Shell Galaxies Research Articles

Minor Mergers and the Formation of Shell Galaxies

AbstractWe present a new mechanism which can explain the observed radial displacement between the stellar and gaseous shells observed around early type galaxies. Our model is based on the well-known phasewrapping process taking into account that a fraction of the interstellar medium is clumpy and consequently it experiences a low dynamical friction during a merging event. Moreover, our recent detection of cold molecular gas associated with the outer shells of Centaurus A is in agreement with the proposed model. This suggests that a considerable fraction of molecular gas in minor mergers may be distributed away from the nuclear regions and remain undetected.

Imaging of the shell galaxies NGC 474 and 7600, and implications for their formation

We present photometric observations of two shell galaxies, NGC 474 and NGC 7600. We examine the photometric colours and surface brightnesses of the shells and their host galaxies, and the isophotal parameters of each galaxy. In the case of NGC 474, we nd that the shell formation is consistent with a merger origin although it is possible that the close companion NGC 470 is contributing to the shell system via mass transfer. NGC 7600 exhibits shell geometry and colours which also favour a merger origin.

Shell Formation in NGC 474

We present broad band photometry (B and R) of the classic shell galaxy NGC 474. Preliminary results indicate that the shells have a similar colour to and follow the same trend of colour with radius as the underlying galaxy.

Star formation history of early-type galaxies in low density environments

The present paper is a companion of two others dedicated one to the measurement of the line–strength indices (Longhetti et al. 1997a) and the second to trace back the star formation history of a sample of early–type galaxies by comparing observed indices to the predictions of new spectro–photometric models (Longhetti et al. 1997b). The sample of 51 early-type galaxies in low density environments is composed of two sub-sets of galaxies: 21 shell galaxies from the Malin & Carter (1983) catalogue (one of which shows double nucleous and has been considered as two separate objects) and 30 members of isolated interacting pairs from the Reduzzi & Rampazzo (1995) catalogue. Most of the objects show fine structures. The paper collects nuclear kinematic data together with the velocity and velocity dispersion curves of the stellar and gaseous components as a function of the distance from the galaxies centres. The galaxies heliocentric systemic velocity compares within km s-1 with RC3 data, while their central velocity dispersion compares within km s-1 , km s-1 and km s-1 with Gonzalez (1993), Davies et al. (1987) and Carter et al. (1988) respectively. The detailed comparison between our velocity and velocity dispersion curves and those from several authors is discussed. 9 out of 22 shell galaxies nuclei show emission lines, 4 of which, using data in the literature, have line ratios characteristic of LINERs. 10 members of pairs out of 30 show emission lines. RR 331a has a Seyfert like nucleus, while for the remaining galaxies the ([O III] λ 5007)/Hβ ratio is characteristic of low ionization regions. In a small fraction of the objects the emission component is detectable outside the central value. None of the objects in the sample shows counter-rotation of the gaseous versus the stellar component. The two components appear associated, although, in two cases there is evidence that gas and stars lie on different planes. This latter phenomenon could be associated to accretion events. Emission lines in the central part of the RR 331a show a secondary component in the emission lines profile. E 2400100 has two nuclei embedded in the main body of the galaxy. The U-shape profile of the stellar velocity profile shows the ongoing interaction of the two nuclei. profile of shell galaxies is, finally, discussed in relation to the hypothesis of the accretion/merging origin of these galaxies.

Star formation history of early-type galaxies in low density environments

This paper is the first of a series (Longhetti et al. 1997a,b) dedicated to the study of the star formation history in early-type galaxies which show fine structures and/or signatures of interaction . It presents nuclear line–strength indices for a sample composed of 21 shell galaxies, from the the Malin & Carter (1983) southern survey, and 30 members of isolated interacting pairs, from the Reduzzi & Rampazzo (1995) catalogue, located in low density environments . The spectral range covers 3700 A 5700 A at 2.1 A FWHM resolution. We measure 16 red ( 4200 A) indices defined by the Lick Group. Measures have been transformed into the Lick-IDS standard” system. The procedure has been tested on a set of 5 elliptical galaxies selected from the Gonzalez (1993) sample. We derive also three blue ( 4200) indices, namely Δ (4000 A) defined by by Hamilton (1985), H+K(CaII) and Hδ /FeI defined by Rose (1984, 1985). Blue indices are correlated to the age of the last starburst occurred in a galaxy (Leonardi & Rose 1996). The determination of these indices, the estimate of the measurement errors and the correction for the galaxies velocity dispersions are discussed in detail. In the Appendix A we present the indices for a set of hot stars ( K) which may be used for extending W92 fitting functions toward high temperatures.

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.

