A minor merger origin for stellar inner discs and rings in spiral galaxies

Abstract

Context. Observations have shown that inner discs and rings (IDs and IRs) are not preferably found in barred galaxies, which indicates that their formation may differ from that described by the traditional bar-origin scenario in many cases. In contrast, the role of minor mergers in producing these inner components (ICs), while often invoked, is still poorly understood. Aims. We investigate the capability of minor mergers to trigger the formation of IDs and IRs in spiral galaxies through collisionless N-body simulations. Methods. We run a battery of minor merger simulations in which both primary and secondary galaxies are modelled as disc-bulgehalo galaxies with realistic density ratios. Different orbits and mass ratios are considered, as well as two different models for the primary galaxy (a Sab or Sc). We then perform a detailed analysis of the morphology, structure, and kinematics of the ICs resulting from the minor merger. Results. All the simulated minor mergers develop thin ICs out of satellite material, supported by rotation. A wide morphological zoo of ICs are obtained (including IDs, IRs, pseudo-rings, nested IDs, spiral patterns, and combinations of them), but all have structural and kinematical properties similar to those observed. The sizes of the resulting ICs are comparable to those observed in real galaxies with the adequate scaling. The existence of the resulting ICs can be deduced from the features that they imprint in the isophotal profiles and kinemetric maps of the final remnant, as in many real galaxies. Weak transitory oval distortions appear in the remnant centre in many cases, but none of them develops a noticeable bar. The realistic density ratios used in the present models ensure that the satellites experience more efficient orbital circularization and disruption than in previous studies. Combined with the disc resonances induced by the encounter, these processes produce highly aligned co-and counter-rotating ICs at the remnant centre. Conclusions. Minor mergers are an efficient mechanism for forming rotationally-supported stellar ICs in spiral galaxies, without requiring either strong dissipation or the development of noticeable bars. The present models indicate that minor mergers can account for the existence of pure-stellar old ICs in unbarred galaxies, and suggest that their role must have been crucial in the formation of ICs and much more complex than just bar triggering.

SUPERMASSIVE BLACK HOLES AND THEIR HOST SPHEROIDS. II. THE RED AND BLUE SEQUENCE IN THE MBH–M*,SPH DIAGRAM

“…In the recent literature, pseudo-and classical bulges have frequently been divided at the Sérsic index n 2 sph = (e.g., Sani et al 2011;Beifiori et al 2012), although, from a selection of hundreds of disk galaxies imaged in the Kband, Graham & Worley (2008) observed no bimodality in the bulge Sérsic indices about n 2 sph = or any other value. While pseudo-bulges are expected to have exponential-like surface brightness profiles (n 1 sph  ), being disky components that formed from their surrounding exponential disks (e.g., Bardeen 1975;Hohl 1975;Combes & Sanders 1981;Combes et al 1990;Pfenniger & Friedli 1991), it has been shown that mergers can create bulges with n 2 sph < (e.g., Eliche-Moral et al 2011;Scannapieco et al 2011;Querejeta et al 2015), just as low-luminosity elliptical galaxies (not built from the secular evolution of a disk) are also well known to have n 2 sph < and even n 1 sph < (e.g., Davies et al 1988;Young & Currie 1994;Jerjen et al 2000). The use of the Sérsic index (in addition to rotation) to identify pseudo-bulges is thus a dangerous practice.…”

Spectro-photometric close pairs in GOODS-S: major and minor companions of intermediate-mass galaxies

“…In addition, residual star formation in early-type (E/S0) galaxies at z 0.7 can also be explained by minor merging (Kaviraj et al 2009. This problem has also been analysed by Eliche-Moral et al (2010a), who have modelled the evolution of luminosity function backwards in time for M 10 11 M galaxies, selected according to their colours (red/blue/total) and their morphologies. They find that the observed luminosity function evolution can be explained naturally by the observed gas-rich and dry major merger rates and that 50-60% of today's E/S0 in this mass range were formed by major mergers at 0.8 < z < 1, with a small number evolution since z = 0.8 (see also Cristóbal-Hornillos et al 2009;van der Wel et al 2009;Ilbert et al 2010;Eliche-Moral et al 2010b).…”

“…This problem has also been analysed by Eliche-Moral et al (2010a), who have modelled the evolution of luminosity function backwards in time for M 10 11 M galaxies, selected according to their colours (red/blue/total) and their morphologies. They find that the observed luminosity function evolution can be explained naturally by the observed gas-rich and dry major merger rates and that 50-60% of today's E/S0 in this mass range were formed by major mergers at 0.8 < z < 1, with a small number evolution since z = 0.8 (see also Cristóbal-Hornillos et al 2009;van der Wel et al 2009;Ilbert et al 2010;Eliche-Moral et al 2010b). The gas-rich major merger fractions assumed by ElicheMoral et al (2010a) are those from L09 for B-band selected galaxies (M B ≤ −20), which were obtained in a similar way than the morphological merger fractions used through present paper.…”

Devasthal Fast Optical Telescope Observations of Wolf–Rayet Dwarf Galaxy Mrk 996

“…This fresh supply of gas can be effectively converted to stars causing nuclear starburst (Mihos & Hernquist 1994). As the new gas can come with different angular momentum, it is not necessary that orbits of fresh gas are aligned with that of the old stellar disk (Eliche-Moral et al 2011, Haynes et al 2000. Therefore, we believe that the current phase of star-burst in Mrk 996 has resulted from a recent tidal encounter, most likely a minor merger with a low-mass companion.…”