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The relation between mergers and AGN activity. Results from radio galaxy and luminous infrared galaxy studies
There is morphological evidence that the activity in powerful radiogalaxies could be triggered by mergers and galaxy interactions. However,nothing is known about the timescales, order of events, and the type ofinteraction involved. It is not yet known whether there exists anevolutionary link between radio galaxies and other merger systems suchas very luminous and ultraluminous infrared galaxies (VLIRGs andULIRGs). Here, we report preliminary results obtained from the analysisof near-ultraviolet and optical spectroscopic observations of samples ofRadio Galaxies and VLIRGs-ULIRGs to investigate, through age-dating oftheir young stellar population, whether an evolutionary link existsbetween VLIRGs/ULIRGs-Radio Galaxies-normal elliptical galaxies. Theseresults will help to understand the genesis events that lead to theformation of radio jet and quasar activity, and they will allow us toplace radio galaxies in the context of hierarchical evolution models forthe population of giant elliptical galaxies.
| The Effects of Interactions on the Structure and Morphology of Elliptical/Lenticular Galaxies in Pairs
We present a structural and photometric analysis of 42elliptical/lenticular galaxies in E/S0 + S pairs observed in the BVRIcolor bands. The aim of the analysis is to empirically determine theeffects of interactions on the galaxies' morphology, structure, andstellar populations as seen from the CAS parameters (light concentrationC, asymmetry A, and clumpiness S). We further compare these values to acontrol sample of 67 mostly isolated noninteracting E/S0 galaxies. Wefind that the paired E/S0 galaxies occupy more scattered loci in CASspace than noninteracting E/S0s and that the structural effects ofinteractions on E/S0s are minor, in contrast to disk galaxies involvedin interactions. This suggests that observational methods forrecognizing interactions, such the CAS methodology of Conselice, wouldnot detect E/S0s involved in interactions (related to early phases ofthe so-called dry mergers), and that the majority of interactinggalaxies identified at high redshift must be gas-dominated systems.However, we find statistical differences in the asymmetry index whencomparing isolated and interacting E/S0s. On average, paired E/S0galaxies have A-values 2.96+/-0.72 times larger than those ofnoninteracting E/S0s. For the subset of presumably strongly interactingE/S0s, A and S can be several times larger than the typical values ofthe isolated E/S0s. We show that the asymmetries are consistent withseveral internal and external morphological distortions. We concludethat the subsample of interacting E/S0s should be dense, gas-poorgalaxies in systems spanning a wide range of interaction stages, withtypical merging timescales >~0.1-0.5 Gyr. We use the observedphenomenology of this subsample to predict the approximate loci of drypremergers in the CAS parameter space.
| Molecular Gas at High Redshift
The Early Universe Molecular Emission Line Galaxies (EMGs) are apopulation of galaxies with only 36 examples that hold great promise forthe study of galaxy formation and evolution at high redshift. Theclassification, luminosity of molecular line emission, molecular mass,far-infrared (FIR) luminosity, star formation efficiency, morphology,and dynamical mass of the currently known sample are presented anddiscussed. The star formation rates derived from the FIR luminosityrange from about 300 to 5000 Mȯ year 1 andthe molecular mass from 4 × 109 to 1 ×1011 Mȯ. At the lower end, these starformation rates, gas masses, and diameters are similar to those of localultraluminous infrared galaxies and represent starbursts in centrallyconcentrated disks, sometimes, but not always, associated with activegalactic nuclei. The evidence for large (>5 kpc) molecular disks islimited. Morphology and several high angular resolution images suggestthat some EMGs are mergers with a massive molecular interstellar mediumin both components. A critical question is whether the EMGs, inparticular those at the higher end of the gas mass and luminositydistribution, represent the formation of massive, giant ellipticalgalaxies in the early Universe. The sample size is expected to growexplosively in the era of the Atacama Large Millimeter Array (ALMA).
