Formation of S0 galaxies through mergers

Abstract

Context. Observations reveal a strong structural coupling between bulge and disc in S0 galaxies, which seems difficult to explain if they have formed from supposedly catastrophic events such as major mergers. Aims. We face this question by quantifying the bulge-disc coupling in dissipative simulations of major and minor mergers that result in realistic S0s. Methods. We have studied the dissipative N-body binary merger simulations from the GalMer database that give rise to realistic, relaxed E/S0 and S0 remnants (67 major and 29 minor mergers). We simulate surface brightness profiles of these S0-like remnants in the K band, mimicking typical observational conditions, to perform bulge-disc decompositions analogous to those carried out in real S0s. Additional components have been included when needed. The global bulge-disc structure of these remnants has been compared with real data. Results. The S0-like remnants distribute in the B/T -r e -h d parameter space consistently with real bright S0s, where B/T is the bulgeto-total luminosity ratio, r e is the bulge effective radius, and h d is the disc scalelength. Major mergers can rebuild a bulge-disc coupling in the remnants after having destroyed the structures of the progenitors, whereas minor mergers directly preserve them. Remnants exhibit B/T and r e /h d spanning a wide range of values, and their distribution is consistent with observations. Many remnants have bulge Sérsic indices ranging 1 < n < 2, flat appearance, and contain residual star formation in embedded discs, a result which agrees with the presence of pseudobulges in real S0s. Conclusions. Contrary to the popular view, mergers (and in particular, major events) can result in S0 remnants with realistically coupled bulge-disc structures in less than ∼3 Gyr. The bulge-disc coupling and the presence of pseudobulges in real S0s cannot be used as an argument against the possible major-merger origin of these galaxies.

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.…”

Formation of S0 galaxies through mergers

“…Specific details on the criteria used to select the sample will be provided elsewhere (Eliche-Moral et al, in prep. ), but Borlaff et al (2014) and Querejeta et al (2015a) contain a summary of how this process is done. Hence, in Sect.…”

“…For the K-band images we simulated the conditions of the NearInfraRed S0 Survey (NIRS0S, Laurikainen et al 2011) to perform the comparisons shown in Querejeta et al (2015a). We added photon noise to the model images to have a signal-tonoise ratio (S /N) of 3 for the limiting magnitude of the images.…”

“…We note that all our minor mergers are onto the same gS0 progenitor. Additionally, our sample is smaller than the one presented in Borlaff et al (2014) and Querejeta et al (2015a) because we excluded the merger remnants classified as E/S0 to focus on the well-defined S0s.…”

“…where R disc corresponds to the disc scale length of each remnant (see Table 3 in Querejeta et al 2015a). In our case, the kinematics in the azimuthal direction (V φ (R) and σ φ (R)) correspond to our measurements of V rot (R) and σ(R) (Sect.…”