摘要：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 3.6 μm 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 108.5 ≲
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
(V3.6
μm), 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 R25.5. Furthermore, we infer the
gravitational potentials from the synthetic bars to obtain the tangential-to-radial force
ratio (QT) 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 (hR) and fainter extrapolated central
surface brightnesses (Σ°); (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 ~10 M⊙
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 V3.6
μm. 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.
