Measurement of non-axisymmetry in centres of advanced mergers of galaxies

We measure the non-axisymmetry in the luminosity distribution in the inner few kpc of the remnants of advanced mergers of galaxies with a view to understand the relaxation in the central regions. For this, we analyze the images from the 2MASS archival data for a selected sample of 12 merging galaxies, which show signs of interaction but have a single nucleus. The central regions are fitted by elliptical isophotes whose centres are allowed to vary to get the best fit. The centres of isophotes show a striking sloshing pattern with a spatial variation of up to 20-30 % within the central 1 kpc. This indicates mass asymmetry and a dynamically unrelaxed behaviour. Next, we Fourier-analyze the galaxy images while keeping the centre constant and measure the deviation from axisymmetry in terms of the fractional Fourier amplitudes (A_1, A_2 etc) as a function of radius. All mergers show a high value of lopsidedness (upto A_1 ~ 0.2) in the central 5 kpc. The m=2 asymmetry is even stronger, with values of A_2 upto ~ 0.3, and in three cases these are shown to represent bars. The corresponding values denoting non-axisymmetry in inner regions of a control sample of eight non-merger galaxies are found to be several times smaller. Surprisingly, this central asymmetry is seen even in mergers where the outer regions have relaxed into a smooth elliptical-like r^{1/4} profile or a spiral-like exponential profile. Thus the central asymmetry is long-lived, estimated to be ~ 1 Gyr, and hence lasts for over 100 local dynamical timescales. These central asymmetries are expected to play a key role in the future dynamical evolution of the central region of a merger, and can help in feeding a central AGN.Comment: 14 pages, 3 figures, accepted for publication in MNRA

Lopsidedness and Sloshing in Centres of Advanced Mergers of Galaxies

We measure the non-axisymmetry in the luminosity distribution in the central few kpc of a sample of advanced mergers of galaxies, by analyzing their 2MASS images. All mergers show a high central asymmetry: the centres of isophotes show a striking sloshing pattern with a spatial variation of upto 30 % within the central 1 kpc; and the Fourier amplitude for lopsidedness (m=1) shows high values upto 0.2 within the central 5 kpc. The central asymmetry is estimated to be long-lived, lasting for ~ a few Gyr or ~ 100 local dynamical timescales. This will significantly affect the dynamical evolution of this region, by helping fuel the central active galactic nucleus, and also by causing the secular growth of the bulge driven by lopsidedness.Comment: 2 pages, 1 figure; to appear in the Proceedings of the IAU Symposium 245, "Formation and Evolution of Galaxy Bulges", held at Oxford, U.K., July 2007, Eds. M. Bureau, E. Athanassoula, B. Barbu

The Escape Fraction of Ionizing Radiation from Galaxies

The escape of ionizing radiation from galaxies plays a critical role in the evolution of gas in galaxies, and the heating and ionization history of the intergalactic medium. We present semi-analytic calculations of the escape fraction of ionizing radiation for both hydrogen and helium from galaxies ranging from primordial systems to disk-type galaxies that are not heavily dust-obscured. We consider variations in the galaxy density profile, source type, location, and spectrum, and gas overdensity/distribution factors. For sufficiently hard first-light sources, the helium ionization fronts closely track or advance beyond that of hydrogen. Key new results in this work include calculations of the escape fractions for He I and He II ionizing radiation, and the impact of partial ionization from X-rays from early AGN or stellar clusters on the escape fractions from galaxy halos. When factoring in frequency-dependent effects, we find that X-rays play an important role in boosting the escape fractions for both hydrogen and helium, but especially for He II. We briefly discuss the implications of these results for recent observations of the He II reionization epoch at low redshifts, as well as the UV data and emission-line signatures from early galaxies anticipated from future satellite missions.Comment: 43 pages, 9 figures, accepted in ApJ, comments welcom

Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization

We examine the significance of the first metal-free stars (Pop III) for the cosmological reionization of HI and HeII. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Pop III stellar cluster for HI and HeII being 1.6 and $10^5$ times stronger respectively than those from a Pop II cluster. For the currently favored cosmology, we find that Pop III stars alone can reionize HI and HeII at redshifts, $z$ of about 9 (4.7) and 5.1 (0.7) for continuous (instantaneous) modes of star formation. More realistic scenarios involving combinations of Pop III and Pop II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these cases; the ionization fraction of HeIII reaches a maximum of about 60 percent at $z$ of about 5.6 if Pop III star formation lasts for $10^9$ yr. Future data on HI reionization can test the amount of small-scale power available to the formation of the first objects, and provide a constraint on values of $\sigma_8$ less than or about 0.7. Since current UV observations indicate an epoch of reionization for HeII at $z$ of about 3, HeII may reionize more than once. Measurements of the HeII Gunn-Peterson effect in the intergalactic medium at redshifts exceeding about 3 may reveal the significance of Pop III stars for HeII reionization, particularly in void regions that may contain relic ionization from early Pop III stellar activity.Comment: 29 pages, 4 figures. Accepted for publication in Ap.J. (Feb. 20, 2003 issue; v. 584); minor revisions, results unchange

Constraints on First-Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background

We examine the constraints on high-redshift star formation, ultraviolet and X-ray pre-ionization, and the epoch of reionization at redshift z_r, inferred from the recent WMAP-5 measurement, tau_e = 0.084 +/- 0.016, of the electron scattering optical depth of the cosmic microwave background (CMB). Half of this scattering can be accounted for by the optical depth, tau_e = 0.04-0.05, of a fully ionized intergalactic medium (IGM) at z < z_GP = 6-7, consistent with Gunn-Peterson absorption in neutral hydrogen. The required additional optical depth, Delta-tau_e = 0.03 +/- 0.02 at z > z_GP, constrains the ionizing contributions of first light sources. WMAP-5 also measured a significant increase in small-scale power, which lowers the required efficiency of star formation and ionization from mini-halos. Early massive stars (UV radiation) and black holes (X-rays) can produce a partially ionized IGM, adding to the residual electrons left from incomplete recombination. Inaccuracies in computing the ionization history, x_e(z), and degeneracies in cosmological parameters (Omega_m, Omega_b, sigma_8, n_s) add systematic uncertainty to the measurement and modeling of $\tau_e$. From the additional optical depth from sources at z > z_GP, we limit the star-formation efficiency, the rate of ionizing photon production for Pop III and Pop II stars, and the photon escape fraction, using standard histories of baryon collapse, minihalo star formation, and black-hole X-ray preionization.Comment: Greatly revised version, based on WMAP-5 results and new models. Accepted for ApJ (2008

Assessment of Iron Overload in Homozygous and Heterozygous Beta Thalassemic Children below 5 Years of Age

Background: Thalassemia is a genetic disease having 3-7% carrier rate in Indians. It is transfusion dependent anemia having high risk of iron overloading. A clinical symptom of iron overload becomes detectable in second decade causing progressive liver, heart and endocrine glands damage. There is a need to assess iron overload in thalassemics below 5 years of age to protect them from complications at later age of life. Aims and objectives: Present study was undertaken to estimate serum iron status and evaluate serum transferrin saturation in both homozygous & heterozygous form of thalassemia as an index of iron overload among children of one to five years of age. Materials and Methods: Clinically diagnosed thirty cases of β thalassemia major & thirty cases of β thalassemia minor having severe anemia, hepatospleenomegaly and between 1 year to 5 years of age were included in study group and same age matched healthy controls were included in the study. RBC indices and HbA, HbA2 and HbF were estimated along with serum iron & serum Total Iron Binding Capacity (TIBC) and serum transferrin levels. Results: Significant difference was observed in hemoglobin levels between control and both beta thalassemia groups. Mean Corpuscular Volume (MCV) and Mean Corpuscular Hemoglobin (MCH) values were reduced. Hemoglobin electrophoresis showed the elevated levels of HbF and HbA2 in both beta thalassemia groups. Among serum iron parameters, serum iron, TIBC and transferrin saturation were elevated whereas serum transferrin levels were low in thalassemia major in children below 5 years of age. Conclusion: Although clinical symptoms of iron overload have been absent in thalassemic children below five years of age, biochemical iron overloading has started at much lower age which is of great concern