Gas-rich Disk Galaxies Research Articles

Impact of Dark Matter Subhalos on Extended H i Disks of Galaxies: Possible Formation of H i Fine Structures and Stars

Recent observations have discovered star formation activities in the extreme outer regions of disk galaxies. However, it remains unclear what physical mechanisms are responsible for triggering star formation in such low-density gaseous environments of galaxies. In order to understand the origin of these outer star-forming regions, we numerically investigate how the impact of dark matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark matter halo influences the dynamical evolution of the outer H I gas disk of the galaxy. We find that if the masses of the subhalos (Msb) in a galaxy with an extended H I gas disk are as large as 10-3Mh, where Mh is the total mass of the galaxy's dark halo, local fine structures can be formed in the extended H I disk. We also find that the gas densities of some apparently filamentary structures can exceed a threshold gas density for star formation and thus be likely to be converted into new stars in the outer part of the H I disk in some models with larger Msb. These results thus imply that the impact of dark matter subhalos (dark impact) can be important for better understanding the origin of recent star formation discovered in the extreme outer regions of disk galaxies. We also suggest that characteristic morphologies of local gaseous structures formed by the dark impact can indirectly prove the existence of dark matter subhalos in galaxies. We discuss the origin of giant H I holes observed in some gas-rich galaxies (e.g., NGC 6822) in the context of the dark impact.

On the Structural Differences between Disk and Dwarf Galaxies

Gas-rich dwarf and disk galaxies overlap in numerous physical quantities, which makes their classification subjective. We report the discovery of a separation of dwarfs and disks into two unique sequences in the mass (luminosity) versus scale length plane. This provides an objective classification scheme for late-type galaxies that only requires optical or near-IR surface photometry of a galaxy. Since the baryonic Tully-Fisher relation for these samples produces a continuous relation between baryonic mass and rotational velocity, we conclude that the difference between dwarfs and disks must be due to their distribution of stellar light such that dwarfs are more diffuse than disk galaxies. This structural separation may be due to a primordial difference between low- and high-mass galaxies or produced by hierarchical mergers in which disks are built up from dwarfs. Structural differences between dwarf and disk galaxies may also be driven by the underlying kinematics, where the strong rotation in disks produces an axially symmetric object that undergoes highly efficient star formation in contrast to the more weakly rotating, more disordered motion of dwarfs that produces a diffuse, triaxial object with a history of inefficient star formation.

A possible formation scenario for the ultramassive cluster W3 in NGC 7252

The intermediate age star cluster W3 (age � 300‐500 Myr) in NGC 7252 is the most luminous star cluster known to date with a dynamical mass estimate of 8±2·10 7 M⊙. With an effective radius of about 17.5 pc and a velocity dispersion of 45 kms −1 this object may be viewed as one of the recently discovered ultra-compact dwarf galaxies (UCDs). Its intermediate age, however, precludes an origin as a stripped nucleated dwarf elliptical galaxy. The galaxy NGC 7252 is a merger remnant from two gas-rich disc galaxies. Interactions between two gas-rich galaxies lead to bursts of intense star formation. The age of the interaction and the age of W3 are in good agreement, suggesting that was W3 probably formed in the starburst. We propose a formation scenario for W3. Observations of interacting galaxies reveal regions of strong star formation forming dozens up to hundreds of star cluster in confined regions of up to several hundred parsec in diameter. The total mass of new stars in these regions can reach 10 7 or even 10 8 M⊙. By means of numerical simulations we have shown that the star clusters in these regions merge on short time-s cales of a few Myr up to a few hundred Myr. We apply this scenario to W3 and predict properties which could be observable and the future evolution of this object. This work lends credence to the notion that at least some of the UCDs may be evolved star cluster complexes formed during early hierarchical mergers.

