The contribution of the Unresolved Extragalactic Radio Sources to the diffuse brightness of the sky was evaluated using the source number - flux measurements available in literature. We first optimized the fitting function of the data based on number counts distribution. We then computed the brightness temperature at various frequencies from 151 MHz to 8440 MHz and derived its spectral dependence. As expected the frequency dependence can be described by a power law with a spectral index $γ\simeq -2.7$, in agreement with the flux emitted by the {\it steep spectrum} sources. The contribution of {\it flat spectrum} sources becomes relevant at frequencies above several GHz. Using the data available in literature we improved our knowledge of the brightness of the unresolved extragalactic radio sources. The results obtained have general validity and they can be used to disentangle the various contributions of the sky brightness and to evaluate the CMB temperature.
Angular fluctuations of stochastic gravitational wave backgrounds (GWB) produced by extragalactic astrophysical sources are calculated. The angular properties of such backgrounds are determined by the large scale structure of Universe (galaxy clustering). The evolution of star formation rate with redshift is taken into account. Fluctuations of the metric strain amplitude associated with such noises at angular scales of about one degree are found to be of order 5-20% slowly growing toward smaller angular scales. This feature can be potentially used to separate astrophysical GWB from cosmologica…
We present a model for estimating the blazar contribution to the extragalactic gamma-ray background (EGRB) in the EGRET energy range. It is based on inverse-Compton scattering as the dominant gamma-ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. Depending on the evolutionary behaviour of radio-loud AGN, our estimate for the point source contribution to the extragalactic background emission ranges from about ~20%-40% for a redshift drop off at Z_{max}=3 to around ~35%-85% for a cutoff at Z_{max}=5. Ro…
We have used VSOP space very long baseline interferometry observations to measure the brightness temperature distribution of a well-defined sub-set of the Pearson-Readhead sample of extragalactic radio sources. VLBI which is restricted to Earth-diameter baselines is not generally sensitive to emitting regions with brightness temperatures greater than approximately $10^{12}$ K, coincidentally close to theoretical estimates of brightness temperature limits, $10^{11} - 10^{12}$ K. We find that a significant proportion of our sample have brightness temperatures greater than $10^{12}$ K; many have …
The large majority of EGRET point sources remain without an identified low-energy counterpart, and a large fraction of these sources are most likely extragalactic. Whatever the nature of the extragalactic EGRET unidentified sources, faint unresolved objects of the same class must have a contribution to the diffuse extragalactic gamma-ray background (EGRB). Understanding this component of the EGRB, along with other guaranteed contributions from known sources, is essential if we are to use this emission to constrain exotic high-energy physics. Here, we follow an empirical approach to estimate wh…
We calculate stochastic gravitational wave background produced by extragalactic merging binary white dwarfs at the LISA frequencies $10^{-3}-10^{-2}$ Hz with account of a strong evolution of global star formation rate in the Universe recently established observationally. We show that for the observed global star formation history and modern cosmological models the extragalactic background is an order of magnitude smaller than the mean Galactic value. An early star formation burst at high redshifts can bring it at a higher level but still a few times lower than the mean Galactic one.
The large majority of EGRET point sources remain to this day without an identified low-energy counterpart. Whatever the nature of the EGRET unidentified sources, faint unresolved objects of the same class must have a contribution to the diffuse gamma-ray background: if most unidentified objects are extragalactic, faint unresolved sources of the same class contribute to the background, as a distinct extragalactic population; on the other hand, if most unidentified sources are Galactic, their counterparts in external galaxies will contribute to the unresolved emission from these systems. Underst…
We show that GLAST should be able to support or rule out the unresolved-blazar hypothesis for the origin of the diffuse, extragalactic gamma-ray background.
