1,942 research outputs found
Squelched Galaxies and Dark Halos
There is accumulating evidence that the faint end of the galaxy luminosity
function might be very different in different locations. The luminosity
function might be rising in rich clusters and flat or declining in regions of
low density. If galaxies form according to the model of hierarchical clustering
then there should be many small halos compared to the number of big halos. If
this theory is valid then there must be a mechanism that eliminates at least
the visible component of galaxies in low density regions. A plausible mechanism
is photoionization of the intergalactic medium at a time before the epoch that
most dwarf galaxies form in low density regions but after the epoch of
formation for similar systems that ultimately end up in rich clusters. The
dynamical timescales are found to accommodate this hypothesis in a flat
universe with Omega_m < 0.4.
If small halos exist but simply cannot be located because they have never
become the sites of significant star formation, they still might have dynamical
manifestations. These manifestations are hard to identify in normal groups of
galaxies because small halos do not make a significant contribution to the
global mass budget. However, it could be entertained that there are clusters of
halos where there are only small systems, clusters that are at the low mass end
of the hierarchical tree. There may be places where only a few small galaxies
managed to form, enough for us to identify and use as test probes of the
potential. It turns out that such environments might be common. Four probable
groups of dwarfs are identified within 5 Mpc and the assumption they are
gravitationally bound suggests M/L_B ~ 300 - 1200 M_sun/L_sun, 6 +/- factor 2
times higher than typical values for groups with luminous galaxies.Comment: Accepted ApJ 569, (April 20), 2002, 12 pages, 6 figures, 1 tabl
Galactic winds and stellar populations in Lyman emitting galaxies at z ~ 3.1
We present a sample of 33 spectroscopically confirmed z ~ 3.1
Ly-emitting galaxies (LAEs) in the Cosmological Evolution Survey
(COSMOS) field. This paper details the narrow-band survey we conducted to
detect the LAE sample, the optical spectroscopy we performed to confirm the
nature of these LAEs, and a new near-infrared spectroscopic detection of the [O
III] 5007 \AA\ line in one of these LAEs. This detection is in addition to two
[O III] detections in two z ~ 3.1 LAEs we have reported on previously (McLinden
et al 2011). The bulk of the paper then presents detailed constraints on the
physical characteristics of the entire LAE sample from spectral energy
distribution (SED) fitting. These characteristics include mass, age,
star-formation history, dust content, and metallicity. We also detail an
approach to account for nebular emission lines in the SED fitting process -
wherein our models predict the strength of the [O III] line in an LAE spectrum.
We are able to study the success of this prediction because we can compare the
model predictions to our actual near-infrared observations both in galaxies
that have [O III] detections and those that yielded non-detections. We find a
median stellar mass of 6.9 10 M and a median star
formation rate weighted stellar population age of 4.5 10 yr. In
addition to SED fitting, we quantify the velocity offset between the [O III]
and Ly lines in the galaxy with the new [O III] detection, finding that
the Ly line is shifted 52 km s redwards of the [O III] line,
which defines the systemic velocity of the galaxy.Comment: 38 pages, 27 figures, 4 tables, Accepted for publication in MNRA
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The Borwein brothers, Pi and the AGM
We consider some of Jonathan and Peter Borweins' contributions to the
high-precision computation of and the elementary functions, with
particular reference to their book "Pi and the AGM" (Wiley, 1987). Here "AGM"
is the arithmetic-geometric mean of Gauss and Legendre. Because the AGM
converges quadratically, it can be combined with fast multiplication algorithms
to give fast algorithms for the -bit computation of , and more
generally the elementary functions. These algorithms run in almost linear time
, where is the time for -bit multiplication. We
outline some of the results and algorithms given in Pi and the AGM, and present
some related (but new) results. In particular, we improve the published error
bounds for some quadratically and quartically convergent algorithms for ,
such as the Gauss-Legendre algorithm. We show that an iteration of the
Borwein-Borwein quartic algorithm for is equivalent to two iterations of
the Gauss-Legendre quadratic algorithm for , in the sense that they
produce exactly the same sequence of approximations to if performed using
exact arithmetic.Comment: 24 pages, 6 tables. Changed style file and reformatted algorithms in
v