A Search for H [CSC]i[/CSC] in Five Elliptical Galaxies with Fine Structure

We report on VLA H I spectral line observations of five early-type galaxies classified as optically peculiar because of the presence of jets, ripples, or other optical fine structure. We detect H I within the primary beam (30' half-power beamwidth) in four of the five systems. However, in only one case is this gas associated with the targeted elliptical galaxy. In the other cases the H I is associated with a nearby gas-rich disk or dwarf galaxy. The one H I detection is for NGC 7626, where we tentatively detect an H I cloud lying between 20 and 40 kpc southwest of the galaxy center. Its origin is unclear. Our failure to detect obvious tidal H I features suggests that if these fine-structure elliptical galaxies are remnants of disk galaxy mergers, either the progenitors were gas-poor or they are well evolved and any gaseous tidal features have dispersed and/or been converted into other phases. Our targeted systems all reside in groups or clusters, and it seems likely that tidal H I is shorter lived in these environments than suggested by studies of more isolated merger remnants.

Radio-selected Galaxies in Very Rich Clusters at [CLC][ITAL]z[/ITAL][/CLC] ≤ 0.25. II. Radio Properties and Analysis

We report on the properties of radio-selected galaxies within 30 very-rich Abell clusters with z . 0.25. The radio, optical, and x-ray data for these clusters were presented in Paper I (Morrison et al. 2002). These radio data sample the ultra-faint (L1.4 ≥ 2 × 10 22 W Hz −1 ) radio galaxy population with MR ≤ −21 using the well-known FIR/radio correlation to link the radio with ongoing star formation within individual cluster galaxies. Spectroscopic redshifts exist for ∼ 96% of the optical identifications. These radio-selected galaxies reveal the ‘active’ galaxy population (starburst and active galactic nuclei) within these rich cluster environments that can be identified regardless of their level of dust obscuration. These new radio data provide the largest sample to date of low-luminosity radio galaxies within rich cluster environments allowing an unbiased search for dusty starbursting galaxies. For all clusters in our sample, we are sensitive to star formation rates (M ≥ 5M⊙) & 5 M⊙yr −1 . We have found that the excess number of low-luminosity ‘starburst’ radio-selected galaxies (SBRG) found by Owen et al. (1999) in Abell 2125 is not indicative of other rich clusters in our sample. The average fraction of SBRG is h fSBRG i = 0.022 ± 0.003. The A2125 fraction is fSBRG = 0.09 ± 0.03 which is significantly different from the sample average at a > 99.99% confidence level. Both A1278 and A1689 are slightly different from the rest of the sample at ∼ 90% confidence level. The bimodal structure of both the x-ray brightness distribution and optical adaptively smoothed images of A1278 and A2125 suggests that ongoing cluster-cluster mergers may be enhancing this SBRG population. The A1689 excess low-luminosity (and high-luminosity) radio galaxy population may be due to interaction with the ICM. The mid-infrared ISOCAM results for A1689’s radio galaxy population suggests that the radio emission for both low- and high-luminosity radio galaxies is AGN in origin except for one radio galaxy. There is a significant spatial distribution difference between the low and high-luminosity (HLRG) radio-selected populations. The SBRG have a core radius of 0.40 ± 0.08 Mpc which is > 3× larger than the HLRG core radius. In addition, 48% of the SBRGs have colors that are bluer than a typical Sab galaxy compared to 4% for the HLRGs. The average absolute magnitude for the SBRG’s is h MR i = −21.93 ± 0.05, while for the HLRG’s it is h MR i = −22.33 ± 0.07, indicating that the SBRG are less optically luminous than their HLRG counterparts. The HLRGs seem to be a subclass of the cluster’s massive red elliptical population, while the SBRGs have a projected radial distribution more like the blue spiral population. Our results indicate that most of the SBRGs are probably gas-rich disk galaxies undergoing & 5 M⊙ yr −1 of star-formation. Subject headings: galaxies: evolution — galaxies: clusters: Abell — galaxies: starburst — radio continuum: galaxies

Ultraluminous Infrared Mergers: Elliptical Galaxies in Formation?

