1,692 research outputs found
Resolving the Radio Source Background: Deeper Understanding Through Confusion
We used the Karl G. Jansky Very Large Array (VLA) to image one primary beam
area at 3 GHz with 8 arcsec FWHM resolution and 1.0 microJy/beam rms noise near
the pointing center. The P(D) distribution from the central 10 arcmin of this
confusion-limited image constrains the count of discrete sources in the 1 <
S(microJy/beam) < 10 range. At this level the brightness-weighted differential
count S^2 n(S) is converging rapidly, as predicted by evolutionary models in
which the faintest radio sources are star-forming galaxies; and ~96$% of the
background originating in galaxies has been resolved into discrete sources.
About 63% of the radio background is produced by AGNs, and the remaining 37%
comes from star-forming galaxies that obey the far-infrared (FIR) / radio
correlation and account for most of the FIR background at lambda = 160 microns.
Our new data confirm that radio sources powered by AGNs and star formation
evolve at about the same rate, a result consistent with AGN feedback and the
rough correlation of black hole and bulge stellar masses. The confusion at
centimeter wavelengths is low enough that neither the planned SKA nor its
pathfinder ASKAP EMU survey should be confusion limited, and the ultimate
source detection limit imposed by "natural" confusion is < 0.01 microJy at 1.4
GHz. If discrete sources dominate the bright extragalactic background reported
by ARCADE2 at 3.3 GHz, they cannot be located in or near galaxies and most are
< 0.03 microJy at 1.4 GHz.Comment: 28 pages including 16 figures. ApJ accepted for publicatio
Radio continuum and far-infrared emission from the galaxies in the Eridanus group
The Eridanus galaxies follow the well-known radio-FIR correlation. Majority
(70%) of these galaxies have their star formation rates below that of the Milky
Way. The galaxies having a significant excess of radio emission are identified
as low luminosity AGNs based on their radio morphologies obtained from the GMRT
observations. There are no powerful AGNs (L{20cm} > 10^{23} W Hz^{-1}) in the
group. The two most far-infrared and radio luminous galaxies in the group have
optical and HI morphologies suggestive of recent tidal interactions. The
Eridanus group also has two far-infrared luminous but radio-deficient galaxies.
It is believed that these galaxies are observed within a few Myr of the onset
of an intense star formation episode after being quiescent for at least a 100
Myr. The upper end of the radio luminosity distribution of the Eridanus
galaxies (L_{20cm} ~ 10^{22} W Hz^{-1}) is consistent with that of the field
galaxies, other groups, and late-type galaxies in nearby clusters.Comment: 16 pages; Accepted for publication in Journal of Astroph. & Astron.
March, 200
The Megamaser Cosmology Project. VII. Investigating disk physics using spectral monitoring observations
We use single-dish radio spectra of known 22 GHz HO megamasers, primarily
gathered from the large dataset observed by the Megamaser Cosmology Project, to
identify Keplerian accretion disks and to investigate several aspects of the
disk physics. We test a mechanism for maser excitation proposed by Maoz & McKee
(1998), whereby population inversion arises in gas behind spiral shocks
traveling through the disk. Though the flux of redshifted features is larger on
average than that of blueshifted features, in support of the model, the
high-velocity features show none of the predicted systematic velocity drifts.
