteristic AGN ratios. This division also separates the mechanism
responsible for the radio emission (star-formation vs. AGN). For both cases we
find a strong, almost identical, correlation between [OIII] and radio
luminosity, with the AGN sample extending toward lower, and the star-formation
sample toward higher luminosities. A clearer separation between the two
subsamples is seen as function of the central velocity dispersion of the host
galaxy. For systems with similar redshifts and velocity dispersions, the
star-formation subsample is brighter than the AGN in the radio by an order of
magnitude. This underlines the notion that the radio emission in star-forming
systems can dominate the emission associated with the AGN.Comment: Accepted for publication in Astronomical Journal; 15 pages, 8 color
figure
Supernovae Rates: A Cosmic History
We discuss the cosmic history of supernovae on the basis of various
assumptions and recent data on the star formation history.
We show that supernova rates as a function of redshift can be used to place
significant constraints on progenitor models, on the star formation history,
and on the importance of dust obscuration.
We demonstrate that it is unlikely that the current observational indications
for the existence of a cosmological constant are merely an artifact of the
dominance of different progenitor classes at different redshift intervals.Comment: Accepted for publication in The Astrophysical Journa
HST Studies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster
We have obtained V and I images of the lone globular cluster that belongs to
the dwarf Local Group irregular galaxy known as WLM. The color-magnitude
diagram of the cluster shows that it is a normal old globular cluster with a
well-defined giant branch reaching to M_V=-2.5, a horizontal branch at
M_V=+0.5, and a sub-giant branch extending to our photometry limit of M_V=+2.0.
A best fit to theoretical isochrones indicates that this cluster has a
metallicity of [Fe/H]=-1.52\pm0.08 and an age of 14.8\pm0.6 Gyr, thus
indicating that it is similar to normal old halo globulars in our Galaxy. From
the fit we also find that the distance modulus of the cluster is 24.73\pm0.07
and the extinction is A_V=0.07\pm0.06, both values that agree within the errors
with data obtained for the galaxy itself by others. We conclude that this
normal massive cluster was able to form during the formation of WLM, despite
the parent galaxy's very small intrinsic mass and size.Comment: 14 pages, 5 figures, 1 tabl
Monitoring the Low-Energy Gamma-Ray Sky Using Earth Occultation with GLAST GBM
Long term all-sky monitoring of the 20 keV – 2 MeV gamma-ray sky using the Earth occultation technique was demonstrated by the BATSE instrument on the Compton Gamma Ray Observatory. The principles and techniques used for the development of an end-to-end earth occultation data analysis system for BATSE can be extended to the GLAST Burst Monitor (GBM), resulting in multiband light curves and time-resolved spectra in the energy range 8 keV to above 1 MeV for known gamma-ray sources and transient outbursts, as well as the discovery of new sources of gamma-ray emission. In this paper we describe the application of the technique to the GBM. We also present the expected sensitivity for the GBM
The star formation history of the Local Group dwarf elliptical galaxy NGC 185: II. Gradients in the stellar population
The star formation history of the dE NGC 185, together with its spatial
variations, has been investigated using new ground-based and
photometry, and synthetic color--magnitude diagrams (CMDs). We find that the
bulk of the stars were formed in NGC 185 at an early epoch of its evolution.
After that, the star formation proceeded at a low rate until the recent past,
the age of the most recent traces of star formation activity detected in the
galaxy being some 100 Myr.
The star formation rate, for old and intermediate ages shows a
gradient in the sense of taking smaller values for higher galactocentric radii.