We report high-quality near-IR spectroscopy of 12 ultraluminous infrared galaxy mergers (ULIRGs). Our new VLT and Keck data provide ~05 resolution, stellar and gas kinematics of these galaxies, most of which are compact systems in the last merger stages. We confirm that ULIRG mergers are in formation. Random motions dominate their stellar dynamics, but significant rotation is common. Gasdynamics and stellar dynamics are decoupled in most systems. ULIRGs fall on or near the fundamental plane of hot stellar systems, and especially on its less evolution-sensitive, reff-σ projection. The ULIRG velocity dispersion distribution, their location in the fundamental plane, and their distribution of vrot sin i/σ closely resemble those of intermediate-mass (~L*), elliptical galaxies with moderate rotation. As a group ULIRGs do not resemble giant ellipticals with large cores and little rotation. Our results are in good agreement with other recent studies indicating that disky ellipticals with compact cores or cusps can form through dissipative mergers of gas-rich disk galaxies while giant ellipticals with large cores have a different formation history.

How Did Globular Clusters Form?

It is suggested that there have been at least two physically distinct epochs of massive cluster formation. The first generation of globular clusters may have formed as halo gas was compressed by shocks driven inward by ionization fronts generated during cosmic reionization at z ~ 6. On the other hand a second generation of massive clusters might have been formed at later times by compression, and subsequent collapse, of giant molecular clouds. In some cases this compression may have been triggered by heating of the interstellar medium via collisions between gas-rich disk galaxies. It is also suggested that the present specific globular cluster frequency in galaxies was mainly determined by the peak rate of star creation, with elevated peak rates of star formation per unit area resulting in high present specific globular cluster frequencies.

Nature of Widely Separated Ultraluminous Infrared Galaxies

In the complete sample of ultraluminous infrared galaxies (ULIRGs) compiled by Kim (1995) about 5% consists of widely separated galaxies which are presumably in the early phase of interaction. This fact is contrary to the conventional view that ULIRGs are in the final stages of the merger of two gas-rich disk galaxies. We have undertaken high resolution CO(J=1-0) observations for the ultraluminous infrared galaxies that have nuclear separations larger than 20 kpc. We have detected the CO emission in 5 out of 6 systems, but only in one component of the ULIRG pairs. 4 of them have LINER spectral type and 1 galaxy has Seyfert II spectral type. In K'-band images these components are also brighter than the other components which have either HII-region spectra or no detectable emission lines. Using the standard conversion factor, the molecular gas content is estimated to be a few times 10 $^{10}$ M$_\odot$, similar to that of the other ultraluminous galaxies. The result indicates that the galaxy containing the molecular gas is also the source of most, if not all, of the huge far-infrared luminosity of the system. The optical and K'-band imaging observations and optical spectra suggest multiple merger scenario for 1 system. If the remaining systems are in an early stage of a binary tidal interaction, the commonly accepted interpretation of the ULIRG phenomenon as the final merger stage of two disk galaxies may need to be re-examined.

Erratum: “Molecular Gas in Infrared-Excess, Optically Selected Quasars and the Connection with Infrared-luminous Galaxies” [Astron. J. [BF]121[/BF], 1893 (2001)

