20 research outputs found
Properties of Nearby Starburst Galaxies Based on their Diffuse Gamma-ray Emission
The physical relationship between the far-infrared and radio fluxes of star
forming galaxies has yet to be definitively determined. The favored
interpretation, the "calorimeter model," requires that supernova generated
cosmic ray (CR) electrons cool rapidly via synchrotron radiation. However, this
cooling should steepen their radio spectra beyond what is observed, and so
enhanced ionization losses at low energies from high gas densities are also
required. Further, evaluating the minimum energy magnetic field strength with
the traditional scaling of the synchrotron flux may underestimate the true
value in massive starbursts if their magnetic energy density is comparable to
the hydrostatic pressure of their disks. Gamma-ray spectra of starburst
galaxies, combined with radio data, provide a less ambiguous estimate of these
physical properties in starburst nuclei. While the radio flux is most sensitive
to the magnetic field, the GeV gamma-ray spectrum normalization depends
primarily on gas density. To this end, spectra above 100 MeV were constructed
for two nearby starburst galaxies, NGC 253 and M82, using Fermi data. Their
nuclear radio and far-infrared spectra from the literature are compared to new
models of the steady-state CR distributions expected from starburst galaxies.
Models with high magnetic fields, favoring galaxy calorimetry, are overall
better fits to the observations. These solutions also imply relatively high
densities and CR ionization rates, consistent with molecular cloud studies.Comment: Accepted to Ap
Diffuse Gamma-Ray Emission from the Starburst Galaxy NGC 253
The starburst galaxy NGC 253 was observed with the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory (CGRO) satellite. We obtain a 2 sigma upper limit to the gamma-ray emission above 100 MeV of 8 x 10(exp -8) photons/sq cm/s. Because of their large gas column densities and supernova rates, nearby starburst galaxies were predicted to have gamma-ray fluxes detectable by EGRET. Our nondetection of gamma-rays from NGC 253 motivates us to reexamine in detail the premise of supernova acceleration of cosmic rays and the effect of enhanced cloud densities, photon densities, and magnetic fields on the high-energy spectra of galaxies. By modeling the expected gamma-ray and synchrotron spectra from NGC 253, we find that up to 20% of the energy from supernovae is transferred to cosmic rays in the starburst, which is consistent with supernova acceleration models. Our calculations match the EGRET and radio data well with a supernova rate of 0.08/yr, a magnetic field B greater than or approximately equal to 5 x 10(exp -5) G, a density n approximately 300/cu cm, a photon density U(sub ph) approximately 200 eV/cu cm, and an escape timescale tau(sub o) less than or approximately equal to 10 Myr
The Structure, Kinematics and Physical Properties of the Molecular Gas in the Starburst Nucleus of NGC 253
We present 5.2" x 2.6" resolution interferometry of CO J=1-0 emission from
the starburst galaxy NGC 253. The high spatial resolution of these new data, in
combination with recent high resolution maps of 13CO, HCN and near-infrared
emission, allow us for the first time to link unambiguously the gas properties
in the central starburst of NGC 253 with its bar dynamics. We confirm that the
star formation results from bar-driven gas flows as seen in "twin peaks"
galaxies. Two distinct kinematic features are evident from the CO map and
position-velocity diagram: a group of clouds rotating as a solid body about the
kinematic center of the galaxy, and a more extended gas component associated
with the near-infrared bar. We model the line intensities of CO, HCN and 13CO
to infer the physical conditions of the gas in the nucleus of NGC 253. The
results indicate increased volume densities around the radio nucleus in a
twin-peaks morphology. Compared with the CO kinematics, the gas densities
appear highest near the radius of a likely inner Linblad resonance, and
slightly lead the bar minor axis. This result is similar to observations of the
face-on, twin-peaks galaxy NGC 6951, and is consistent with models of starburst
generation due to gas inflow along a bar.Comment: To appear in the ApJ, 28 pages, 12 figure file
The Faint End Slopes Of Galaxy Luminosity Functions In The COSMOS 2-Square Degree Field
We examine the faint-end slope of the rest-frame V-band luminosity function
(LF), with respect to galaxy spectral type, of field galaxies with redshift
z<0.5, using a sample of 80,820 galaxies with photometric redshifts in the
Cosmic Evolution Survey (COSMOS) field. For all galaxy spectral types combined,
the LF slope, alpha, ranges from -1.24 to -1.12, from the lowest redshift bin
to the highest. In the lowest redshift bin (0.02<z<0.1), where the magnitude
limit is M(V) ~ -13, the slope ranges from ~ -1.1 for galaxies with early-type
spectral energy distributions (SEDs), to ~ -1.9 for galaxies with
low-extinction starburst SEDs. In each galaxy SED category (Ell, Sbc, Scd/Irr,
and starburst), the faint-end slopes grow shallower with increasing redshift;
in the highest redshift bin (0.4<z<0.5), the slope is ~ -0.5 and ~ -1.3 for
early-types and starbursts respectively. The steepness of alpha at lower