| Mixed-Morphology Pairs as a Breeding Ground for Active Nuclei
Mixed-morphology pairs offer a simplification of the interactionequation that involves a gas-rich fast rotator paired with a gas-poorslow rotator. In past low-resolution IRAS studies it was assumed thatthe bulk of the far-infrared (FIR) emission originated in the spiralcomponent. However, our Infrared Space Observatory studies revealed asurprising number of early-type components with significant IR emission,some of which turned out to show active nuclei. This motivated us tolook at the current statistics of active nuclei in mixed pairs using theradio-FIR continuum correlation as a diagnostic. We find a clear excessof early-type components with radio continuum emission and activenuclei. We suggest that they arise more often in mixed pairs viacross-fueling of gas from the spiral companion. This fuel is moreefficiently channeled into the nucleus of the slow-rotating receptor. Ina sample of 112 mixed-morphology pairs from the Karachentsev catalog, wefind that about 25%-30% of detected mixed pairs show a displacement fromthe radio-FIR relation defined by normal star-forming galaxies. Thelatter objects show excess radio continuum emission, while others extendthe relation to unusually high radio and FIR flux levels. Many of theoutliers or extreme emitters involve an early-type component with anactive nucleus. The paired E/S0 galaxies in the sample exhibit asignificant excess detection fraction and a marginal excess luminositydistribution compared to those of isolated unpaired E/S0 galaxies.
| Molecular gas properties of 12-μm Seyfert galaxies - I. The southern sample
We have used the 15-m Swedish European Southern Observatory (ESO)Submillimetre Telescope (SEST) to observe the J= 1 -> 0 and J= 2-> 1 transition lines of CO in 30 Southern hemisphere Seyfertgalaxies from the extended 12-μm sample of Rush, Malkan &Spinoglio. We detected CO J= 1 -> 0 in 16 out of the 30 Seyfertgalaxies and CO J= 2 -> 1 in 17 out of the 30 Seyfert galaxies. Fromthe observed spectra, various CO gas properties have been derivedincluding the luminosity of the CO gas and, using a standard conversionfactor, the H2 mass. The average H2 gas mass forSeyfert 1 galaxies was 3 × 109 Msolar for COJ= 1 -> 0 and 1 × 109 Msolar for CO J= 2-> 1, while in comparison the H2 gas mass for Seyfert 2type galaxies was 11 × 109 Msolar for CO J=1 -> 0 and 3 × 109 MsolarCO J= 2 -> 1.From this small sample of Seyfert galaxies, we tentatively support theconclusion that type 2 Seyfert galaxies contain more molecular gas thantheir type 1 counterparts.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Galaxy Interaction and the Starburst-Seyfert Connection
Galaxy interactions are studied in terms of the starburst-Seyfertconnection. The starburst requires a high rate of gas supply. Since theefficiency for supplying the gas is high in a galaxy interaction,although the companion is not necessarily discernible, Seyfert galaxieswith circumnuclear starbursts are expected to be interacting. Since thelarge amounts of circumnuclear gas and dust obscure the broad-lineregion, they are expected to be observed as Seyfert 2 galaxies. Theactive galactic nucleus itself does not require a high rate of gassupply. Seyfert galaxies without circumnuclear starbursts are notnecessarily expected to be interacting even at the highest luminosities.They are not necessarily expected to evolve from Seyfert galaxies withcircumnuclear starbursts. We derive these and other theoreticalexpectations and confirm them with statistics on observational data ofmagnitude-limited samples of Seyfert galaxies.
| An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample
The importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies.