The initial results of a millimeter (CO) survey of infrared-excess, optically selected quasars from the Palomar-Green (PG) Bright Quasar Survey with redshifts in the range 0.04 z 0.17 are presented. These observations represent the Ðrst step toward establishing with a complete sample whether or not quasi-stellar objects (QSOs) reside in molecular gasErich galaxies, as well as determining how the infrared and molecular gas properties of QSOs compare with those of ultraluminous infrared galaxies (ULIGs), which are a possible evolutionary precursor of QSOs. The sample consists of QSOs having absolute blue magnitudes and infrared excesses (i.e., infrared to ““ big blue bump II M B [[22.0 luminosity), km)[ 0.36, in which the contribution to the bolometric lumiL IR(8E1000 km)/L bbb(0.1E1.0 nosity of infrared thermal dust emission for all PG QSOs is typically 20%E40%. Six out of ten of the QSOs observed are detected in the CO (1] 0) emission line ; two detections conÐrm previous, less sensitive detections of CO (1] 0) in PG 1613]658 and 0838]770, and four additional QSOs are detected for the Ðrst time (PG 1119]120, 1351]640, 1415]451, and 1440]356). These six detections, plus two previous detections of CO in I Zw 1 and Mrk 1014, bring the total number of 0.04 z 0.17 infraredexcess PG QSOs detected in CO to date to eight and provide possible evidence that, in addition to fueling star formation, molecular gas may also serve as a primary source of fuel for QSO activity. Both the eight QSOs detected in CO and the four QSOs with nondetections have high infrared-to-CO luminosity ratios, relative to most infrared-luminous galaxies of the same The placement of L IR/L CO @ , L IR these QSOs on the plane may be due to signiÐcant contributions from dust heated by the L IR/L CO @ EL IR QSO in their host galaxies, due to dust heated by massive stars formed with high efficiency (i.e., per unit molecular gas mass) relative to most infrared-luminous galaxies, or a combination of both. If the observed high values of are primarily due to dust heating by QSOs, a signiÐcant fraction of L IR/L CO @ ULIGs with similar values of may also contain buried active galactic nuclei. Alternatively, if L IR/L CO @ high is due primarily to star formation, then an enhanced star formation rate may be intimately L IR/L CO @ connected to the QSO phenomenon. A comparison of the infrared and CO luminosities of the eight detected and four undetected QSOs with the optical morphologies of their host galaxies shows that the three QSOs with and similar to ULIGs appear to reside in morphologically disturbed galaxies L IR L CO @ (i.e., ongoing major mergers involving two or more gas-rich disk galaxies), whereas the host galaxies of the remaining eight QSOs with lower and appear to be a mixture of barred spiral host galaxies, L IR L CO @ elliptical galaxies, galaxies with an indeterminate classiÐcation, and at least one ongoing major merger.

Arp 119: A High‐Speed Galaxy Collision with Episodic Star Formation

Colliding galaxies are excellent laboratories for studying galactic evolution and global star formation. Computer simulations of galaxy collisions, in which at least one galaxy has a significant gaseous component, show the production of density enhancements and shock waves in the interstellar medium. These high-density regions coincide with the locations of recent, large-scale star formation in observations of some real colliding galaxies. Thus, combined n-body/hydrodynamic computer simulations can be used to explore the history and conditions of star forming regions in colliding galaxies. The galaxy system Arp 119 (CPG 29) contains a southern member, Arp 119S (Mrk 984), that has an extremely disturbed appearance, and a northern member, Arp 119N, a gas-poor elliptical. The morphology of both members can be fit well by a simulation in which a gas-rich disk galaxy has been impacted by an equal mass elliptical that had a trajectory approximately perpendicular to the plane of the disk and passed through the disk slightly off-center. We compare multi-wavelength observations of the Arp 119 system with a combined n-body/SPH simulation of colliding galaxies. We find that the progression of recent, large-scale star formation in this galaxy can be accounted for by a single, outwardly propagating collision-induced density wave in the gas.

Molecular Gas in Infrared-Excess, Optically Selected and the Quasars Connection with Infrared-Luminous Galaxies

