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Cold gas and star formation in a merging galaxy sequence
We explore the evolution of the cold gas (molecular and neutralhydrogen) and star formation activity during galaxy interactions, usinga merging galaxy sequence comprising both pre- and post-mergercandidates. Data for this study come from the literature, but aresupplemented by some new radio observations presented here. First, weconfirm that the ratio of far-infrared luminosity to molecular hydrogenmass (LFIRM(H2); star formation efficiency)increases close to nuclear coalescence. After the merging of the twonuclei there is evidence that the star formation efficiency declinesagain to values typical of ellipticals. This trend can be attributed toM(H2) depletion arising from interaction induced starformation. However, there is significant scatter, likely to arise fromdifferences in the interaction details (e.g., disc-to-bulge ratio,geometry) of individual systems. Secondly, we find that the centralmolecular hydrogen surface density, ΣH2,increases close to the final stages of the merging of the two nuclei.Such a trend, indicating gas inflows caused by gravitationalinstabilities during the interaction, is also predicted by numericalsimulations. Furthermore, there is evidence for a decreasing fraction ofcold gas mass from early interacting systems to merger remnants,attributed to neutral hydrogen conversion into other forms (e.g., stars,hot gas) and molecular hydrogen depletion resulting from ongoing starformation. The evolution of the total-radio to blue-band luminosityratio, reflecting the total (disc and nucleus) star formation activity,is also investigated. Although this ratio is on average higher than thatfor isolated spirals, we find a marginal increase along the mergingsequence, attributed to the relative insensitivity of disc starformation to interactions. However, a similar result is also obtainedfor the nuclear radio emission, although galaxy interactions arebelieved to significantly affect the activity (star formation, AGN) inthe central galaxy regions. Nevertheless, the nuclear-radio to blue-bandluminosity ratio is significantly elevated compared with that forisolated spirals. Finally, we find that the FIR-radio flux ratiodistribution of interacting galaxies is consistent with star formationbeing the main energizing source.
| Luminous Infrared Galaxies. III. Multiple Merger, Extended Massive Star Formation, Galactic Wind, and Nuclear Inflow in NGC 3256
We report detailed evidence for multiple merger, extended massive starformation, galactic wind, and circular/noncircular motions in theluminous infrared galaxy NGC 3256, based on observations ofhigh-resolution imaging (Hubble Space Telescope, ESO NTT), and extensivespectroscopic data (more than 1000 spectra, collected at EstaciónAstrofísica de Bosque Alegre, Complejo Astronómico elLeoncito, Cerro Tololo InterAmerican Observatory, and IUEobservatories). We find in a detailed morphological study (resolution~15 pc) that the extended massive star formation process detectedpreviously in NGC 3256 shows extended triple asymmetrical spiral arms(r~5 kpc), emanating from three different nuclei. The main opticalnucleus shows a small spiral disk (r~500 pc), which is a continuation ofthe external one and reaches the very nucleus. The core shows blueelongated structure (50 pc×25 pc) and harbors a blue stellarcluster candidate (r~8 pc). We discuss this complex morphology in theframework of an extended massive star formation driven by a multiplemerger process (models of Hernquist et al. and Taniguchi et al.). Westudy the kinematics of this system and present a detailed Hαvelocity field for the central region(40''×40'' rmax~30''~5kpc), with a spatial resolution of 1" and errors of +/-15 kms-1. The color and isovelocity maps show mainly (1) akinematic center of circular motion with ``spider'' shape, locatedbetween the main optical nucleus and the close (5") mid-IR nucleus and(2) noncircular motions in the external parts. We obtained three``sinusoidal rotation curves'' (from the Hα velocity field) aroundposition angle (P.A.) ~55°, ~90°, and ~130°. In the mainoptical nucleus we found a clear ``outflow component'' associated withgalactic winds plus an ``inflow radial motion.'' The outflow componentwas also detected in the central and external regions (r<=5-6 kpc).The main axis of the inflow region (P.A.~80deg) ispractically perpendicular to the ouflow axis (atP.A.