Disk-disk Mergers Research Articles

Structure, kinematics and chemical enrichment patterns after major gas-rich disc-disc mergers

We used an N-body smoothed particle hydrodynamics algorithm, with a detailed treatment of star formation, supernovae feedback, and chemical enrichment, to perform eight simulations of mergers between gas-rich disc galaxies. We vary the mass ratio of the progenitors, their rotation axes, and their orbital parameters and analyze the kinematic, structural, and chemical properties of the remnants. Six of these simulations result in the formation of a merger remnant with a disc morphology as a result of the large gas-fraction of the remnants. We show that stars formed during the merger (a sudden starburst occur in our simulation and last for 0.2-0.3 Gyr) and those formed after the merger have different kinematical and chemical properties. The first ones are located in thick disc or the halo. They are partially supported by velocity dispersion and have high [alpha/Fe] ratios even at metallicities as high as [Fe/H]=-0.5. The former ones -- the young component -- are located in a thin disc rotationally supported and have lower [alpha/Fe] ratios. The difference in the rotational support of both components results in the rotation of the thick disc lagging that of the thin disc by as much as a factor of two, as recently observed.We find that, while the kinematic and structural properties of the merger remnant depends strongly upon the orbital parameters of the mergers, there is a remarkable uniformity in the chemical properties of the mergers. This suggests that general conclusions about the chemical signature of gas-rich mergers can be drawn.

Dynamical Models of Elliptical Galaxies inz= 0.5 Clusters. I. Data‐Model Comparison and Evolution of Galaxy Rotation

We present spatially resolved stellar rotation velocity and velocity dispersion profiles form Keck/LRIS absorption-line spectra for 25 galaxies, mostly visually classified ellipticals, in three clusters at z=0.5. We interpret the kinematical data and HST photometry using oblate axisymmetric two-integral f(E,Lz) dynamical models based on the Jeans equations. This yields good fits, provided that the seeing and observational characteristics are carefully modeled. The fits yield for each galaxy the dynamical M/L and a measure of the galaxy rotation rate. Paper II addresses the implied M/L evolution. Here we study the rotation-rate evolution by comparison to a sample of local elliptical galaxies of similar present-day luminosity. The brightest galaxies in the sample all rotate too slowly to account for their flattening, as is also observed at z=0. But the average rotation rate is higher at z=0.5 than locally. This may be due to a higher fraction of misclassified S0 galaxies (although this effect is insufficient to explain the observed strong evolution of the cluster S0 fraction with redshift). Alternatively, dry mergers between early-type galaxies may have decreased the average rotation rate over time. It is unclear whether such mergers are numerous enough in clusters to explain the observed trend quantitatively. Disk-disk mergers may affect the comparison through the so-called progenitor bias, but this cannot explain the direction of the observed rotation-rate evolution. Additional samples are needed to constrain possible environmental dependencies and cosmic variance in galaxy rotation rates. Either way, studies of the internal stellar dynamics of distant galaxies provide a valuable new approach for exploring galaxy evolution.

Axisymmetric orbit models of N-body merger remnants: a dependency of reconstructed mass on viewing angle

We model mock observations of collisionless N-body disc-disc mergers with the same axisymmetric orbit superposition program that has been used to model elliptical galaxies in Coma. The remnants sample representatively the shape distribution of disc-disc mergers, including the most extreme cases, like highly prolate, maximally triaxial and dominantly oblate objects. The aim of our study is to better understand how the assumption of axial symmetry affects reconstructed masses and stellar motions of systems which are intrinsically not axisymmetric, whether the axisymmetry assumption then leads to a bias and how such a potential bias can be recognized in models of real galaxies. The mass recovery at the half-light radius depends on viewing angle and intrinsic shape: edge-on views allow to reconstruct total masses with an accuracy between 20 per cent (triaxial/prolate remnants) and 3 per cent (oblate remnant). Masses of highly flattened, face-on systems are underestimated by up to 50 per cent. Deviations in local mass densities can be larger where remnants are strongly triaxial or prolate. Luminous mass-to-light ratios are sensitive to box orbits in the remnants. Box orbits cause the central value of the Gauss-Hermite parameter H 4 to vary with viewing angle. Reconstructed luminous mass-to-light ratios, as well as reconstructed central masses, follow this variation. Luminous mass-to-light ratios are always underestimated (up to a factor of 2.5). Respective dark haloes in the models can be overestimated by about the same amount, depending again on viewing angle. Reconstructed velocity anisotropies ft depend on viewing angle as well as on the orbital composition of the remnant and are mostly accurate to about A/? = 0.2. Larger deviations can occur towards the centre or the outer regions, respectively. We construct N-body realizations of the Schwarzschild models to discuss chaotic orbits and the virial equilibrium in our models. In this study we explore the extreme limits of axisymmetric models. Apparently flattened, rotating ellipticals of intermediate mass are likely close to both, axial symmetry and edge-on orientation. Our results imply that Schwarzschild models allow a reconstruction of their masses and stellar anisotropies with high accuracy.