The initial results of a millimeter (CO) survey of infrared-excess, optically selected quasars from the Palomar-Green (PG) Bright Quasar Survey with redshifts in the range 0.04 0.36, in which the contribution to the bolometric luminosity of infrared thermal dust emission for all PG QSOs is typically 20%–40%. Six out of ten of the QSOs observed are detected in the CO (1 → 0) emission line; two detections confirm previous, less sensitive detections of CO (1 → 0) in PG 1613+658 and 0838+770, and four additional QSOs are detected for the first time (PG 1119+120, 1351+640, 1415+451, and 1440+356). These six detections, plus two previous detections of CO in I Zw1 and Mrk 1014, bring the total number of 0.04 < z < 0.17 infrared-excess PG QSOs detected in CO to date to eight and provide possible evidence that, in addition to fueling star formation, molecular gas may also serve as a primary source of fuel for QSO activity. Both the eight QSOs detected in CO and the four QSOs with nondetections have high infrared-to-CO luminosity ratios, LIR/L, relative to most infrared luminous galaxies of the same LIR. The placement of these QSOs on the LIR/L–LIR plane may be due to significant contributions from dust heated by the QSO in their host galaxies, due to dust heated by massive stars formed with high efficiency (i.e., per unit molecular gas mass) relative to most infrared luminous galaxies, or a combination of both. If the observed high values of LIR/L are primarily due to dust heating by QSOs, a significant fraction of ULIGs with similar values of LIR/L may also contain buried active galactic nuclei. Alternatively, if high LIR/L is due primarily to star formation, then an enhanced star formation rate may be intimately connected to the QSO phenomenon. A comparison of the infrared and CO luminosities of the eight detected and four undetected QSOs with the optical morphologies of their host galaxies shows that the three QSOs with LIR and L similar to ULIGs appear to reside in morphologically disturbed galaxies (i.e., ongoing major mergers involving two or more gas-rich disk galaxies), whereas the host galaxies of the remaining eight QSOs with lower LIR and L appear to be a mixture of barred spiral host galaxies, elliptical galaxies, galaxies with an indeterminate classification, and at least one ongoing major merger.

Molecular Gas in the Powerful Radio Galaxies 3C 31 and 3C 264: Major or Minor Mergers?

We report the detection of 12CO (1 → 0) and 12CO (2 → 1) emission from the central regions (5-10 kpc) of the two powerful radio galaxies 3C 31 and 3C 264. Their individual CO emission exhibits a double-horned line profile that is characteristic of an inclined rotating disk with a central depression at the rising part of its rotation curve. The inferred disk or ring distributions of the molecular gas are consistent with the observed presence of dust disks or rings detected optically in the cores of both galaxies. For a CO-to-H2 conversion factor similar to that of our Galaxy, the corresponding total mass in molecular hydrogen gas is (1.3 ± 0.2) × 109 M☉ in 3C 31 and (0.31 ± 0.06) × 109 M☉ in 3C 264. Despite their relatively large molecular gas masses and other peculiarities, both 3C 31 and 3C 264, as well as many other powerful radio galaxies in the (revised) 3C catalog, are known to lie within the fundamental plane of normal elliptical galaxies. We reason that if their gas originates from the mergers of two gas-rich disk galaxies, as has been invoked to explain the molecular gas in other radio galaxies, then both 3C 31 and 3C 264 must have merged a long time (a few billion years or more) ago, but their remnant elliptical galaxies have only recently (last tens of millions of years or less) become active in radio. Instead, we argue that the cannibalism of gas-rich galaxies provides a simpler explanation for the origin of molecular gas in the elliptical hosts of radio galaxies. Given the transient nature of their observed disturbances, these galaxies probably become active in radio soon after the accretion event, when sufficient molecular gas agglomerates in their nuclei.

Dwarf Galaxy Formation Induced by Galaxy Interactions

Growing evidence is being accumulated that some gas-rich dwarf galaxies are formed from material liberated by galaxy collisions and/or mergers. Also, gas-poor dwarf elliptical galaxies are often found in the central regions of clusters of galaxies. These observations suggest strongly that the formation of most dwarf galaxies is linked to galaxy interactions. Therefore, now seems like the right time to investigate the formation efficiency of such tidal dwarf galaxies. Adopting the galaxy interaction scenario proposed by Silk & Norman, we find that if only a few dwarf galaxies are formed in each galaxy collision, we are able to explain the observed morphology-density relations for both dwarf and giant galaxies in the field, groups of galaxies, and clusters of galaxies. It seems worthwhile noting that tidal dwarf formation may be coupled with the transformation from gas-rich disk galaxies to early-type galaxies such as S0 and elliptical galaxies.

