sukye

astro-ph 0801w4

NICMOS Measurements of the Near Infrared Background
http://arxiv.org/abs/0801.3825
he near infrared background is now spatially resolved and is dominated by galaxies in the redshift range between 0.5 and 1.5

Estimating the Redshift Distribution of Faint Galaxy Samples
http://arxiv.org/abs/0801.3822
assigns weights to galaxies in a spectroscopic subsample such that the weighted distributions of photometric observables (e.g., multi-band magnitudes) match the corresponding distributions for the photometric sample. The weights are estimated using a nearest-neighbor technique that ensures stability in sparsely populated regions of color-magnitude space. The derived weights are then summed in redshift bins to create the redshift distribution.

Optical monitoring of the z=4.40 quasar Q 2203+292
http://arxiv.org/abs/0801.3729

Confirmation of a correlation between the X-ray luminosity and spectral slope of AGNs in the Chandra Deep Fields
http://arxiv.org/abs/0801.3599
0.1 < z < 4

Large Scale Structures at High Redshift in the GOODS Field
http://arxiv.org/abs/0801.3557
GOODS-South, catalogue of overdensities, up to z ~ 2.5

*Detecting quasars at very high redshift with next generation X-ray telescopes*
http://arxiv.org/abs/0801.3482
modelling of the evolution of the optical and X-ray AGN luminosity function at 2 < z < 6 based on a CDM merger-driven model

Difference Imaging of Lensed Quasar Candidates in the SDSS Supernova Survey Region
http://arxiv.org/abs/0801.3432
Difference imaging provides a new way to discover gravitationally lensed quasars because few non-lensed sources will show spatially extended, time variable flux.

Can very massive stars avoid Pair-Instability Supernovae?
http://arxiv.org/abs/0801.3397
magnetic fields provide the strong coupling that is lacking in standard evolution metal-free models and our 150 Msol Population III model avoids indeed the pair-instability explosion.

Astroparticle Physics: Puzzles and Discoveries
http://arxiv.org/abs/0801.3028

astro-ph 0801w3

http://arxiv.org/abs/0801.2759
Spitzer Uncovers AGN Missed by Optical Surveys in 7 Late-type Galaxies
Our findings add to the growing evidence that black holes do form and grow in low-bulge environments and that they are significantly more common than optical studies indicate.

http://arxiv.org/abs/0801.2694
Milli-arcsecond astrophysics with VSI, the VLTI spectro-imager in the ELT era
This instrument will provide information complementary to what is obtained at the same time with ALMA at different wavelengths and the extreme large telescopes.

http://arxiv.org/abs/0801.2484
Quantifying systematic uncertainties in supernova cosmology
While the tool is primarily aimed for studies and optimization of future instruments, we use the “Gold'’ data-set in Riess et al. (2007) to show examples of potential systematic uncertainties that could exceed the quoted statistical uncertainties.

http://arxiv.org/abs/0801.2383
Observational Constraints on the Dependence of Radio-Quiet Quasar X-ray Emission on Black Hole Mass and Accretion Rate
alpha_ox increases with increasing M_BH and L_UV / L_Edd, and decreases with increasing L_X / L_Edd.

http://arxiv.org/abs/0801.2141
Mapping warm molecular hydrogen with Spitzer’s Infrared Array Camera (IRAC)
David Neufeld, Yuan Yuan (JHU)

probability ABC

A priori probability is the probability estimate prior to receiving new information.

Posterior probability is a revised probability that takes into account new available information. For example, let there be two urns, urn A having 5 black balls and 10 red balls and urn B having 10 black balls and 5 red balls. Now if an urn is selected at random, the probability that urn A is chosen is 0.5. This is the a priori probability. If we are given an additional piece of information that a ball was drawn at random from the selected urn, and that ball was black, what is the probability that the chosen urn is urn A? Posterior probability takes into account this additional information and revises the probability downward from 0.5 to 0.333 according to Bayes’ theorem, because a black ball is more probable from urn B than urn A.