Line-of-sight velocity distributions of elliptical galaxies from collisionless mergers

Abstract We analyse the skewness of the line-of-sight velocity distributions in model elliptical galaxies built through collisionless galaxy mergers. We build the models using large N-body simulations of mergers between either two spiral or two elliptical galaxies. Our aim is to investigate whether the observed ranges of skewness coefficient (h3) and the rotational support (V/σ), as well as the anticorrelation between h3 and V, may be reproduced through collisionless mergers. Previous attempts using N-body simulations failed to reach V/σ≈ 1–2 and corresponding high h3 values, which suggested that gas dynamics and ensuing star formation might be needed in order explain the skewness properties of ellipticals through mergers. Here we show that high V/σ and high h3 are reproduced in collisionless spiral–spiral mergers whenever a central bulge allows the discs to retain some of their original angular momentum during the merger. We also show that elliptical–elliptical mergers, unless merging from a high-angular momentum orbit, reproduce the strong skewness observed in non-rotating, giant, boxy ellipticals. The behaviour of the h3 coefficient therefore associates rapidly-rotating discy ellipticals to disc-disc mergers, and associates boxy, slowly rotating giant ellipticals to elliptical–elliptical mergers, a framework generally consistent with the expectations of hierarchical galaxy formation.

Elliptical galaxies from mergers of discs

We analyse N-body galaxy merger experiments involving disc galaxies. Mergers of disc-bulge-halo models are compared to those of bulge-less, disc-halo models to quantify the effects of the central bulge on the merger dynamics and on the structure of the remnant. Our models explore galaxy mass ratios 1:1 through 3:1, and use higher bulge mass fraction than previous studies. A full comparison of structural and dynamical properties to observations is carried out. The presence of central bulges results in longer tidal tails, oblate final intrinsic shapes, surface brightness profiles with higher Sersic index, steeper rotation curves, and oblate-rotator internal dynamics. Mergers of bulge-less galaxies do not generate long-lasting tidal tails, and their strong triaxiality seems inconsistent with observations; these remnants show shells, which we do not find in models including central bulges. Giant ellipticals with boxy isophotes and anisotropic dynamics cannot be produced by the mergers modeled here; they could be the result of mergers between lower luminosity ellipticals, themselves plausibly formed in disc-disc mergers.

Revisiting the Globular Cluster System of the Merger Remnant Elliptical Galaxy NGC 3610

We have obtained Keck spectra of six candidate globular clusters (GCs) in the dynamically young elliptical galaxy NGC 3610, supplementing our previous Keck spectra of eight confirmed GCs. Five of our new candidates are confirmed to be GCs. Of the 13 GCs, 11 are located within the K-band effective radius of 7 kpc. Two of these 13 clusters are found to be young (~2 Gyr) and very metal-rich ([Z/H] ~ +0.5), three are old and metal-poor, and the remaining eight clusters are old and metal-rich. The ages of the young clusters are consistent with a recent spectroscopic age estimate of 1.6 ± 0.5 Gyr for the galaxy itself and suggest that these clusters formed in the disk-disk merger that likely created NGC 3610. Intriguingly, both young GCs have [α/Fe] ~ +0.3, while the majority of the old clusters are not α-enhanced, in contrast to Galactic and M31 GCs, and contrary to predictions of nucleosynthetic calculations. The two old subpopulations of GCs can be attributed to the merger progenitors. The relative numbers of old and new metal-rich GCs are poorly constrained because of the expected differences in radial distributions of the two subpopulations. However, based on our spectroscopic results and a comparison of the Hubble Space Telescope color-magnitude diagram with stellar population models, we argue that more than half of the metal-rich GCs are likely to be old.