Moreover, recent star formation is detected in the central
pc only, where the youngest, 100 Myr old population is
found. The luminous blue {\it stars} discovered by Baade (1951) in the center
of NGC 185 are discussed using new CCD images in and Baade's original
photographic plates, reaching the conclusion that most of them are in fact star
clusters. A consistent picture arises in which the gas observed in the central
region of NGC 185 would have an internal origin. The rate at which evolved
stars return gas to the ISM is enough to seed the recent star formation
observed in the center of the galaxy and the SN rate is probably low enough to
allow the galaxy to retain the gas not used in the new stellar generations.Comment: 34 pages, 17 figures, 5 Tables, to be published in AJ October 9
On the stellar populations in NGC 185 and NGC 205, and the nuclear star cluster in NGC 205 from Hubble Space Telescope observations
[Abridged] We present a first detailed analysis of resolved stellar
populations in the dwarf galaxies NGC 185 and NGC 205 based on archival V- and
I-band WFPC2 pointings. For NGC 185 we deduce that star formation was probably
still active about 4 x 10^8 yr ago. Key abundance-related results are: (1) We
identify ancient stars with [Fe/H] <~ -1.5 dex by a well-defined horizontal
branch (HB). (2) We find a prominent RGB/ faint-AGB clump/ bump- like feature
with the same mean V-band magnitude as the HB, within uncertainties; from a
comparison with theory, ancient stars have [Fe/H] ~ -1.5 dex, with a higher
abundance level for intermediate-age stars. (3) From colour information we
infer that the median [Fe/H] > -1.11 +/- 0.08 dex for ancient stars. For NGC
205, we record (m-M)o = 24.76 +/- 0.1 mag, based on the RGB I-band tip
magnitude method. We find that stars were probably still forming less than 3 x
10^8 yr ago, which is compatible with star formation triggered by an
interaction with M31. Key abundance-related results are: (1) The RGB/ faint-AGB
is significantly skewed to redder values than that of a control field in the
outskirts of M31; it probably results from a relatively narrow metallicity and
or age range for a significant fraction of the dwarf's stars. (2) For ancient
stars we infer from colour information that the median [Fe/H] > -1.06+/-0.04
dex. We briefly compare the stellar populations of NGC 205, NGC 185 and NGC
147.
Finally, we find an apparent blue excess in the outer region of the nuclear
star cluster in NGC 205. It is as compact as a typical galactic globular
cluster, but is quite bright (10^6 L_solar,R); and by matching its blue colour
with models, its stellar population is young, up to a few times 10^8 yr old.Comment: To appear in the May edition of the Astronomical Journal. Some
figures have been degraded in quality for the purpose of submissio
The Biot-Savart operator and electrodynamics on subdomains of the three-sphere
We study steady-state magnetic fields in the geometric setting of positive
curvature on subdomains of the three-dimensional sphere. By generalizing the
Biot-Savart law to an integral operator BS acting on all vector fields, we show
that electrodynamics in such a setting behaves rather similarly to Euclidean
electrodynamics. For instance, for current J and magnetic field BS(J), we show
that Maxwell's equations naturally hold. In all instances, the formulas we give
are geometrically meaningful: they are preserved by orientation-preserving
isometries of the three-sphere.
This article describes several properties of BS: we show it is self-adjoint,
bounded, and extends to a compact operator on a Hilbert space. For vector
fields that act like currents, we prove the curl operator is a left inverse to
BS; thus the Biot-Savart operator is important in the study of curl
eigenvalues, with applications to energy-minimization problems in geometry and
physics. We conclude with two examples, which indicate our bounds are typically
within an order of magnitude of being sharp.Comment: 24 pages (was 28 pages) Revised to include a new introduction, a
detailed example, and results about helicity; other changes for readabilit
HI Density Distribution Driven by Supernovae: A Simulation Study
We model the complex distribution of atomic hydrogen (HI) in the interstellar
medium (ISM) assuming that it is driven entirely by supernovae (SN). We develop
and assess two different models. In the first approach, the simulated volume is
randomly populated with non-overlapping voids of a range of sizes. This may
relate to a snapshot distribution of supernova-remnant voids, although somewhat
artificially constrained by the non-overlap criterion. In the second approach,
a simplified time evolution (considering momentum conservation as the only
governing constraint during interactions) is followed as SN populate the space
with the associated input mass and energy.
We describe these simulations and present our results in the form of images
of the mass and velocity distributions and the associated power spectra. The
latter are compared with trends indicated by available observations. In both
approaches, we find remarkable correspondence with the observed statistical
description of well-studied components of the ISM, wherein the spatial spectra
have been found to show significant deviations from the Kolmogorov spectrum.
One of the key indications from this study, regardless of whether or not the
SN-induced turbulence is the dominant process in the ISM, is that the apparent
non-Kolmogorov spectral characteristics (of HI and/or electron column density
across thick or thin screens) needed to explain related observations may not at
all be in conflict with the underlying turbulence (i.e. the velocity structure)
being of Kolmogorov nature. We briefly discuss the limitations of our
simulations and the various implications of our results.Comment: To appear in Astrophysical Journal. 21 pages, 6 figure
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