Diego G. Lambas

THE ARECIBO LEGACY FAST ALFA SURVEY. I. SCIENCE GOALS, SURVEY DESIGN, AND STRATEGY

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map ~7000 deg2 of the high Galactic latitude sky visible from Arecibo, providing a H I line spectral database covering the redshift range between -1600 and 18,000 km s-1 with ~5 km s-1 resolution. Exploiting Arecibos large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H I mass function in the local universe and will provide a census of H I in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H I diameter function, and enable a first wide-area blind search for local H I tidal features, H I absorbers at z < 0.06, and OH megamasers in the redshift range 0.16 < z < 0.25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H I line sources out to z ~ 0.06, including several hundred with H I masses M < 107.5 M⊙.

Galaxy pairs in the 2dF survey – I. Effects of interactions on star formation in the field

We study galaxy pairs (GPs) in the field selected from the 100-K public release of the Two Degree Field (2dF) galaxy redshift survey. Our analysis provides a well-defined sample of 1258 GPs, a large data base suitable forstatistical studies of galaxy interactions in the local Universe, z ≤ 0.1. GPs were selected by radial velocity (AV) and projected separation (rp) criteria determined by analysing the star-formation activity within neighbours. We have excluded pairs in high-density regions by removing galaxies in groups and clusters. We analyse the star-formation activity in the pairs as a function of both relative projected distance and relative radial velocity. We found power-law relations for the mean star-formation birth parameter and equivalent widths of the galaxies in pairs as a function of rp and Δ V. We find that star formation in GPs is significantly enhanced over that of isolated galaxies with similar redshifts in the field for r p < 25 h - 1 kpc and AV < 100 km s - 1 . We detected that, when compared to isolated galaxies of similar luminosity and redshift distribution, the effects of having a companion are more significant on the star-formation activity of bright galaxies in pairs, unless the pairs are formed by similar luminosity galaxies. In this case, the star formation is enhanced in both components. The ratio between the fractions of star-forming galaxies in pairs and in isolation is a useful tool to unveil the effects of having a close companion. We found that about 50 per cent of GPs do not show signs of important star-formation activity (independently of their luminosities), supporting the hypothesis that the internal properties of the galaxies play a crucial role in the triggering of star formation by interactions.

Anisotropy in the distribution of satellites around primary galaxies in the 2dF Galaxy Redshift Survey: the Holmberg effect

We have analysed a sample of satellite and primary galaxies in the 2dF Galaxy Redshift Survey. In our study we find strong statistical evidence of the Holmberg effect (that is, a tendency for satellites to avoid regions along the line defined by the primary plane) within 500 kpc of the projected distance to the primary. This effect is present only when we restrict our analysis to objects with radial velocity relative to the primary |Δν| < 160 km s -1 , which corresponds approximately to the mean of the distribution. We explore the dependence of this anisotropy on spectral type (η), colours and luminosities of both primaries and satellites, finding that objects with a low present-day star formation rate present the most significant effect.

Active galactic nuclei and galaxy interactions

We perform a statistical analysis of AGN host characteristics and nuclear activity for AGNs in pairs and without companions. Our study concerns a sample of AGNs derived from the SDSS-DR4 data by Kauffmann et al (2003) and pair galaxies obtained from the same data set by Alonso et al. (2006). An eye-ball classification of images of 1607 close pairs (rp < 25 kpc h , ∆V < 350 km s) according to the evidence of interaction through distorted morphologies and tidal features provides us with a more confident assessment of galaxy interactions from this sample. We notice that, at a given luminosity or stellar mass content, the fraction of AGNs is larger for pair galaxies exhibiting evidence for strong interaction and tidal features which also show sings of strong star formation activity. Nevertheless, this process accounts only for a ∼ 10% increase of the fraction of AGNs. As in previous works, we find AGN hosts to be redder and with a larger concentration morphological index than non-AGN galaxies. This effect does not depend whether AGN hosts are in pairs or in isolation. The OIII luminosity of AGNs with strong interaction features is found to be significantly larger than that of other AGNs, either in pairs or in isolation. Estimations of the accretion rate, L[OIII]/MBH , show that AGNs in merging pairs are actively feeding their black holes, regardless of their stellar masses. We also find that the luminosity of the companion galaxy seems to be a key parameter in the determination of the black hole activity. At a given host luminosity, both the OIII luminosity and the L[OIII]/MBH are significantly larger in AGNs with a bright companion (Mr < −20) than otherwise.

