A population of intermediate-mass black holes in dwarf starburst galaxies up to redshift=1.5

We study a sample of $\sim$50,000 dwarf starburst and late-type galaxies drawn from the COSMOS survey with the aim of investigating the presence of nuclear accreting black holes (BHs) as those seed BHs from which supermassive BHs could grow in the early Universe. We divide the sample into five complete redshift bins up to $z=1.5$ and perform an X-ray stacking analysis using the \textit{Chandra} COSMOS-Legacy survey data. After removing the contribution from X-ray binaries and hot gas to the stacked X-ray emission, we still find an X-ray excess in the five redshift bins that can be explained by nuclear accreting BHs. This X-ray excess is more significant for $z<0.5$. At higher redshifts, these active galactic nuclei could suffer mild obscuration, as indicated by the analysis of their hardness ratios. The average nuclear X-ray luminosities in the soft band are in the range 10$^{39}-10^{40}$ erg s$^{-1}$. Assuming that the sources accrete at $\geq$ 1\% the Eddington rate, their BH masses would be $\leq$ 10$^{5}$ M$_{\odot}$, thus in the intermediate-mass BH regime, but their mass would be smaller than the one predicted by the BH-stellar mass relation. If instead the sources follow the correlation between BH mass and stellar mass, they would have sub-Eddington accreting rates of $\sim$ 10$^{-3}$ and BH masses 1-9 $\times$ 10$^{5}$ M$_{\odot}$. We thus conclude that a population of intermediate-mass BHs exists in dwarf starburst galaxies, at least up to $z$=1.5, though their detection beyond the local Universe is challenging due to their low luminosity and mild obscuration unless deep surveys are employed.Comment: 10 pages, 7 figures, ApJ in pres

The diffuse X-ray background

The deepest observations of the X-ray background approach the surface brightness of the truly diffuse component generated by Thomson scattering of cosmic X-ray photons. Available estimates of the electron density and the X-ray luminosity density of AGNs as a function of cosmological epoch are used to calculate the integral scattered X-ray background component. It is shown that the scattered component constitutes 1.0 - 1.7 % of the total background, depending on the AGN cosmic evolution. Albeit this is a minute fragment of the total flux, it becomes a perceptible fraction of the still unresolved part of the background and should be taken into account in the future rigorous assessments of the X-ray background structure. This diffuse component at energies < 1 keV sums up with the emission by WHIM to 3 - 4 %. Consequently, one should expect that integrated counts of discrete sources account for just 96 - 97 % for soft background and ~99 % at higher energies.Comment: 5 pages, 2 figures, AA in prin

Distance to G14.33-0.64 in the Sagittarius Spiral Arm: H2O Maser Trigonometric Parallax with VERA

We report on trigonometric parallax measurements for the Galactic star forming region G14.33-0.64 toward the Sagittarius spiral arm. We conducted multi-epoch phase-referencing observations of an H2O maser source in G14.33-0.64 with the Japanese VLBI array VERA. We successfully detected a parallax of 0.893+/-0.101 mas, corresponding to a source distance of 1.12+/-0.13 kpc, which is less than half of the kinematic distance for G14.33-0.64. Our new distance measurement demonstrates that the Sagittarius arm lies at a closer distance of ~1 kpc, instead of previously assumed ~2-3 kpc from kinematic distances. The previously suggested deviation of the Sagittarius arm toward the Galactic center from the symmetrically fitted model (Taylor & Cordes 1993) is likely due to large errors of kinematic distances at low galactic longitudes. G14.33-0.64 most likely traces the near side of the Sagittarius arm. We attempted fitting the pitch angle of the arm with other parallax measurements along the arm, which yielded two possible pitch angles of i=34.7+/-2.7 degrees and i=11.2+/-10.5 degrees. Our proper motion measurements suggest G14.33-0.64 has no significant peculiar motion relative to the differential rotation of the Galaxy (assumed to be in a circular orbit), indicating that the source motion is in good agreement with the Galactic rotation.Comment: 14 pages, 7 figures, to appear in PASJ Vol. 62, No.

Occupation of X-ray selected galaxy groups by X-ray AGN

We present the first direct measurement of the mean Halo Occupation Distribution (HOD) of X-ray selected AGN in the COSMOS field at z < 1, based on the association of 41 XMM and 17 C-COSMOS AGN with member galaxies of 189 X-ray detected galaxy groups from XMM and Chandra data. We model the mean AGN occupation in the halo mass range logM_200[Msun] = 13-14.5 with a rolling-off power-law with the best fit index alpha = 0.06(-0.22;0.36) and normalization parameter f_a = 0.05(0.04;0.06). We find the mean HOD of AGN among central galaxies to be modelled by a softened step function at logMh > logMmin = 12.75 (12.10,12.95) Msun while for the satellite AGN HOD we find a preference for an increasing AGN fraction with Mh suggesting that the average number of AGN in satellite galaxies grows slower (alpha_s < 0.6) than the linear proportion (alpha_s = 1) observed for the satellite HOD of samples of galaxies. We present an estimate of the projected auto correlation function (ACF) of galaxy groups over the range of r_p = 0.1-40 Mpc/h at = 0.5. We use the large-scale clustering signal to verify the agreement between the group bias estimated by using the observed galaxy groups ACF and the value derived from the group mass estimates. We perform a measurement of the projected AGN-galaxy group cross-correlation function, excluding from the analysis AGN that are within galaxy groups and we model the 2-halo term of the clustering signal with the mean AGN HOD based on our results.Comment: Accepted for publication in The Astrophysical Journa