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تسجيل مستخدم جديدCross-Correlation between Damped Ly-alpha Systems and Ly-break Galaxies in Cosmological SPH Simulations
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Tae Song Lee
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We calculate the cross-correlation function (CCF) between damped Ly-a systems (DLAs) and Lyman break galaxies (LBGs) using cosmological hydrodynamic simulations at z=3. We compute the CCF with two different methods. First, we assume that there is one DLA in each dark matter halo if its DLA cross section is non-zero. In our second approach we weight the pair-count by the DLA cross section of each halo, yielding a cross-section-weighted CCF. We also compute the angular CCF for direct comparison with observations. Finally, we calculate the auto-correlation functions of LBGs and DLAs, and their bias against the dark matter distribution. For these different approaches, we consistently find that there is good agreement between our simulations and observational measurements by Cooke et al. and Adelberger et al. Our results thus confirm that the spatial distribution of LBGs and DLAs can be well described within the framework of the concordance Lambda CDM model. We find that the correlation strengths of LBGs and DLAs are consistent with the actual observations, and in the case of LBGs it is higher than would be predicted by low-mass galaxy merger models.
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In 2000, we started the program ``Building the Bridge between Damped Ly-alpha Absorbers and Lyman-Break Galaxies: Ly-alpha Selection of Galaxies at the European Southern Observatorys Very Large Telescope. This project is an attempt to use Ly-alpha se
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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.
We study the average Ly$alpha$ emission associated with high-$z$ strong (log $N$(H I) $ge$ 21) damped Ly$alpha$ systems (DLAs). We report Ly$alpha$ luminosities ($L_{rm Lyalpha}$) for the full as well as various sub-samples based on $N$(H I), $z$, $(
r-i)$ colours of QSOs and rest equivalent width of Si II$lambda$1526 line (i.e., $W_{1526}$). For the full sample, we find $L_{rm Lyalpha}$$< 10^{41} (3sigma) rm erg s^{-1}$ with a $2.8sigma$ level detection of Ly$alpha$ emission in the red part of the DLA trough. The $L_{rm Lyalpha}$ is found to be higher for systems with higher $W_{1526}$ with its peak, detected at $geq 3sigma$, redshifted by about 300-400 $rm km s^{-1}$ with respect to the systemic absorption redshift, as seen in Lyman Break Galaxies (LBGs) and Ly$alpha$ emitters. A clear signature of a double-hump Ly$alpha$ profile is seen when we consider $W_{1526} ge 0.4$ AA and $(r-i) < 0.05$. Based on the known correlation between metallicity and $W_{1526}$, we interpret our results in terms of star formation rate (SFR) being higher in high metallicity (mass) galaxies with high velocity fields that facilitates easy Ly$alpha$ escape. The measured Ly$alpha$ surface brightness requires local ionizing radiation that is 4 to 10 times stronger than the metagalactic UV background at these redshifts. The relationship between the SFR and surface mass density of atomic gas seen in DLAs is similar to that of local dwarf and metal poor galaxies. We show that the low luminosity galaxies will contribute appreciably to the stacked spectrum if the size-luminosity relation seen for H I at low-$z$ is also present at high-$z$. Alternatively, large Ly$alpha$ halos seen around LBGs could also explain our measurements.
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21-cm emission line maps are available as part of an extensive survey with the Westerbork telescope (WHISP). We find that expected luminosities, impact parameters between quasars and DLA host galaxies, and metal abundances are in good agreement with the observed properties of DLAs and DLA galaxies. The measured redshift number density of z=0 gas above the DLA limit is dN/dz=0.045 +/- 0.006, which compared to higher redshift measurements implies that there is no evolution in the comoving density of DLAs along a line of sight between z=1.5 and z=0, and a decrease of only a factor of two from z=4 to the present time. We conclude that the local galaxy population can explain all properties of low redshift DLAs.
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