Bayes’ Theorem:
Bayes theorem is a formula for revising a priori probabilities after receiving new information. The revised probabilities are called posterior probabilities. For example, consider the probability that you will develop a specific cancer in the next year. An estimate of this probability based on general population data would be a prior estimate; a revised (posterior) estimate would be based on both on the population data and the results of a specific test for cancer.
The best way to understand the terms is to look at an example. Consider a screening test for intestinal tumors. Let Ai = A1 = the event “tumor present”, “B” the event “screening test positive” and “A2″ the event “tumor not present” with no further A’s.
If you have a tumor, the screening test has an 85% chance of catching it — P(B|A1) = .85. However, it also has a 10% chance of falsely indicating “tumor present” when there is no tumor P(B|A2) = .10. The probability of a person having a tumor is .02 P(A1) = .02.
If the screening test is positive, what is the probability that you have a tumor?
.02*.85/(.02*.85+.98*.10)
= .017/(.017+ .098)
= .148

Cosmology with Active Nuclei as Probes

http://www.astr.ua.edu/keel/galaxies/qsoevolve.html

It has long been seductive to try probing the world model of cosmology by trying to use AGN as standard candles or standard measuring rods. Radio galaxies seen at 2.2 microns have a very well-behaved Hubble diagram, and for a while it appeared that radio QSOs with flat-spectrum core sources showed a neat relation between luminosity and equivalent width of C IV l 1549 emission (the Baldwin effect, as in Nature 273, 431, 1978, which seems to have been the result of sheer bad luck in object selection and has just about gone away with recent data collections).

We can also use AGNs to probe the properties of galaxies along our line of sight to them, galaxies that are in many cases individually impossible to detect directly, by studying their influence on the propagation of light from the distant AGN. This may be in the form of gravitational lensing (which we’ve already treated) or absorption-line systems. Aside from the Lyman a forest, these include metal-line systems, damped Lyman a systems, and Lyman-limit systems. All three of these are telling us about some kind of gas-rich galaxy. Much of the controversy is how we are to associate them with the sorts of galaxies seen in our neighborhood to derive evolutionary changes.

Metal-line systems have been reviewed in several chapters of QSO Absorption Lines: Probing the Universe, and are considered important enough to have driven one of the few initial Key Projects for HST. Various species are seen in absorption depending on the absorber redshift zabs < zem: Lyman a, Mg I, Mg II, Fe II, C I, C IV, various species of Si, N, O. Complex velocity structure may be present; sometimes associated H I absorption at (1+z) × 21 cm may be found. The column densities involved are hard to assess because of uncertainties in ionization structure. Some similarity is found with the hot ISM in our galactic halo, but we sample different regions for these kinds of observations and the ionization match is still poor. There is some evidence that the absorbers are starburst galaxies because (1) there is a problem with spatial extent for normal galaxies, and starbursts are known to blow gas away in winds, and (2) the ambient QSO UV radiation field is not strong enough to ionize the gas as strongly as the absorption-line ionization levels suggest. In fact, the nearby galaxies identified with QSO absorption lines of Ca II and Na I are systematically high in SFR for their morphological types (Caulet and Keel, should have been published years ago). Also, objects at intermediate redshift z~ 0.5 for which Mg II can be seen from the ground and optical identification is possible look like some sort of starbursts. It is still not clear whether we are dealing with dwarf or luminous galaxies as the dominant absorbers. The evolution in comoving density with redshift is at most mild, unlike the Lyman a forest (essentially metal-free) systems.

Damped Lyman a systems} have much higher column densities in absorption, so that the H I resonance line is in a different part of the curve of growth. These are taken to be genuine disk galaxies, and extensive surveys (for example Wolfe et al 1986 ApJSupp 61, 249) have been carried out in hopes of watching spiral disks evolve. Straightforward assumptions indicate that these contain most of the neutral gas in teh Universe for a wide range of redshift. Metal lines are associated with these systems as well. A sample is shown by Turnshek et al 1989 ApJ 344, 567, by permission of the AAS):

Note the broad Lyman a absorption trough. A fascinating recent finding is that the metal abundances in these systems increase systematically with cosmic time, although they do not reach the levels seen in galactic stars of similar age (Lu et al. 1996 ApJSuppl 107, 475).

Many of these are also Lyman-limit systems, in which the optical depth at the Lyman edge is unity or greater. These are recognized by a step down in flux at the Lyman edge at the absorber (not QSO) redshift, frequently to zero detectable flux. The Milky Way is a Lyman limit absorber from our vantage point in every dirction at z=0. Various treatments are given by Tytler (1982 Nature 298, 427), Bechtold et al 1984 (ApJ 281, 76) and Lanzetta (1988 ApJ 332, 96). An example is shown by Sargent, Steidel, and Boksenberg (1989, ApJSuppl 69, 703, courtesy of the AAS):

Lanzetta found that these systems seem to have constant comoving space density, and that the Mg II metal-line systems seem to be disappearing relative to Lyman-limit systems.

These systems may also be sought in BL Lac objects, avoiding confusion with emission-line or BAL structure but introducing the problem that we may not know the AGN redshift except as the redshift at which the Lyman a forest disappears.