2016A&A...596A..84D

Context. Models of galaxy formation in a cosmological framework need to be tested against observational constraints, such as the average stellar density profiles (and their dispersion) as a function of fundamental galaxy properties (e.g. the total stellar mass). Simulation models predict that the torques produced by stellar bars efficiently redistribute the stellar and gaseous material inside the disk, pushing it outwards or inwards depending on whether it is beyond or inside the bar corotation resonance radius. Bars themselves are expected to evolve, getting longer and narrower as they trap particles from the disk and slow down their rotation speed.
Aims. We use 3.6µm photometry from the Spitzer Survey of Stellar Structure in Galaxies (S⁴G) to trace the stellar distribution in nearby disk galaxies (z~=0) with total stellar masses 10^8.5≲M_*/M_☉≲10¹¹ and mid-IR Hubble types -3≲T≲10. We characterize the stellar density profiles (Σ_*), the stellar contribution to the rotation curves (V_3.6µm), and the m=2 Fourier amplitudes (A₂) as a function of M_* and T. We also describe the typical shapes and strengths of stellar bars in the S⁴G sample and link their properties to the total stellar mass and morphology of their host galaxy.
Methods. For 1154 S⁴G galaxies with disk inclinations lower than 65°, we perform a Fourier decomposition and rescale their images to a common frame determined by the size in physical units, by their disk scalelength, and for 748 barred galaxies by both the length and orientation of their bars. We stack the resized density profiles and images to obtain statistically representative average stellar disks and bars in bins of M_* and T. Based on the radial force profiles of individual galaxies we calculate the mean stellar contribution to the circular velocity. We also calculate average A₂ profiles, where the radius is normalized to R_25.5. Furthermore, we infer the gravitational potentials from the synthetic bars to obtain the tangential-to-radial force ratio (Q_T) and A₂ profiles in the different bins. We also apply ellipse fitting to quantitatively characterize the shape of the bar stacks.
Results. For M_*≳10⁹M_☉, we find a significant difference in the stellar density profiles of barred and non-barred systems: (i) disks in barred galaxies show larger scalelengths (h_R) and fainter extrapolated central surface brightnesses (Σ_deg); (ii) the mean surface brightness profiles (Σ_*) of barred and non-barred galaxies intersect each other slightly beyond the mean bar length, most likely at the bar corotation; and (iii) the central mass concentration of barred galaxies is higher (by almost a factor 2 when T≲5) than in their non-barred counterparts. The averaged Σ_* profiles follow an exponential slope down to at least ∼10M_☉ pc^–2, which is the typical depth beyond which the sample coverage in the radial direction starts to drop. Central mass concentrations in massive systems (≳10¹⁰M_☉) are substantially larger than in fainter galaxies, and their prominence scales with T. This segregation also manifests in the inner slope of the mean stellar component of the circular velocity: lenticular (S0) galaxies present the most sharply rising V_3.6µm. Based on the analysis of bar stacks, we show that early- and intermediate-type spirals (0≲T<5) have intrinsically narrower bars than later types and S0s, whose bars are oval-shaped. We show a clear agreement between galaxy family and quantitative estimates of bar strength. In early- and intermediate-type spirals, A₂ is larger within and beyond the typical bar region among barred galaxies than in the non-barred subsample. Strongly barred systems also tend to have larger A₂ amplitudes at all radii than their weakly barred counterparts.
Conclusions. Using near-IR wavelengths (S⁴G 3.6 µm), we provide observational constraints that galaxy formation models can be checked against. In particular, we calculate the mean stellar density profiles, and the disk(+bulge) component of the rotation curve (and their dispersion) in bins of M_* and T. We find evidence for bar-induced secular evolution of disk galaxies in terms of disk spreading and enhanced central mass concentration. We also obtain average bars (2D), and we show that bars hosted by early-type galaxies are more centrally concentrated and have larger density amplitudes than their late-type counterparts.