We find rapid intra-day variability in the maser spectrum of ESO 558-G009 that
is likely the result of interstellar scintillation, for which we favor a nearby
( pc) scattering screen. In a search for reverberation in six
well-sampled sources, we find that any radially-propagating signal must be
contributing 10% of the total variability. We also set limits on the
magnetic field strengths in seven sources, using strong flaring events to check
for the presence of Zeeman splitting. These limits are typically 200--300 mG
(), but our most stringent limits reach down to 73 mG for the galaxy
NGC 1194.Comment: Accepted for publication in Ap
The Megamaser Cosmology Project. X. High Resolution Maps and Mass Constraint for SMBHs
We present high resolution (sub-mas) VLBI maps of nuclear H2O megamasers for
seven galaxies. In UGC6093, the well-aligned systemic masers and high-velocity
masers originate in an edge-on, flat disk and we determine the mass of the
central SMBH to be M_SMBH = 2.58*10^7Msun(+-7%). For J1346+5228, the
distribution of masers is consistent with a disk, but the faint high-velocity
masers are only marginally detected, and we constrain the mass of the SMBH to
be in the range 1.5-2.0*10^7Msun. The origin of the masers in Mrk1210 is less
clear, as the systemic and high-velocity masers are misaligned and show a
disorganized velocity structure. We present one possible model in which the
masers originate in a tilted, warped disk, but we do not rule out the
possibility of other explanations including outflow masers. In NGC6926, we
detect a set of redshifted masers, clustered within a pc of each other, and a
single blueshifted maser about 4.4pc away, an offset that would be unusually
large for a maser disk system. Nevertheless, if it is a disk system, we
estimate the enclosed mass to be M_SMBH<4.8*10^7 Msun . For NGC5793, we detect
redshifted masers spaced about 1.4pc from a clustered set of blueshifted
features. The orientation of the structure supports a disk scenario as
suggested by Hagiwara et al.(2001). We estimate the enclosed mass to be M
SMBH<1.3*10^7 Msun. For NGC2824 and J0350-0127, the masers may be associated
with pc or sub-pc scale jets or outflows.Comment: Accepted by Ap
The VLBA Calibrator Search for the BeSSeL Survey
We present the results of a survey of radio continuum sources near the
Galactic plane using the Very Long Baseline Array (VLBA). Our observations are
designed to identify compact extragalactic sources of milliarcsecond size that
can be used for parallax measurements in the Bar and Spiral Structure Legacy
Survey. We selected point sources from the NVSS and CORNISH catalogs with flux
densities above 30 mJy and within 1.5\degr of known maser targets. Of the
1529 sources observed, 199 were detected. For sources detected on 3 or more
baselines, we determined accurate positions and evaluated their quality as
potential calibrators. Most of the 1330 sources that were not detected with the
VLBA are probably of extragalactic origin.Comment: 27 pages, 3 figures, 3 tables. Table 3 is available on the homepage
of the BeSSeL survey:
http://www.mpifr-bonn.mpg.de/staff/abrunthaler/BeSSeL/index.shtm
Physical Conditions and Star Formation Activity in the Intragroup Medium of Stephan's Quintet
New multi-band observations of the famous compact group of galaxies Stephan's
Quintet (SQ) are presented and analyzed. These include far infrared (FIR)
images at 60 and 100 (ISOPHOT C-100 camera), radio continuum
images at 1.4 GHz (VLA B-array) and 4.86 GHz (VLA C-array), and long-slit
optical spectrographs (Palomar telescope). With these new data, we aim
to learn more about the X-ray/radio ridge in the middle of the intragroup
medium (IGM) and the IGM starburst SQ-A, both are likely to be caused by the
high speed collision ( km s) between the intruder galaxy NGC
7318b ( km s) and the IGM ( km s).Comment: 31 pages text, 17 figures. Accepted by ApJ. A PS file including all
figures can be found in
http://spider.ipac.caltech.edu/staff/cxu/preprints/sq/apj_sq.ps.g
The Complexity of Nash Equilibria in Simple Stochastic Multiplayer Games
We analyse the computational complexity of finding Nash equilibria in simple
stochastic multiplayer games. We show that restricting the search space to
equilibria whose payoffs fall into a certain interval may lead to
undecidability. In particular, we prove that the following problem is
undecidable: Given a game G, does there exist a pure-strategy Nash equilibrium
of G where player 0 wins with probability 1. Moreover, this problem remains
undecidable if it is restricted to strategies with (unbounded) finite memory.
However, if mixed strategies are allowed, decidability remains an open problem.
One way to obtain a provably decidable variant of the problem is restricting
the strategies to be positional or stationary. For the complexity of these two
problems, we obtain a common lower bound of NP and upper bounds of NP and
PSPACE respectively.Comment: 23 pages; revised versio
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