Spectra Of Extragalactic Sources Research Articles

Thermal Pressures in the Interstellar Medium away from Stellar Environments*

Abstract Interstellar thermal pressures can be measured using C i absorption lines that probe the pressure-sensitive populations of the fine-structure levels of its ground state. In a survey of C i absorption toward Galactic hot stars, Jenkins & Tripp found evidence of small amounts (∼0.05%) of gas at high pressures ( p / k ≫ 10 4 cm − 3 K ) mixed with a more general presence of lower pressure material exhibiting a log-normal distribution that spanned the range 10 3 ≲ p / k ≲ 10 4 cm − 3 K . In this paper, we study Milky Way C i lines in the spectra of extragalactic sources instead of Galactic stars and thus measure the pressures without being influenced by regions where stellar mass loss and H ii region expansions could create localized pressure elevations. We find that the distribution of low pressures in the current sample favors slightly higher pressures than the earlier survey, and the fraction of gaseous material at extremely high pressures is about the same as that found earlier. Thus we conclude that the earlier survey was not appreciably influenced by the stellar environments, and the small amounts of high pressure gas indeed exist within the general interstellar medium.

Cascades from primary gamma rays and nuclei as a source of background in searches for oscillations between photons and axion-like particles

Methods of searching for oscillations of photons to axion-like particles (γ → ALP) in spectra of extragalactic sources are considered. It is shown that the secondary component from the cascades initiated by primary gamma rays or nuclei can produce an appreciable background in searches for ALPs in the optically thick region of the spectrum. Experiments with a high energy threshold (~1 TeV and higher) are insufficient for searching for γ → ALP oscillations; low-energy instruments (E < 100 GeV) like CTA should also be used.

Identifying the spectra of ultrahigh energy cosmic rays from extragalactic sources

The energy spectra of extragalactic sources of cosmic rays are calculated by solving an inverse problem of the transport of cosmic rays with energies of 1018–1021 eV in a Universe filled with background electromagnetic radiation. Calculations are performed using cosmic-ray spectra measured on Earth in Auger experiments. It is assumed that protons and iron nuclei dominate in the composition of a source.

A cosmological distance scale program

A program for identifying the cosmological distance scale for different models of the red shift in the spectra of extragalactic sources from the observed red shifts and stellar magnitudes, combined with the database on supernovae, radio galaxies and quasars, is developed. The Friedmann–Mattig and generalized interpolation models in combination with models of the intrinsic red shift of the sources are taken as the main parameters.

TESTING RELATIVITY AT HIGH ENERGIES USING SPACEBORNE DETECTORS

The Gamma-Ray Large Area Space Telescope (GLAST), to be launched in the fall of 2007, will measure the spectra of distant extragalactic sources of high energy γ-rays, particularly active galactic nuclei and γ-ray bursts. GLAST can look for energy-dependent γ-ray propagation effects from such sources as a signal of Lorentz invariance violation (LIV). These sources should also exhibit the high energy cutoffs predicted to be the result of intergalactic annihilation interactions with low energy photons having a flux level as determined by various astronomical observations. Such annihilations result in electron–positron pair production above a threshold energy given by 2me in the center-of-momentum frame of the system, assuming Lorentz invariance. If Lorentz invariance is violated, this threshold can be significantly raised, changing the predicted absorption turnover in the observed spectrum of the sources. Stecker and Glashow have shown that the existence of such absorption features in the spectra of extragalactic sources puts constraints on LIV. Such constraints have important implications for some quantum gravity and large extra dimension models. Future spaceborne detectors dedicated to measuring γ-ray polarization can look for birefringence effects as a possible signal of loop quantum gravity. As shown by Coleman and Glashow, a much smaller amount of LIV has potential implications for possibly suppressing the "GZK cutoff" predicted to be caused by the interactions of cosmic rays having multijoule energies with photons of the 2.7 K cosmic background radiation in intergalactic space. Owing to the rarity of such ultrahigh energy cosmic rays, their spectra are best studied by a UV-sensitive satellite detector which looks down on a large volume of the Earth's atmosphere to study the nitrogen fluorescence tracks of giant air showers produced by these ultrahigh energy cosmic rays. We discuss here, in particular, a two-satellite mission called OWL, which would be suited for making such studies.

The AGN-Starburst Connection in the Mid-Infrared

We present a new method based on mid-IR (5 — 16 µm) emission for classifying the origin of the mid-IR activity in galaxies. Our method was devefoped using a set of 33 ISOCAM spectra of extragalactic sources which exhibit diverse mid-IR spectral features. Detailed analysis of our sample enables us to identify the general properties of mid-IR emission from active galactic nuclei (AGNs), starbursts, and quiescent star forming regions. The benefits of this mid-IR classification for identifying obscured or faint AGNs and for estimating the star formation intensity in star-forming regions are investigated.