Morphological Classification of galaxies by Artificial Neural Networks

We explore a method for automatic morphological classification of galaxies by an Artificial Neural Network algorithm. The method is illustrated using 13 galaxy parameters measured by machine (ESO-LV), and classified into five types (E, S0, Sa + Sb, Sc + Sd and Irr). A simple Backpropagation algorithm allows us to train a network on a subset of the catalogue according to human classification, and then to predict, using the measured parameters, the classification for the rest of the catalogue. We show that the neural network behaves in our problem as a Bayesian classifier, i.e. it assigns the a posteriori probability for each of the five classes considered. The network highest probability choice agrees with the catalogue classification for 64 percent of the galaxies. If either the first or the second highest probability choice of the network is considered, the success rate is 90 per cent. The technique allows uniform and more objective classification of very large extragalactic data sets

APM z>4 QSO Survey: Distribution and Evolution of High Column Density HI Absorbers

Eleven candidate damped Lya absorption systems were identified in 27 spectra of the quasars from the APM z>4 survey covering the redshift range 2.83.5). High resolution echelle spectra (0.8A FWHM) have been obtained for three quasars, including 2 of the highest redshift objects in the survey. Two damped systems have confirmed HI column densities of N(HI) >= 10^20.3 atoms cm^-2, with a third falling just below this threshold. We have discovered the highest redshift damped Lya absorber known at z=4.383 in QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z>3. We combine this high redshift sample with other quasar samples covering the redshift range 0.008 < z < 4.7 to study the redshift evolution and the column density distribution function for absorbers with log N(HI)>=17.2. In the HI column density distribution f(N)=kN^-beta we find evidence for breaks in the power law, flattening for 17.221.2. The column density distribution function for the data with log N(HI)>=20.3 is better fit with the form f(N)=(f*/N*)(N/N*)^-beta exp(-N/N*). Significant redshift evolution in the number density per unit redshift is evident in the higher column density systems with an apparent decline in N(z) for z>3.5.Comment: To appear in MNRAS. Latex file (10 pages of text) plus 14 separate postscript figure files. Requires mn.sty. Postscript version with figures embedded is available at http://www.ociw.edu/~lisa/publications.htm

Evolution of Neutral Gas at High Redshift -- Implications for the Epoch of Galaxy Formation

Though observationally rare, damped Lya absorption systems dominate the mass density of neutral gas in the Universe. Eleven high redshift damped Lya systems covering 2.84 QSO Survey, extending these absorption system surveys to the highest redshifts currently possible. Combining our new data set with previous surveys we find that the cosmological mass density in neutral gas, omega_g, does not rise as steeply prior to z~2 as indicated by previous studies. There is evidence in the observed omega_g for a flattening at z~2 and a possible turnover at z~3. When combined with the decline at z>3.5 in number density per unit redshift of damped systems with column densities log N(HI)>21 atoms cm^-2, these results point to an epoch at z>3 prior to which the highest column density damped systems are still forming. We find that over the redshift range 2<z<4 the total mass in neutral gas is marginally comparable with the total visible mass in stars in present day galaxies. However, if one considers the total mass visible in stellar disks alone, ie excluding galactic bulges, the two values are comparable. We are observing a mass of neutral gas comparable to the mass of visible disk stars. Lanzetta, Wolfe & Turnshek (1995) found that omega_g(z~3.5) was twice omega_g(z~2), implying a much larger amount of star formation must have taken place between z=3.5 and z=2 than is indicated by metallicity studies. This created a `cosmic G-dwarf problem'. The more gradual evolution of omega_g we find alleviates this. These results have profound implications for theories of galaxy formation.Comment: To appear in MNRAS. Latex file (4 pages of text) plus 3 separate postscript figure files. Requires mn.sty. Postscript version with figures embedded is available at http://www.ociw.edu/~lisa/publications.htm

Automated classification of stellar spectra - I. Initial results with artificial neural networks

We have initiated a project to classify stellar spectra automatically from high-dispersion objective prism plates. The automated technique presented here is a simple backpropagation neural network, and is based on the visual classification work of Houk. The plate material (Houk's) is currently being digitized, and contains ≈ 105 stars down to V ≈ 11 at ≈ 2-Å resolution from ≈ 3850 to 5150 Å. For this first paper in the series we report on the results of 575 stars digitized from 6 plates. We find that even with the limited data set now in hand we can determine the temperature classification to better than 1.7 spectral subtypes from B3 to M4. Our current sample size provides insufficient training set material to generate luminosity and metallicity classifications. Our eventual aims in this project are (1) to create a large and homogeneous digital stellar spectral library; (2) to create a well-understood and robust automatic classification algorithm which can determine temperatures, luminosities and metallicities for a wide variety of spectral types; (3) to use these data, supplemented by deeper plate material, for the study of Galactic structure and chemical evolution; and (4) to find unusual or new classes of objects

The evolution of Omega(HI) and the epoch of formation of damped Lyman-alpha absorbers

