The KX method for producing K-band flux-limited samples of quasars

The longstanding question of the extent to which the quasar population is affected by dust extinction, within host galaxies or galaxies along the line of sight, remains open. More generally, the spectral energy distributions of quasars vary significantly and flux-limited samples defined at different wavelengths include different quasars. Surveys employing flux measurements at widely separated wavelengths are necessary to characterise fully the spectral properties of the quasar population. The availability of panoramic near-infrared detectors on large telescopes provides the opportunity to undertake surveys capable of establishing the importance of extinction by dust on the observed population of quasars. We introduce an efficient method for selecting K-band, flux-limited samples of quasars, termed ``KX'' by analogy with the UVX method. This method exploits the difference between the power-law nature of quasar spectra and the convex spectra of stars: quasars are relatively brighter than stars at both short wavelengths (the UVX method) and long wavelengths (the KX method). We consider the feasibility of undertaking a large-area KX survey for damped Ly-alpha galaxies and gravitational lenses using the planned UKIRT wide-field near-infrared camera.Comment: 6 pages, 3 figures, to appear in MNRA

A joint model for the emission and absorption properties of damped Lyman alpha absorption systems

The recently discovered population of ultra-faint extended line emitters can account for the majority of the incidence rate of Damped Lyman Alpha systems (DLAs) at z ~ 3 if the line emission is interpreted as Ly alpha. We show here that a model similar to that proposed by Haehnelt, Steinmetz, & Rauch (2000), which explains the incidence rate and kinematics of DLAs in the context of $\Lambda$CDM models for structure formation, also reproduces the size distribution of the new population of faint Ly alpha emitters for plausible parameters. This lends further support to identification of the emitters with the hitherto elusive population of DLA host galaxies. The observed incidence rate of DLAs together with the observed space density and size distribution of the emitters suggest a duty cycle of ~ 0.2 - 0.4 for the Ly alpha emission from DLA host galaxies. We further show that Ly alpha cooling is expected to contribute little to the Ly alpha emission for the majority of emitters. This leaves centrally concentrated star formation at a rate of a few tenths M_sun/yr, surrounded by extended Ly alpha halos with radii up to 30-50 kpc, as the most plausible explanation for the origin of the emission. Both the luminosity function of Ly alpha emission and the velocity width distribution of low ionization absorption require that galaxies inside Dark Matter (DM) halos with virial velocities < 50 - 70 km/s contribute little to the incidence rate of DLAs at z ~ 3, suggesting that energy and momentum input due to star formation efficiently removes gas from these halos. Galaxies with DM halos with virial velocities of 100 - 150 km/s appear to account for the majority of DLA host galaxies. DLA host galaxies at z ~ 3 should thus become the building blocks of typical present-day galaxies.Comment: 9 Pages, 4 Figures. Submitted to MNRA

Detection of Lyman-alpha emission from a DLA galaxy: Possible implications for a luminosity-metallicity relation at z = 2-3

In an ongoing programme to identify a sample of high z DLA galaxies we have found the long sought for case of a Ly-alpha emitter seen in the centre of a broad DLA trough. This is the predicted `textbook case' of an intervening DLA galaxy if DLA galaxies are small, but would not be expected if intervening high redshift DLA galaxies have large gaseous disks. The Ly-alpha flux is 5.4{+2}{-0.8} * 1e-17 ergs/s/cm2 similar to what has been found in previously known high z DLA galaxies. The impact parameter is found to be 0.3+-0.3 arcsec. This is smaller than what was found in previous cases but still consistent with random sight-lines through absorbers with mean impact parameter ~1 arcsec. Of the 24 DLAs targeted in the NICMOS imaging survey five have now been identified as Ly-alpha emitters. The DLA galaxies with detected Ly-alpha emission tend to have higher interstellar metallicities than those with undetected Ly-alpha emission. This is plausibly explained as a consequence of a positive correlation between the Ly-alpha line luminosities of the galaxies and their metallicities, although the present sample is too small for a definitive conclusion. The available observations of high-redshift DLA galaxies are also consistent with a negative correlation between Ly-alpha equivalent widths and metallicities, as seen in nearby star-forming galaxies and usually attributed to the preferential absorption of Ly-alpha photons by dust grains.Comment: Accepted for publication in A&A-Letter

