113 research outputs found
A GALEX Ultraviolet Imaging Survey of Galaxies in the Local Volume
We present results from a GALEX ultraviolet (UV) survey of a complete sample
of 390 galaxies within ~11 Mpc of the Milky Way. The UV data are a key
component of the composite Local Volume Legacy (LVL), an
ultraviolet-to-infrared imaging program designed to provide an inventory of
dust and star formation in nearby spiral and irregular galaxies. The ensemble
dataset is an especially valuable resource for studying star formation in dwarf
galaxies, which comprise over 80% of the sample. We describe the GALEX survey
programs which obtained the data and provide a catalog of far-UV (~1500
Angstroms) and near-UV (~2200 Angstroms) integrated photometry. General UV
properties of the sample are briefly discussed. We compute two measures of the
global star formation efficiency, the SFR per unit HI gas mass and the SFR per
unit stellar mass, to illustrate the significant differences that can arise in
our understanding of dwarf galaxies when the FUV is used to measure the SFR
instead of H-alpha. We find that dwarf galaxies may not be as drastically
inefficient in coverting gas into stars as suggested by prior H-alpha studies.
In this context, we also examine the UV properties of late-type dwarf galaxies
that appear to be devoid of star formation because they were not detected in
previous H-alpha narrowband observations. Nearly all such galaxies in our
sample are detected in the FUV, and have FUV SFRs that fall below the limit
where the H-alpha flux is robust to Poisson fluctuations in the formation of
massive stars. The UV colors and star formation efficiencies of
H-alpha-undetected, UV-bright dwarf irregulars appear to be relatively
unremarkable with respect to those exhibited by the general population of
star-forming galaxies.Comment: submitted to ApJS, revised per referee's comments; accepted Oct. 30
w/o further revision; 37 pages; figure 6 omitted due to size; figure
available from http://users.obs.carnegiescience.edu/jlee/paper
A panchromatic study of BLAST counterparts: total star-formation rate, morphology, AGN fraction and stellar mass
We carry out a multi-wavelength study of individual galaxies detected by the
Balloon-borne Large Aperture Submillimeter Telescope (BLAST) and identified at
other wavelengths, using data spanning the radio to the ultraviolet (UV). We
develop a Monte Carlo method to account for flux boosting, source blending, and
correlations among bands, which we use to derive deboosted far-infrared (FIR)
luminosities for our sample. We estimate total star-formation rates for BLAST
counterparts with z < 0.9 by combining their FIR and UV luminosities. Star
formation is heavily obscured at L_FIR > 10^11 L_sun, z > 0.5, but the
contribution from unobscured starlight cannot be neglected at L_FIR < 10^11
L_sun, z < 0.25. We assess that about 20% of the galaxies in our sample show
indication of a type-1 active galactic nucleus (AGN), but their submillimeter
emission is mainly due to star formation in the host galaxy. We compute stellar
masses for a subset of 92 BLAST counterparts; these are relatively massive
objects, with a median mass of ~10^11 M_sun, which seem to link the 24um and
SCUBA populations, in terms of both stellar mass and star-formation activity.
The bulk of the BLAST counterparts at z<1 appear to be run-of-the-mill
star-forming galaxies, typically spiral in shape, with intermediate stellar
masses and practically constant specific star-formation rates. On the other
hand, the high-z tail of the BLAST counterparts significantly overlaps with the
SCUBA population, in terms of both star-formation rates and stellar masses,
with observed trends of specific star-formation rate that support strong
evolution and downsizing.Comment: Accepted for publication in the Astrophysical Journal. 44 pages, 11
figures. The SED template for the derivation of L_FIR has changed (added new
figure) and the discussion on the stellar masses has been improved. The
complete set of full-color postage-stamps can be found at
http://blastexperiment.info/results_images/moncelsi
Mid-IR Luminosities and UV/Optical Star Formation Rates at z<1.4
UV continuum and mid-IR emission constitute two widely used star formation
indicators at intermediate and high redshifts. We study 2430 galaxies with
z<1.4 in the Extended Groth Strip with MIPS 24 mic observations from FIDEL,
spectroscopy from DEEP2, and UV, optical, and near-IR photometry from AEGIS.