Atomic hydrogen, star formation and feedback in the lowest mass Blue Compact Dwarf galaxies
We present the results from a search for HI emission from a sample of newly
discovered dwarf galaxies in the M81 group. HI is detected in three galaxies,
all of which are classified as BCDs. The HI masses of these galaxies are ~ 10^6
M_sun, making these some of the lowest mass BCDs known. For these three
galaxies FUV images (from GALEX) and H-alpha images (from the Russian 6m BTA
telescope) are available.The H-alpha emission is very faint, and, in principle
could be produced by a single O star. Further, in all cases we find offsets
between the peak of the FUV emission and that of the H-alpha emission. Offsets
between the most recent sites of star formation (i.e. those traced by H-alpha)
and the older sites (i.e. those traced by FUV) would be natural if the star
formation is stochastic. In spite of the expectation that the effects of
mechanical feedback from star formation would be most directly seen in the
smallest galaxies with low gravitational potentials, we only see tentative
evidence of outflowing HI gas associated with the star forming region in one of
the galaxies.Comment: 9 pages, 5 figures, 5 tables. Accepted for publication in MNRAS. The
definitive version is available at www.blackwell-synergy.co
Their Day in Court: Assessing Guilty Plea Rates Among Terrorists
Individuals who are charged for traditional crimes are substantially more likely to plead guilty than individuals who are charged under the same statutes but who are officially involved in terrorism (Smith & Damphousse, 1998). Relying on a structuralâcontextual theory framework, a quantitative analysis not only confirmed that terrorists plead guilty more often than traditional offenders but that the defendantâs age and number of counts in the indictment are important predictors. Directions for future research are suggested.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
The Incidence of Highly-Obscured Star-Forming Regions in SINGS Galaxies
Using the new capabilities of the Spitzer Space Telescope and extensive
multiwavelength data from the Spitzer Infrared Nearby Galaxies Survey (SINGS),
it is now possible to study the infrared properties of star formation in nearby
galaxies down to scales equivalent to large HII regions. We are therefore able
to determine what fraction of large, infrared-selected star-forming regions in
normal galaxies are highly obscured and address how much of the star formation
we miss by relying solely on the optical portion of the spectrum. Employing a
new empirical method for deriving attenuations of infrared-selected
star-forming regions we investigate the statistics of obscured star formation
on 500pc scales in a sample of 38 nearby galaxies. We find that the median
attenuation is 1.4 magnitudes in H-alpha and that there is no evidence for a
substantial sub-population of uniformly highly-obscured star-forming regions.
The regions in the highly-obscured tail of the attenuation distribution
(A_H-alpha > 3) make up only ~4% of the sample of nearly 1800 regions, though
very embedded infrared sources on the much smaller scales and lower
luminosities of compact and ultracompact HII regions are almost certainly
present in greater numbers. The highly-obscured cases in our sample are
generally the bright, central regions of galaxies with high overall attenuation
but are not otherwise remarkable. We also find that a majority of the galaxies
show decreasing radial trends in H-alpha attenuation. The small fraction of
highly-obscured regions seen in this sample of normal, star-forming galaxies
suggests that on 500pc scales the timescale for significant dispersal or break
up of nearby, optically-thick dust clouds is short relative to the lifetime of
a typical star-forming region.Comment: Accepted for publication in ApJ; emulateapj style, 30 pages, 18
figures (compressed versions), 3 table
Warm Dust and Spatially Variable PAH Emission in the Dwarf Starburst Galaxy NGC 1705
We present Spitzer observations of the dwarf starburst galaxy NGC 1705
obtained as part of SINGS. The galaxy morphology is very different shortward
and longward of ~5 microns: short-wavelength imaging shows an underlying red
stellar population, with the central super star cluster (SSC) dominating the
luminosity; longer-wavelength data reveals warm dust emission arising from two
off-nuclear regions offset by ~250 pc from the SSC. These regions show little
extinction at optical wavelengths. The galaxy has a relatively low global dust
mass (~2E5 solar masses, implying a global dust-to-gas mass ratio ~2--4 times
lower than the Milky Way average). The off-nuclear dust emission appears to be
powered by photons from the same stellar population responsible for the
excitation of the observed H Alpha emission; these photons are unassociated
with the SSC (though a contribution from embedded sources to the IR luminosity
