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The Far-Infrared Spectrum of Arp 220
Infrared Space Observatory Long Wavelength Spectrometer gratingobservations of the ultraluminous infrared galaxy Arp 220 showsabsorption in molecular lines of OH, H2O, CH, NH, andNH3, as well as in the [O I] 63 μm line and emission inthe [C II] 158 μm line. We have modeled the continuum and theemission/absorption of all observed features by means of a nonlocalradiative transfer code. The continuum from 25 to 1300 μm is modeledas a warm (106 K) nuclear region that is optically thick in thefar-infrared, attenuated by an extended region (2") that is heatedmainly through absorption of nuclear infrared radiation. The molecularabsorption in the nuclear region is characterized by high excitation dueto the high-infrared radiation density. The OH column densities are hightoward the nucleus (2-6×1017 cm-2) and theextended region (~2×1017 cm-2). TheH2O column density is also high toward the nucleus(2-10×1017 cm-2) and lower in the extendedregion. The column densities in a halo that accounts for the absorptionin the lowest lying lines are similar to what are found in the diffuseclouds toward the star-forming regions in the Sgr B2 molecular cloudcomplex near the Galactic center. Most notable are the high columndensities found for NH and NH3 toward the nucleus, withvalues of ~1.5×1016 cm-2 and~3×1016 cm-2, respectively, whereas theNH2 column density is lower than ~2×1015cm-2. A combination of photodissociation regions (PDRs) inthe extended region and hot cores with enhanced H2Ophotodissociation and a possible shock contribution in the nuclei mayexplain the relative column densities of OH and H2O, whereasthe nitrogen chemistry may be strongly affected by cosmic-rayionization. The [C II] 158 μm line is well reproduced by our modelsand its ``deficit'' relative to the C II/far-IR ratio in normal andstarburst galaxies is suggested to be mainly a consequence of thedominant non-PDR component of far-infrared radiation, although ourmodels alone cannot rule out extinction effects in the nuclei.Based on observations with the Infrared Space Observatory, an ESAproject with instruments funded by ESA Member States (especially theprincipal investigator countries: France, Germany, Netherlands, and theUnited Kingdom) and with the participation of ISAS and NASA.
| The Role of Mergers in Galaxy Evolution
The evolution of galaxies has been driven by galaxy-galaxy collisions onall cosmological time-scales, from the primordial universe to thepresent. In this context, nearby mergers serve as local analogs to gaininsight into the physical processes that lead to the formation andtrans-formation of galaxies in the more distant universe. Here I reviewmultiwavelength observations -with particular emphasis on recent resultsobtained with ISO- of mergers of massive galaxies driving the formationof: 1) luminous infrared galaxies, 2) elliptical galaxy cores, 3)luminous dust-enshrouded extranuclear starbursts, 4) symbiotic galaxies,and 5) tidal dwarf galaxies. The most important implication for futurestudies on the formation of galaxies at early cosmological timescales isthat the most luminous galaxies in the local universe (z<=0.1) emitthe bulk of their energy in the mid and far-infrared, and therefore,their analogs in the more distant universe would be invisible in theultraviolet and optical wavelength rest-frames.
| The Surroundings of Disturbed, Active Galaxies
The brightest apparent magnitude examples of ultraluminous infraredgalaxies (ULIRGs) are studied here in X-ray, optical, infrared, andradio wavelengths. It is found that they have associated materialreaching out to apparent diameters of the order of 1° on the sky.Gas, dust, X-ray material, and quasars appear to be ejected from theactive nuclei with all objects nearer than their redshift distances.