Although about 40% of the soft X-ray background emission in 0.4 to 1 keV range has extragalactic origins and thus is totally blocked by the Galactic absorption in midplane directions, it decreases at most by about 20 % in midplane. Suzaku observation of the direction, (l, b) = (235, 0), showed an OVII Kalpha emission intensity comparable with that of the MBM-12 on cloud Suzaku observation, but revealed a narrow bump peaked at ~ 0.9 keV. The latter component is partly filling the decrease of the extragalactic component in midplane. The feature can be well represented by a thin thermal emission …
Several models which have been constructed to explain the faint galaxy excess in observed number counts are used to predict the intensity of the extragalactic background light (EBL). Special attention is given to irregular and dwarf galaxies, which seem to be more common in the universe than once thought, and to low surface brightness galaxies (LSB), which can in principle be altogether missed from galaxy counts. The nature of the latter objects is still unclear, but some plausible models predict that LSB galaxies can increase the intensity of the EBL by a factor of up to 5 from a standard, no…
I review the constraints imposed by the observed extragalactic background light (EBL) on the history of the stellar birthrate in galaxies. At faint magnitudes, the logarithmic slope of the galaxy counts is flatter than 0.4 in all seven UBVIJHK optical bandpasses of the Hubble Deep Field-South imaging survey. The integration of the number counts provides a lower limit to the surface brightness of the optical extragalactic sky of 15 nW/m2/sr, comparable to the intensity of the far-IR background from COBE data. If the initial mass function has a Salpeter slope with a lower mass cutoff consistent …
We describe the properties of the blazars detected by EGRET and summarize the results on the calculations of the evolution and luminosity function of these sources. Of the large number of possible origins of extragalactic diffuse gamma-ray emission, it has been postulated that active galaxies might be one of the most likely candidates. However, some of our recent analyses indicate that only 25 percent of the diffuse extragalactic emission measured by SAS-2 and EGRET can be attributed to unresolved gamma-ray blazars. Therefore, other sources of diffuse extragalactic gamma-ray emission must exis…
We have used the Very Large Array to search for compact milliarcsecond-size radio sources near methanol masers in high-mass star-forming regions. Such sources are required for Very Long Baseline Interferometry phase-referencing observations. We conducted pointed observations of 234 compact sources found in the NVSS survey and find 92 sources with unresolved components and synchrotron spectral indexes. These sources are likely the cores of AGNs and, thus, good candidates for astrometric calibrators.
To empirically calibrate the IR surface brightness fluctuation (SBF) distance scale and probe the properties of unresolved stellar populations, we measured fluctuations in 65 galaxies using NICMOS on the Hubble Space Telescope. The early-type galaxies in this sample include elliptical and S0 galaxies and spiral bulges in a variety of environments. Absolute fluctuation magnitudes in the F160W (1.6 micron) filter were derived for each galaxy using previously-measured I-band SBF and Cepheid variable star distances. F160W SBFs can be used to measure distances to early-type galaxies with a relative…
The practice of measuring galaxy distances from their spatial fluctuations in surface brightness is now a decade old. While several past articles have included some review material, this is the first intended as a comprehensive review of the surface brightness fluctuation (SBF) method. The method is conceptually quite simple, the basic idea being that nearby (but unresolved) star clusters and galaxies appear "bumpy", while more distant ones appear smooth. This is quantified via a measurement of the amplitude of the Poisson fluctuations in the number of unresolved stars encompassed by a CCD pix…
We present an empirical assessment of the use of broadband optical colours as age indicators for unresolved extragalactic clusters and investigate stochastic sampling effects on integrated colours. We use the integrated properties of Galactic open clusters as models for unresolved extragalactic clusters. The population synthesis code Starburst99 (Leitherer et al. 1999) and four optical colours were used to estimate how well we can recover the ages of 62 well-studied Galactic open clusters with published ages. We provide a method for estimating the ages of unresolved clusters and for reliably d…
We detect a dip of 20-45% in the surface brightness and number counts of NVSS sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of approximately 1-10 degrees. Together with independent all-sky wavelet analyses, our results suggest that the dip in extragalactic brightness and number counts and the WMAP cold spot are physically related, i.e., that the coincidence is neither a statistical anomaly nor a WMAP foreground correction problem. If the cold spot does originate from structures at modest redshifts, as we suggest, then there is no remai…
We review the observations of rapid flux density variations in compact radio sources, and discuss the inverse Compton limit to the maximum brightness temperature of incoherent synchrotron sources in comparison with recent VLBA observations. The apparent agreement of the theoretical brightness temperature limit due to inverse Compton cooling and the brightness temperatures observed by early VLBI observations appears to have been fortuitous. VLBA observations have greatly improved the quality of the data, but many of the early issues remain unresolved.