of the off-nuclear regions cannot be ruled out). Low-resolution IRS
spectroscopy shows moderate-strength PAH emission in the 11.3 micron band in
the eastern peak; no PAH emission is detected in the SSC or the western dust
emission complex. There is significant diffuse 8 micron emission after scaling
and subtracting shorter wavelength data; the spatially variable PAH emission
strengths revealed by the IRS data suggest caution in the interpretation of
diffuse 8 micron emission as arising from PAH carriers alone. The metallicity
of NGC 1705 falls at the transition level of 35% solar found by Engelbracht and
collaborators; the fact that a system at this metallicity shows spatially
variable PAH emission demonstrates the complexity of interpreting diffuse 8
micron emission. A radio continuum non-detection, NGC 1705 deviates
significantly from the canonical far-IR vs. radio correlation. (Abridged)Comment: ApJ, in press; please retrieve full-resolution version from
http://www.astro.wesleyan.edu/~cannon/pubs.htm
Galactic-Scale Outflow and Supersonic Ram-Pressure Stripping in the Virgo Cluster Galaxy NGC 4388
The Hawaii Imaging Fabry-Perot Interferometer (HIFI) on the University of
Hawaii 2.2m telescope was used to map the Halpha and [O III] 5007 A
emission-line profiles across the entire disk of the edge-on Sb galaxy NGC
4388. We confirm a rich complex of highly ionized gas that extends ~4 kpc above
the disk of this galaxy. Low-ionization gas associated with star formation is
also present in the disk. Evidence for bar streaming is detected in the disk
component and is discussed in a companion paper (Veilleux, Bland-Hawthorn, &
Cecil 1999; hereafter VBC). Non-rotational blueshifted velocities of 50 - 250
km/s are measured in the extraplanar gas north-east of the nucleus. The
brighter features in this complex tend to have more blueshifted velocities. A
redshifted cloud is also detected 2 kpc south-west of the nucleus. The velocity
field of the extraplanar gas of NGC 4388 appears to be unaffected by the
inferred supersonic (Mach number M ~ 3) motion of this galaxy through the ICM
of the Virgo cluster. We argue that this is because the galaxy and the high-|z|
gas lie behind a Mach cone with opening angle ~ 80 degrees. The shocked ICM
that flows near the galaxy has a velocity of ~ 500 km/s and exerts insufficient
ram pressure on the extraplanar gas to perturb its kinematics. We consider
several explanations of the velocity field of the extraplanar gas. Velocities,
especially blueshifted velocities on the N side of the galaxy, are best
explained as a bipolar outflow which is tilted by > 12 degrees from the normal
to the disk. The observed offset between the extraplanar gas and the radio
structure may be due to buoyancy or refractive bending by density gradients in
the halo gas. Velocity substructure in the outflowing gas also suggests an
interaction with ambient halo gas.Comment: 29 pages including 5 figures, Latex, requires aaspp4.sty, to appear
in ApJ, 520 (July 20, 1999 issue
Sexual selection protects against extinction
Reproduction through sex carries substantial costs, mainly because only half of sexual adults produce offspring. It has been theorised that these costs could be countered if sex allows sexual selection to clear the universal fitness constraint of mutation load. Under sexual selection, competition between (usually) males, and mate choice by (usually) females create important intraspecific filters for reproductive success, so that only a subset of males gains paternity. If reproductive success under sexual selection is dependent on individual condition, which depends on mutation load, then sexually selected filtering through âgenic captureâ could offset the costs of sex because it provides genetic benefits to populations. Here, we test this theory experimentally by comparing whether populations with histories of strong versus weak sexual selection purge mutation load and resist extinction differently. After evolving replicate populations of the flour beetle Tribolium castaneum for ~7 years under conditions that differed solely in the strengths of sexual selection, we revealed mutation load using inbreeding. Lineages from populations that had previously experienced strong sexual selection were resilient to extinction and maintained fitness under inbreeding, with some families continuing to survive after 20 generations of sib Ă sib mating. By contrast, lineages derived from populations that experienced weak or non-existent sexual selection showed rapid fitness declines under inbreeding, and all were extinct after generation 10. Multiple mutations across the genome with individually small effects can be difficult to clear, yet sum to a significant fitness load; our findings reveal that sexual selection reduces this load, improving population viability in the face of genetic stress
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