| Azimuthal and Kinematic Segregation of Neutral and Molecular Gas in Arp 118: The Yin-Yang Galaxy NGC 1144
We present new high-resolution H I observations of the disk of thecollisional infrared luminous(LIR=2.2×1011Lsolar) galaxy NGC1144, which reveal an apparent large-scale azimuthal and kinematicsegregation of neutral hydrogen relative to the molecular gasdistribution. Even among violently collisional galaxies, the CO/H Iasymmetry in NGC 1144 is unusual, both in the inner regions and in theouter disk. We suggest that we are observing Arp 118 at a specialmoment, shortly after a high-speed collision between NGC 1144 and itselliptical companion NGC 1143. H I emission with an average molecularfraction fmol<0.5 is observed on one side (northwest) ofthe rotating disk of NGC 1144, while the other side (southeast) isdominated by dense molecular complexes in which fmol isalmost unity. The interface region between the warm- and cool-clouddominated regions lies on a deep spiral-like dust lane that we identifyas a shock wave responsible for the relative shift in the dominance of HI and H2 gas. A strong shock being fed by diffuse H I cloudswith unusually large (>400 km s-1) rotational velocities canexplain (1) the CO/H I asymmetries, (2) a large velocity jump (185 kms-1) across the arm as measured by H I absorption against aradio bright continuum source that straddles the arm, and (3) theasymmetric distribution of star formation and off-nuclear molecular gasresulting from likely streaming motions associated with the strongshock. The new results provide for the first time a coherent picture ofArp 118's many peculiarities and underline the potentially complexchanges in the gas phase that can accompany large gravitationalperturbations of gas-rich galaxies.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| Multiwavelength Insights into Mixed-Morphology Binary Galaxies. I. ISOCAM, ISOPHOT, and Hα Imaging
We present Hα and ISO mid- and far-IR observations for a sample ofmixed-morphology galaxy pairs that reveal both the stellar andnonstellar signatures of the interaction process. A mixed-morphologypair is perhaps the simplest form of galaxy-galaxy interaction becauseit is expected to involve only a single rapidly rotating gas-richcomponent paired with a gas-poor elliptical or lenticular galaxy. Aprimary assumption that we address is whether spirals are the only IRemitter in these mixed (E+S) pairs. Our observations reveal that many ofthe early-type galaxies exhibit weak (low equivalent width) emission, asoften observed in field elliptical galaxies. These are the classicalmixed-morphology pairs. However, some of the early-type components,especially the lenticular galaxies, show evidence for significant starformation, with Hα equivalent widths and 15 μm luminositiescomparable to or exceeding those of their often much larger spiralcompanions. Our sample contains five Seyfert 2 nuclei, of which threecan be described as companions on the end of a spiral arm. The Seyfertnucleus is often accompanied by a starburst region, while other suchcompanions currently show only the starburst component. These pairs areamong the best candidates for direct interaction fuelling of bothstarbursts and active galactic nuclei.Based on observations with the Infrared Space Observatory (ISO), an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands and the United Kingdom) withthe participation of ISAS and NASA.
| Comparisons of Infrared Colors and Emission-line Intensities between Two types of Seyfert 2 Galaxies
We study the relation between the infrared colors, [OIII] emissionlines, gaseous absorbing column density (NH),and thedetectability of the polarized (hidden) broad-line region (HBLR) in alarge sample of 75 Seyfert 2 galaxies (Sy2s). From the indicators ofstar-formation activity, f60/f100 andLFIR/LB, we find some evidence that the Sy2swithout HBLR show higher star-formation activities than those with HBLR,in agreement with previous prediction. Also, we confirm that the HBLRSy2s tend to have a larger luminosity ratio of the core to the hostgalaxy, suggesting that the HBLR Sy2s display more powerful AGNactivity. However, the level of obscuration found in previous papers isnearly indistinguishable between the two types of Sy2s. The resultssupport the statement that the non-HBLR Sy2s, with a weaker corecomponent and a stronger star-formation activity component, areintrinsically different from the HBLR Sy2s, which are Sy1 systems with ahidden powerful AGN core and a low star-formation activity. Theindications are that the non-HBLR Sy2s might be at an earlierevolutionary phase than the HBLR Sy2s.