Chemical evolution using smooth particle hydrodynamical cosmological simulations – I. Implementation, tests and first results

We develop a model to implement metal enrichment in a cosmological context based on the hydrodynamical AP3MSPH code described by Tissera, Lambas and Abadi (1997). The star formation model is based on the Schmidt law and has been modified in order to describe the transformation of gas into stars in more detail. The enrichment of the interstellar medium due to supernovae I and II explosions is taken into account by assuming a Salpeter Initial Mass Function and different nucleosynthesis models. The different chemical elements are mixed within the gaseous medium according to the Smooth Particle Hydrodynamics technique. Gas particles can be enriched by different neighbouring particles at the same time. We present tests of the code that assess the effects of resolution and model parameters on the results. We show that the main effect of low numerical resolution is to produce a more effective mixing of elements, resulting in abundance relations with less dispersion. We have performed cosmological simulations in a standard Cold Dark Matter scenario and we present results of the analysis of the star formation and chemical properties of the interstellar medium and stellar population of the simulated galactic objects. We show that these systems reproduce abundance ratios for primary and secondary elements of the interstellar medium, and the correlation between the (O/H) abundance and the gas fraction of galaxies. We find that star formation efficiency, the relative rate of supernovae II to supernovae I and life-time of binary systems as well as the stellar nucleosynthesis

The Luminosity Function of Galaxies in Clusters

By means of statistical analyses, we compute the luminosity function of galaxies in clusters and explore its dependence on cluster global properties. The data consist of two different samples of southern clusters with published redshifts, taken from the Abell cluster catalog and the APM cluster catalog. Galaxy assignment to the clusters and their corresponding apparent magnitudes were derived from the Edinburgh-Durham Southern Galaxy Catalogue. We have considered two limiting absolute magnitudes, M < -16 and M < -17, comprising 16 and 55 clusters, respectively. The galaxy luminosity function in clusters derived in this work shows a large excess of faint galaxies compared with previous field determinations. In spite of their different identification procedures, we obtain similar galaxy luminosity functions in Abell and APM clusters. A Schechter function with parameters M* -20.0 ± 0.1 and α -1.4 ± 0.1 provides a satisfactory fit to the galaxy luminosity function of the sample of 55 clusters. We also find a flatter galaxy luminosity function in poor clusters (α -1.2). We have computed errors through a Monte Carlo procedure that considers different possible background corrections and errors in the photometry. Besides providing reliable values of the uncertainties in the luminosity function fitting parameters, this method poses confident limits on the effects of possible systematics. Our results strongly argue against a universal galaxy luminosity function. The high relative frequency of faint galaxies in rich clusters reported in this work may provide constraints on models for the origin and evolution of galaxies.

Galaxy pairs in the 2dF survey – II. Effects of interactions on star formation in groups and clusters

We assess the effects of galaxy-galaxy interactions on star formation (SF) by analysing pairs in groups and clusters of galaxies with virial masses in the range 10 13 -10 15 M ○. extracted from the 2dF Galaxy Redshift Survey (2dFGRS). We find that galaxy pairs with relative separation r p < 15 h - kpc show significant SF activity in comparison with other group member galaxies. Similar results were found for galaxy pairs in the field (rp < 25 h -1 kpc) so that the nature of SF driven by galaxy interactions is nearly independent of environment, in spite of the general lower level of SF activity in systems of galaxies. The above results reflect, on the one hand, the local nature of SF induced by tidal interactions and, on the other, the role played by the internal properties of galaxies. By using a 2dFGRS mock catalogue we estimate the contamination by spurious pairs, finding that our statistics are dominated by real pairs, in particular for close relative separations, for which our study indicates significant SF activity. We obtain a similar radial and relative velocity distribution of the pairs with respect to the group centre compared with those of other typical group members, so that galaxy pairs have no particular location and dynamics within groups. Except for pairs with small relative separations, which show significant SF activity, we find that pairs in groups are systematically redder and have a lower present-day SF activity than other group members.