Superdisks in Radio Galaxies

We present evidence for a striking new feature of powerful radio galaxies: gigantic disklike structures of thermal gas and dust oriented roughly perpendicular to the radio jets and situated around the elliptical host galaxy. These superdisks, or fat pancakes, appear to have a typical diameter of at least 75 kpc and a width of ~25 kpc. Quite plausibly, they are tidally stretched remnants of gas rich disk galaxies, or damped Lyman-a clouds, captured by the massive elliptical, and heated by its hot coronal gas. Observational manifestations of the superdisks include the sharp, quasi-linear, edges of the radio lobes on the side facing the central elliptical, detected in at least a dozen well mapped radio galaxies at low to moderate redshifts. For radio galaxies with z > 1.8, evidence for superdisks is based on the apparent asymmetry of diffuse Lya emission associated with the radio lobes, which is a very sensitive tracer of dust. This asymmetry can be understood if the brighter Lya emission is associated with the radio lobe on the near side of the nucleus and hence not obscured by the dust in the superdisk. Superdisks can provide more consistent explanations for some of the best established correlations among the radio source properties: namely, the Laing-Garrington effect and the correlated radio-optical asymmetry. They are also likely to be responsible for part of the low-frequency flux variability of compact radio sources. We argue they may provide an improved explanation of Lya absorption dips in the emission profiles of distant radio galaxies. Additional methods of searching for superdisks and testing this scenario are proposed.

The Minor‐Merger–driven Nuclear Activity in Seyfert Galaxies: A Step toward the Simple Unified Formation Mechanism of Active Galactic Nuclei in the Local Universe

Seyfert nuclei are typical active galactic nuclei (AGNs) in the local universe, which are thought to be powered by gas accretion onto a supermassive black hole (SMBH). The dynamical effect exerted either by non-axisymmetric gravitational potential (such as a stellar bar) or by interaction with other galaxies has been often considered to cause the efficient gas fueling. However, recent systematic studies for large samples of Seyfert nuclei have shown that; 1) Seyfert nuclei do not prefer barred galaxies as their hosts, and 2) only $\simeq$ 10% of Seyfert galaxies have companion galaxies. These findings raise again a question; What is important in the fueling of nuclear activity ? Here we suggest that an alternative fueling mechanism, the minor-merger (i.e., mergers between a gas-rich disk galaxy and a bound, less-massive satellite galaxy) driven fueling appears consistent with almost all important observational properties of Seyfert galaxies. Nucleated (i.e., either a SMBH or a dense nuclear star cluster) galaxies and satellites seem necessary to ensure that the gas in the host disk is surely fueled into the very inner region (e.g., $\ll$ 1 pc). Taking account that local quasars may be formed by major mergers between/among galaxies, we propose that all the AGNs in the local universe were made either by minor or by major mergers.

Fueling Nuclear Starbursts

We present a numerical simulation of two merging equal-mass, gas-rich disk galaxies. Special emphasis is given to an accurate treatment of the interstellar medium physics and star formation with its feedback. We will explain how the negative feedback from young stars restricts the bulk of the star formation during the merger-induced starburst to the nucleus.

HI in Karachentsev Objects Properties of new nearby Dwarf Galaxies

AbstractThis is a report on HI observations of newly discovered nearby dwarf galaxies, most of which of low surface brightness, from the first section of the Karachentsev catalog. Observations were performed using the 100-m radiotelescope at Effelsberg, the Nançay radiotelescope, and the compact array of the Australia Telescope. We observed 220 galaxies with a detection rate of about 60%. 35 of the observed galaxies are located within the Local Volume (i.e., within 10 Mpc). The smallest detected galaxies have diameters around 0.3 kpc and HI-masses of a few times 106 solar masses. We confirm a correlation between HI column density and surface brightness of gas-rich disk galaxies.

