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تسجيل مستخدم جديدThe sources of extended continuum emission towards Q0151+048A : The host galaxy and the Damped Ly-alpha Absorber
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We present deep imaging in the U, B and I bands obtained under excellent seeing conditions of the double quasar Q0151+048A,B and of the Damped Ly-alpha (DLA) absorbing galaxy at z(abs) = 1.9342 named S4. We analyse the data employing two separate and independent methods. First we deconvolve the images using the MCS algorithm, secondly we decompose the images via an object based iteration process where we fit models to objects without any attempt to improve the resolution of the data. Our detailed analysis of the images reveals, somewhat surprisingly, that extended objects centred on the quasars themselves are much brighter continuum sources than the DLA galaxy. Due to the complexity caused by the many superimposed objects, we are unable to certify whether or not continuum emission from the DLA galaxy is detected. Continuum emission from the extended objects centred on the positions of the quasars is clearly seen, and the objects are tentatively identified as the ``host galaxies of the quasars. The flux of those host galaxies is of order 2--6% of the quasar flux, and the light profile of the brighter of the two is clearly best fit with a de Vaucouleurs profile. We discuss two alternative interpretations of the origin of the extended flux: i) the early stage of a massive elliptical galaxy in the process of forming the bulk of its stars, and ii) quasar light scattered by dust.
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The number of damped Ly-alpha absorbers (DLAs) currently known is about 100, but our knowledge of their sizes and morphologies is still very sparse as very few have been detected in emission. Here we present narrow-band and broad-band observations of
a DLA in the field of the quasar pair Q0151+048A (qA) and Q0151+048B (qB). These two quasars have very similar redshifts z_em = 1.922, 1.937, respectively, and an angular separation of 3.27 arcsec. The spectrum of qA contains a DLA at z_abs = 1.9342 (close to the emission redshift) which shows an emission line in the trough, detected at 4 sigma. Our narrow-band image confirms this detection and we find Ly-alpha emission from an extended area covering 6x3 arcsec^2, corresponding to 25x12h^-2 kpc^2 (q0=0.5, H0 = 100h km s^-1). The total Ly-alpha luminosity from the DLA is 1.2 x 10^43 h^-2 erg s^-1, which is a factor of several higher than the Ly-alpha luminosity found from other DLAs. The narrow-band image also indicates that qB is not covered by the DLA. This fact, together with the large equivalent width of the emission line from the Ly-alpha cloud, the large luminosity, and the 300 km s^-1 blueshift relative to the DLA, can plausibly be explained if qB is the sourceof a Lyman-limit system. We also consider the relation between DLAs and Lyman-break galaxies (LBGs). If DLAs are gaseous disks surrounding LBGs, and if the apparent brightnesses and impact parameters of the few identified DLAs are representative of the brighter members of the population, then the luminosity distribution of DLAs is nearly flat, and we would expect that some 70% of the galaxy counterparts to DLAs at z=3 are fainter than m_R=28.
Observations of damped Ly-alpha absorbers (DLA) indicate that the fraction of hydrogen in its neutral form (HI) is significant by mass at all redshifts. This gas represents the reservoir of material that is available for star formation at late times.
As a result, observational identification of the systems in which this neutral hydrogen resides is an important missing ingredient in models of galaxy formation. Precise identification of DLA host mass via traditional clustering studies is not practical owing to the small numbers of known systems being spread across sparsely distributed sight lines. However following the completion of reionization, 21cm surface brightness fluctuations will be dominated by neutral hydrogen in DLAs. Observations of these fluctuations will measure the combined clustering signal from all DLAs within a large volume. We show that measurement of the spherically averaged power-spectrum of 21cm intensity fluctuations due to DLAs could be used to measure the galaxy bias for DLA host galaxies when combined with an independent measurement of the cosmological HI mass density from quasar absorption studies. Utilising this technique, the low frequency arrays now under construction could measure the characteristic DLA host mass with a statistical precision as low as 0.3 dex at z~4. In addition, high signal-to-noise observations of the peculiar-motion induced anisotropy of the power-spectrum would facilitate measurement of both the DLA host mass and the cosmic HI density directly from 21cm fluctuations. By exploiting this anisotropy, a second generation of low frequency arrays with an order of magnitude increase in collecting area could measure the values of cosmic HI density and DLA host mass, with uncertainties of a few percent and a few tens of percent respectively.
