1,459 research outputs found
Spectroscopy of Low Surface Brightness Galaxies with the Hobby-Eberly Telescope
We have obtained low resolution spectra of nineteen red and blue low surface
brightness galaxies, using the Marcario Low Resolution Spectrograph on the 9.2m
Hobby-Eberly Telescope. These galaxies form a very heterogeneous class, whose
spectra qualitatively resemble those of high surface brightness galaxies
covering the full range of spectra seen in galaxies of Hubble types from E to
Irr. We use a combination of emission line (EW(Halpha), NII/Halpha) and
absorption line (Mgb, Hbeta, ) based diagnostics to investigate the
star-formation and chemical enrichment histories of these galaxies. These are
diverse, with some galaxies having low metallicity and very young mean stellar
ages, and other galaxies showing old, super-solar metallicity stellar
populations. In contrast with some previous studies which found a strong trend
of decreasing metallicity with decreasing central surface brightness, we find a
population of galaxies with low surface brightness and near-solar metallicity.
Correlations between several of the gas phase and stellar population age and
metallicity indicators are used to place contraints on plausible evolutionary
scenarios for LSB galaxies. The redshift range spanned by these galaxies is
broad, with radial velocities from 3400 km/s to more than 65000 km/s. A subset
of the sample galaxies have published HI redshifts and gas masses based on
observations with the Arecibo 305m single-dish radio telescope, which place
these galaxies far off of the mean Tully-Fisher relation. Our new optical
redshifts do not agree with the published HI redshifts for these galaxies. Most
of the discrepancies can be explained by beam confusion in the Arecibo
observations, causing erroneous HI detections for some of the galaxies.Comment: 31 pages, 12 figures, 7 tables. Uses emulateapj5.sty and
onecolfloat5.sty, which are included. Accepted for publication in the
Astronomical Journa
Quantified HI Morphology VII: star-formation and tidal influence on local dwarf HI morphology
Scale-invariant morphology parameters applied to atomic hydrogen maps (HI) of
galaxies can be used to quantify the effects of tidal interaction or
star-formation on the ISM. Here we apply these parameters, Concentration,
Asymmetry, Smoothness, Gini, M20, and the GM parameter, to two public surveys
of nearby dwarf galaxies, the VLA-ANGST and LITTLE-THINGS survey, to explore
whether tidal interaction or the ongoing or past star-formation is a dominant
force shaping the HI disk of these dwarfs.
Previously, HI morphological criteria were identified for ongoing
spiral-spiral interactions. When we apply these to the Irregular dwarf
population, they either select almost all or none of the population. We find
that only the Asymmetry-based criteria can be used to identify very isolated
dwarfs (i.e., these have a low tidal indication). Otherwise, there is little or
no relation between the level of tidal interaction and the HI morphology. We
compare the HI morphology to three star-formation rates based on either Halpha,
FUV or the resolved stellar population, probing different star-formation
time-scales.
The HI morphology parameters that trace the inequality of the distribution,
the Gini, GM, and M20 parameters, correlate weakly with all these
star-formation rates. This is in line with the picture that local physics
dominates the ISM appearance and not tidal effects. Finally, we compare the
SDSS measures of star-formation and stellar mass to the HI morphological
parameters for all four HI surveys. In the two lower-resolution HI surveys
(12"), there is no relation between star-formation measures and HI morphology.
