Dark matter and galaxy formation is an early and ongoing, more general interest. What constitutes the dark matter (DM) and how does it affects galaxy formation and large scale structure? Recent observational evidence increasingly suggests that cold (i.e. nonrelativistic) dark matter (CDM) is responsible for the unseen mass in the universe. Examples of CDM would be black holes, axions, Jupiters, brown dwarfs, etc. The big question is to determine whether it is MACHOS (stars or planets), black holes of mass 10^4-0.1 solar masses or even smaller, primordial black holes (PBHs), or is it exotic particles (axions, wimps, photinos etc.)? One of the earliest quantitative investigation of the importance of the gravitational effects of CDM on the baryonic fluctuations which give rise to galaxies was presented in Meszaros (1974) and (1975) (also Meszaros (1980)). These papers discussed the evolution of the density contrast in what came to be known as cold dark matter fluctuations in the cosmological radiation background, as it enters the horizon and goes through the matter-radiaton equilibrium and the recombination epochs, and its effect on the formation of galaxies and large scale structure mediated by such cold dark matter. This is referred to in some books and articles as the "Mészáros effect" , which has become incorporated in all subsequent CDM calculations of structure evolution, determining the so-called transfer function of the original fluctuation spectrum. Large scale structure observations, modeling and simulations are now a major activity in cosmology, but the actual nature of the dark matter still remains elusive.
With the recent discovery of GRB afterglows, the distance scale of these objects has been confirmed to be cosmological, from ground-based optical determinations of redshifts for a significant number of bursts (see, e.g. Greiner's GRB data page). Significant progress in this area is being made with the launch of the Swift satellite, and the planned JWST (James Webb Space Telescope), together with other existing space facilities such as Chandra and XMM Newton . These provide extremely interesting prospects for doing cosmology at much higher redshifts than currently studied. The first generation of GRB associated with pop. III (first generation, metal-free stars) may in principle be detectable from redshifts as large as z ~ 20-30.
Due to the shallow decay of the infared light curves for these high redshift bursts (Tanvir et al. 2009, Nature, Vol. 461, p 1254), they can be easily seen and studied at times that are well after (~hours) the initial GRB (e.g. see Figure on left for GRB 090423). JANUS is a proposed mission, with Penn State involvement, that will capitalize on this to study the high redshift universe by using distant GRBs. Penn State faculty working primarily on GRBs include Peter Mészáros, Derek Fox, John Nousek and David Burrows, Abe Falcone, with at least five or six other faculty involved part-time. The nucleus of the observational activity centers on the Swift team, in close interaction with the Penn State GRB theory team.
The X-ray flux of GRB afterglows at redshifts in the range 3-30, including the effects of intergalactic He II absorption, was discussed by Meszaros and Rees (2003) . Because of the increasing K-corrections the X-ray fluxes do not decrease much with redshift beyond z ~ 4-5, and are easily detectable out to z~30 with SWIFT in the first day or so. Calculations by Gou, Meszaros, Abel and Zhang (2003) indicate that the forward shock X-ray emission of GRB afterglows is detectable by Chandra and Swift out redshifts of z ~13 and 30 respectively. In the near-IR, the prompt reverse shock emission in the K and M bands is detectable by the planned James Webb Space Telescope, even one day after the trigger, out to z ~ 15 and 30 respectively (figure on the left).
The redshifts detected by Swift are, statistically, approximately a factor of 2 larger (e.g. Bagoly, et al 2006) than those detected based on localizations by previous satellites. A very exciting result in this respect was the detection by Swift of the most distant GRB so far, GRB 050904 (Cusumano, et al, 2006), at a redshift z=6.29, comparable ot that of the most distant quasars. This burst had a bright (~ 9th magnitude) near-IR prompt flash, and its X-ray luminosity exceeded that of the brightest X-ray quasars at the same redshift for a whole day (figure on the right). This emphasizes the significant prospects for using such bursts to investigate the high redshift intergalactic medium and the conditions during the formation of the first stars and galaxies. We have made a complete analysis of the available X-ray, NIR, and radio afterglow data on GRB 050904 to date (Gou, Fox and Meszaros, 2007), utilizing afterglow models that incorporate a range of physical effects not previously considered in addressing this or any other GRB afterglow. In particular, using the Markov Chain Monte Carlo method, we investigated the possibility that the early flares are due to synchrotron and inverse Compton emission from the reverse shock regions of the outflow. Model fits were made to various cases for the energetics, the radiation parameters and the external medium density. The best-fit values shows that the density and the kinetic energy of the burst are both above the typical values of the low-redshift GRBs.
Other ways of investigating high redshift GRB may be through their radio afterglows. Ioka and Meszaros investigated the radio afterglows of GRBs and hypernovae (HNe) at high redshifts, and quantified their detectability, as well as their potential usefulness for 21 cm absorption line studies of the intergalactic medium (IGM) and intervening structures. The radio afterglows of GRBs would be detectable out to z ~30, while the energetic HNe could be detectable out to z ~ 20 even by the current Very Large Array (VLA).
