We describe a new program for determining photometric redshifts, dubbed EAZY. The program is optimized for cases where spectroscopic redshifts are not available, or only available for a biased subset of the galaxies. The code combines features from various existing codes: it can fit linear combinations of templates, it includes optional flux- and redshift-based priors, and its user interface is modeled on the popular HYPERZ code. A novel feature is that the default template set, as well as the default functional forms of the priors, are not based on (usually highly biased) spectroscopic samples, but on semi-analytical models. Furthermore, template mismatch is addressed by a novel rest-frame template error function. This function gives different wavelength regions different weights, and ensures that the formal redshift uncertainties are realistic. We introduce a redshift quality parameter, Q_z, that provides a robust estimate of the reliability of the photometric redshift estimate. Despite the fact that EAZY is not "trained" on spectroscopic samples, the code (with default parameters) performs very well on existing public datasets. For K-selected samples in CDF-South and other deep fields we find a 1-sigma scatter in dz/(1+z) of 0.034, and we provide updated photometric redshift catalogs for the FIRES, MUSYC, and FIREWORKS surveys.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 11 figures. Code available at http://www.astro.yale.edu/eazy

To constrain the nature of the very first stars, we investigate the collapse and fragmentation of primordial, metal-free gas clouds. We explore the physics of primordial star formation by means of threedimensional simulations of the dark matter and gas components, using smoothed particle hydrodynamics, under a wide range of initial conditions, including the initial spin, the total mass of the halo, the redshift of virialization, the power spectrum of the DM fluctuations, the presence of HD cooling, and the number of particles employed in the simulation. We find characteristic values for the temperature, T ∼ a few 100 K, and the density, n ∼ 10 3 − 10 4 cm −3 , characterising the gas at the end of the initial free-fall phase. These values are rather insensitive to the initial conditions. The corresponding Jeans mass is M J ∼ 10 3 M ⊙ . The existence of these characteristic values has a robust explanation in the microphysics of H 2 cooling, connected to the minimum temperature that can be reached with the H 2 coolant, and to the critical density at which the transition takes place between levels being populated according to NLTE, and according to LTE.In all cases, the gas dissipatively settles into an irregular, central configuration which has a filamentary and knotty appearance. The fluid regions with the highest densities are the first to undergo runaway collapse due to gravitational instability, and to form clumps with initial masses ∼ 10 3 M ⊙ , close to the characteristic Jeans scale. These results suggest that the first stars might have been quite massive, possibly even very massive with M ⋆ ∼ > 100 M ⊙ .After a gas element has undergone runaway collapse, and has reached densities in excess of 10 8 cm −3 , a sink particle is created. This procedure allows us to follow the evolution of the overall system beyond the point where the first nonlinear region would otherwise force the calculation to a halt. These later evolutionary stages, during which the clumps grow in mass due to accretion and merging with other clumps, are quite sensitive to the initial conditions. The key process in building up very massive clumps, with masses up to a few times 10 4 M ⊙ , is merging between clumps. Since the merging rate sensitively depends on the density of the gas, halos with the highest degree of central concentration are able to assemble the most massive clumps. Among these are halos with a low spin (λ ≃ 0.01), and with DM fluctuations imprinted according to a white-noise spectrum.

Recent studies have shown that massive quiescent galaxies at high redshift are much more compact than present-day galaxies of the same mass. Here we compare the radial stellar density profiles and the number density of a sample of massive galaxies at z ∼ 2.3 to nearby massive elliptical galaxies. We confirm that the average stellar densities of the z ∼ 2.3 galaxies within the effective radius, ρ(< r e ), are two orders of magnitude higher than those of local elliptical galaxies of the same stellar mass. However, we also find that the densities measured within a constant physical radius of 1 kpc, ρ(< 1 kpc), are higher by a factor of 2-3 only. This suggests that inside-out growth scenarios are plausible, in which the compact high redshift galaxies make up the centers of normal nearby ellipticals. The compact galaxies are common at high redshift, which enables us to further constrain their evolution by requiring that the number density of their descendants does not exceed constraints imposed by the z = 0 galaxy mass function. We infer that size growth must be efficient, with (r 1+2 /r 1 ) ∼ (M 1+2 /M 1 ) 2 . A simple model where compact galaxies with masses ∼ 10 11 M ⊙ primarily grow through minor mergers produces descendants with the approximate sizes, stellar densities, and number density of elliptical galaxies with masses 2 − 3 × 10 11 M ⊙ in the local Universe. We note that this model also predicts evolution in the M BH − σ relation, such that the progenitors of elliptical galaxies have lower black hole masses at fixed velocity dispersion. The main observational uncertainty is the conversion from light to mass; measurements of kinematics are needed to calibrate the masses and stellar densities of the high redshift galaxies.

