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تسجيل مستخدم جديدFirst observational support for overlapping reionized bubbles generated by a galaxy overdensity
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We present the analysis of deep HST multi-band imaging of the BDF field specifically designed to identify faint companions around two of the few Ly-alpha emitting galaxies spectroscopically confirmed at z~7 (Vanzella et al. 2011). Although separated by only 4.4 proper Mpc these galaxies cannot generate HII regions large enough to explain visibility of their Ly-alpha line, thus requiring a population of fainter ionizing sources in their vicinity. We use deep HST and VLT-Hawk-I data to select z~7 Lyman break galaxies around the emitters. We select 6 new robust z~7 LBGs at Y~26.5-27.5 whose average spectral energy distribution is consistent with the objects being at the redshift of the close-by Ly-alpha emitters. The resulting number density of z~7 LBGs in the BDF field is a factor ~3-4 higher than expected in random pointings of the same size. We compare these findings with cosmological hydrodynamic plus radiative transfer simulations of a universe with a half neutral IGM: we find that indeed Ly-alpha emitter pairs are only found in completely ionized regions characterized by significant LBG overdensities. Our findings match the theoretical prediction that the first ionization fronts are generated within significant galaxy overdensities and support a scenario where faint, normal star-forming galaxies are responsible for reionization.
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We present deep spectroscopic follow-up observations of the Bremer Deep Field (BDF) where the two $zsim$7 bright Ly$alpha$ emitters (LAE) BDF521 and BDF3299 were previously discovered by Vanzella et al. (2011) and where a factor of $sim$3-4 overdensi
ty of faint LBGs has been found by Castellano et al. (2016). We confirm a new bright Ly$alpha$ emitter, BDF2195, at the same redshift of BDF521, $z=7.008$, and at only $sim$90 kpc physical distance from it, confirming that the BDF area is likely an overdense, reionized region. A quantitative assessment of the Ly$alpha$ fraction shows that the number of detected bright emitters is much higher than the average found at z$sim$7, suggesting a high Ly$alpha$ transmission through the inter-galactic medium (IGM). However, the line visibility from fainter galaxies is at odds with this finding, since no Ly$alpha$ emission is found in any of the observed candidates with $M_{UV}>$-20.25. This discrepancy can be understood either if some mechanism prevents Ly$alpha$ emission from fainter galaxies within the ionized bubbles from reaching the observer, or if faint galaxies are located outside the reionized area and bright LAEs are solely responsible for the creation of their own HII regions. A thorough assessment of the nature of the BDF region and of its sources of re-ionizing radiation will be made possible by JWST spectroscopic capabilities.
We present an approximate, analytical calculation of the reionized spectra $C_l^{XX}$ of cosmic microwave background radiation (CMB) anisotropies and polarizations generated by relic gravitational waves (RGWs). Three simple models of reionization are
explored, whose visibility functions are fitted by gaussian type of functions as approximations. We have derived the analytical polarization $beta_l$ and temperature anisotropies $alpha_l$, both consisting of two terms proportional to RGWs at the decoupling and at the reionization as well. The explicit dependence of $beta_l$ and $alpha_l$ upon the reionization time $eta_r$, the duration $Deltaeta_r$, and the optical depth $kappa_r$ are demonstrated. Moreover, $beta_l$ and $alpha_l$ contain $kappa_r$ in different coefficients, and the polarization spectra $C_l^{EE}$ are $C_l^{BB}$ are more sensitive probes of reionization than $C_l^{TT}$. These results facilitate examination of the reionization effects, in particular, the degeneracies of $kappa_r$ with the normalization amplitude and with the initial spectral index of RGWs. It is also found that reionization also causes a $kappa_r$-dependent shift $Delta lsim 20$ of the zero multipole $l_0$ of $C_l^{TE}$, an effect that should be included in order to detect the traces of RGWs. Compared with numerical results, the analytical $C_l^{XX}$ as approximation have the limitation. For the primary peaks in the range $lsimeq (30, 600)$, the error is $le 3%$ in three models. In the range $l < 20$ for the reionization bumps, the error is $le 15%$ for $C_l^{EE}$ and $C_l^{BB}$ in the two extended reionization models, and $C_l^{TT}$ and $C_l^{TE}$ have much larger departures for $l<10$. The bumps in the sudden reionization model are too low.