redshift could be qualitatively explained by large numbers of faint dwarf
galaxies, perhaps of low surface brightness, which are not detected at higher
redshifts.Comment: 24 pages including 5 figures, accepted to ApJ
Diffuse Gamma-Ray Emission from Starburst Galaxies and M31
We present a search for high energy gamma-ray emission from 9 nearby
starburst galaxies and M31 with the EGRET instrument aboard CGRO. Though the
diffuse gamma-ray emission from starburst galaxies was suspected to be
detectable, we find no emission from NGC 253, M82 nor from the average of all 9
galaxies. The 2 sigma upper limit for the EGRET flux above 100 MeV for the
averaged survey observations is 1.8 x 10-8 ph cm-2 s-1. From a model of the
expected radio and gamma-ray emission, we find that the magnetic field in the
nuclei of these galaxies is > 25 micro Gauss, and the ratio of proton and
electron densities is < 400. The EGRET limits indicate that the rate of massive
star formation in the survey galaxies is only about an order of magnitude
higher than in the Milky Way. The upper limit to the gamma-ray flux above 100
MeV for M31 is 1.6 x 10-8 ph cm-2 s-1. At the distance of M31, the Milky Way
flux would be over twice this value, indicating higher gamma-ray emissivities
in our Galaxy. Therefore, since the supernova rate of the Milky Way is higher
than in M31, our null detection of M31 supports the theory of the supernova
origin of cosmic rays in galaxies.Comment: 17 pages, plus 1 Postscript figure, AAS Latex macros v4.0, accepted
for publication in ApJ Main Journa
The faint-end slopes of galaxy luminosity functions in the COSMOS field
We examine the faint-end slope of the rest-frame V-band luminosity function (LF), with respect to galaxy spectral type, of field galaxies with redshift z < 0.5, using a sample of 80,820 galaxies with photometric redshifts in the 2 deg^2 Cosmic Evolution Survey (COSMOS) field. For all galaxy spectral types combined, the LF slope ranges from –1.24 to –1.12, from the lowest redshift bin to the highest. In the lowest redshift bin (0.02 < z < 0.1), where the magnitude limit is MV ≾ − 13, the slope ranges from α ~ − 1.1 for galaxies with early-type spectral energy distributions (SEDs) to α ~ − 1.9 for galaxies with low-extinction starburst SEDs. In each galaxy SED category (early-type, Sbc, Scd+Irr, and starburst), the faint-end slopes grow shallower with increasing redshift; in the highest redshift bin (0.4 < z < 0.5), α ~ − 0.5 and –1.3 for early types and starbursts, respectively. The steepness of α at lower redshifts could be qualitatively explained by LF evolution, or by large numbers of faint dwarf galaxies, perhaps of low surface brightness, that are not detected at higher redshifts
GA4GH: International policies and standards for data sharing across genomic research and healthcare.
The Global Alliance for Genomics and Health (GA4GH) aims to accelerate biomedical advances by enabling the responsible sharing of clinical and genomic data through both harmonized data aggregation and federated approaches. The decreasing cost of genomic sequencing (along with other genome-wide molecular assays) and increasing evidence of its clinical utility will soon drive the generation of sequence data from tens of millions of humans, with increasing levels of diversity. In this perspective, we present the GA4GH strategies for addressing the major challenges of this data revolution. We describe the GA4GH organization, which is fueled by the development efforts of eight Work Streams and informed by the needs of 24 Driver Projects and other key stakeholders. We present the GA4GH suite of secure, interoperable technical standards and policy frameworks and review the current status of standards, their relevance to key domains of research and clinical care, and future plans of GA4GH. Broad international participation in building, adopting, and deploying GA4GH standards and frameworks will catalyze an unprecedented effort in data sharing that will be critical to advancing genomic medicine and ensuring that all populations can access its benefits
A Stacking Survey of Gamma-ray Pulsars
We report on a likelihood stacking search for gamma-ray pulsars at 525 high
latitude locations that coincide with known radio pulsar positions. We
significantly detect a stacked signal over the background. Stacking their
likelihood profiles in spectral parameter space implies a pulsar-like spectral
index and a characteristic flux one order of magnitude below the Fermi-LAT
sensitivity. The same procedures performed on empty control fields imply a
false detection rate as much as 50\%, although the stacked spectra of the
control fields are distinctly softer than those of the pulsars. This study also
probes a unique region of parameter space populated by older, transitional, and
recycled (millisecond) pulsars. Many of these sources have lower rotational
energy loss rates implying that the empirical gamma-ray "death line" could be
predominantly a sensitivity limit. These new stacking results sensitively probe
an unexplored population of low luminosity and low spin-down power pulsars. The
millisecond pulsar luminosity function measured for the stack of sub-threshold
millisecond pulsars constrains their contribution to the Galactic center GeV
excess.Comment: Submitted to ApJ. Second revision. 15 pages, 11 figures, and 2 table