Gas Accretion in NGC 3656 (ARP 155)

view Abstract Citations (11) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gas Accretion in NGC 3656 (ARP 155) Balcells, Marc ; Sancisi, Renzo Abstract We study the gas distribution in the recent merger remnant NGC 3656, a perturbed elliptical. The total gaseous mass content is a few times 10^9^M_sun_. The galaxy contains comparable amounts of atomic and molecular hydrogen. Roughly 90% of the gas mass lies at or near the prominent minor axis dust lane of the galaxy. There are also strong indications of an H I extension to the south, which projects onto a luminosity condensation bound by a prominent optical shell. Most likely, the galaxy is still accreting gas from the southern extension. This system may be a precursor of polar ring or shell galaxies such as Cen A. Publication: The Astronomical Journal Pub Date: March 1996 DOI: 10.1086/117850 Bibcode: 1996AJ....111.1053B Keywords: GALAXIES: INDIVIDUAL: NGC 3656; GALAXIES: INTERACTIONS; ACCRETION full text sources ADS | data products SIMBAD (1) NED (1)

Peculiar Kinematics in the Core of NGC 474

We present absorption line profile analysis of NGC 474, an elliptical with prominent, irregular shells. Profiles are parameterized with Gauss-Hermite polynomials (van der Marel & Franx ApJ 407 1993; Rix & White MNRAS 254 1992). The fastest rotation (∼ 50 km s–1) and steepest central velocity gradient along the intermediate photometric axis rules out the possibility that NGC 474 is a face-on S0 (Schombert & Wallin AJ 94 1987) and suggests that it is triaxial. The asymmetry of the LOSVDs (h3 up to 0.08) indicates the presence of a subsystem with rapid, ordered rotation. The minor axis velocity curve shows a kinematic feature at 3–4″ east of the nucleus, with no associated h3 or h4 features. Non-parametric LOSVD analysis (unresolved Gaussian decomposition, Kuijken & Merrifield MNRAS 264 1993) reveals a double-peaked profile at that location. In all position angles line-profiles are distinctly pointy for radii up to 2″, and are consistent with zero further out. We have found similar central positive h4 terms in the shell galaxy NGC 2865 (Hau et al. MNRAS in prep). Cores with pointy LOSVDs are uncommon in ellipticals (Bender, Saglia & Gerhard MNRAS 269 1994). Positive h4 terms might contain important clues on the shell-formation mechanism in ellipticals.

A search for secondary nuclei in shell galaxies

A search for secondary nuclei in shell galaxies

Segregation of gas and stars in shell galaxies

view Abstract Citations (85) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Segregation of Gas and Stars in Shell Galaxies Weil, Melinda L. ; Hernquist, Lars Abstract Using a code which is capable of evolving composite systems of collisionless matter and gas, we explore mergers like those thought responsible for the shells seen around many elliptical galaxies. If a small companion containing both gas and stars is accreted by a more massive primary the stellar and gaseous debris are rapidly segregated: while the stars are free to oscillate back and forth in the primary's potential, thereby forming shells, the oppositely directed flows near the center of the primary effectively dissipate the orbital kinetic energy of the gas. Consequently the gas settles into compact disks or rings in the nucleus of the primary depending on orbital parameters. We note implications of these findings for the production of polar rings and the onset of nuclear activity in galaxies. Publication: The Astrophysical Journal Pub Date: March 1993 DOI: 10.1086/172347 Bibcode: 1993ApJ...405..142W Keywords: Elliptical Galaxies; Galactic Evolution; Interacting Galaxies; Shell Galaxies; Astronomical Models; Galactic Nuclei; Interstellar Matter; Astrophysics; GALAXIES: ELLIPTICAL AND LENTICULAR; CD; GALAXIES: INTERACTIONS; GALAXIES: ISM; GALAXIES: NUCLEI full text sources ADS | data products SIMBAD (7)

Starbursts in the nuclei of shell galaxies

THE luminosity profile of elliptical galaxies for the most part follows a smooth distribution of the de Vaucouleurs form1, but recent observations have shown that most ellipticals possess fine structure, including sharp-edged 'shells'2–5 and 'ripples'6–8. This suggests that the underlying dynamical structure is not fully relaxed, and Schweizer's proposal6 that these features result from the accretion of matter from a less massive companion galaxy has been amply confirmed by simulations of galaxy encounters and mergers9–14. A likely supply of shell-forming material is dwarf spheroidal and disk galaxies, which tend to be gas-rich, but shell galaxies do not seem to have any notable excess of gas15,16. Here we report simulations of shell-forming galaxy mergers in which gas and stars are tracked separately. We find that the two components are effectively segregated: the stars form sharp-edged features by oscillating back and forth in their orbits, but most of the gas settles into a compact disk or ring in the nucleus of the primary. Shocks in the gas are likely to lead to star formation, perhaps accounting for the presence of young stars in the nuclei of some shell galaxies; if the primary galaxy contains a central black hole, evolution of the new supply of gas may yield an active galactic nucleus. Mergers between large galaxies and gas-rich satellites may therefore be a way of activating dormant elliptical galaxies.