From 1979--1996 the Green Bank Interferometer was used by the Naval Research Laboratory to monitor the flux density from 146 compact radio sources at frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate intrinsic variations on times of a year or more from more rapid interstellar scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2 GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS variation is much stronger at 2 than at 8 GHz. We characterize the ISS variation by an rms amplitude and a timescale and examine the statistics of th…
(Abridged) Statistical analysis of the unresolved light in the Hubble Deep Field (HDF) strongly constrains possible sources of the optical Extragalactic Background Light (EBL). To test for the statistical signature of previously undetected sources, we estimate the auto, cross, and color correlations of the ``sky'' in the HDF that remains after masking objects brighter than I=30 mag. This measurement yields the most stringent limits to date on small-scale structure in the night sky; analysis of shallower imaging would be dominated by galaxies now detected by the HDF. Unless there is a truly uni…
"Diffuse" gamma rays consist of several components: truly diffuse emission from the interstellar medium, the extragalactic background, whose origin is not firmly established yet, and the contribution from unresolved and faint Galactic point sources. One approach to unravel these components is to study the diffuse emission from the interstellar medium, which traces the interactions of high energy particles with interstellar gas and radiation fields. Because of its origin such emission is potentially able to reveal much about the sources and propagation of cosmic rays. The extragalactic backgrou…
A significant fraction of non-baryonic or baryonic dark matter in galactic halos may consist of MASsive Compact Objects (MASCOs) with mass M=10^{1-4}M_{sun}. Possible candidates for such compact objects include primordial black holes or remnants of primordial (Population III) stars. We propose a method for directly detecting MASCOs in extragalactic halos, using the VLBI techniques with extremely high resolution. If a galactic halo comprising a large number of MASCOs produces multiple images of a background radio-loud QSO by gravitational lensing, then a high-resolution radio map of each macro-…
We present the results of an Australia Telescope Compact Array (ATCA) survey for intra-day variability (IDV) of the total and polarized flux densities of 118 compact, flat spectrum, extragalactic radio sources from the Parkes 2.7 GHz Survey. A total of 22 total flux density IDV sources were discovered and 15 sources were found to show IDV of their polarized flux density. We discuss the statistical properties of the IDV sources, including the distribution of source modulation indices, and the dependence of the variability amplitude on source spectral index and on Galactic position. We suggest i…
We decompose the COBE/DIRBE observations of the near-IR sky brightness (minus zodiacal light) into Galactic stellar and interstellar medium (ISM) components and an extragalactic background. This empirical procedure allows us to estimate the 4.9 micron cosmic infrared background (CIB) as a function of the CIB intensity at shorter wavelengths. A weak indication of a rising CIB intensity at wavelengths > 3.5 micron hints at interesting astrophysics in the CIB spectrum, or warns that the foreground zodiacal emission may be incompletely subtracted. Subtraction of only the stellar component from the…
We report on an ongoing effort to image active galactic nuclei simultaneously observed at 2.3 and 8.6 GHz in the framework of a long-term VLBI project RDV (Research and Development - VLBA) started in 1994 aiming to observe compact extragalactic radio sources in the astrometric/geodetic mode. Observations of bright extragalactic sources are carried out bi-monthly making up to six sessions per year with participation of all ten VLBA antennas and up to nine additional (geodetic and EVN) radio telescopes. Analysis of single-epoch results for 370 quasars, BL Lacs and radio galaxies is presented. We…
We have conducted low-frequency radio observations with the Giant Metrewave Radio Telescope (GMRT) of 40 new hard X-ray sources discovered by the INTEGRAL satellite. This survey was conducted in order, to study radio emissions from these sources, to provide precise position and to identify new microquasar candidates. From our observations we find that 24 of the X-ray sources have radio candidates within the INTEGRAL error circle. Based on the radio morphology, variability and information available from different wavelengths, we categorize them as seventeen Galactic sources (4 unresolved, 7 ext…
Although kilometer-scale neutrino detectors such as IceCube are discovery instruments, their conceptual design is very much anchored to the observational fact that Nature produces protons and photons with energies in excess of 10^20 eV and 10^13 eV, respectively. The puzzle of where and how Nature accelerates the highest energy cosmic particles is unresolved almost a century after their discovery. From energetics considerations we anticipate order 10~100 neutrino events per kilometer squared per year pointing back at the source(s) of both galactic and extragalactic cosmic rays. In this context…
Clusters of galaxies produce negative features at wavelengths $λ> 1.25$ mm in CMB maps, by means of the thermal SZ effect, while point radio sources produce positive peaks. This fact implies that a distribution of unresolved SZ clusters could be detected using the negative asymmetry introduced in the odd-moments of the brightness map (skewness and higher), or in the probability distribution function (PDF) for the fluctuations, once the map has been filtered in order to remove the contribution from primordial CMB fluctuations from large scales. This property provides a consistency check to the …