We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in 1996 May and June. The spectra consist of a dominant soft component below ~2 keV and a power-law-like continuum extending to at least ~800 keV. We interpret them as emission from an optically-thick, cold accretion disc and from an optically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of total available power is dissipated in the corona. We model the soft component by multi-colour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCA/RXTE data yield the most probable black hole mass of about 10 solar masses and an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure dominated, accretion-disc solution branch. The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50 keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. The compactness of the corona is between 2 and 7, and a presence of a significant population of electron-positron pairs is ruled out. We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuum is accompanied by a broad iron K-alpha line.Comment: 18 pages, 12 figures, 2 landscape tables in a separate file. Accepted to MNRA

In order to constrain the initial mass function of the first generation of stars (Population III), we investigate the fragmentation properties of metal-free gas in the context of a hierarchical model of structure formation. We investigate the evolution of an isolated 3 j peak of mass that collapses at using smoothed 6 2 # 10 M z Ӎ 30, coll particle hydrodynamics. We find that the gas dissipatively settles into a rotationally supported disk that has a very filamentary morphology. The gas in these filaments is Jeans unstable with . Fragmentationleads to the formation of high-density ( cm ) clumps that subsequently grow in mass by accreting the 8 Ϫ3 n 1 10 surrounding gas and by merging with other clumps up to masses of ∼10 4 M , . This suggests that the very first stars were rather massive. We explore the complex dynamics of the merging and tidal disruption of these clumps by following their evolution over a few dynamical times.

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

A B S T R A C TRecent theoretical investigations have suggested that the formation of the very first stars, forming out of metal-free gas, was fundamentally different from the present-day case. The question then arises which effect was responsible for this transition in the star formation properties. In this paper, we study the effect of metallicity on the evolution of the gas in a collapsing dark matter mini-halo. We model such a system as an isolated 3s peak of mass 2 Â 10 6 M ( that collapses at z coll . 30, using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of either Z ¼ 10 24 Z ( or 10 23 Z ( . We assume that H 2 has been radiatively destroyed by the presence of a soft UV background. Metals therefore provide the only viable cooling at temperatures below 10 4 K. We find that the evolution proceeds very differently for the two cases. The gas in the lower metallicity simulation fails to undergo continued collapse and fragmentation, whereas the gas in the higher metallicity case dissipatively settles into the centre of the dark matter halo. The central gas, characterized by densities n H * 10 4 cm 23 , and a temperature, T . 90 K, that closely follows that of the cosmic microwave background, is gravitationally unstable and undergoes vigorous fragmentation. We discuss the physical reason for the existence of a critical metallicity, Z crit , 5 Â 10 24 Z ( , and its possible dependence on redshift. Compared with the pure H/He case, the fragmentation of the Z ¼ 10 23 Z ( gas leads to a larger relative number of low-mass clumps.

Following the detection of strong TeV γ-ray flares from the BL Lac object 1ES 1959+650 with the Whipple 10 m Cherenkov telescope on May 16 and 17, 2002, we performed intensive Target of Opportunity (ToO) radio, optical, X-ray and TeV γ-ray observations from -2correlation properties. Although the X-ray and γ-ray fluxes seemed to be correlated in general, we found an "orphan" γ-ray flare that was not accompanied by an X-ray flare. While we detected optical flux variability with the Boltwood and Abastumani observatories, the data did not give evidence for a correlation between the optical flux variability with the observed X-ray and γ-ray flares. Within statistical errors of about 0.03 Jy at 14.5 GHz and 0.05 Jy at 4.8 GHz, the radio fluxes measured with the University of Michigan Radio Astrophysical Observatory (UMRAO) stayed constant throughout the campaign; the mean values agreed well with the values measured on May 7 and June 7, 2002 at 4.9 GHz and 15 GHz with the Very Large Array (VLA), and, at 4.8 GHz with archival flux measurements. After describing in detail the radio, optical, X-ray and γ-ray light curves and Spectral Energy Distributions (SEDs) we present initial modeling of the SED with a simple Synchrotron Self-Compton (SSC) model. With the addition of another TeV blazar with good broadband data, we consider the set of all TeV blazars to begin to look for a connection of the jet properties to the properties of the central accreting black hole thought to drive the jet. Remarkably, the temporal and spectral X-ray and γ-ray emission characteristics of TeV blazars are very similar, even though the masses estimates of their central black holes differ by up to one order of magnitude.