582 research outputs found
Optical SETI: A Spectroscopic Search for Laser Emission from Nearby Stars
We have searched for nonastrophysical emission lines in the optical spectra
of 577 nearby F, G, K, and M main-sequence stars. Emission lines of
astrophysical origin would also have been detected, such as from a
time--variable chromosphere or infalling comets. We examined ~20 spectra per
star obtained during four years with the Keck/HIRES spectrometer at a
resolution of 5 km/s, with a detection threshold 3% of the continuum flux
level. We searched each spectrum from 4000-5000 angstroms for emission lines
having widths too narrow to be natural from the host star, as well as for lines
broadened by astrophysical mechanisms. We would have detected lasers that emit
a power, P>60 kW, for a typical beam width of ~0.01 arcsec (diffraction-limit
from a 10-m aperture) if directed toward Earth from the star. No lines
consisstent with laser emission were found.Comment: 27 pages, 11 figures, uses aastex.st
Neutrino-induced deuteron disintegration experiment
Cross sections for the disintegration of the deuteron via neutral-current
(NCD) and charged-current (CCD) interactions with reactor antineutrinos are
measured to be 6.08 +/- 0.77 x 10^(-45) cm-sq and 9.83 +/- 2.04 x 10^(-45)
cm-sq per neutrino, respectively, in excellent agreement with current
calculations. Since the experimental NCD value depends upon the CCD value, if
we use the theoretical value for the CCD reaction, we obtain the improved value
of 5.98 +/- 0.54 x 10^(-45) for the NCD cross section. The neutral-current
reaction allows a unique measurement of the isovector-axial vector coupling
constant in the hadronic weak interaction (beta). In the standard model, this
constant is predicted to be exactly 1, independent of the Weinberg angle. We
measure a value of beta^2 = 1.01 +/- 0.16. Using the above improved value for
the NCD cross section, beta^2 becomes 0.99 +/- 0.10.Comment: 22pages, 9 figure
Hundreds of Low-Mass Active Galaxies in the Galaxy And Mass Assembly (GAMA) Survey
We present an entirely new sample of 388 low-mass galaxies () that have spectroscopic signatures indicating the presence of
massive black holes (BHs) in the form of active galactic nuclei (AGNs) or tidal
disruption events (TDEs). Of these, 70 have stellar masses in the dwarf galaxy
regime with . We identify the
active galaxies by analyzing optical spectra of a parent sample of 23,000
low-mass emission-line galaxies in the Galaxy and Mass Assembly (GAMA) Survey
Data Release 4, and employing four different diagnostics based on narrow
emission line ratios and the detection of high-ionization coronal lines. We
find that 47 of the 388 low-mass active galaxies exhibit broad H in
their spectra, corresponding to virial BH masses in the range with a median BH mass of . Our sample extends to higher redshifts () than previous samples of AGNs in low-mass/dwarf galaxies based
on Sloan Digital Sky Survey spectroscopy, which can be attributed to the
spectroscopic limit of GAMA being magnitudes deeper. Moreover, our
multi-diagnostic approach has revealed low-mass active galaxies spanning a wide
range of properties, from blue star-forming dwarfs to luminous "miniquasars"
powered by low-mass BHs. As such, this work has implications for BH seeding and
AGN feedback at low masses.Comment: Accepted for publication in The Astrophysical Journa
Inelastic Scattering of Tritium-Source Antineutrinos on Electrons of Germanium Atoms
Processes of the inelastic magnetic and weak scattering of
tritium-beta-source antineutrinos on the bound electrons of a germanium atom
are considered. The results obtained by calculating the spectra and cross
sections are presented for the energy-transfer range between 1 eV and 18 keV.Comment: Latex, 7 pages, 8 ps figure
Super star clusters in Haro 11: Properties of a very young starburst and evidence for a near-infrared flux excess
We have used multi-band imaging to investigate the nature of the extreme
starburst environment in Haro 11 galaxy. The central starburst region has been
observed in 8 HST wavebands and at 2.16 micron at the ESO-VLT. We constructed
integrated spectral energy distributions (SEDs) for about 200 star clusters and
compared them with single stellar population models in order to derive ages,
masses and extinctions of thestar clusters. The present starburst has lasted
for 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such
an extremely young cluster population, Haro 11 represents a unique opportunity
to investigate the youngest phase of the cluster formation process and
evolution in starburst systems. Extinction tends to diminish as function of the
cluster age, but the spread is large and for clusters in partial embedded
phases ( 10^4 Msun), very young (1-3 Myr)
clusters is missing, either because they are embedded inthe parental molecular
cloud and heavily extinguished, or because of blending. Almost half of the
cluster sample is affected by flux excesses at wavelengths 8000 \AA which
cannot be explained by simple stellar evolutionary models. Fitting SED models
over all wavebands leads to systematic overestimates of cluster ages and
incorrect masses for the stellar population supplying the light in these
clusters. We show that the red excess affects also the HST F814W filter, which
is typically used to constrain cluster physical properties. The clusters which
show the red excess are younger than 40 Myr; we propose possible physical
explanations for the phenomenon. Finally, we estimate that Haro 11 hasproduced
bound clusters at a rate almost a factor of 10 higher than the massive and
regular spirals, like the Milky Way. (Abriged)Comment: Accepted for publication in MNRAS. 23 pages, 23 figure
The Massive Star Clusters in the Dwarf Merger ESO 185-IG13: is the Red Excess Ubiquitous in Starbursts?
We have investigated the starburst properties of the luminous blue compact
galaxy ESO 185-IG13. The galaxy has been imaged with the high resolution
cameras onboard to the Hubble Space Telescope. From the UV to the IR, the data
reveal a system shaped by hundreds of young star clusters, and fine structures,
like a tidal stream and a shell. The presence of numerous clusters and the
perturbed morphology indicate that the galaxy has been involved in a recent
merger event. Using previous simulations of shell formation in galaxy mergers
we constrain potential progenitors of ESO 185-IG13. The analysis of the star
cluster population is used to investigate the properties of the present
starburst and to date the final merger event, which has produced hundreds of
clusters younger than 100 Myr. We have found a peak of cluster formation only
3.5 Myr old. A large fraction of these clusters will not survive after 10-20
Myr, due to the "infant mortality" caused by gas expulsion. However, this
sample of clusters represents an unique chance to investigate the youngest
phases of cluster evolution. As already observed in the analog blue compact
galaxy Haro 11, a fraction of young clusters are affected by a flux excess at
wavelengths longer than 8000 \AA. Ages, masses, and extinctions of clusters
with this NIR excess are estimated from UV and optical data. We discuss
similarities and differences of the observed NIR excess in ESO 185-IG13
clusters with other cases in the literature. The cluster ages and masses are
used to distinguish among the potential causes of the excess. We observe, as in
Haro 11, that the use of the IR and the (commonly used) I band data results in
overestimates of age and mass in clusters affected by the NIR excess. This has
important implications for a number of related studies of star clusters.Comment: 23 pages, 16 figures, accepted for publication in MNRA
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