An indirect way of measuring GRB redshifts is based on the fact that the soft tail of the gamma-ray burst spectra show excesses from the exact power-law dependence. Bagoly, Csabai, Meszaros A, Meszaros P, Horvath, Balazs & Vavrek (2003) have shown that this departure can be detected in the peak flux ratios of different BATSE DISCSC energy channels. This effect allows to estimate the redshift of the bright long gamma-ray bursts in the BATSE Catalog. A verification of these redshifts is obtained for the 8 GRB which have both BATSE DISCSC data and measured optical spectroscopic redshifts. There is a good correlation between the measured and estimated redshifts, and the average error is Delta z ~ 0.33. The method is similar to that of the photometric redshift estimation of galaxies in the optical range, hence it can be called the "gamma photometric redshift estimation". The estimated redshifts for the long bright gamma-ray bursts are up to z ~ 4. For the the faint long bursts - which should be up to z ~ 20 - the redshifts cannot be determined unambiguously with this method. Other related indirect redshift determination methods are based on the variability of the gamma-ray light-curve (Fenimore & Ramirez-Ruiz 2000, Reichart et al 2000) and on the time-lags between hard and soft pulses (Norris and Bonell, 2000).
Another method for gleaning statistical information on the GRB distances is the use of the brightness distribution, i.e. the number of sources with peak flux greater than a certain value, log N(>F) vs. log F. This distribution is expected to have a slope of -3/2 in the brightness limited Euclidean case, and is observed to have that only at the brighter end, flattening at the fainter end to something that appears to be -0.9 or so. This could be due either to cosmological redshift effects, or to having reached a limiting maximum redshift, or may be due to having reached a maximum redshift plus having a luminosity function of slope which reproduces the low-brightness slope. Work on the GRB distribution and related statistical questions has been going on in collaboration with Lajos Balazs (Konkoly Observatory, Budapest), Attila Meszaros (Charles University, Prague- not a relative), Istvan Horvath (Bolyai Military University, Budapest) and Daniel Reichart (former Penn State undergrad, now assistant professor, U. North Carolina). D. Reichart and P. M. investigated chi-squared fits of cosmological number counts with a more general luminosity function shape, arbitrary power law slopes and arbitrary cosmologies, including corrections for incompleteness of the data. In addition, the fits included information on the time dilations as a function of peak flux. Using the 3B data set, these fits show that the faintest bursts could be at redshifts in excess of 5 (ApJ, 1997)
I. Horvath, P. M. and A. M. carried out detailed chi-squared fits of relativistic k=0 cosmological number count distributions under the above assumptions to the 2B BATSE and the PVO catalogue of gamma bursts. The results indicate that for standard candles the 2B/PVO data are compatible with a comoving constant source density, but to within one standard deviation are also compatible with density evolution steeper by up to two powers in (1+z) or also significantly flatter. For a nontrivial power law luminosity function slope of about 1.9 (which mimics the -0.9 low brightness source count slope) the range of maximum to minimum luminosity may be as large as 100, compatible with having 90% of the observed sources with a luminosity within one decade of each other (Ap.J., 1996)
A. M. and P. M. derived the exact analytic expressions for the cosmological number counts of bursting sources, and calculated the mean redshifts and time dilations as a function of the flux range, as well as the dispersion of these quantities, for various values of the density evolution and the luminosity function dispersion. Time dilation values of 2.25, 1.75 and 1.35 as reported in the literature are found to imply redshifts which, while large, are within one standard deviation of conventional redshifts associated with galaxy formation. This is particularly so in the case of a power law luminosity function, but is also true for standard candles. Extensions of these analytical expressions to the case of an open cosmology with K-corrections are also discussed (Ap.J., 1996)
P. Meszaros and A. Meszaros calculated analytic expressions in three different asymptotic regimes of the number counts of bursting or steady sources in a relativistic k=0 cosmology, under the assumption of a (1+z) power law density evolution and either a standard candle or a power law luminosity function. These analytic expressions are useful for modeling the data of gamma ray burst sources, in particular the number distribution of objects per peak flux interval. An approximate eye fit to the BATSE 2B catalogue of gamma bursts is sufficient for estimating, based on these expressions, the value of the standard candle luminosity, or alternatively the slope of the luminosity function, the minimum source luminosity and a limit on the maximum luminosity (Ap.J., 1995)
We are interested in various aspects of models of the diffuse cosmological X-ray background, in particular its origin, spatial fluctuations, and what it can tell us about large scale structure. More recently, the emphasis is shifting toward the gamma-ray background.
In a series of papers, P.M. and collaborators (A. Meszaros, Z. Bagoly, H. Bi) investigated the effects of the very large scale structure (voids and superclusters) on the spatial fluctuations of the cosmic X-ray background (CXB). This can be modeled as a redshift dependence of the mass contrast inside and outside structures, which varies as the structures form and become bound, and assuming a relationship between this and the birth of X-ray emitting objects (e.g. AGNs) which contribute to the CXB. The main effect is given by the distance fluctuations of the large structures, with the varying density contrast adding a small correction. We concluded that, independently of whether one uses a hot or cold dark matter prescription, the discrete sources contributing to the CXB in constant comoving density structures may have present separations of order 30 Mpc, but structures larger than about 60 Mpc may only be one-dimensional, and any great attractors may only be present at low redshifts below 0.5, otherwise they would have considerably exceeded the fluctuation upper limits of HEAO-1.