| The Far-Infrared Energy Distributions of Seyfert and Starburst Galaxies in the Local Universe: Infrared Space Observatory Photometry of the 12 Micron Active Galaxy Sample
New far-infrared photometry with ISOPHOT aboard the Infrared SpaceObservatory (ISO) is presented for 58 galaxies with homogeneouspublished data for another 32 galaxies, all belonging to the 12 μmgalaxy sample-in total, 29 Seyfert 1 galaxies, 35 Seyfert 2 galaxies,and 12 starburst galaxies, or about half of the 12 μm active galaxysample, plus 14 normal galaxies for comparison. ISO and InfraredAstronomical Satellite (IRAS) data are used to define color-colordiagrams and spectral energy distributions (SEDs). Thermal dust emissionat two temperatures (one cold at 15-30 K and one warm at 50-70 K) canfit the 60-200 μm SED, with a dust emissivity law proportional to theinverse square of the wavelength. Seyfert 1 galaxies and Seyfert 2galaxies are indistinguishable longward of 100 μm, while, as alreadyseen by IRAS, the former have flatter SEDs shortward of 60 μm. A mildanticorrelation is found between the [200-100] color and the ``60 μmexcess.'' We infer that this is due to the fact that galaxies with astrong starburst component and thus a strong 60 μm flux have asteeper far-infrared turnover. In non-Seyfert galaxies, increasing theluminosity corresponds to increasing the star formation rate, whichenhances the 25 and 60 μm emission. This shifts the peak emissionfrom around 150 μm in the most quiescent spirals to shorter than 60μm in the strongest starburst galaxies. To quantify these trendsfurther, we identified with the IRAS colors three idealized infraredSEDs: pure quiescent disk emission, pure starburst emission, and pureSeyfert nucleus emission. Even between 100 and 200 μm, the quiescentdisk emission remains much cooler than the starburst component. Seyfertgalaxies have 100-200 μm SEDs ranging from pure disks to purestarbursts, with no apparent contribution from their active nuclei atthose wavelengths. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, the Netherlands, and the United Kingdom) with theparticipation of ISAS and NASA.
| Mid-Infrared and CO Observations of the Infrared/X-Ray Luminous Seyfert 1 Galaxy NGC 985: The Making or Breaking of a ULIRG?
We describe Infrared Space Observatory (ISO)1 and BIMA observations of the z=0.04 Seyfert 1ring galaxy NGC 985 which suggest close parallels with some quasar hostgalaxies. NGC 985 contains two closely spaced nuclei embedded in anR1/4-law stellar bulge and an outer ring, evidence of anongoing merger. The system contains ~1.8×1010Msolar of highly disturbed molecular gas which lies in anasymmetric barlike structure with the peak in observed CO columndensities significantly offset from the compact double nucleus. Incontrast to this, the ISO observations show strong dust emissioncentered on the active galactic nucleus (AGN), located in one of the twonuclei. Fainter CO, mid-infrared (MIR), and radio continuum emissionprovides a glimpse of the complexities of star formation in the outerring. An analysis of the kinematics of the main CO emission revealsevidence for two dynamically distinct molecular components within NGC985. The first is a set of isolated supergiant molecular clouds (SGMCs)which are concentrated within 9-10 kpc of the active nucleus. Althoughrandomly distributed about the center, the clouds may form part of aclumpy highly disturbed disk which may be either just forming arounddouble nucleus (the making of an ultraluminous infrared galaxy [ULIRG])or alternatively in the process of being disrupted, perhaps as a resultof a powerful nuclear outflow (the breaking of a ULIRG). A second majorconcentration of CO lies offset from the double nucleus in a dynamicallycoherent ridge of emission in which powerful star formation isoccurring. We tentatively associate CO emission with two out of six UVabsorption lines seen in the blue wing of the very broad Lyαemission. Such an association would imply a complex interrelationshipbetween the nuclear CO cloud population in colliding systems andAGN-driven winds.