The morphology of the two high-resolution HI surveys (6"), the Asymmetry,
Smoothness, Gini, M20, and GM, do show a link to the total star-formation, but
a weak one.Comment: 26 figures, 4 tables, two appendices. Third appendix (HI maps of all
galaxies) omitted. Accepted by MNRA
Local Group dSph radio survey with ATCA (III): Constraints on Particle Dark Matter
We performed a deep search for radio synchrotron emissions induced by weakly
interacting massive particles (WIMPs) annihilation or decay in six dwarf
spheroidal (dSph) galaxies of the Local Group. Observations were conducted with
the Australia Telescope Compact Array (ATCA) at 16 cm wavelength, with an rms
sensitivity better than 0.05 mJy/beam in each field. In this work, we first
discuss the uncertainties associated with the modeling of the expected signal,
such as the shape of the dark matter (DM) profile and the dSph magnetic
properties. We then investigate the possibility that point-sources detected in
the proximity of the dSph optical center might be due to the emission from a DM
cuspy profile. No evidence for an extended emission over a size of few arcmin
(which is the DM halo size) has been detected. We present the associated bounds
on the WIMP parameter space for different annihilation/decay final states and
for different astrophysical assumptions. If the confinement of electrons and
positrons in the dSph is such that the majority of their power is radiated
within the dSph region, we obtain constraints on the WIMP annihilation rate
which are well below the thermal value for masses up to few TeV. On the other
hand, for conservative assumptions on the dSph magnetic properties, the bounds
can be dramatically relaxed. We show however that, within the next 10 years and
regardless of the astrophysical assumptions, it will be possible to
progressively close in on the full parameter space of WIMPs by searching for
radio signals in dSphs with SKA and its precursors.Comment: 17 pages, 6 figure panels. Companion papers: arXiv:1407.5479 and
arXiv:1407.5482. v3: minor revision, matches published versio
The evolution of the stellar populations in low surface brightness galaxies
We investigate the star formation history and chemical evolution of low
surface brightness (LSB) disk galaxies by modelling their observed
spectro-photometric and chemical properties using a galactic chemical and
photometric evolution model incorporating a detailed metallicity depen dent set
of stellar input data. For a large fraction of the LSB galaxies in our sample,
observed properties are best explained by models incorporating an exponentially
decreasing global star formation rate (SFR) ending at a present-day gas
fraction (M_{gas}/(M_{gas}+M_{stars}) = 0.5 for a galaxy age of 14 Gyr. For
some galaxies small amplitude star formation bursts are required to explain the
contribution of the young (5-50 Myr old) stellar population to the galaxy
integrated luminosity. This suggests that star formation has proceeded in a
stochastic manner.
The presence of an old stellar population in many late-type LSB galaxies
suggests that LSB galaxies roughly follow the same evolutionary history as HSB
galaxies, except at a much lower rate. In particular, our results imply that
LSB galaxies do not form late, nor have a delayed onset of star formation, but
simply evolve slowly.Comment: To be published in A&
A research note on accountability and institutional clarity: how two dimensions of clarity of responsibility moderate accountability mechanisms
The Effect of a Single Supernova Explosion on the Cuspy Density Profile of a Small-Mass Dark Matter Halo
Some observations of galaxies, and in particular dwarf galaxies, indicate a
presence of cored density profiles in apparent contradiction with cusp profiles
predicted by dark matter N-body simulations. We constructed an analytical
model, using particle distribution functions (DFs), to show how a supernova
(SN) explosion can transform a cusp density profile in a small-mass dark matter
halo into a cored one. Considering the fact that a SN efficiently removes
matter from the centre of the first haloes, we study the effect of mass removal
through a SN perturbation in the DFs. We found that the transformation from a
cusp into a cored profile is present even for changes as small as 0.5% of the
total energy of the halo, that can be produced by the expulsion of matter
caused by a single SN explosion.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Evidence for a clumpy, rotating gas disk in a submillimeter galaxy at z=4
We present Karl G. Jansky Very Large Array (VLA) observations of the CO(2-1)
emission in the z=4.05 submillimeter galaxy (SMG) GN20. These high-resolution
data allow us to image the molecular gas at 1.3 kpc resolution just 1.6 Gyr
after the Big Bang. The data reveal a clumpy, extended gas reservoir, 14 +/- 4
kpc in diameter, in unprecedented detail. A dynamical analysis shows that the
data are consistent with a rotating disk of total dynamical mass 5.4 +/- 2.4 X
10^11 M_sun. We use this dynamical mass estimate to constrain the CO-to-H_2
mass conversion factor (alpha_CO), finding alpha_CO=1.1 +/- 0.6 M_sun (K km
s^-1 pc^2)^-1. We identify five distinct molecular gas clumps in the disk of
GN20 with masses a few percent of the total gas mass, brightness temperatures
of 16-31K, and surface densities of >3,200-4,500 X (alpha_CO/0.8) M_sun pc^-2.