Effects of galaxy interactions in different environments

We analyse star formation rates (SFRs) derived from photometric and spectroscopic data of galaxies in pairs in different environments using the 2-degree field galaxy redshift survey (2dFGRS) and the Sloan digital sky survey (SDSS). The two samples comprise several thousand pairs, suitable to explore into detail the dependence of star formation activity in pairs on orbital parameters and global environment. We use the projected galaxy density derived from the fifth brightest neighbour of each galaxy, with a convenient luminosity threshold to characterize environment in both surveys in a consistent way. Star formation activity is derived through the η parameter in 2dFGRS and through the SFR normalized to the total mass in stars, SFR/M*, given by Brinchmann et al. in the SDSS-second data release (SDSS-DR2). For both galaxy pair catalogs, the star formation birth rate parameter is a strong function of the global environment and orbital parameters. Our analysis on SDSS pairs confirms previous results found with the 2dFGRS where suitable thresholds for the star formation activity induced by interactions are estimated at a projected distance r p = 100 h -1 kpc and a relative velocity A V = 350 km s -1 . We observe that galaxy interactions are more effective at triggering important star formation activity in low- and moderate-density environments with respect to the control sample of galaxies without a close companion. Although close pairs have a larger fraction of actively star-forming galaxies, they also exhibit a greater fraction of red galaxies with respect to those systems without a close companion, an effect that may indicate that dust stirred up during encounters could affect colours and, partially, obscure tidally induced star formation.

The mass—metallicity relation of interacting galaxies

We study the mass-metallicity relation of galaxies in pairs and in isolation taken from the Sloan Digital Sky Survey-Data Release 4 (SDSS-DR4) using the stellar masses and oxygen abundances derived by Tremonti et al. Close galaxy pairs, defined by projected separation rp < 25 kpc h −1 and radial velocityV < 350 km s −1 , are morphologically classified ac- cording to the strength of the interaction signs. We find that only for pairs showing signs of strong interactions, the mass-metallicity relation differs significantly from that of galaxies in isolation. In such pairs, the mean gas-phase oxygen abundances of galaxies with low stellar masses (M∗ � 10 9 Mh −1 ) exhibit an excess of 0.2 dex. Conversely, at larger masses (M∗ � 10 10 Mh −1 ) galaxies have a systematically lower metallicity, although with a smaller differ- ence (−0.05 dex). Similar trends are obtained if g-band magnitudes are used instead of stellar masses. In minor interactions, we find that the less massive member is systematically enriched, while a galaxy in interaction with a comparable stellar mass companion shows a metallicity decrement with respect to galaxies in isolation. We argue that metal-rich starbursts triggered by a more massive component, and inflows of low-metallicity gas induced by comparable or less massive companion galaxies, provide a natural scenario to explain our findings.

Shapes of clusters and groups of galaxies: comparison of model predictions with observations

We study the properties of the 3-dimensional and projected shapes of haloes using high resolution numerical simulations and observational data where the latter comes from the 2PIGG (Eke et al. 2004) and SDSS-DR3GC group catalogues (Merchan & Zandivarez 2005). We investigate the dependence of halo shape on characteristics such as mass and number of members. In the 3-dimensional case, we find a significant corre- lation between the mass and halo shape; massive systems are more prolate than small haloes. We detect a source of strong systematics in estimates of the triaxiality of a halo, which is found to be a strong function of the number of members; �CDM haloes usually characterised by triaxial shapes, slightly bent toward prolate forms, appear more oblate when taking only a small subset of the halo particles. The ellipticities of observed 2PIGG and SDSS-DR3GC groups are found to be strongly dependent on the number of group members, so that poor groups appear more elon- gated than rich ones. However, this is again an artifact caused by poor statistics and not an intrinsic property of the galaxy groups, nor an effect from observational bi- ases. We interpret these results with the aid of a GALFORM mock 2PIGG catalogue. When comparing the group ellipticities in mock and real catalogues, we find an excel- lent agreement between the trends of shapes with number of group members. When carefully taking into account the effects of low number statistics, we find that more massive groups are consistent with more elongated shapes. Finally, our studies find no significant correlations between the shape of observed 2PIGG or SDSS-DR3GC groups with the properties of galaxy members such as colour or spectral type index.