~160deg). We analyze in detail the physical conditions inthe giant H II regions located in the asymmetric spiral arms, the twomain optical nuclei, and the outflow component (using long-slitspectroscopy, plus standard models of photoionization, shocks, andstarbursts). We present four detailed emission-line ratios (NII/Hα, S II/Hα, S II/S II), and FWHM (Hα) maps for thecentral region (30''×30''rmax~22''~4 kpc), with a spatial resolution of 1".In the central region (r~5-6 kpc) we detected that the nuclear starburstand the extended giant H II regions (in the spiral arms) have verysimilar properties, i.e., high metallicity and low-ionization spectra,with Teff=35,000 K, solar abundance, a range ofTe~6000-7000 K, and Ne~100-1000 cm-3.The nuclear and extended outflow shows properties typical of galacticwind/shocks, associated with the nuclear starburst. We suggest that theinteraction between dynamical effects, the galactic wind (outflow),low-energy cosmic rays, and the molecular+ionized gas (probably in theinflow phase) could be the possible mechanism that generate the``similar extended properties in the massive star formation, at a scaleof 5-6 kpc!'' We have also studied the presence of the closemerger/interacting systems NGC 3256C (at ~150 kpc, ΔV=-100 kms-1) and the possible association between the NGC 3256 and3263 groups of galaxies. In conclusion, these results suggest that NGC3256 is the product of a multiple merger, which generated an extendedmassive star formation process with an associated galactic wind plus anuclear inflow. Therefore, NGC 3256 is another example in which therelation between mergers and extreme starburst (and the powerfulgalactic wind, ``multiple'' Type II supernova explosions) play animportant role in the evolution of galaxies (the hypothesis of Rieke etal., Joseph et al., Terlevich et al., Heckman et al., and Lípariet al.). Based on observations obtained at the Hubble Space Telescope(HST; Wide Field Planetary Camera 2 [WFPC2] and NICMOS) satellite;International Ultraviolet Explorer (IUE) satellite; European SouthernObservatory (ESO, NTT); Chile, Cerro Tololo Inter-American Observatory(CTIO), Chile; Complejo Astronómico el Leoncito (CASLEO),Argentina; Estación Astrofísica de Bosque Alegre(BALEGRE), Argentina.
| Helium and hydrogen excitation in starburst galaxies - infrared and optical line ratios
We present infrared spectra for a heterogeneous sample of 22 starburstgalaxies. Line fluxes are measured for the prominent HeI 2.058-μm andBrgamma lines, and their ratio is compared with theoretical predictionsof the behaviour of the HeI 2.058 mum/Brgamma line ratio with stellareffective temperature. The observed line ratios are lower than thosefound in individual compact HII regions. For a subset of these galaxiesoptical measurements of HeI/Hβ were derived from the literature.The combination of optical and infrared HeI/HI line ratios suggestseffective temperatures for the UV field of at least 40000K, consistentwith existing results from metal-line excitation.
| The He I 2.06 microns/Br-gamma ratio in starburst galaxies - an objective constraint on the upper mass limit to the initial mass function
The use of the He I 2.06 microns/Br-gamma ratio as a constraint on themassive stellar population in star-forming galaxies is developed. Atheoretical relationship between the He I 2.06 microns/Br-gamma ratioand the effective temperature of the exciting star in H II regions isderived. The effects of collisional excitation and dust within thenebula on the ratio are also considered. It is shown that the He I 2.06microns/Br-gamma ratio is a steep function of the effective temperature,a property which can be used to determine the upper mass limit of theinitial mass function (IMF) in galaxies. This technique is reliable forupper mass limits less than about 40 solar masses. New near-infraredspectra of starburst galaxies are presented. The He I 2.06microns/Br-gamma ratios observed imply a range of upper mass limits from27 to over 40 solar masses. There is also evidence that the upper masslimit is spatially dependent within a given galaxy. These resultssuggest that the upper mass limit is not a uniquely defined parameter ofthe IMF and probably varies with local physical conditions.
| The velocity field of clusters of galaxies within 100 megaparsecs. I - Southern clusters
Distances from the Tully-Fisher relation have been measured with theParkes radio telescope for eight additional southern clusters ofgalaxies in the redshift range 2500-5000 km/s. Most of them have large(average 1000 km/s) positive peculiar velocities in a comoving frame inwhich the microwave background radiation shows no dipole anisotropy. Infive out of eight cases these measurements have a significance at orabove the 2-sigma level. Outflow is seen on both sides of the Galacticplane. The source of this large-scale flow, if gravitational in orgin,lies at or beyond the limit of the present sample.
| Southern Galaxy Catalogue.
Not Available
| A List of Peculiar Galaxies, Interacting Pairs, Groups and Clusters South of Declination -43°
The coordinates, dimensions and short descriptions of 186 objects aregiven after a search of the Maksutov plate collection in theObservatorio Astronómico, Universidad de Chile, Santiago
| Corrections to the Reference Catalogue of Bright Galaxies (Third List)
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972PASP...84..462C&db_key=AST
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