We explore here an scenario for massive black hole formation driven by stellar collisions in galactic nuclei, proposing a new formation regime of global instability in nuclear stellar clusters triggered by runaway stellar collisions. Using order of m
agnitude estimations, we show that observed nuclear stellar clusters avoid the regime where stellar collisions are dynamically relevant over the whole system, while resolved detections of massive black holes are well into such collision-dominated regime. We interpret this result in terms of massive black holes and nuclear stellar clusters being different evolutionary paths of a common formation mechanism, unified under the standard terminology of being both central massive objects. We propose a formation scenario where central massive objects more massive than $rm sim 10^8 , Msun$, which also have relaxation times longer that their collision times, will be too dense (in virial equilibrium) to be globally stable against stellar collisions and most of its mass will collapse towards the formation of a massive black hole. Contrarily, this will only be the case at the core of less dense central massive objects leading to the formation of black holes with much lower black hole efficiencies $rm epsilon_{BH} = frac{M_{BH}}{M_{CMO}}$, with these efficiencies $rm epsilon_{BH}$ drastically growing for central massive objects more massive than $rm sim 10^7 , Msun$, approaching unity around $rm M_{CMO} sim 10^8 , Msun$. We show that the proposed scenario successfully explains the relative trends observed in the masses, efficiencies, and scaling relations between massive black holes and nuclear stellar clusters.
We report the discovery of a galaxy overdensity around a Compton-thick Fanaroff-Riley type II (FRII) radio galaxy at z=1.7 in the deep multiband survey around the z=6.3 QSO SDSS J1030+0524. Based on a 6hr VLT/MUSE and on a 4hr LBT/LUCI observation, w
e identify at least eight galaxy members in this structure with spectroscopic redshift z=1.687-1.699, including the FRII galaxy at z=1.699. Most of the identified overdensity members are blue, compact galaxies that are actively forming stars at rates of $sim$8-60 $M_{odot}$ yr$^{-1}$. Based on a 500ks Chandra ACIS-I observation we found that the FRII nucleus hosts a luminous QSO ($L_{2-10keV}=1.3times10^{44}$ erg s$^{-1}$, intrinsic and rest-frame) that is obscured by Compton-thick absorption ($N_H=1.5pm0.6times 10^{24}$ cm$^{-2}$). Our Chandra observation, the deepest so far for a distant FRII within a galaxy overdensity, revealed significant diffuse X-ray emission within the region covered by the overdensity. In particular, X-ray emission extending for $sim$240 kpc is found around the Eastern lobe of the FRII. Four out of the six MUSE star forming galaxies in the overdensity are distributed in an arc-like shape at the edge of this diffuse X-ray emission. The probability of observing by chance four out of the six $z=1.7$ sources at the edge of the diffuse emission is negligible. In addition, these four galaxies have the highest specific star formation rates of the MUSE galaxies in the overdensity and lie above the main sequence of field galaxies of equal stellar mass at z=1.7. We propose that the diffuse X-rays originate from an expanding bubble of gas that is shock-heated by the FRII jet, and that star formation is promoted by the compression of the cold interstellar medium of the galaxies around the bubble, which may be remarkable evidence of positive AGN feedback on cosmological scales. [shortened version]
Analysis of galaxies with overlapping images offers a direct way to probe the distribution of dust extinction and its effects on the background light. We present a catalog of 1990 such galaxy pairs selected from the Sloan Digital Sky Survey (SDSS) by
volunteers of the Galaxy Zoo project. We highlight subsamples which are particularly useful for retrieving such properties of the dust distribution as UV extinction, the extent perpendicular to the disk plane, and extinction in the inner parts of disks. The sample spans wide ranges of morphology and surface brightness, opening up the possibility of using this technique to address systematic changes in dust extinction or distribution with galaxy type. This sample will form the basis for forthcoming work on the ranges of dust distributions in local disk galaxies, both for their astrophysical implications and as the low-redshift part of a study of the evolution of dust properties. Separate lists and figures show deep overlaps, where the inner regions of the foreground galaxy are backlit, and the relatively small number of previously-known overlapping pairs outside the SDSS DR7 sky coverage.
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