Extragalactic Background Light and Gamma-Ray Attenuation

Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN) and gamma ray bursts (GRBs) give upper limits on the extragalactic background light (EBL) from the UV to the mid-IR that are only a little above the lower limits from observed galaxies. These upper limits now rule out some EBL models and purported observations, with improved data likely to provide even stronger constraints. We present EBL calculations both based on multiwavelength observations of thousands of galaxies and also based on semi-analytic models, and show that they are consistent with these lower limits from observed galaxies and with the gamma-ray upper limit constraints. Such comparisons "close the loop" on cosmological galaxy formation models, since they account for all the light, including that from galaxies too faint to see. We compare our results with those of other recent works, and discuss the implications of these new EBL calculations for gamma ray attenuation. Catching a few GRBs with groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov detectors could provide important new constraints on the high-redshift star formation history of the universe.Comment: 12 pages, 8 multi-panel figures, Invited talk at the 25th Texas Symposium on Relativistic Astrophysics, Heidelberg December 6-10, 201

Diffuse Extragalactic Background Radiation

Attenuation of high--energy gamma rays by pair--production with UV, optical and IR background photons provides a link between the history of galaxy formation and high--energy astrophysics. We present results from our latest semi-analytic models (SAMs), based upon a $\Lambda$CDM hierarchical structural formation scenario and employing all ingredients thought to be important to galaxy formation and evolution, as well as reprocessing of starlight by dust to mid- and far-IR wavelengths. Our models also use results from recent hydrodynamic galaxy merger simulations. These latest SAMs are successful in reproducing a large variety of observational constraints such as number counts, luminosity and mass functions, and color bimodality. We have created 2 models that bracket the likely ranges of galaxy emissivities, and for each of these we show how the optical depth from pair--production is affected by redshift and gamma-ray energy. We conclude with a discussion of the implications of our work, and how the burgeoning science of gamma-ray astronomy will continue to help constrain cosmology.Comment: 12 pages, 8 figures, to be published in the Proceedings of the 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, held July 2008 in Heidelberg, German

Modeling Gamma-Ray Attenuation in High-Redshift GeV Spectra

We present two models for the cosmological UV background light, and calculate the opacity of GeV gamma--rays out to redshift 9. The contributors to the background include 2 possible quasar emissivities, and output from star--forming galaxies as determined by recent a semi--analytic model (SAM) of structure formation. The SAM used in this work is based upon a hierarchical build-up of structure in a $\Lambda$CDM universe and is highly successful in reproducing a variety of observational parameters. Above 1 Rydberg energy, ionizing radiation is subject to reprocessing by the IGM, which we treat using our radiative transfer code, CUBA. The two models for quasar emissivity differing above z = 2.3 are chosen to match the ionization rates observed using flux decrement analysis and the higher values of the line-of-sight proximity effect. We also investigate the possibility of a flat star formation rate density at z $>5$. We conclude that observations of gamma--rays from 10 to 100 GeV by Fermi (GLAST) and the next generation of ground based experiments should confirm a strongly evolving opacity from $1<$ z $<4$. Observation of attenuation in the spectra of gamma--ray bursts at higher redshift could constrain emission of UV radiation at these early times, either from a flat or increasing star-formation density or an unobserved population of sources.Comment: 4 pages, 7 figures, To be published in the Proceedings of the 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, held July 2008 in Heidelberg, German

Antenatal depression, treatment with selective serotonin reuptake inhibitors, and neonatal brain structure: A propensity-matched cohort study

The aim of this propensity-matched cohort study was to evaluate the impact of prenatal SSRI exposure and a history of maternal depression on neonatal brain volumes and white matter microstructure. SSRI-exposed neonates (n = 27) were matched to children of mothers with no history of depression or SSRI use (n=54). Additionally, neonates of mothers with a history of depression, but no prenatal SSRI exposure (n=41), were matched to children of mothers with no history of depression or SSRI use (n=82). Structural magnetic resonance imaging and diffusion weighted imaging scans were acquired with a 3T Siemens Allegra scanner. Global tissue volumes were characterized using an automatic, atlas-moderated expectation maximization segmentation tool. Local differences in gray matter volumes were examined using deformation-based morphometry. Quantitative tractography was performed using an adaptation of the UNC-Utah NA-MIC DTI framework. SSRI-exposed neonates exhibited widespread changes in white matter microstructure compared to matched controls. Children exposed to a history of maternal depression but no SSRIs showed no significant differences in brain development compared to matched controls. No significant differences were found in global or regional tissue volumes. Additional research is needed to clarify whether SSRIs directly alter white matter development or whether this relationship is mediated by depressive symptoms during pregnancy

GeV Gamma-Ray Attenuation and the High-Redshift UV Background

We present new calculations of the evolving UV background out to the epoch of cosmological reionization and make predictions for the amount of GeV gamma-ray attenuation by electron-positron pair production. Our results are based on recent semi-analytic models of galaxy formation, which provide predictions of the dust-extinguished UV radiation field due to starlight, and empirical estimates of the contribution due to quasars. We account for the reprocessing of ionizing photons by the intergalactic medium. We test whether our models can reproduce estimates of the ionizing background at high redshift from flux decrement analysis and proximity effect measurements from quasar spectra, and identify a range of models that can satisfy these constraints. Pair-production against soft diffuse photons leads to a spectral cutoff feature for gamma rays observed between 10 and 100 GeV. This cutoff varies with redshift and the assumed star formation and quasar evolution models. We find only negligible amounts of absorption for gamma rays observed below 10 GeV for any emission redshift. With observations of high-redshift sources in sufficient numbers by the Fermi Gamma-ray Space Telescope and new ground-based instruments it should be possible to constrain the extragalactic background light in the UV and optical portion of the spectrum.Comment: 19 pages, 12 figures, Accepted for publication in MNRAS, this version includes minor correction

