Type-II surface brightness profiles in edge-on galaxies produced by flares

Previous numerical studies had apparently ruled out the possibility that flares in galaxy discs could give rise to the apparent breaks in their luminosity profiles when observed edge-on. However the studies have not, until now, analyzed this hypothesis systematically using realistic models for the disc, the flare, and the bulge. We revisit this theme by analyzing a series of models which sample a wide range of observationally based structural parameters for these three components. We have considered realistic distributions of bulge to disc ratios, morphological parameters of bulges and discs, vertical scale heights of discs and their radial gradients defining the flare for different morphological types and stellar mass bins, based on observations. The surface brightness profiles for the face-on and edge-on views of each model were simulated to find out whether the flared disc produces a Type-II break in the disc profile when observed edge-on, and if so under what conditions. Contrary to previous claims, we find that discs with realistic flares can produce significant breaks in discs when observed edge-on. Specifically a flare with the parameters of that of the Milky Way would produce a significant break of the disc at a Rbreak of ~8.6 kpc if observed edge-on. Central bulges have no significant effects on the results. These simulations show that flared discs can explain the existence of many Type-II breaks observed in edge-on galaxies, in a range of galaxies with low-to-intermediate break strength values of -0.25<S<-0.1.Comment: Published in Astronomy & Astrophysics. 5 pages, 5 figures. Language corrections by the journal included in this new versio

Photometric scaling relations of antitruncated stellar discs in S0-Scd galaxies

It has been recently found that the characteristic photometric parameters of antitruncated discs in S0 galaxies follow tight scaling relations. We investigate if similar scaling relations are satisfied by galaxies of other morphological types. We have analysed the trends in several photometric planes relating the characteristic surface brightness and scalelengths of the breaks and the inner and outer discs of local antitruncated S0-Scd galaxies, using published data and fits performed to the surface brightness profiles of two samples of Type-III galaxies in the R and Spitzer 3.6 microns bands. We have performed linear fits to the correlations followed by different galaxy types in each plane, as well as several statistical tests to determine their significance. We have found that: 1) the antitruncated discs of all galaxy types from Sa to Scd obey tight scaling relations both in R and 3.6 microns, as observed in S0s; 2) the majority of these correlations are significant accounting for the numbers of the available data samples; 3) the trends are clearly linear when the characteristic scalelengths are plotted on a logarithmic scale; and 4) the correlations relating the characteristic surface brightnesses of the inner and outer discs and the breaks with the various characteristic scalelengths significantly improve when the latter are normalized to the optical radius of the galaxy. The results suggest that the scaling relations of Type-III discs are independent of the morphological type and the presence (or absence) of bars within the observational uncertainties of the available datasets, although larger and deeper samples are required to confirm this. The tight structural coupling implied by these scaling relations impose strong constraints on the mechanisms proposed for explaining the formation of antitruncated stellar discs in the galaxies across the whole Hubble Sequence (Abridged).Comment: Accepted for publication in Astronomy & Astrophysics, 18 pages, 12 figures, 7 table

Formation of S0 galaxies through mergers. Evolution in the Tully-Fisher relation since z∼1z\sim1

(Abridged version) We explore whether a scenario that combines an origin by mergers at $z\sim$1.8-1.5 with a subsequent passive evolution of the resulting S0 remnants since $z \sim$0.8-1 is compatible with observational data of S0s in the Tully-Fisher relation (TFR). We studied a set of major and minor merger experiments from the GalMer database that generate massive S0 remnants. We analysed the location of these remnants in the photometric and stellar TFRs assuming that they correspond to $z\sim0.8$ galaxies. We then estimated their evolution in these planes over the last 7 Gyr. The results were compared with data of real S0s and spirals at different redshifts. We also tested how the use of Vcirc or Vrot,max affects the results. We found that just after $\sim$1-2 Gyr of coalescence, major mergers generate S0 remnants that are outliers of the local photometric and stellar TFRs at $z\sim0.8$. After $\sim$4-7 Gyr of passive evolution in isolation, the S0 remnants move towards the local TFR, although the initial scatter among them persists. This scatter is sensitive to the indicator used for the rotation velocity: Vcirc values yield a lower scatter than when Vrot,max values are considered instead. In the planes involving Vrot,max, a clear segregation of the S0 remnants in terms of the spin-orbit coupling of the model is observed, in which the remnants of retrograde encounters overlap with local S0s hosting counter-rotating discs. The location of the S0 remnants at $z\sim 0$ agrees well with the observed distribution of local S0 galaxies in the $\sigma_0$-$M_K$, Vcirc-$\sigma_0$ and Vrot,max-$\sigma_0$ planes. Thus, massive S0 galaxies may have been formed through major mergers that occurred at high redshift and have later evolved towards the local TFR through passive evolution in relative isolation, a mechanism that would also contribute to the scatter observed in this relation.Comment: 19 pages, 15 figures. Accepted for publication in A&

Evolution along the sequence of S0 Hubble types induced by dry minor mergers. II - Bulge-disk coupling in the photometric relations through merger-induced internal secular evolution