Shells and isophotal distortions in elliptical galaxies

An improved shell-finding algorithm is described and images of eight previously unknown shell galaxies visible from the northern hemisphere are presented. The data were obtained in the R-band using the 1-m Jacobus Kapteyn Telescope, located in the Canary Islands. Most of the galaxies were selected on the basis of independent evidence for a merger or interaction, such as extensive dust or kinematically decoupled cores. The detection of shells and other fine structures is achieved by subtracting a smooth elliptical model of the underlying galaxy and then inspecting the residual map. The isophotal distortions resulting from these structures are quantified using the fourth cosine term of the isophotal intensity. In particular, it is suggested that shells may be responsible for boxy isophotes in some cases

Stellar populations in shell galaxies

view Abstract Citations (23) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar Populations in Shell Galaxies McGaugh, Stacy S. ; Bothun, Gregory D. Abstract UBV surface photometry of the shell galaxies Arp 230, NGC 7010, and Arp 223 is presented. We find all to be the result of mergers. In Arp 230, the merger of two spirals induced a burst of star formation which has declined in strength since the collision. The remnant seems likely to become elliptical. NGC 7010 has very red shells, perhaps redder than the galaxy itself. Evolution of the stellar content of these shells may be important. Arp 223 has shell colors which are slightly bluer than the main body of the galaxy, consistent with an origin in an Sb. Our data indicate that the timescale since the interaction is typicaly ~ 1 Gyr. Moreover, those ellipticals that have been formed by mergers should have complex stellar populations with a component formed in the collision. This extra component might be detectable by the methods of empirical population synthesis, at least to some age limit, making it possible to investigate the relative numbers of ellipticals formed in this manner by methods other than morphology. Publication: The Astronomical Journal Pub Date: October 1990 DOI: 10.1086/115580 Bibcode: 1990AJ....100.1073M Keywords: Galactic Structure; Star Distribution; Star Formation; Astronomical Photometry; Color-Color Diagram; Ultraviolet Radiation; Astronomy; GALAXIES: INTERACTIONS; GALAXIES: STELLAR CONTENT full text sources ADS | data products SIMBAD (9) NED (3)

Do elliptical galaxies have thick disks?

AbstractWe discuss new evidence which supports the existence of thick disks in elliptical/S0 galaxies. Numerical simulations of weak interactions with thick disk systems produce shell structures very similar in appearance to those observed in many shell galaxies. We think this model presents a more plausible explanation for the formation of shell structures in elliptical/S0 galaxies than does the merger model and, if correct, supports the existence of thick disks in elliptical/S0 galaxies.

Caustic Waves in Galaxy Disks Produced in Collisions with Low Mass Companions

At this meeting much attention has been focussed on interactions and mergers between roughly equal mass galaxies. On the contrary, I will begin by mentioning a few justifications for studying collisions with relatively low mass companions, specifically, less than about one third of the mass of the target galaxy. The first is simply that such collisions are likely to be common, given that the galaxy luminosity function is broad. The second reason is that such collisions have evidently been less well studied than collisions between nearly equal partners. However, there are a few important exceptions to this generalization, including the sinking satellite problem (e.g. Quinn and Goodman 1986), and the collisional model for the formation of shell galaxies in which a companion of negligible mass is completely disrupted(e.g. Dupraz and Combes 1986, Hernquist and Quinn 1988). The third, and potentially most important reason, is that the effects of a collision with a low-mass companion are less extreme (at least from the big galaxy’s point of view!). Thus, these effects are closer to the theorist’s ideal of a “small perturbation”. This is important for both conceptually understanding the effects of the collision, and for justifying the use of approximate numerical techniques (e.g. restricted three-body) to study them.

Formation of shell galaxies. II - Nonspherical potentials

The formation of shells through interactions of low-mass companions with nonspherical galaxies is investigated using the restricted three-body method. We find that sharp-edged features remain a common outcome of such encounters, regardless of the detailed structure of the more massive galaxy. The morphology and alignment of the resulting shell system are sensitive to the form of the potential of the primary galaxy; however, they depend also on the orbital parameters of the collision and on the nature of the companion. The latter two considerations imply that it will be difficult to infer the intrinsic shapes of luminous ellipticals from observations of shells. 22 refs.