| Spectropolarimetry of a complete infrared-selected sample of Seyfert 2 galaxies
We report the results of a spectropolarimetric survey of a completefar-infrared-selected sample of Seyfert 2 galaxies. We have foundpolarized broad Hα emission in one new source, NGC 5995. In thesample as a whole, there is a clear tendency for galaxies in which wehave detected broad Hα in polarized light to have warmmid-far-infrared colours(F60μm/F25μm<~4), in agreement with ourprevious results. However, a comparison of the optical, radio and hardX-ray properties of these systems leads us to conclude that this is asecondary consequence of the true mechanism governing our ability to seescattered light from the broad-line region. We find a strong trend forgalaxies showing such emission to lie above a critical value of therelative luminosity of the active core to the host galaxy (as measuredfrom the [OIII] 5007-Å equivalent width) which varies as afunction of the obscuring column density as measured from hard X-rayobservations. The warmth of the infrared colours is then largely due toa combination of the luminosity of the active core, the obscuring columnand the relative importance of the host galaxy in powering thefar-infrared emission, and not solely orientation as we inferred in ourprevious paper. Our data may also provide an explanation as to why themost highly polarized galaxies, which appear to have tori that arelargely edge-on, are also the most luminous and have the most easilydetectable scattered broad Hα.
| High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - II. The properties of compact radio components
We discuss the properties of compact nuclear radio components in Seyfertgalaxies from the extended 12-μm AGN sample of Rush et al. Our mainresults can be summarized as follows. Type 1 and type 2 Seyferts producecompact radio components which are indistinguishable in strength andaspect, indicating that their central engines are alike, as proposed bythe unification model. Infrared IRAS fluxes are more closely correlatedwith low-resolution radio fluxes than high-resolution radio fluxes,suggesting that they are dominated by kiloparsec-scale, extranuclearemission regions; extranuclear emission may be stronger in type 2Seyferts. Early-type Seyfert galaxies tend to have stronger nuclearradio emission than late-type Seyfert galaxies. V-shaped extendedemission-line regions, indicative of `ionization cones', are usuallyfound in sources with large, collimated radio outflows. Hidden broadlines are most likely to be found in sources with powerful nuclear radiosources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm fluxdensities have well-matched properties.
| The unified model and the Seyfert 2 infrared dichotomy
An optical spectropolarimetric study has shown that the detectability ofpolarized broad Hα in Seyfert 2 galaxies is correlated with theIRAS f60f25 flux ratio where only those Seyfert 2swith `warm' IRAS colours show polarized broad-line emission. It wassuggested that those Seyfert 2s with `cool' IRAS colours have highlyinclined tori which obscure the broad-line scattering screen. I presenthere hard X-ray observations inconsistent with this picture, showingthat the derived column densities of warm and cool Seyfert 2 galaxiesare statistically the same. I classify the Bright Galaxy Sample toproduce a non-Seyfert comparison. The analysis of the properties ofthese galaxies with the Seyfert 2s suggests that the IRASf60f25 flux ratio is consistent with implying therelative strength of galactic to Seyfert emission. I show that this newpicture can account for the absence of polarized broad Hα in thecool Seyfert 2s.
| Arp 119: A High-Speed Galaxy Collision With Episodic Star Formation
Colliding galaxies are excellent laboratories for studying galacticevolution and global star formation. Computer simulations of galaxycollisions, in which at least one galaxy has a significant gaseouscomponent, show the production of density enhancements and shock wavesin the interstellar medium. These high-density regions coincide with thelocations of recent, large-scale star formation in observations of somereal colliding galaxies. Thus, combined n-body/hydrodynamic computersimulations can be used to explore the history and conditions ofstar-forming regions in colliding galaxies. We compare multiwavelengthobservations of the Arp 119 system with a combined n-body/SPH simulationof colliding galaxies. Most of the observations used here are gleanedfrom the literature. Additionally, we obtained new near-infrared (J- andH-band) observations of this system, using the NIRIM camera at the MountLaguna observatory. These new data add information about the underlying,old stellar population. Arp 119 (CPG 29) is comprised of a southernmember, Arp 119S (Mrk 984), which has an extremely disturbed appearance,and a northern member, Arp 119N, a gas-poor elliptical. The morphologyof both members can be fitted well by a simulation in which a gas-richdisk galaxy has been impacted by an equal-mass elliptical that had atrajectory approximately perpendicular to the plane of the disk andpassed through the disk slightly off-center. From our comparison, wefind that the progression of recent large-scale star formation in thisgalaxy can be accounted for by a single outwardly propagatingcollision-induced density wave in the gas. We deduce that the starformation rate in this density wave was not a smooth function of timebut that, so far, three major episodes of star formation have occurredat roughly 25-30 Myr intervals. An initial burst took place very soonafter the peak of the collision between the two galaxies, and the latestburst is still ongoing. The fit of the simulations to the observationsindicates that this collision occurred approximately 71 Myr ago. At thecurrent, projected separation of 53 kpc (assuming H0=75 kms-1 Mpc-1), we obtain a current relative spacevelocity between the two galaxies of approximately 850 kms-1. This is strong evidence that the collision partner wasArp 119N and not some currently more distant galaxy. Furthermore, thehigh relative velocity of the pair and the paucity of gas currently tobe found in the elliptical may explain the very high velocity gasobserved in the greatly disturbed disk galaxy, Arp 119S.