Galaxy mergers are considered as questionable mechanisms for the evolution of lenticular galaxies (S0's), on the basis that even minor ones induce structural changes that are difficult to reconcile with the strong bulge-disk coupling observed in the photometric scaling relations of S0's. We check if the evolution induced onto S0's by dry intermediate and minor mergers can reproduce their photometric scaling relations, analysing the bulge-disk decompositions of the merger simulations presented in Eliche-Moral et al. (2012). The mergers induce an evolution in the photometric planes compatible with the data of S0's, even in those ones indicating a strong bulge-disk coupling. The mergers drive the formation of the observed photometric relation in some cases, whereas they induce a slight dispersion compatible with data in others. Therefore, this evolutionary mechanism tends to preserve these scaling relations. In those photometric planes where the morphological types segregate, the mergers always induce evolution towards the region populated by S0's. The structural coupling of the bulge and the disk is preserved or reinforced because the mergers trigger internal secular processes in the primary disk that induce significant bulge growth, even although these models do not induce bars. Intermediate and minor mergers can thus be considered as plausible mechanisms for the evolution of S0's attending to their photometric scaling relations, as they can preserve and even strengthen any pre-existing structural bulge-disk coupling, triggering significant internal secular evolution (even in the absence of bars or dissipational effects). This means that it may be difficult to isolate the effects of pure internal secular evolution from those of the merger-driven one in present-day early-type disks (abridged).Comment: Accepted for publication in Astronomy & Astrophysics, 13 pages, 8 figures. Definitive version after proofs. Added references and corrected typo

Evolution of the anti-truncated stellar profiles of S0 galaxies since z=0.6z=0.6 in the SHARDS survey: I - Sample and Methods

The controversy about the origin of the structure of S0--E/S0 galaxies may be due to the difficulty of comparing surface brightness profiles with different depths, photometric corrections and PSF effects (almost always ignored). We aim to quantify the properties of Type-III (anti-truncated) discs in a sample of S0 galaxies at 0.2<z<0.6. In this paper, we present the sample selection and describe in detail the methods to robustly trace the structure in their outskirts and correct for PSF effects. We have selected and classified a sample of 150 quiescent galaxies at 0.2<z<0.6 in the GOODS-N field. We perform a quantitative structural analysis of 44 S0-E/S0 galaxies. We corrected their surface brightness profiles for PSF distortions and analysed the biases in the structural and photometric parameters when the PSF correction is not applied. Additionally, we have developed Elbow, an automatic statistical method to determine whether a possible break is significant - or not - and its type and made it publicly available. We found 14 anti-truncated S0-E/S0 galaxies in the range 0.2<z<0.6 (~30% of the final sample). This fraction is similar to the those reported in the local Universe. In our sample, ~25% of the Type-III breaks observed in PSF-uncorrected profiles are artifacts, and their profiles turn into a Type I after PSF correction. PSF effects also soften Type-II profiles. We found that the profiles of Type-I S0 and E/S0 galaxies of our sample are compatible with the inner profiles of the Type-III, in contrast with the outer profiles. We have obtained the first robust and reliable sample of 14 anti-truncated S0--E/S0 galaxies beyond the local Universe, in the range 0.2<z<0.6. PSF effects significantly affect the shape of the surface brightness profiles in galaxy discs even in the case of the narrow PSF of HST/ACS images, so future studies on the subject should make an effort to correct them.Comment: Accepted for publishing in Astronomy & Astrophysics. 75 pages, 57 figure

Formation of S0 galaxies through mergers. Bulge-disc structural coupling resulting from major mergers

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. We face this question by quantifying the bulge-disc coupling in dissipative simulations of major and minor mergers that result in realistic S0s. 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. The global bulge-disc structure of these remnants has been compared with real data, and they distribute in the B/T - r_e - h_d parameter space consistently with real bright S0s, where B/T is the bulge-to-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 Sersic 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. 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. In conclusion, 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.Comment: 23 pages, accepted for publication in Astronomy and Astrophysics (version after minor language corrections

Evolution induced by dry minor mergers onto fast-rotator S0 galaxies

We analysed collisionless N-body simulations of intermediate and minor dry mergers onto S0s to test whether these mergers can generate S0 galaxies with kinematics intermediate between fast and slow rotators. We find that minor mergers induce a lower decrease of the global rotational support than encounters of lower mass ratios, which results in S0s with properties intermediate between fast and slow rotators. The resulting remnants are intrinsically more triaxial, less flattened, and span the whole range of apparent ellipticities up to $\epsilon_\mathrm{e} \sim 0.8$. They do not show lower apparent ellipticities in random projections than initially; on the contrary, the formation of oval distortions and the disc thickening increase the percentage of projections at $0.4 < \epsilon_\mathrm{e} < 0.7$. In the experiments with S0b progenitor galaxies, minor mergers tend to spin up the bulge and to decrease slightly its intrinsic ellipticity, whereas in the cases of primary S0c galaxies they keep the rotational support of the bulge nearly constant and decrease significantly its intrinsic ellipticity. The remnant bulges remain nearly spherical ($B/A \sim C/A > 0.9$), but exhibit a wide range of triaxialities ($0.20 < T < 1.00$). In the plane of global anisotropy of velocities ($\delta$) vs. intrinsic ellipticity ($\epsilon_\mathrm{e,intr}$), some of our models extend the linear trend found in previous major merger simulations towards higher $\epsilon_\mathrm{e,intr}$ values, while others depart from it. This is consistent with the wide dispersion exhibited by real S0s in this diagram compared with ellipticals, which follow the linear trend drawn by major merger simulations. The different trends exhibited by ellipticals and S0 galaxies in the $\delta$ - $\epsilon_\mathrm{e}$ diagram may be pointing to the different role played by major mergers in the build-up of each morphological type.Comment: Corrected typos. 20 pages, 14 figures. Accepted for publishing in A&