We present a study of the evolution of the column density distribution, f(N,z), and total neutral hydrogen mass in high-column density quasar absorbers using candidates from a recent high-redshift survey for damped Lyman-alpha (DLA) and Lyman limit system (LLS) absorbers. The observed number of LLS (N(HI)> 1.6 * 10^{17} atom/cm^2) is used to constrain f(N,z) below the classical DLA Wolfe et al. (1986) definition of 2 * 10^{20} atom/cm^2. The joint LLS-DLA analysis shows unambiguously that f(N,z) deviates significantly from a single power law and that a Gamma-law distribution of the form f(N,z)=(f_*/N_*)(N/N_*)^{-Beta} exp(-N/N_*) provides a better description of the observations. These results are used to determine the amount of neutral gas contained in DLAs and in systems with lower column density. Whilst in the redshift range 2 to 3.5, ~90% of the neutral HI mass is in DLAs, we find that at z>3.5 this fraction drops to only 55% and that the remaining 'missing' mass fraction of the neutral gas lies in sub-DLAs with N(HI) 10^{19} - 2 * 10^{20} atom/cm^2. The characteristic column density, N_*, changes from 1.6 * 10^{21} atom/cm^2 at z3.5, supporting a picture where at z>3.5, we are directly observing the formation of high column density neutral hydrogen DLA systems from lower column density units. Moreover since current metallicity studies of DLA systems focus on the higher column density systems they may be giving a biased or incomplete view of global galactic chemical evolution at z>3. After correcting the observed mass in HI for the ``missing'' neutral gas the comoving mass density now shows no evidence for a decrease above z=2. (abridged)Comment: Replaced to match version published in MNRAS. One figure and appendix added, analysis and conclusions unchange

Soft X-ray Absorption by High-Redshift Intergalactic Helium

The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has been measured with HST in four quasars over the last few years, over the redshift range 2.4 < z < 3.2. These observations have indicated that the HeII reionization may not have been completed until z\simeq 2.8, and that large fluctuations in the intensity of the HeII-ionizing background were present before this epoch. The detailed history of HeII reionization at higher redshifts is, however, model-dependent and difficult to determine from these observations, because the IGM can be completely optically thick to Lya photons when only a small fraction of the helium remains as HeII. In addition, finding quasars in which the HeII Lya absorption can be observed becomes increasingly difficult at higher redshift, owing to the large abundance of hydrogen Lyman limit systems. It is pointed out here that HeII in the IGM should also cause detectable continuum absorption in the soft X-rays. The spectrum of a high-redshift source seen behind the IGM when most of the helium was HeII should recover from the HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic absorption will generally be stronger, but not by a large factor; the intergalactic HeII absorption can be detected as an excess over the expected Galactic absorption from the 21cm HI column density. In principle, this method allows a direct determination of the fraction of helium that was singly ionized as a function of redshift, if the measurement is done on a large sample of high-redshift sources over a range of redshift.Comment: accepted to The Astrophysical Journal Letter

Starburst Galaxies in Cluster-feeding Filaments Unveiled by Spitzer

We report the first direct detection with Spitzer of galaxy filaments. Using Spitzer and ancillary optical data, we have discovered two filamentary structures in the outskirts of the cluster Abell 1763. Both filaments point toward Abell 1770, which lies at the same redshift as Abell 1763 (z = 0.23), at a projected distance of ~13 Mpc. The X-ray cluster emission is elongated along the same direction. Most of the far-infrared emission is powered by star formation. According to the optical spectra, only one of the cluster members is classified as an active galactic nucleus. Star formation is clearly enhanced in galaxies along the filaments: the fraction of starburst galaxies in the filaments is more than twice than that in other cluster regions. We speculate that these filaments are feeding the cluster Abell 1763 by the infall of galaxies and galaxy groups. Evidence for one of these groups is provided by the analysis of galaxy kinematics in the central cluster region

NICMOS Snapshot Survey of Damped Lyman Alpha Quasars

We image 19 quasars with 22 damped Lyman alpha (DLA) systems using the F160W filter and the Near-Infrared Camera and Multiobject Spectrograph aboard the Hubble Space Telescope, in both direct and coronagraphic modes. We reach 5 sigma detection limits of ~H=22 in the majority of our images. We compare our observations to the observed Lyman-break population of high-redshift galaxies, as well as Bruzual & Charlot evolutionary models of present-day galaxies redshifted to the distances of the absorption systems. We predict H magnitudes for our DLAs, assuming they are producing stars like an L* Lyman-break galaxy (LBG) at their redshift. Comparing these predictions to our sensitivity, we find that we should be able to detect a galaxy around 0.5-1.0 L* (LBG) for most of our observations. We find only one new possible candidate, that near LBQS0010-0012. This scarcity of candidates leads us to the conclusion that most DLA systems are not drawn from a normal LBG luminosity function nor a local galaxy luminosity function placed at these high redshifts.Comment: 31 pages, 8 figures, Accepted for Feb. 10 issue of Ap

APM z >> 4 QSO survey: spectra and intervening absorption systems

The APM multicolor survey for bright z > 4 objects, covering 2500 deg^2 of sky to m(R)~19, resulted in the discovery of thirty-one quasars with z > 4. High signal-to-noise optical spectrophotometry at 5A resolution has been obtained for the twenty-eight quasars easily accessible from the northern hemisphere. These spectra have been surveyed to create new samples of high redshift Lyman-limit systems, damped Lyman-alpha absorbers, and metal absorption systems (e.g. CIV and MgII). In this paper we present the spectra, together with line lists of the detected absorption systems. The QSOs display a wide variety of emission and absorption line characteristics, with 5 exhibiting broad absorption lines and one with extremely strong emission lines (BR2248-1242). Eleven candidate damped Ly-alpha absorption systems have been identified covering the redshift range 2.83.5). An analysis of the measured redshifts of the high ionization emission lines with the low ionization lines shows them to be blueshifted by 430+/-60 km/s. In a previous paper (Storrie-Lombardi et. al. 1994) we discussed the redshift evolution of the Lyman limit systems catalogued here. In subsequent papers we will discuss the properties of the Ly-alpha forest absorbers and the redshift and column density evolution of the damped Ly-alpha absorbers