| An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. II. Morphologies, Star Formation, and Faint Companions
We analyze the optical properties of ~300 galaxies within and aroundthree prominent voids of the Center for Astrophysics Redshift Survey. Wedetermine CCD morphologies and Hα equivalent widths from ourimaging and spectroscopic survey. We also describe a redshift survey of250 neighboring galaxies in the imaging survey fields. We assess themorphology-density relation, EW(Hα)-density relation, and theeffects of nearby companions for galaxies in low-density environmentsselected with a smoothed large-scale (5 h-1 Mpc) galaxynumber density n. Both the morphological mix and the Hα line widthdistribution of galaxies at modest underdensities, 0.5R=16.13, demonstrates that the incidence ofa close companion in redshift space is insensitive to global densityover the range we investigate (0.163 σ) fromΔcz>~200 km s-1 at 0.5-1 at n<=0.5n. In the lowest densityenvironments, galaxies with companions clearly (~4 σ) havestronger star formation than comparable galaxies at larger globaldensity (0.5-1 kpc and 1000 km s-1) varies little over theentire density range. These results, combined with the luminosity- andcolor-density relations of this sample (Paper I), suggest that theformation and evolution of field galaxies are insensitive to large-scaleunderdensity down to a threshold of roughly half the mean density. Thedifferences in galaxy properties at the lowest global densities we canexplore (n<=0.5n) may be explained by (1) a relative scarcity of thesmall-scale primordial density enhancements needed to form massiveearly-type/absorption-line galaxies and (2) present-day galaxyencounters that are relatively more effective because of the lowervelocity dispersion on small scales (<~200 h-1 kpc) weobserve in these regions. In the voids, where the luminous galaxiespresumably formed more recently, there should be more gas and dustpresent for active star formation triggered by nearby companions.
| Galaxy collisions.
Theories of how galaxies, the fundamental constituents of large-scalestructure, form and evolve have undergone a dramatic paradigm shift inthe last few decades. Earlier views were of rapid, early collapse andformation of basic structures, followed by slow evolution of the stellarpopulations and steady buildup of the chemical elements. Currenttheories emphasize hierarchical buildup via recurrent collisions andmergers, separated by long periods of relaxation and secularrestructuring. Thus, collisions between galaxies are now seen as aprimary process in their evolution. This article begins with a briefhistory; we then tour parts of the vast array of collisional forms thathave been discovered to date. Many examples are provided to illustratehow detailed numerical models and multiwaveband observations haveallowed the general chronological sequence of collisional morphologiesto be deciphered, and how these forms are produced by the processes oftidal kinematics, hypersonic gas dynamics, collective dynamical frictionand violent relaxation. Galaxy collisions may trigger the formation of alarge fraction of all the stars ever formed, and play a key role infueling active galactic nuclei. Current understanding of the processesinvolved is reviewed. The last decade has seen exciting new discoveriesabout how collisions are orchestrated by their environment, howcollisional processes depend on environment, and how these environmentsdepend on redshift or cosmological time.
| Redshifts of Galaxies around ARP 220 and Serendipitous Discovery of Three Star-forming Dwarf Galaxies at Redshift Z~0.5
We present redshift measurements of four faint galaxies around thearchetypal ultraluminous infrared galaxy Arp 220. These galaxies havesignificantly higher redshifts (z~0.036-0.091) than that of Arp 220(z~=0.018). Therefore, we conclude that they are background objects notphysically related to Arp 220. Three of these faint galaxies located tothe south of Arp 220 are a group of galaxies (or the brightest membersin a cluster of galaxies) at z~=0.09, as has been suggested previouslyon the basis of their associated soft X-ray emission. We also report theserendipitous discovery of three additional galaxies at redshift z~0.5,found along one of the slit positions. All three galaxies exhibit an [OII] lambda3727 emission line. The spectrum of the brightest galaxy(m_R~=24.4) shows other strong emission lines: Mg II lambda2798,Hβ, [O III] lambda4959, and[O III] lambda5007. The emission-lineproperties of these galaxies as well as their intrinsically lowluminosities (M_R>=-18.4) indicate that they are star-forming dwarfgalaxies.
| Redshifts of 31 bright galaxies
Optical spectroscopy of 31 bright galaxies, most of which are among theUGC galaxies detected at 2380 MHz by Dressel and Condon (1978), isreported. The observations were made at the McDonald Observatory withthe 2.7-m NASA reflector (except for UGC 3915, 4752, 5507, and 9357, forwhich the 2.1-m Struve reflector was used).
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