The data are coupled with stellar population models and Bayesian SED fitting to
estimate dust-corrected SFRs. In order to probe the dust heating from stellar
populations of various ages, the derived SFRs were averaged over various
timescales--from 100 Myr for "current" SFR to 1--3 Gyr for long-timescale SFRs.
These SED-based UV/optical SFRs are compared to total infrared luminosities
extrapolated from 24 mic observations. We find that for the blue, actively star
forming galaxies the correlation between the IR luminosity and the UV/optical
SFR shows a decrease in scatter when going from shorter to longer SFR-averaging
timescales. We interpret this as the greater role of intermediate age stellar
populations in heating the dust than what is typically assumed. This holds over
the entire redshift range. Many so-called green valley galaxies are simply
dust-obscured actively star-forming galaxies. However, there exist 24
mic-detected galaxies, some with L>10^11 L_sun, yet with little current star
formation. For them a reasonable amount of dust absorption of stellar light is
sufficient to produce the observed levels of IR. In our sample optical and
X-ray AGNs do not contribute on average more than ~50% to the mid-IR
luminosity, and we see no evidence for a large population of "IR excess"
galaxies (Abridged).Comment: Accepted for publication in ApJ. Content identical to arXiv version
1. No color figure
LHC Signatures of a Minimal Supersymmetric Hidden Valley
We investigate the LHC signals of a minimal supersymmetric hidden valley. Our
theory consists of the supersymmetric Standard Model along with a light hidden
U(1)_x gauge multiplet and a pair of hidden chiral superfields that
spontaneously break the new Abelian gauge symmetry near a GeV. The visible and
hidden sectors interact exclusively through supersymmetric gauge kinetic
mixing. We perform a thorough examination of the hidden decay cascades
initiated by the lightest Standard Model superpartner and we study the range of
LHC signals they can produce. In particular, we find parameter regions that
give rise to missing energy, single and multiple lepton jets, and displaced
vertices. Given the simplicity of the underlying theory and the broad range of
collider signals it can produce, we propose that this model is a useful
benchmark for LHC studies of (supersymmetric) hidden valleys.Comment: 45 pages, 15 figures; typos corrected but conclusions unchange
The WiggleZ Dark Energy Survey: Survey Design and First Data Release
The WiggleZ Dark Energy Survey is a survey of 240,000 emission line galaxies
in the distant universe, measured with the AAOmega spectrograph on the 3.9-m
Anglo-Australian Telescope (AAT). The target galaxies are selected using
ultraviolet photometry from the GALEX satellite, with a flux limit of NUV<22.8
mag. The redshift range containing 90% of the galaxies is 0.2<z<1.0. The
primary aim of the survey is to precisely measure the scale of baryon acoustic
oscillations (BAO) imprinted on the spatial distribution of these galaxies at
look-back times of 4-8 Gyrs. Detailed forecasts indicate the survey will
measure the BAO scale to better than 2% and the tangential and radial acoustic
wave scales to approximately 3% and 5%, respectively.
This paper provides a detailed description of the survey and its design, as
well as the spectroscopic observations, data reduction, and redshift
measurement techniques employed. It also presents an analysis of the properties
of the target galaxies, including emission line diagnostics which show that
they are mostly extreme starburst galaxies, and Hubble Space Telescope images,
which show they contain a high fraction of interacting or distorted systems. In
conjunction with this paper, we make a public data release of data for the
first 100,000 galaxies measured for the project.Comment: Accepted by MNRAS; this has some figures in low resolution format.