| The Mid-Infrared View of Star Formation in Collisional Ring Galaxies
We present mid-infrared images and spectra of Arp 10 and Arp 118, twocollisional ring galaxies observed as part of the ISOCAM GT programCAMACTIV (P.I.: I.F. Mirabel). The observations reveal the distributionof hot dust in the galaxies and enable us to probe the mechanismsresponsible for the heating of the ISM. Unlike the peculiar mid-infraredcolours recently discovered in the Cartwheel, the prime example of acollisional ring, Arp 10 has colours typical of those found in spiralarms of late type spirals. Similarly for Arp 118 (NGC 1144), themid-infrared emission is associated with the regions of star formationin the ring. Moreover, a hot continuum in the 5.1-6.7 μm range, whichis a typical mid-infrared signature of an AGN, is clearly detected fromthe Seyfert 2 nucleus of the galaxy.
| Nuclear obscuration and scattering in Seyfert 2 galaxies
We study the relationship between gaseous absorbing column density (N_Ha), infrared colors and detectability of the broad lines in a largesample of Seyfert 2 galaxies(Sy2s). We confirm that Sy2s withoutpolarized broad lines tend to have cooler 60 mu m/25 mu m colors; thiscorrelation was previously ascribed to the effect of obscuration towardsthe nuclear region. We find some evidence that Sy2s without polarizedbroad lines have larger absorbing column density (N_H a) and that afraction of them are characterized by dust lanes crossing their nuclei.However, we find that the IR colors do not correlate with N_H a, indisagreement with the obscuration scenario. Also, Sy2s without polarizedbroad lines follow the same radio-FIR relation as normal and starburstgalaxies, at variance with Sy2s with polarized broad lines. Theseresults indicate that the lack of broad lines in the polarized spectrumof Sy2s is mostly due to the contribution/dilution from the host galaxyor from a circumnuclear starburst, though a lesser extent to theobscuration toward the nuclear region also plays a role.
| High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - I. The observations
We present 8.4-GHz VLA A-configuration observations of 87 sources fromthe mid-infrared-selected AGN sample of Rush et al. These0.25-arcsec-resolution observations allow elongated radio structurestens of pc in size to be resolved, and enable radio components smallerthan 3.5arcsec to be isolated from diffuse galactic disc emission. Whencombined with previous data, matched radio observations covering 90percent of the sample have been collected, and these represent the largestsubarcsecond-resolution radio imaging survey of a homogeneously selectedsample of Seyfert galaxies to date. We use our observations to identifyfive radio-loud AGN in the sample. The nature of the radio emission fromSeyfert nuclei will be discussed in subsequent papers.