Rudak and Meszaros considered the effects upon the X-ray background of the scattering by dust in intervening young galaxies, assuming the X-rays arise in discrete sources. This effect has been previously observed to produce X-ray halos around the image of distant point sources, but its effect upon the background had not been explored. The effective angle of scattering is 1-10 arcminutes at 1-3 KeV, over which scale it can give a significant reduction of the background spatial fluctuations. As a result, the number of sources needed to explain the smoothness observed with the IPC can be reduced to values of about 1000/sq.deg. The effect may be much stronger in the direction of distant clusters, or for dustier disk galaxies. This has implications also for the increasing scarcity of optical QSO at very high redshifts, and may be used for putting limits on a possible IR background.
Ricker (former PSU Honors scholar, now at Univ. of Chicago) and Meszaros investigated the contribution of starburst galaxies and reflection-dominated AGN to the diffuse X-ray background. The contribution of the former cannot be larger than about 30% at energies below about 15 KeV, and do not affect the diffuse spectrum at higher energies. Reflection-dominated AGN were modeled with a detailed code including scattering and metallic absorption lines, as well as a varying covering factor. The conclusion is that, in their simple form, such models do not satisfy a statistical chi-squared fit with sufficient significance, in the context of relativistic cosmological models, and additional ingredients to the model are necessary to model the phenomenon.
Research sponsor: NASA, NSF
Meszaros, P (1974), ``The Behavior of Point Masses in an Expanding Cosmological Substratum", Astron. Astrophys. 37, 225
Meszaros, P (1975), ``Primeval Black Holes and Galaxy Formation", Astron. Astrophys. 38, 5
Meszaros, P (1980) "Small perturbations in a flat radiation-matter universe and the effect of black hole formation" ApJ 238:781
Gou, L-J, Fox, D.B. and Meszaros, P (2007) "Modeling GRB 050904: Autopsy of a Massive Stellar Explosion at z=6.29", ApJ in press (astro-ph/0612256)
Cusumano, G, et al (2006), ``Detection of a huge explosion in the early Universe", Nature 440:164 (astro-ph/0509737)
Bagoly, Z, et al (2006), ``The Swift satellite and the redshifts of long gamma-ray bursts", Astron.Ap., 453:797 (astro-ph/0604326)
Ioka, K and Meszaros, P (2005), "Radio Afterglows of Gamma-Ray Bursts and Hypernovae at High Redshift", ApJ 619:684 (astro-ph/0408487)
Meszaros P and Rees MJ (2003), ``Gamma-ray bursts as X-ray depth-gauges of the Universe", Ap.J.(Lett.), 591:L91 (astro-ph/0305115)
Gou L.J., Meszaros, P., Abel, T. & Zhang, B. (2003), ``Detectability of Long GRB Afterglows from Very High Redshifts", Ap.J., 604:508 (astro-ph/0307489)
Bagoly Z, Csabai I, Meszaros A, Meszaros P, Horvath I, Balazs LG & Vavrek R (2003), ``Gamma Photometric Redshifts for Long Gamma-Ray Bursts", Astron.Ap., 398, 919 (astro-ph/0211539)
``Constraint on the Redshift and Luminosity Distributions of Gamma-Ray Bursts in an Einstein-de Sitter Universe", Reichart. D.E. & Meszaros, P., 1997, Ap.J., 483, 597
Meszaros, A. and Meszaros, P., ``Cosmological Evolution and Luminosity Function Effects on Number Counts, Redshift and Time Dilation of Bursting Sources", Ap.J., 466, 29 (1996)
Horvath, I., Meszaros, P. and Meszaros, A., ``Cosmological Brightness Distribution Fits of Gamma Ray Burst Sources", Ap.J., 470, 56 (1996)
Meszaros, P and Meszaros, A., ``The Brightness Distribution of Gamma-ray Bursters in Relativistic Cosmologies", Ap.J., 449, 9 (1995)
Ricker, P.M. and \Mesz, P., ``Starburst and Reflection-Dominated AGN Contributions to the Cosmic X-ray Background", Ap.J., 418, 49 (1993)
Rudak, B. and Meszaros, P., ``Dust from Early Galaxies and the X-ray Background Radiation", Ap.J., 371, 29 (1991).
Bi, H.G., Meszaros, A. and Meszaros, P., ``On the Large Scale Structure of X-ray Background Sources", Astron.Astrophys., 243, 16 (1991)
Bagoly, Z., Meszaros, A. and Meszaros, P., ``Cosmological Constraints on the Clustering of X-ray Background Sources", Ap.J., 333, 54-63 (1988)
Meszaros, A. and Meszaros, P., ``Large Scale Structure of the Universe: Constraints from the X-ray Background", Ap.J. , 327, 25-33 (1988)