Chandraobservations of the Mice

Presented here are high spatial and spectral resolution Chandra X-ray observations of the famous interacting galaxy pair, the Mice, a system similar to, though less evolved than, the well-known Antennae galaxies. Previously unpublished ROSAT HRI data of the system are also presented. Starburst-driven galactic winds outflowing along the minor axis of both galaxies (but particularly the northern) are observed, and spectral and spatial properties, and energetics are presented. That such a phenomenon can occur in such a rapidly-evolving and turbulent system is surprising, and this is the first time that the very beginning - the onset, of starburst-driven hot gaseous outflow in a full-blown disk-disk merger has been seen. Point source emission is seen at the galaxy nuclei, and within the interaction-induced tidal tails. Further point source emission is associated with the galactic bar in the southern system. A comparison of the source X-ray luminosity function and of the diffuse emission properties is made with the Antennae and other galaxies, and evidence of a more rapid evolution of the source population than the diffuse component is found. No evidence for variability is found between the Chandra and previous observations.

H [CSC]i[/CSC] in the Shell Elliptical Galaxy NGC 3656

Very Large Array** neutral hydrogen observations of the shell elliptical galaxy NGC 3656 reveal an edge-on, warped minor-axis gaseous disk (M ~ 2 × 109 M⊙) extending 7 kpc. H I is also found outside the optical image, on two complexes to the northeast and northwest that seem to trace one or two tidal tails, or possibly an outer broken H I disk or ring. These complexes link with the outer edges of the inner disk and appear displaced with respect to the two optical tails in the galaxy. The disk kinematics is strongly lopsided, suggesting recent or ongoing accretion. Integral-field optical fiber spectroscopy at the region of the bright southern shell of NGC 3656 has provided a determination of the stellar velocities of the shell. The shell, at 9 kpc from the center, has traces of H I with velocities bracketing the stellar velocities, providing evidence for a dynamical association of H I and stars at the shell. Within the errors the stars have systemic velocity, suggesting a possible phase-wrapping origin for the shell. We probed a region of 40' × 40' (480 kpc × 480 kpc) × 1160 km s-1 down to an H I mass sensitivity (6 σ) of 3 × 107 M⊙ and detect five dwarf galaxies with H I masses ranging from 2 × 108 to 2 × 109 M⊙, all within 180 kpc of NGC 3656 and all within the velocity range (450 km s-1) of the H I of NGC 3656. The dwarfs had been previously cataloged, but none had a known redshift. For the NGC 3656 group to be bound requires a total mass of (3–7.4) × 1012 M⊙, yielding a mass-to-light ratio from 125 to 300. The overall H I picture presented by NGC 3656 supports the hypothesis of a disk-disk merger origin or possibly an ongoing process of multiple mergers with nearby dwarfs.

The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies

We have mapped the neutral atomic gas (HI) in the three disk-disk merger systems NGC 520, Arp 220, and Arp 299. These systems differ from the majority of the mergers mapped in HI, in that their stellar and gaseous tidal features do not coincide. In particular, they exhibit large stellar tidal features with little if any accompanying neutral gas and large gas-rich tidal features with little if any accompanying starlight. On smaller scales, there are striking anti-correlations where the gaseous and stellar tidal features appear to cross. We explore several possible causes for these differences, including dust obscuration, ram pressure stripping, and ionization effects. No single explanation can account for all of the observed differences. The fact that each of these systems shows evidence for a starburst driven superwind expanding in the direction of the most striking anti-correlations leads us to suggest that the superwind is primarily responsible for the observed differences, either by sweeping the features clear of gas via ram pressure, or by excavating a clear sightline towards the starburst and allowing UV photons to ionize regions of the tails.

Making Spirals with Counter-Rotating Disks: The Case of NGC 4550

A single merger scenario for making galaxies such as NGC 4550 possessing equal coplanar counter-rotating stellar disks is investigated by collisionless N-body technique. The scenario is successful in producing an axisymmetric disk made of two almost equal counter-rotating populations. The final disk shows a clear bimodal line profile in the outer part, which demonstrates that disk-disk mergers do not always produce ellipticals.

Shells, Ripples and Tails

It has been known since the early simulations of Wright (1972), and Toomre & Toomre (1972), that interactions between galaxies can give rise to quite spectacular morphological features, including spiral structure in disk galaxies, and extensive tails. It appears that long tidal tails only arise from interactions involving disk galaxies (Toomre & Toomre 1972), and thus the presence of two opposed long tidal tails in a number of disturbed galaxies such as the Antennae (Whitmore & Schweizer 1995), NGC 3921 (Schweizer 1996), and NGC 7252 (Schweizer 1982), has led to the interpretation of these galaxies as disk-disk mergers in progress. These systems can be modeled rather successfully, as the work of Hibbard & Mihos (1995) has shown.