| Mid-infrared diagnostics to distinguish AGNs from starbursts
We present new mid-infrared (MIR) diagnostics to distinguish emission ofactive galactic nuclei (AGN) from that originating in starburst regions.Our method uses empirical spectroscopic criteria based on the fact thatMIR emission from star forming or active galaxies arises mostly from HIIregions, photo-dissociation regions (PDRs) and AGNs. The analysis of thestrength of the 6.2 mu m Unidentified Infrared Band (UIB) and the MIRcontinuum shows that UIBs are very faint or absent in regions harboringthe intense and hard radiation fields of AGNs and pure HII regions,where the UIB carriers could be destroyed. The MIR signature of AGNs isthe presence of an important continuum in the 3-10 mu m band whichoriginates from very hot dust heated by the intense AGN radiation field.Using these two distinct spectral properties found in our MIR templates,we build diagnostic diagrams which provide quantitative estimates of theAGN, PDR and HII region contribution in a given MIR spectrum. This newMIR classification can be used to reveal the presence of AGNs highlyobscured by large columns of dust. Based on observations made with ISO,an ESA project with instruments funded by ESA Member States (especiallythe PI countries: France, Germany, the Netherlands and the UnitedKingdom) and with the participation of ISAS and NASA.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| Morphology of the 12 Micron Seyfert Galaxies. II. Optical and Near-Infrared Image Atlas
We present 263 optical and near-infrared (NIR) images for 42 1s and 48Seyfert 2s, selected from the Extended 12 μm Galaxy Sample.Elliptically averaged profiles are derived from the images, andisophotal radii and magnitudes are calculated from these. We also reportvirtual aperture photometry that, judging from comparison with previouswork, is accurate to roughly 0.05 mag in the optical, and 0.07 mag inthe NIR. Our B-band isophotal magnitude and radii, obtained from ellipsefitting, are in good agreement with those of Third Reference Catalogueof Bright Galaxies. When compared with the B band, V, I, J, and Kisophotal diameters show that the colors in the outer regions of Seyfertgalaxies are consistent with the colors of normal spirals. Differencesin the integrated isophotal colors and comparison with a simple modelshow that the active nucleus+bulge are stronger and redder in the NIRthan in the optical. Finally, roughly estimated Seyfert disk surfacebrightnesses are significantly brighter in B and K than those in normalspirals of similar morphological type.
| The H I and Ionized Gas Disk of the Seyfert Galaxy NGC 1144=Arp 118: A Violently Interacting Galaxy with Peculiar Kinematics
We present observations of the distribution and kinematics of neutraland ionized gas in NGC 1144, a galaxy that forms part of the Arp 118system. Ionized gas is present over a huge spread in velocity (1100 kms-1) in the disk of NGC 1144, but H I emission is detectedover only one-third of this velocity range, in an area that correspondsto the NW half of the disk. In the nuclear region of NGC 1144, a jump invelocity in the ionized gas component of 600 km s-1 isobserved. Faint, narrow H I absorption lines are also detected againstradio sources in the SE part of the disk of NGC 1144, which includesregions of massive star formation and a Seyfert nucleus. The peculiar HI distribution, which is concentrated in the NW disk, seems to be theinverse of the molecular distribution that is concentrated in the SEdisk. Although this may partly be the result of the destruction of H Iclouds in the SE disk, there is circumstantial evidence that the entireH I emission spectrum of NGC 1144 is affected by a deep nuclearabsorption line covering a range of 600 km s-1 and is likelyblueshifted with respect to the nucleus. In this picture, a high columndensity H I stream is associated with the nuclear ionized gas velocitydiscontinuity, and the absorption effectively masks any H I emissionthat would be present in the SE disk of NGC 1144.
| An X-Ray Survey of Galaxies in Pairs
Results are reported from the first survey of X-ray emission fromgalaxies in pairs. The sample consists of 52 pairs of galaxies from theCatalog of Paired Galaxies whose coordinates overlap the ROSAT PositionSensitive Proportional Counter pointed observations. The mean observedlogl_X for early-type pairs is 41.35+/-0.21, while the mean logl_Xpredicted using the l_X-l_b relationship for isolated early-typegalaxies is 42.10+/-0.19. With 95% confidence, the galaxies in pairs areunderluminous in the X-ray, compared with isolated galaxies, for thesame l_b. A significant fraction of the mixed pair sample also appearssimilarly underluminous. A spatial analysis shows that the X-rayemission from pairs of both types typically has an extent of ~10-50 kpc,much smaller than the group intergalactic medium, and thus likelyoriginates from the galaxies. CPG 564, the most X-ray luminousearly-type pair, 4.7x10^42 ergs s^-1, is an exception. The extent of itsX-ray emission, greater than 169 kpc, and HWHM, ~80 kpc, is comparableto that expected from an intergalactic medium. The sample shows only aweak correlation, ~81% confidence, between l_X and l_b, presumably dueto variations in gas content within the galaxies. No correlation betweenl_X and the pair velocity difference (Deltav), separation (Deltar), orfar-infrared luminosity (l_fir) is found, although the detection rate islow, 22%.