The components of mid- and far-infrared emission from S0 and early-type shell galaxies

The IRAS database has been used to study detections of about 150 early-type elliptical and S0 galaxies exhibiting a shell structure. No strong evidence for the expected enhancement of either star formation rates or heating of the interstellar medium is found. It is suggested that for some of the sample galaxies either a contribution from warm dust surrounding evolved stars or emission from an active nucleus may be significant.

Spectra of shell ellipticals: redshifts, velocity dispersions and evidence for recent nuclear star formation

Nuclear spectra have been obtained for 100 of the 137 galaxies in the Malin & Carter catalogue. The objects were selected purely on the grounds of position on the sky. Redshifts are presented for all, and velocity dispersions for most of these systems. Many of these galaxies have spectra atypical of elliptical or lenticular galaxies. A-type stars, suggestive of recent star formation in the galactic nucleus, are probably required in 15 to 20 per cent, and 25 per cent have emission lines. About 10 per cent have colours, absolute magnitudes and spectra similar to the post-starbust galaxies in high redshift clusters. Assuming bursts of star formation have been triggered in the shell galaxies by merger events which also created the shells, we hypothesize that the more distant post-starburst galaxies may have also undergone merger or accretion events.

Formation of shell galaxies. I - Spherical potentials

view Abstract Citations (215) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Formation of Shell Galaxies. I. Spherical Potentials Hernquist, Lars ; Quinn, P. J. Abstract The formation of shells through interactions of low-mass systems with spherical galaxies is investigated using the restricted three-body method. It is shown that such encounters lead to the development of sharp-edged features under a much wider set of conditions than considered by Quinn. In particular (i) collisions involving low-mass spheroids, in addition to low-mass disks, can produce shells and would naturally account for the absence of gas and dust in some elliptical shell galaxies; (ii) shells can be formed from companions on nonradial orbits, as well as radial orbits, and tend to be rather confused, representative of the majority of observed shell galaxies; (iii) mass transfer that occurs during roughly parabolic encounters can also generate shells. Shells appear to be a general consequence of interactions of spatially compact and/or dynamically cold systems with large galaxies. The variety of structures exhibited by the present simulations resembles the observed shell galaxies. However, owing to the number of free parameters and the chaotic nature of most observed shell systems, it seems unlikely in retrospect that shells will prove widely useful as diagnostics of the potential of the host galaxies. Possible exceptions remain such unusually regular systems as NGC 3923. Publication: The Astrophysical Journal Pub Date: August 1988 DOI: 10.1086/166592 Bibcode: 1988ApJ...331..682H Keywords: Computational Astrophysics; Elliptical Galaxies; Galactic Evolution; Galactic Structure; Spherical Shells; Astronomical Photometry; Mass Transfer; Radial Velocity; Three Body Problem; Astrophysics; GALAXIES: INTERNAL MOTIONS; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (13) NED (12) Related Materials (1) Part 2: 1989ApJ...342....1H

The shell system around NGC 3923 and its implications for the potential of the galaxy

A study of the shell galaxy NGC 3923 is presented. It is mainly based on CCD and photographic observations made at the CFHT, ESO, and AAT from 1982 to 1986. New shells have been discovered. The geometry of 22 shells is investigated in detail with an estimate of the uncertainties. The interleaving of the shells is not a clear as it has been claimed: it is well defined for the outer shells but the distances of the inner shells present a certain symmetry relative to the center. Shells have a roughly constant ellipticity of E = 1. +/- 0.4 [ E = 10 x ( 1 - b/a)]. It has been suggested by some authors that shell ellipticities are related to the elliptical potential; a brief study of current models of elliptical galaxies shows that the shell ellipticities are of the same magnitude as the equipotential ellipticities, but no clear relation id visible. Profiles were derived for 19 shells, and a simple model was fitted to these profiles. The shell thickness is typically of 0.17 kpc for the inner shells. The total B luminosity of the shell system is ~ 5 % (+/- 1 %) of the luminosity of the galaxy. The outermost shells are much more luminous than the inner ones: the outer four shells contribute ~85% of the total luminosity. A dust lane has been discovered on deep CCD exposures. If it is in a stable configuration its orientation is in contradiction with some previous studies based on numerical simulations which concluded that NGC 3923 was a prolate system. A photometric study of the galaxy is presented. The inferred central M/L ratio, is close to 10 which does not depart from the values usually found in ellipticals. Observations in the 21 cm hydrogen line with the Parkes Radio Telescope gives an upper limit of the H I content of 7 x 10^8 solar mass. The results of the present study seem compatible with the merging hypothesis but dynamical friction has to be invoked to account for the presence of the innermost shells we have observed.