arXiv subjects

Kari Helgason

Publications and source records attributed to Kari Helgason.

7 recordsLinked to original sources

arXiv2019

A new measurement of the Hubble constant and matter content of the Universe using extragalactic background light $γ$-ray attenuation

The Hubble constant $H_{0}$ and matter density $Ω_{m}$ of the Universe are measured using the latest $γ$-ray attenuation results from Fermi-LAT and Cherenkov telescopes. This methodology is based upon the fact that the extragalactic background light supplies opacity for very high energy photons via photon-photon interaction. The amount of $γ$-ray attenuation along the line of sight depends on the expansion rate and matter content of the Universe. This novel strategy results in a value of $H_{0}=67.4_{-6.2}^{+6.0}$~km~s$^{-1}$~Mpc$^{-1}$ and $Ω_{m}=0.14_{-0.07}^{+0.06}$. These estimates are independent and complementary to those based on the distance ladder, cosmic microwave background (CMB), clustering with weak lensing, and strong lensing data. We also produce a joint likelihood analysis of our results from $γ$ rays and these from more mature methodologies, excluding the CMB, yielding a combined value of $H_{0}=66.6\pm 1.6$~km~s$^{-1}$~Mpc$^{-1}$ and $Ω_{m}=0.29\pm 0.02$.

astro-ph.CO↗

arXiv2018

The SPLASH and Chandra COSMOS Legacy Survey: the Cross Power Between Near-Infrared and X-Ray Background Fluctuations

We study the source-subtracted near-infrared and X-ray background fluctuations of the COSMOS field using data from the Spitzer SPLASH program ($\sim$1272 hours) and Chandra COSMOS Legacy Survey (4.6 Ms). The new auto power spectra of the cosmic infrared and X-ray background fluctuations reach maximum angular scales of $\sim$ 3000$''$ and $\sim$ 5000$''$, respectively. We measure the cross power spectra between each infrared and X-ray band and calculate the mean power above 20$''$. We find that the soft X-ray band is correlated with 3.6 and 4.5$μ$m at $\sim$ 4 $σ$ significance level. The significance between hard X-ray and the 3.6$μ$m (4.5$μ$m) band is $\sim$2.2 $σ$ ($\sim$3.8$σ$). The combined infrared (3.6 + 4.5$μ$m) data are correlated with the X-ray data in soft ([0.5-2] keV), hard ([2-7] keV) and broad ([0.5-7] keV) bands at $\sim$5.6$σ$, $\sim$4.4$σ$ and $\sim$6.6$σ$ level, respectively. We compare the new measurements with existing models for the contributions from known populations at $z$$<$7, which are not subtracted. The model predictions are consistent with the measurements but we cannot rule out contributions from other components, such as Direct Collapse Black Holes (DCBH). However, the stacked cross-power spectra, combining other available data, show excess fluctuations about an order of magnitude on average at $\sim$4$σ$ confidence at scales within $\sim$300$''$. By studying the X-ray SED of the cross-power signal, assuming no significant variation from the infrared, we find that its shape is consistent with DCBHs.

astro-ph.GA↗

arXiv2017

Probing the EBL evolution at high redshift using GRBs detected with the Fermi-LAT

The extragalactic background light (EBL), from ultraviolet to infrared wavelengths, is predominantly due to emission from stars, accreting black holes and reprocessed light due to Galactic dust. The EBL can be studied through the imprint it leaves, via $γ$-$γ$ absorption of high-energy photons, in the spectra of distant $γ$-ray sources. The EBL has been probed through the search for the attenuation it produces in the spectra of BL Lacertae (BL Lac) objects and individual $γ$-ray bursts (GRBs). GRBs have significant advantages over blazars for the study of the EBL especially at high redshifts. Here we analyze a combined sample of twenty-two GRBs, detected by the Fermi Large Area Telescope between 65 MeV and 500 GeV. We report a marginal detection (at the ~2.8 $σ$ level) of the EBL attenuation in the stacked spectra of the source sample. This measurement represents a first constraint of the EBL at an effective redshift of ~1.8. We combine our results with prior EBL constraints and conclude that Fermi-LAT is instrumental to constrain the UV component of the EBL. We discuss the implications on existing empirical models of EBL evolution.

astro-ph.HE↗

arXiv2014

The Contribution of z<6 Sources to the Spatial Coherence in the Unresolved Cosmic near-Infrared and X-ray Backgrounds