The Metal-rich Globular Cluster NGC6553: Observations with WFPC2, STIS, and NICMOS

We present a HST study of the metal-rich globular cluster NGC6553 using WFPC2, NICMOS and STIS. Our primary motivation is to calibrate the STIS broad-band LP magnitude against $V_{555}$ and $I_{814}$ magnitudes for stars of known metallicity and absolute (visual) magnitude, for application to our study of LMC globular clusters. NGC6553 has been shown in earlier studies to have a very unusual colour-magnitude diagram, so we also use our data to investigate the reddening, distance, luminosity function and structure of this cluster. We deduce a higher metallicity and smaller distance modulus than did some previous studies, but emphasise that very large patchy extinction on small angular scales prohibits accurate determination of the parameters of this cluster. The horizontal branch of NGC6553 in ($V,V-I$) is tilted at an angle close to that of the reddening vector. We show that extinction does not, however, explain the tilt, which is presumably a metallicity effect. The colour-magnitude diagram shows an apparent second turnoff some 1.5 magnitudes fainter than that of the cluster. We show that this is most likely the background Galactic bulge: however, in that case, the colour-magnitude diagram of NGC6553 is not a good match to that of the field bulge population. The cluster is probably more metal-rich than is the mean field bulge star.Comment: 29 pages (Latex), 13 figs (PS, in document), 10 figs (JPEG format, outside document, degraded from original to save download time), accepted for pub. in A

Semi-Analytic Modelling of Galaxy Formation: The Local Universe

Using semi-analytic models of galaxy formation, we investigate galaxy properties such as the Tully-Fisher relation, the B and K-band luminosity functions, cold gas contents, sizes, metallicities, and colours, and compare our results with observations of local galaxies. We investigate several different recipes for star formation and supernova feedback, including choices that are similar to the treatment in Kauffmann, White & Guiderdoni (1993) and Cole et al. (1994) as well as some new recipes. We obtain good agreement with all of the key local observations mentioned above. In particular, in our best models, we simultaneously produce good agreement with both the observed B and K-band luminosity functions and the I-band Tully-Fisher relation. Improved cooling and supernova feedback modelling, inclusion of dust extinction, and an improved Press-Schechter model all contribute to this success. We present results for several variants of the CDM family of cosmologies, and find that models with values of $\Omega_0 \simeq 0.3$--0.5 give the best agreement with observations.Comment: 26 pages, LaTeX, MNRAS format, 23 inlined postscript figures. Accepted for publication in MNRAS. Revised version contains substantial changes including improved models. High resolution figures, original version, and summary of changes may be found at http://www.fiz.huji.ac.il/~rachels/papers/sp.htm

Semi-analytic modeling of the EBL and consequences for extragalactic gamma-ray spectra

Attenuation of high-energy gamma rays by pair-production with UV, optical and IR extragalactic background light (EBL) photons provides a link between the history of galaxy formation and high-energy astrophysics. We present results from our latest semi-analytic models (SAMs), which employ the main ingredients thought to be important to galaxy formation and evolution, as well as an improved model for reprocessing of starlight by dust to mid- and far-IR wavelengths. These SAMs are based upon a Lambda-CDM hierarchical structural formation scenario, and are successful in reproducing a large variety of observational constraints such as number counts, luminosity and mass functions, and color bimodality. Our fiducial model is based upon a WMAP5 cosmology, and treats dust emission using empirical templates. This model predicts a background flux considerably lower than optical and near-IR measurements that rely on subtraction of zodiacal and galactic foregrounds, and near the lower bounds set by number counts of resolvable sources at a large number of wavelengths. We also show the results of varying cosmological parameters and dust attenuation model used in our SAM. For each EBL prediction, we show how the optical depth due to electron-positron pair-production is affected by redshift and gamma-ray energy, and the effect of gamma-ray absorption on the spectra of a variety of extragalactic sources. We conclude with a discussion of the implications of our work, comparisons to other models and key measurements of the EBL and a discussion of how the burgeoning science of gamma-ray astronomy will continue to help constrain cosmology. The low EBL flux predicted by our fiducial model suggests an optimistic future for further studies of distant gamma-ray sources.Comment: 23 pages, 11 figures, 3 tables, accepted by MNRAS; this preprint matches accepted versio

Galaxy Properties from the Ultra-violet to the Far-Infrared: Lambda-CDM models confront observations

We combine a semi-analytic model of galaxy formation with simple analytic recipes describing the absorption and re-emission of starlight by dust in the interstellar medium of galaxies. We use the resulting models to predict galaxy counts and luminosity functions from the far-ultraviolet to the sub-mm, from redshift five to the present, and compare with an extensive compilation of observations. We find that in order to reproduce the rest-UV and optical luminosity functions at high redshift, we must assume an evolving normalization in the dust-to-metal ratio, implying that galaxies of a given bolometric luminosity (or metal column density) must be less extinguished than their local counterparts. In our best-fit model, we find remarkably good agreement with observations from rest-frame 1500 Angstroms to 250 microns. At longer wavelengths, most dramatically in the sub-mm, our models underpredict the number of bright galaxies by a large factor. The models reproduce the observed total IR luminosity function fairly well. We show the results of varying several ingredients of the models, including various aspects of the dust attenuation recipe, the dust emission templates, and the cosmology. We use our models to predict the integrated Extragalactic Background Light (EBL), and compare with an observationally-motivated EBL model and with other available observational constraints.Comment: 27 pages, 17 figures, 1 table, accepted to MNRAS, this version matches accepted manuscrip