Virial mass estimates indicate they could be self-gravitating, and we constrain
their CO-to-H_2 mass conversion factor to be <0.2-0.7 M_sun (K km s^-1
pc^2)^-1. A multiwavelength comparison demonstrates that the molecular gas is
concentrated in a region of the galaxy that is heavily obscured in the
rest-frame UV/optical. We investigate the spatially-resolved gas excitation and
find that the CO(6-5)/CO(2-1) ratio is constant with radius, consistent with
star formation occuring over a large portion of the disk. We discuss the
implications of our results in the context of different fueling scenarios for
SMGs.Comment: 15 pages, 9 figures, accepted for publication in Ap
Constraints on Warm Dark Matter from Cosmological Reionization
We study the constraints that high-redshift structure formation in the
universe places on warm dark matter (WDM) dominated cosmological models. We
modify the extended Press-Schechter formalism to derive the halo mass function
in WDM models. We show that our predictions agree with recent numerical
simulations at low redshift over the halo masses of interest. Applying our
model to galaxy formation at high redshift, we find that the loss of power on
small scales, together with the delayed collapse of low-mass objects, results
in strong limits on the root-mean-square velocity dispersion v_rms of the WDM
particles at z=0. For fermions decoupling while relativistic, these limits are
equivalent to constraints on the mass m_X of the particles. The presence of a 4
billion solar mass black hole at z=5.8, believed to power the quasar SDSS
1044-1215, implies m_X > 0.5 keV (or v_rms < 0.10 km/s), assuming that the
quasar is unlensed and radiating at or below the Eddington limit. Reionization
by z=5.8 also implies a limit on m_X. If high-redshift galaxies produce
ionizing photons with an efficiency similar to their z=3 counterparts, we find
m_X > 1.2 keV (or v_rms < 0.03 km/s). However, given the uncertainties in
current measurements from the proximity effect of the ionizing background at
redshift 3, values of m_X as low as 0.75 keV (v_rms = 0.06 km/s) are not ruled
out. The limit weakens further if, instead, the ionizing-photon production
efficiency is greater at high z, but this limit will tighten considerably if
reionization is shown in the future to have occurred at higher redshifts. WDM
models with m_X 0.04 km/s) produce a low-luminosity cutoff in
the high-redshift galaxy luminosity function which is directly detectable with
the Next Generation Space Telescope (abridged).Comment: 38 pages, 10 figures, to appear in ApJ. One figure added, some
discussion revise
On the Neutral Gas Content and Environment of NGC 3109 and the Antlia Dwarf Galaxy
As part of a continuing survey of nearby galaxies, we have mapped the neutral
gas content of the low surface brightness, Magellanic-type galaxy NGC 3109 ---
and its environment, including the Antlia dwarf galaxy --- at unprecedented
velocity resolution and brightness sensitivity. The HI mass of NGC 3109 is
measured to be (3.8 +/- 0.5) x 10^8 Msun. A substantial warp in the disk of NGC
3109 is detected in the HI emission image in the form of an extended low
surface brightness feature. We report a positive detection in HI of the nearby
Antlia dwarf galaxy, and measure its total neutral gas mass to be (6.8 +/- 1.4)
x 10^5 Msun. We show the warp in NGC 3109 to lie at exactly the same radial
velocity as the gas in the Antlia dwarf galaxy and speculate that Antlia
disturbed the disk of NGC 3109 during a mild encounter ~1 Gyr in the past. HI
data for a further eight galaxies detected in the background are presented.Comment: Accepted for publication in A
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