Full resolution PDF version (7MB) available at
http://www.physics.uq.edu.au/people/mjd/pub/wigglez1.pdf The WiggleZ home
page is at http://wigglez.swin.edu.au
Single-Scale Natural SUSY
We consider the prospects for natural SUSY models consistent with current
data. Recent constraints make the standard paradigm unnatural so we consider
what could be a minimal extension consistent with what we now know. The most
promising such scenarios extend the MSSM with new tree-level Higgs interactions
that can lift its mass to at least 125 GeV and also allow for flavor-dependent
soft terms so that the third generation squarks are lighter than current bounds
on the first and second generation squarks. We argue that a common feature of
almost all such models is the need for a new scale near 10 TeV, such as a scale
of Higgsing or confinement of a new gauge group. We consider the question
whether such a model can naturally derive from a single mass scale associated
with supersymmetry breaking. Most such models simply postulate new scales,
leaving their proximity to the scale of MSSM soft terms a mystery. This
coincidence problem may be thought of as a mild tuning, analogous to the usual
mu problem. We find that a single mass scale origin is challenging, but suggest
that a more natural origin for such a new dynamical scale is the gravitino
mass, m_{3/2}, in theories where the MSSM soft terms are a loop factor below
m_{3/2}. As an example, we build a variant of the NMSSM where the singlet S is
composite, and the strong dynamics leading to compositeness is triggered by
masses of order m_{3/2} for some fields. Our focus is the Higgs sector, but our
model is compatible with a light stop (with the other generation squarks heavy,
or with R-parity violation or another mechanism to hide them from current
searches). All the interesting low-energy mass scales, including linear terms
for S playing a key role in EWSB, arise dynamically from the single scale
m_{3/2}. However, numerical coefficients from RG effects and wavefunction
factors in an extra dimension complicate the otherwise simple story.Comment: 32 pages, 3 figures; version accepted by JHE
Simplified Models for LHC New Physics Searches
This document proposes a collection of simplified models relevant to the
design of new-physics searches at the LHC and the characterization of their
results. Both ATLAS and CMS have already presented some results in terms of
simplified models, and we encourage them to continue and expand this effort,
which supplements both signature-based results and benchmark model
interpretations. A simplified model is defined by an effective Lagrangian
describing the interactions of a small number of new particles. Simplified
models can equally well be described by a small number of masses and
cross-sections. These parameters are directly related to collider physics
observables, making simplified models a particularly effective framework for
evaluating searches and a useful starting point for characterizing positive
signals of new physics. This document serves as an official summary of the
results from the "Topologies for Early LHC Searches" workshop, held at SLAC in
September of 2010, the purpose of which was to develop a set of representative
models that can be used to cover all relevant phase space in experimental
searches. Particular emphasis is placed on searches relevant for the first
~50-500 pb-1 of data and those motivated by supersymmetric models. This note
largely summarizes material posted at http://lhcnewphysics.org/, which includes
simplified model definitions, Monte Carlo material, and supporting contacts
within the theory community. We also comment on future developments that may be
useful as more data is gathered and analyzed by the experiments.Comment: 40 pages, 2 figures. This document is the official summary of results
from "Topologies for Early LHC Searches" workshop (SLAC, September 2010).
Supplementary material can be found at http://lhcnewphysics.or
Comparison of H-alpha and UV Star Formation Rates in the Local Volume: Systematic Discrepancies for Dwarf Galaxies
(abridged) Using a complete sample of ~300 star-forming galaxies within 11
Mpc, we evaluate the consistency between star formation rates (SFRs) inferred
from the far ultraviolet (FUV) non-ionizing continuum and H-alpha nebular
emission, assuming standard conversion recipes in which the SFR scales linearly
with luminosity at a given wavelength. Our analysis probes SFRs over 5 orders
of magnitude, down to ultra-low activities on the order of ~0.0001 M_sun/yr.
The data are drawn from the 11 Mpc H-alpha and Ultraviolet Galaxy Survey
(11HUGS), which has obtained H-alpha fluxes from ground-based narrowband
imaging, and UV fluxes from imaging with GALEX. For normal spiral galaxies
(SFR~1 M_sun/yr), our results are consistent with previous work which has shown
that FUV SFRs tend to be lower than H-alpha SFRs before accounting for internal
dust attenuation, but that there is relative consistency between the two
tracers after proper corrections are applied. However, a puzzle is encountered
at the faint end of the luminosity function. As lower luminosity dwarf
galaxies, roughly less active than the Small Magellanic Cloud, are examined,
H-alpha tends to increasingly under-predict the SFR relative to the FUV.
Although past studies have suggested similar trends, this is the first time
this effect is probed with a statistical sample for galaxies with SFR~<0.1
M_sun/yr. A range of standard explanations does not appear to be able to
account for the magnitude of the systematic. Some recent work has argued for an
IMF which is deficient in high mass stars in dwarf and low surface brightness
galaxies, and we also consider this scenario.Comment: 29 pages, 10 figures, 2 tables, accepted for publication in Ap
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