261 research outputs found
Reddening Behaviors of Galaxies in the SDSS Photometric System
We analyze the behaviors of reddening vectors in the SDSS photometric system
for galaxies of different morphologies, ages, and redshifts. As seen in other
photometric systems, the dependence of reddening on the spectral energy
distribution (SED) and the nonlinearity of reddening are likewise
non-negligible for the SDSS system if extinction is significant (~> 1 mag).
These behaviors are most significant for the g filter, which has the largest
bandwidth-to-central wavelength ratio among SDSS filters. The SDSS colors
involving adjacent filters show greater SED-dependence and nonlinearity. A
procedure for calculating the correct amount of extinction from an observed
color excess is provided. The relative extinctions between (i.e., the
extinction law for) SDSS filters given by Schlegel et al., which were
calculated with an older version of filter response functions, would
underestimate the amount of extinction in most cases by ~5 to 10 % (maximum ~20
%). We recommend A/A_{5500} values of 1.574, 1.191, 0.876, 0.671 & 0.486 for
the u, g, r, i, & z filters, respectively, as a representative extinction law
for the SDSS galaxies with a small extinction (i.e., for cases where the
nonlinearity and SED-dependence of the reddening is not important). The
dependence of reddening on redshift at low extinction is the largest for colors
involving the g filter as well, which is due to the Balmer break.Comment: Published in PASP, 119, 1449 (Dec. 2007
Simultaneous dual-frequency radio observations of S5 0716+714: A search for intraday variability with the Korean VLBI Network
This study aims to search for the existence of intraday variability (IDV) of
BL Lac object S5 0716+714 at high radio frequencies for which the interstellar
scintillation effect is not significant. Using the 21-meter radio telescope of
the Korean VLBI Network (KVN), we present results of multi-epoch simultaneous
dual-frequency radio observations. Single-dish observations of S5 0716+714 were
simultaneously conducted at 21.7 GHz (K-band) and 42.4 GHz (Q-band), with a
high cadence of 30-60 minute intervals.We observed four epochs between December
2009 and June 2010. Over the whole set of observation epochs, S5 0716+714
showed significant inter-month variations in flux density at both the K- and
Q-bands, with modulation indices of approximately 19% for the K-band and
approximately 36% for the Q-band. In all epochs, no clear intraday variability
was detected at either frequency. The source shows monotonic flux density
increase in epochs 1 and 3 and monotonic flux density decrease in epochs 2 and
4. In the flux density increasing phases, the flux densities at the Q-band
increase more rapidly. In the decreasing phase, no significant flux density
difference is seen at the two frequencies. The situation could be different
close to flux density peaks that we did not witness in our observations. We
find an inverted spectrum with mean spectral indices of -0.57+-0.13 in epoch 1
and -0.15+-0.11 in epoch 3. On the other hand, we find relatively steep indices
of +0.24+-0.14 and +0.17+-0.18 in epochs 2 and 4, respectively. We conclude
that the frequency dependence of the variability and the change of the spectral
index are caused by source-intrinsic effects rather than by any extrinsic
scintillation effect.Comment: 6 pages and 4 figures and 4 table
Topology of Luminous Red Galaxies from the Sloan Digital Sky Survey
We present measurements of the genus topology of luminous red galaxies (LRGs)
from the Sloan Digital Sky Survey (SDSS) Data Release 7 catalog, with
unprecedented statistical significance. To estimate the uncertainties in the
measured genus, we construct 81 mock SDSS LRG surveys along the past light cone
from the Horizon Run 3, one of the largest N-body simulations to date that
evolved 7210^3 particles in a 10815 Mpc/h size box. After carefully modeling
and removing all known systematic effects due to finite pixel size, survey
boundary, radial and angular selection functions, shot noise and galaxy
biasing, we find the observed genus amplitude to reach 272 at 22 Mpc/h
smoothing scale with an uncertainty of 4.2%; the estimated error fully
incorporates cosmic variance. This is the most accurate constraint of the genus
amplitude to date, which significantly improves on our previous results. In
particular, the shape of the genus curve agrees very well with the mean
topology of the SDSS LRG mock surveys in the LCDM universe. However, comparison
with simulations also shows small deviations of the observed genus curve from
the theoretical expectation for Gaussian initial conditions. While these
discrepancies are mainly driven by known systematic effects such as those of
shot noise and redshift-space distortions, they do contain important
cosmological information on the physical effects connected with galaxy
formation, gravitational evolution and primordial non-Gaussianity. We address
here the key role played by systematics on the genus curve, and show how to
accurately correct for their effects to recover the topology of the underlying
matter. In a forthcoming paper, we provide an interpretation of those
deviations in the context of the local model of non-Gaussianity.Comment: 23 pages, 18 figures. APJ Supplement Series 201
The Stream-Stream Collision after the Tidal Disruption of a Star Around a Massive Black Hole
A star can be tidally disrupted around a massive black hole. It has been
known that the debris forms a precessing stream, which may collide with itself.
The stream collision is a key process determining the subsequent evolution of
the stellar debris: if the orbital energy is efficiently dissipated, the debris
will eventually form a circular disk (or torus). In this paper, we have
numerically studied such stream collision resulting from the encounter between
a 10^6 Msun black hole and a 1 Msun normal star with a pericenter radius of 100
Rsun. A simple treatment for radiative cooling has been adopted for both
optically thick and thin regions. We have found that approximately 10 to 15% of
the initial kinetic energy of the streams is converted into thermal energy
during the collision. The angular momentum of the incoming stream is increased
by a factor of 2 to 3, and such increase, together with the decrease in kinetic
energy, significantly helps the circularization process. Initial luminosity
burst due to the collision may reach as high as 10^41 erg/sec in 10^4 sec,
after which the luminosity increases again (but slowly this time) to a steady
value of a few 10^40 erg/sec in a few times of 10^5 sec. The radiation from the
system is expected to be close to Planckian with effective temperature of
\~10^5K.Comment: 19 pages including 12 figures; Accepted for publication in Ap
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