Tightness of the Color‐Magnitude Relation of Elliptical Galaxies and the Epoch of Major Galaxy Merging

We investigate a one-zone chemophotometric evolution model of disk-disk galaxy mergers in order to clarify whether galaxy mergers with a widely spread merging epoch can reproduce reasonably well the observed small scatter of the color-magnitude (C-M) relation in cluster ellipticals at low and intermediate redshift (z < 1). We consider that merger-progenitor disks begin to consume interstellar gas at a moderate rate from z ~ 5 and then merge to form an elliptical with a secondary starburst at z = zmerge. We find that even if the epoch of galaxy merging is rather extended (0.3 < zmerge < 3.0), the dispersion in the rest-frame U-V color among galaxy mergers is well within the observed one (~0.05 mag at z = 0). We also find that the zmerge is required to be within a certain range to keep the observed C-M relation tight at a given z. For example, the required range of zmerge in galaxy mergers between Sa disks is 1.3 < zmerge < 3.0 for cluster ellipticals at z = 0.895, 0.9 < zmerge < 3.0 for z = 0.55, and 0.3 < zmerge < 3.0 for z = 0. The main reason for the derived small scatter is that younger stellar populations, which are formed during the secondary starburst of galaxy mergers, are formed preferentially from more metal-enriched interstellar gas. This result reinforces Worthey's suggestion of 1996 that the age-metallicity conspiracy, which means that younger stellar populations are preferentially more metal-enriched, can operate to keep the tight C-M relation. These numerical results imply that the observed small scatter in the C-M relation at low and intermediate redshift (z < 1) does not necessarily require the coevality of elliptical galaxies in clusters or their formation at high z, which has been conventionally believed in the classical, passive evolution picture.

The Local Merger Rate of Disk Galaxies

view Abstract Citations (44) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Local Merger Rate of Disk Galaxies Keel, William C. ; Wu, Wentao Abstract We consider the local population of galaxy merger remnants, more specifically the remnants of disk-disk mergers for which there are diagnostic features (such as tails) that might be catalogued in a fairly complete manner. We estimate the current rate of mergers for such galaxies by comparing the luminosity function for mergers with those of spiral-spiral pairs (their most common precursors) and for all disk galaxies, incorporating the effects of changing crossing time with luminosity. These are connected via the time scale for mergers, which we estimate from the dynamical properties of our sample and published n-body models. We derive a merger rate for spirals in pairs, at the present epoch, of 4.2 per Hubble time (2.8 over the age of the Universe at critical density, or extrapolated to all spirals, 0.33 per Hubble time). Given this large difference, depending on which precursor population is considered, a two-component model seems most appropriate to assess the effects of mergers on galaxy evolution - one for galaxies originally in pairs or small groups, and a second for galaxies which have never been in environments so susceptible to merging. Distinguishing the initial contributions of each to the overall galaxy populations will require samples at high redshift. We also evaluate the history of several indicators of observable star formation, using the dynamical state of each remnant as an age estimate. With large scatter, we see evidence that star formation builds up to the time of merger rather slowly, with a rapid decline thereafter. For most systems, dust seems to be so important in obscuring much of the starburst's optical radiation that the luminosity increase of a merger over its initial components does not have a profound effect on either our derived merger rate or on higher-redshift counts. This rapid fading means that mergers per se can be at most a minor contributor to the Butcher-Oemler effect; most blue galaxies in systems fated to merge will be observed during the interactions that precede the final merging event. Further observations of the dynamical state of high-redshift systems are clearly needed to assess the time- integrated impact of merging on the overall galaxy population; the crude estimates given per Hubble time must be lower limits under most conditions. An Appendix describes the luminosity function of "field" galaxies of various morphological types, derived from a new analysis of the CfA survey in the B_t_^0^ system. Publication: The Astronomical Journal Pub Date: July 1995 DOI: 10.1086/117501 Bibcode: 1995AJ....110..129K Keywords: GALAXIES: EVOLUTION; GALAXIES; INTERACTIONS; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (39) NED (39)

A near-infrared imaging survey of interacting galaxies - The disk-disk merger candidates subset

We present and discuss near-infrared imaging obtained for systems believed to be advanced disk-disk mergers. These systems were chosen from a sample of approximately 170 objects from the Arp Atlas of Peculiar Galaxies which have been imaged in the JHK bands as part of an investigation into the stellar component of interacting galaxies. Of the eight remnants which show optical signs of a disk-disk merger, the near-infrared surface brightness profiles are well-fitted by an r^1/4^ law over all measured radii in four systems, and out to radii of ~ 3 kpc in three systems. These K band profiles indicate that most of the remnants in the sample either have finished or are in the process of relaxing into a mass distribution like that of normal elliptical galaxies.