Ages and Metallicities of Young Globular Clusters in the Merger Remnant NGC 7252

Ultraviolet-to-visual spectra of eight young star clusters in the merger remnant and protoelliptical galaxy NGC 7252, obtained with the 4 m Blanco Telescope on Cerro Tololo, are presented. These clusters lie at projected distances of 3–15 kpc from the center and move with a velocity dispersion of 140 ± 35 km s-1 in the line of sight. Seven of the clusters show strong Balmer absorption lines in their spectra [EW(Hβ) = 6–13 A], while the eighth lies in a giant H II region and shows no detectable absorption features. Based on comparisons with model cluster spectra computed by Bruzual & Charlot and Bressan, Chiosi, & Tantalo, six of the absorption-line clusters have ages in the narrow range of 400–600 Myr, indicating that they formed early in the recent merger. These clusters, and probably also the seventh absorption-line cluster, are globular clusters, as judged by their small effective radii and ages corresponding to ~102 core crossing times. The one emission-line object is 10 Myr old and may be a nascent globular cluster or an OB association. The mean metallicities measured for three clusters are solar to within about ±0.15 dex, suggesting that the merger of two likely Sc galaxies in NGC 7252 formed a globular cluster system with a bimodal metallicity distribution. Since NGC 7252 itself shows the characteristics of a 0.5–1 Gyr old protoelliptical galaxy, its second-generation solar-metallicity globular clusters provide direct evidence that giant elliptical galaxies with bimodal globular cluster systems can form through major mergers of gas-rich disk galaxies. A puzzling property of the observed young globular clusters are the high masses of (1–35)M(ω Cen) implied by their luminosities and ages (for an assumed Salpeter IMF). A spectrum of a candidate superluminous globular cluster in the elliptical galaxy NGC 1700, obtained with the Hiltner Telescope at MDM Observatory, shows this object to be a foreground star.

A CCD Study of the Environment of Seyfert Galaxies. II. Testing the Interaction Hypothesis

An analysis of the environment of a sample of 33 CfA Seyfert galaxies and a control sample of 45 nonactive galaxies matched in luminosity, redshift, and morphology to the Seyfert galaxies as reported in Paper I is presented. The covariance function amplitudes of the Seyfert and control samples are not statistically significantly different from one another and from the general field. Moreover, the companion frequency of the Seyfert galaxies, the probability of finding a companion galaxy brighter than -17.5 in R within 50 kpc (0.30 ? 0.11), is not statistically significantly different from that for the nonactive control sample (0.23 ? 0.09). The mean environment of Seyfert 1 galaxies is found to be different from that of Seyfert 2 galaxies at greater than the 95% confidence level, in the sense that the latter have a larger covariance amplitude. Such evidence is problematic for the Unified Model, which attributes spectroscopic differences between the classes to purely geometric effects on the order of parsec scales. It cannot, however, account for differences on the order of 100 kpc scales. It is argued that triggering of activity in galactic nuclei may involve a variety of mechanisms and may depend on the luminosity of the class. That is, while there is excellent evidence that QSOs, radio galaxies, and BL Lac objects inhabit environments significantly richer than the field, the same does not seem to be true for Seyfert galaxies and perhaps for LINERs. Finally, because a significant fraction of Seyfert host galaxies show little or no evidence for a recent merger, it is suggested that minor mergers, mergers that involve a gas-rich disk galaxy and a bound companion or satellite galaxy, may play a significant role in triggering activity in Seyfert galaxies.

Sinking Satellite Disk Galaxies. I. Shell Formation Preceded by Cessation of Star Formation

Detailed numerical simulations have been carried out on the sinking of a gas-rich disk galaxy into a large elliptial (spherical) galaxy. Both elliptical and spiral galaxies have been modeled as self-gravitating particle systems. The interstellar gas component in the disk has been modeled as a system of inelastic cloud particles dissipating kinetic energy in mutual collisions. Star formation processes and gas consumption owing to star formation are included in the numerical code. Sinking on a radial or slightly retrograde orbit has been found to produce regular shell structures (i.e., without loops or filaments) made from the disk material. In this shell formation, no significant segregation of the stellar and gaseous components occurs. Global distribution of the gas clouds is similar to that of the stars, though the latter make clearer shells. Star formation is turned off well before the shells develop, because gas clouds are widely scattered and the gas density is much decreased. We propose that the poststarburst nuclei often observed in shell galaxies are not necessarily the result of preceding starbursts, as is widely believed, but could be the result of this drastic truncation of star formation activity.