We present spectroscopic observations of six high redshift ($z_{rm em}$ $>$ 2) quasars, which have been selected for their Lyman $alpha$ (Ly$alpha$) emission region being only partially covered by a strong proximate ($z_{rm abs}$ $sim$ $z_{rm em}$) c
oronagraphic damped Ly$alpha$ system (DLA). We detected spatially extended Ly$alpha$ emission envelopes surrounding these six quasars, with projected spatial extent in the range 26 $le$ $d_{rm Lyalpha}$ $le$ 51 kpc. No correlation is found between the quasar ionizing luminosity and the Ly$alpha$ luminosity of their extended envelopes. This could be related to the limited covering factor of the extended gas and/or due to the AGN being obscured in other directions than towards the observer. Indeed, we find a strong correlation between the luminosity of the envelope and its spatial extent, which suggests that the envelopes are probably ionized by the AGN. The metallicity of the coronagraphic DLAs is low and varies in the range $-$1.75 $<$ [Si/H] $<$ $-$0.63. Highly ionized gas is observed to be associated with most of these DLAs, probably indicating ionization by the central AGN. One of these DLAs has the highest AlIII/SiII ratio ever reported for any intervening and/or proximate DLA. Most of these DLAs are redshifted with respect to the quasar, implying that they might represent infalling gas probably accreted onto the quasar host galaxies through filaments.
We used HST/STIS to obtain the spectrum of molecular hydrogen associated with the damped Ly$alpha$ system at $z_{rm abs}=1.7765$ toward the quasar Q1331+170 at $z_{rm em}=2.084$. Strong ${rm H}_2$ absorption was detected, with a total ${rm H}_2$ colu
mn density of $N({rm H}_2)=(4.45pm 0.36)times 10^{19} {rm cm^{-2}}$.The molecular hydrogen fraction is $f_{{rm H}_2}=frac{2N_{rm H_2}}{N_{rm HI}+2N_{rm H_2}}=(5.6pm 0.7)%$, which is the greatest value reported so far in any redshifted damped Ly$alpha$ system. This results from the combined effect of a relatively high dust-to-gas ratio, a low gas temperature, and an extremely low ambient UV radiation field. Based on the observed population of $J$ states, we estimate the photo-absorption rate to be $R_{rm abs}=(7.6pm 2.4)times 10^{-13} {rm s^{-1}}$, corresponding to a local UV radiation field of $J(1000{rm AA})approx 2.1times 10^{-3} J_{1000{rm AA},odot}$, where $J_{1000{rm AA},odot}$ is the UV intensity at $1000 AA$ in the solar neighborhood. This is comparable with the metagalactic UV background intensity at this redshift, and implies an extremely low star formation rate in the absorbers environment. The observed CO-to-H$_2$ column density ratio is $frac{N_{rm CO}}{N_{rm H_2}}<2.5times 10^{-7}$, which is similar to the value measured for diffuse molecular clouds in the Galactic ISM. Finally, applying the inferred physical conditions to the observed C I fine structure excitation (Songaila {it et al.} 1994), we estimate the cosmic microwave background temperature to be $T_{rm CMB}=(7.2pm 0.8) {rm K}$ at $z=1.77654$, consistent with the predicted value of $7.566 {rm K}$ from the standard cosmology.
[Abridged] We test the reliability of a method to measure the mean halo mass of Damped Ly-alpha absorbers (DLAs). The method is based on measuring the ratio of the cross-correlation between DLAs and galaxies to the auto-correlation of the galaxies th
emselves ($w_{rm dg}/w_{rm gg}$), which is (in linear theory) the ratio of their bias factor. This is shown to be true irrespective of the galaxy redshift distribution, provided that one uses the same galaxies for the two correlation functions. The method is applicable to all redshifts. Here, we focus on z=3 DLAs and we demonstrate that the method robustly constrains the mean DLA halo mass using smoothed particle hydrodynamics (SPH) cosmological simulations. If we use the bias formalism of Mo & White with the DLA and galaxy mass distributions of these simulations, we predict a bias ratio of 0.771. Direct measurement from the simulations of $w_{rm dg}/w_{rm gg}$ st yields a ratio of 0.73+/-0.08, in excellent agreement with that prediction. Equivalently, inverting the measured correlation ratio to infer a mean DLA halo mass yields (log. averaging, in solar units) <log(M_DLA)> =11.13+/-013, in excellent agreement with the true value in the simulations: 11.16. The cross- correlation method thus appears to yield a robust estimate of the average host halo mass even though the DLAs and the galaxies occupy a broad mass spectrum of halos, and massive halos contain multiple galaxies with DLAs. We show that the inferred mean DLA halo mass is independent of the galaxy sub-sample used, i.e. the cross-correlation technique is also reliable. Our results imply that the cross-correlation length between DLAs and LBGs is predicted to be, at most, 2.85 Mpc. Future observations will soon distinguish models in which DLAs are in low mass halos from those in which DLAs are in massive halos.
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