A spatial clustering signal has been established in Spitzer/IRAC measurements of the unresolved Cosmic near-Infrared Background (CIB) out to large angular scales, ~1 deg. This CIB signal, while significantly exceeding the contribution from the remaining known galaxies, was further found to be coherent at a highly statistically significant level with the unresolved soft Cosmic X-ray Background (CXB). This measurement probes the unresolved CXB to very faint source levels using deep near-IR source subtraction. We study contributions from extragalactic populations at low to intermediate redshifts to the measured positive cross-power signal of the CIB fluctuations with the CXB. We model the X-ray emission from AGN, normal galaxies and hot gas residing in virialized structures, calculating their CXB contribution including their spatial coherence with all infrared emitting counterparts. We use a halo model framework to calculate the auto and cross-power spectra of the unresolved fluctuations based on the latest constraints of the halo occupation distribution and the biasing of AGN, galaxies and diffuse emission. At small angular scales (<1'), the 4.5mic vs 0.5-2 keV coherence can be explained by shot noise from galaxies and AGN. However, at large angular scales (~10 arcmin) we find that the net contribution from the modeled populations is only able to account for ~3% of the measured CIBxCXB cross-power. The discrepancy suggests that the CIBxCXB signal originates from the same unknown source population producing the CIB clustering signal out to ~1 deg.

astro-ph.CO↗

arXiv2012

Reconstructing the γ-ray Photon Optical Depth of the Universe to z~4 from Multiwavelength Galaxy Survey Data

We reconstruct γ-ray opacity of the Universe out to z<3-4 using an extensive library of 342 observed galaxy luminosity function surveys extending to high redshifts. We cover the whole range from UV to mid-IR (0.15-25mic) providing for the first time a robust empirical calculation of the γγ-optical depth out to several TeV. Here, we use the same database as Helgason et al. 2012 where the EBL was reconstructed from luminosity functions out to 4.5mic and was shown to recover observed galaxy counts to high accuracy. We extend our earlier library of LFs to 25mic such that it covers the energy range of pair production with γ-rays 1) in the entire Fermi/LAT energy range, and 2) at higher TeV energies probed by ground-based Cherenkov telescopes. In the absence of significant contributions to the cosmic diffuse background from unknown populations, such as the putative Population III era sources, the Universe appears to be largely transparent to γ-rays at all Fermi/LAT energies out to z~2 whereas becoming opaque to TeV photons already at z<0.2 and reaching τ~10 at z=1. Comparing with the currently available Fermi/LAT GRB and blazar data shows that there is room for significant emissions originating in the first stars era.

astro-ph.CO↗

arXiv2012

Can the Near-IR Fluctuations Arise from Known Galaxy Populations?

Spatial Fluctuations in the Cosmic Infrared Background have now been measured out to sub-degree scales showing a strong clustering signal from unresolved sources. We attempt to explain these measurement by considering faint galaxy populations at z<6 as the underlying sources for this signal using 233 measured UV, optical and NIR luminosity functions (LF) from a variety of surveys covering a wide range of redshifts. We populate the lightcone and calculate the total emission redshifted into the near-IR bands in the observer frame and recover the observed optical and near-IR galaxy counts to a good accuracy. Using a halo model for the clustering of galaxies with an underlying LCDM density field, we find that fluctuations from known galaxy populations are unable to account for the large scale CIB clustering signal seen by HST/NICMOS, Spitzer/IRAC and AKARI/IRC and continue to diverge out to larger angular scales. Our purely empirical reconstruction shows that known galaxy populations are not responsible for the bulk of the fluctuation signal seen in the measurements and suggests an unknown population of very faint and highly clustered sources dominating the signal.

astro-ph.CO↗

arXiv2012

Reconstructing the Near-IR Background Fluctuations from known Galaxy Populations using Multiband Measurements of Luminosity Functions

We model fluctuations in the Cosmic Infrared Background (CIB) arising from known galaxy populations using 233 measured UV, optical and NIR luminosity functions (LF) from a variety of surveys spanning a wide range of redshifts. We compare best-fit Schechter parameters across the literature and find clear indication of evolution with redshift. Providing fitting formulae for the multi-band evolution of the LFs out to z~5, we calculate the total emission redshifted into the near-IR bands in the observer frame and recover the observed optical and near-IR galaxy counts to a good accuracy. Our empirical approach, in conjunction with a halo model describing the clustering of galaxies, allows us to compute the fluctuations of the unresolved CIB and compare the models to current measurements. We find that fluctuations from known galaxy populations are unable to account for the large scale CIB clustering signal seen by Spitzer/IRAC and AKARI/IRC and continue to diverge out to larger angular scales. This holds true even if the LFs are extrapolated out to faint magnitudes with a steep faint-end slope all the way to z=8. We also show that removing resolved sources to progressively fainter magnitude limits, isolates CIB fluctuations to increasingly higher redshifts. Our empirical approach suggests that known galaxy populations are not responsible for the bulk of the fluctuation signal seen in the measurements and favors a very faint population of highly clustered sources.

astro-ph.CO↗