| Nearby Active and Starburst Galaxies as Seen with ISOCAM
We present a set of spectral characteristics distinguishing opticallyselected active galactic nuclei (AGNs) from starbursts in the mid-IR. InAGN spectra, the Unidentified Infrared Bands (UIBs) are not detectedwhich is probably due to the destruction of their carriers by the UV-Xradiation field. In addition, a non-negligible continuum below 10 μmcommonly associated with emission from very hot dust is exclusivelypresent in AGNs. On the contrary, starburst spectra present clearlydetected UIBs with a faint underlying continuum between 5-10 μm.Based on these features, we build a mid-IR diagnostic diagram toidentify emission induced by the AGN and that associated with the starformation activity. This diagnostic based on the mid-IR continuum andthe UIB intensity allows us to estimate the relative importance of thethree components (HII regions, photo-dissociation regions (PDRs), andAGN) contained in the integrated mid-IR emission of galaxies. Ourdiagnostic diagram can be applied to reveal the presence of veryobscured active nuclei behind large columns of dust.
| The AGNs and their host galaxies as seen in the mid-infrared
We present a new diagnostic diagram based on ISOCAM spectra (5-16 μm)to distinguish the emission induced by the active galactic nucleus (AGN)from that associated with the star formation activity. This diagnosticbased on the mid-IR continuum and the Unidentified Infrared Band (UIB)intensity allows us to estimate the relative importance of the threecomponents (HII regions, diffuse/photo-dissociation regions (PDRs), andAGN) contained in galaxy mid-infrared emission. In AGN spectra, weconfirm the absence of UIBs presumably photodissociated by the X-UVradiation field. In addition, a non-negligible continuum below 9 μmcommonly associated with emission from hot dust is present in AGNs. Adiagnostic diagram derived from these two results can be used fordetecting obscured AGNs embedded in a large concentration of dust.
| The Supernova Rate in Starburst Galaxies
We conducted an optical CCD search for supernovae in a sample of 142bright [m(B) <= 16 mag], nearby (z<=0.03) starburst galaxies overthe period 1988 December to 1991 June, to a limiting R-band magnitude of18. Five supernovae were found, in all cases outside the host galaxy'snucleus. We determine supernova rates (in supernova units or SNU) in theextranuclear regions to be 0.7 h^2 SNU for Type Ia, 0.7 h^2 SNU for TypeIb/c, and ~0.6 h^2 SNU for Type II, with large uncertainties but upperlimits of 2.2 h^2, 2.5 h^2, and 1.7 h^2 SNU, respectively. These ratesare similar to those measured in ``normal'' galaxies. We found noevidence for a supernova-induced brightening in any galactic nucleusand, with a few reasonable assumptions, can place upper limits of 9 h^2,12 h^2, and 7 h^2 SNU on the rates of unobscured supernovae Types Ia,Ib/c, and II, respectively, inside the nuclei.
| Astrophysics in 1997
Martian marvels, a gamma-ray burster with a redshift, Galileo converseswith Ganymede, a record galactic redshift of 4.92, and much else. Fiscal1997 was definitely an exciting year for astronomers. We have tried hardto hit all the obvious highlights, but also to report more gradualprogress on traditional problems of understanding planets, stars,galaxies, and the universe. Though the year was saddened by the loss ofmany valued colleagues, we nevertheless indulge in occasionalsoupçons of frivolity.
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