Very high energy gamma-rays from blazars traversing cosmological distances through the metagalactic radiation field can convert into electron-positron pairs in photon-photon collisions. The converted gamma-rays initiate electromagnetic cascades driven by inverse-Compton scattering off the microwave background photons. Using a model for the time-dependent metagalactic radiation field consistent with all currently available far-infrared-to-optical data, we calculate the cascade contribution from faint, unresolved high- and low-peaked blazars to the extragalactic gamma-ray background as measured …
Dusty, starforming galaxies and active galactic nuclei that contribute to the integrated background intensity at far-infrared wavelengths trace the large-scale structure. Below the point source detection limit, correlations in the large-scale structure lead to clustered anisotropies in the unresolved component of the far-infrared background (FIRB). The angular power spectrum of the FIRB anisotropies could be measured in large-area surveys with the Spectral and Photometric Imaging Receiver (SPIRE) on the upcoming Herschel observatory. To study statistical properties of these anisotropies, the c…
Very high energy gamma-rays(E>20GeV) from blazars traversing cosmological distances through the metagalactic radiation field can convert to electron-positron pairs in photon-photon collisions. The converted gamma-rays initiate electromagnetic cascades driven by inverse-Compton scattering off the microwave background photons. The cascades shift the injected gamma ray spectrum to MeV-GeV energies. Randomly oriented magnetic fields rapidly isotropize the secondary electron-positron beams resulting from the beamed blazar gamma ray emission, leading o faint gamma-ray halo. Using a model for the tim…
Sub-arcsecond observations using the Very Large Array (VLA) are presented for low-luminosity water maser in M82, M51, and NGC4051. New maser features have been detected within the M82 starburst complex. They are largely associated with star-forming activity, such as optically identified starburst-driven winds, H II regions, or the early phase of star formation in the galaxy. The water maser in M51 consists of blueshifted and redshifted features relative to thesystemic velocity of the galaxy. The redshifted features are measured to the northwest of the nuclear radio source, while the location o…
The spectral shape of the unresolved emission from different classes of gamma-ray emitters can be used to disentangle the contributions from these populations to the extragalactic gamma-ray background (EGRB). We present a calculation of the unabsorbed spectral shape of the unresolved blazar contribution to the EGRB starting from the spectral index distribution (SID) of resolved EGRET blazars derived through a maximum-likelihood analysis accounting for measurement errors. In addition, we explicitly calculate the uncertainty in this theoretically predicted spectral shape, which enters through th…
Context: The Deep Extragalactic VLBI-Optical Survey (DEVOS) aims at constructing a large sample of compact radio sources up to two orders of magnitude fainter than those studied in other Very Long Baseline Interferometry (VLBI) surveys. Optical identification of the objects is ensured by selecting them from the Sloan Digital Sky Survey (SDSS) list. Aims: While continuing to build up the DEVOS data base, we investigated how the VLBI detection rate could be enhanced by refining the initial selection criteria introduced in the first paper of this series. Methods: We observed 26 sources in two adj…
We report on the results of a two epoch study of the low power Compact Symmetric Object 4C 31.04. Observations performed with the VLBA at 5 GHz in 1995 and 2000 have yielded images of this source at milliarcsecond angular resolution. A central core is detected, with bright compact hot spots and extended lobes on both sides. Model-fitting and other analysis of the data (brightness profile, difference map) clearly indicate that the source is expanding. We estimate the velocity of this expansion to be (0.085 +/- 0.016) mas/yr, i.e. (0.33 +/- 0.06) c in both hot spots. Assuming a constant expansio…
VLBA observations at 18 cm are presented for 28 Compact Steep-spectrum radio Sources (CSSs) from the B3-VLA CSS sample. These sources were unresolved in previous VLA observations at high frequencies or their brightness distribution was dominated by an unresolved steep spectrum component. More than half of them also showed a low frequency turnover in their radio spectrum. The VLBA images display in most cases a compact symmetric structure. Only in a minority of cases complex morphologies are present.
If X-rays observed from any extragalactic radio jets are due to inverse Compton scattering on the cosmic microwave background (CMB) radiation, then such a source will be detectable with the same surface brightness anywhere in the more distant universe. Chandra observations imply that such systems do exist, and will therefore serve as Cosmic Beacons out to the redshift at which they form.
We have used VLBA fringe visibility data obtained at 15 GHz to examine the compact structure in 250 extragalactic radio sources. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which …
The Arecibo Legacy Fast ALFA (ALFALFA) survey is a program aimed at obtaining a census of HI-bearing objects over a cosmologically significant volume of the local universe. When complete in ~3-4 years, it will cover 7000 square degrees of high latitude sky using the 305m telescope and the seven-beam Arecibo L-band feed array (ALFA). As of May 1, 2008, almost 60% of the required observations are complete and a catalog exists in preliminary form for 25% of the final sky area. ALFALFA is detecting about twice as many HI sources as predicted based on previously published HI mass functions and shou…