Star formation and chemical evolution of damped Lyman α\alpha systems

In this paper, we investigate the star formation and chemical evolution of damped Lyman-$\alpha$ systems (DLAs) based on the disc galaxy formation model which is developed by Mo, Mao & White. We propose that the DLAs are the central galaxies of less massive dark haloes present at redshifts $z\sim 3$, and they should inhabit haloes of moderately low circular velocity. The empirical Schmidt law of star formation rates, and closed box model of chemical evolution that an approximation known as instantaneous recycling is assumed, are adopted. In our models, when the predicted distribution of metallicity for DLAs is calculated, two cases are considered. One is that, using the closed box model, empirical Schmidt law and star formation time, the distribution of metallicity can be directly calculated. The other is that, when the simple gravitational instability of a thin isothermal gas disc as first discussed by Toomre is considered, the star formation occurs only in the region where the surface density of gas satisfies the critical value, not everywhere of a gas disc. We assume that star formation in each DLA lasts for a period of 1 Gyr from redshifts $z=3$. There is only one output parameter in our models, i.e. the stellar yield, which relates to the time of star formation history and is obtained by normalizing the predicted distribution of metallicity to the mean value of 1/13 $Z_{\odot}$ as presented by Pettini et al. The predicted metallicity distribution is consistent with the current (rather limited) observational data. A random distribution of galactic discs is taken into account.Comment: 7 pages, 6 figures, will appear in MNRAS, 2001 (21 April), 322, 927-93

Uncovering strong MgII absorbing galaxies: Imaging below the Lyman limit

The nature of the galaxies that give rise to absorption lines, such as damped Lyman-alpha systems (DLAs) or strong MgII lines, in quasar spectra is difficult to investigate in emission. Taking advantage of the total absorption of the QSO light bluewards of the Lyman limit of two DLAs at z>3.4, we look for the continuum emission from intervening galaxies at z~2 that are identified via strong metal absorption lines. The MgII absorbers have equivalent width large enough to be potential DLA systems. Deep images are obtained with the FOcal Reducer and Spectrograph (FORS1) on the Very Large Telescope for the fields towards SDSS J110855+120953 and SDSS J140850+020522. These quasars have MgII absorption lines at z=1.87 (W_r(MgII)=2.46 A) and z=1.98 (W_r(MgII)=1.89 A), respectively, and each QSO has two intervening higher redshift DLAs at z>3. The U and R bands of FORS1 lie blue and redwards of the Lyman limit of the background DLAs, allowing us to search for emission from the foreground galaxies directly along the lines of sight to the QSOs. No galaxies are found close to the sight line of the QSO to a point source limit of U_AB=28.0. In both fields, the closest objects lie at an impact parameter of 5 arcsec corresponding to 40 kpc in projection at z=2, and have typical colours of star forming galaxies at that redshift. However, the currently available data do not allow us to confirm if the galaxies lie at the same redshifts as the absorption systems. A more extended structure is visible in the SDSS J14085+020522 field at an impact parameter of 0.8 arcsec or 7 kpc. If these objects are at z~2 their luminosities are 0.03-0.04 L* in both fields. The star formation rates estimated from the UV flux are 0.5-0.6 M_sun yr^-1. (Abbreviated).Comment: 10 pages, Accepted for publication in A&

New search strategy for high z intervening absorbers: GRB021004, a pilot study

We present near-infrared narrow- and broad-band imaging of the field of GRB021004, performed with ISAAC on the UT1 of the ESO Very Large Telescope. The narrow-band filters were chosen to match prominent emission lines at the redshift of the absorption-line systems found against the early-time afterglow of GRB021004: [OIII] at z=1.38 and Halpha at z=1.60, respectively. For the z=1.38 system we find an emission-line source at an impact parameter of 16", which is somewhat larger than the typical impact parameters of a sample of MgII absorbers at redshifts around unity. Assuming that this tentative redshift-identification is correct, the star formation rate of the galaxy is 13 +- 2 Msun/year. Our study reaches star-formation rate limits (5 sigma) of 5.7 Msun/year at z=1.38, and 7.7 Msun/year at z=1.60. These limits correspond to a depth of roughly 0.13 L*. Any galaxy counterpart of the absorbers nearer to the line of sight either has to be fainter than this limit or not be an emission-line source.Comment: 4 pages, 3 figures, accepted for publication in A&A letter

The Cryogenic Refractive Indices of S-FTM16, a Unique Optical Glass for Near-Infrared Instruments

The Ohara glass S-FTM16 is of considerable interest for near-infrared optical designs because it transmits well through the K band and because negative S-FTM16 elements can be used to accurately achromatize positive calcium fluoride elements in refractive collimators and cameras. Glass manufacturers have sophisticated equipment to measure the refractive index at room temperature, but cannot typically measure the refractive index at cryogenic temperatures. Near-infrared optics, however, are operated at cryogenic temperatures to reduce thermal background. Thus we need to know the temperature dependence of S-FTM16's refractive index. We report here our measurements of the thermal dependence of S-FTM16's refractive index between room temperature and ~77 K. Within our measurement errors we find no evidence for a wavelength dependence or a nonlinear temperature term so our series of measurements can be reduced to a single number. We find that Delta n_{abs} / Delta T = -2.4x10^{-6} K^{-1} between 298 K and ~77 K and in the wavelength range 0.6 micron to 2.6 micron. We estimate that the systematic error (which dominates the measurement error) in our measurement is 10%, sufficiently low for most purposes. We also find the integrated linear thermal expansion of S-FTM16 between 298 K and 77 K is -0.00167 m m^{-1}.Comment: 8 pages, including 9 figures. Uses emulateapj.cls. Accepted for publication in PAS

Extended Ly-alpha emission from a damped Ly-alpha absorber at z = 1.93, and the relation between DLAs and Lyman-break galaxies

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.Comment: 10 pages, 6 figures. MNRAS, in pres

Directly imaging damped Lyman-alpha galaxies at z>2. I: Methodology and First Results

We present the methodology for, and the first results from, a new imaging program aimed at identifying and characterizing the host galaxies of damped Lyman-alpha absorbers (DLAs) at z>2. We target quasar sightlines with multiple optically-thick HI absorbers and use the higher-redshift system as a "blocking filter" (via its Lyman-limit absorption) to eliminate all far-ultraviolet (FUV) emission from the quasar. This allows us to directly image the rest-frame FUV continuum emission of the lower-redshift DLA, without any quasar contamination and with no bias towards large impact parameters. We introduce a formalism based on galaxy number counts and Bayesian statistics with which we quantify the probability that a candidate is the DLA host galaxy. This method will allow the identification of a bona fide sample of DLAs that are too faint to be spectroscopically confirmed. The same formalism can be adopted to the study of other quasar absorption line systems (e.g. MgII absorbers). We have applied this imaging technique to two QSO sightlines. For the z~2.69 DLA towards J073149+285449, a galaxy with impact parameter b=1.54"=11.89 kpc and implied star formation rate (SFR) of ~5 M/yr is identified as the most reliable candidate. In the case of the z~2.92 DLA towards J211444-005533, no likely host is found down to a 3-sigma SFR limit of 1.4 M/yr. Studying the HI column density as a function of the impact parameter, including 6 DLAs with known hosts from the literature, we find evidence that the observed HI distribution is more extended than what is generally predicted from numerical simulation.Comment: 22 pages, 13 figures. Accepted for publication in MNRAS. Typos correcte

A first estimate of radio halo statistics from large-scale cosmological simulation

We present a first estimate based on a cosmological gasdynamics simulation of galaxy cluster radio halo counts to be expected in forthcoming low-frequency radio surveys. Our estimate is based on a FLASH simulation of the LCDM model for which we have assigned radio power to clusters via a model that relates radio emissivity to cluster magnetic field strength, intracluster turbulence, and density. We vary several free parameters of this model and find that radio halo number counts vary by up to a factor of two for average magnetic fields ranging from 0.2 to 3.1 uG. However, we predict significantly fewer low-frequency radio halos than expected from previous semi-analytic estimates, although this discrepancy could be explained by frequency-dependent radio halo probabilities as predicted in reacceleration models. We find that upcoming surveys will have difficulty in distinguishing models because of large uncertainties and low number counts. Additionally, according to our modeling we find that expected number counts can be degenerate with both reacceleration and hadronic secondary models of cosmic ray generation. We find that relations between radio power and mass and X-ray luminosity may be used to distinguish models, and by building mock radio sky maps we demonstrate that surveys such as LOFAR may have sufficient resolution and sensitivity to break this model degeneracy by imaging many individual clusters.Comment: 18 pages, 14 figures, revised from referee comments, ApJ accepted, public catalog available at http://sipapu.astro.illinois.edu/http://sipapu.astro.illinois.edu/foswiki/bin/view/Main/RadioHaloMap