Characterization of galactic bars from 3.6μm S⁴G imaging

Abstract

Context. Stellar bars play an essential role in the secular evolution of disk galaxies because they are responsible for the redistribution of matter and angular momentum. Dynamical models predict that bars become stronger and longer in time, while their rotation speed slows down. Aims. We use the Spitzer Survey of Stellar Structure in Galaxies (S 4 G) 3.6 µm imaging to study the properties (length and strength) and fraction of bars at z = 0 over a wide range of galaxy masses (M * ≈ 10 8 −10 11 M ) and Hubble types (−3 ≤ T ≤ 10). Methods. We calculated gravitational forces from the 3.6 µm images for galaxies with a disk inclination lower than 65• . We used the maximum of the tangential-to-radial force ratio in the bar region (Q b ) as a measure of the bar-induced perturbation strength for a sample of ∼600 barred galaxies. We also used the maximum of the normalized m = 2 Fourier density amplitude (A max 2 ) and the bar isophotal ellipticity (ε) to characterize the bar. Bar sizes were estimated i) visually; ii) from ellipse fitting; iii) from the radii of the strongest torque; and iv) from the radii of the largest m = 2 Fourier amplitude in the bar region. By combining our force calculations with the H  kinematics from the literature, we estimated the ratio of the halo-to-stellar mass (M h /M * ) within the optical disk and by further using the universal rotation curve models, we obtained a first-order model of the rotation curve decomposition of 1128 disk galaxies. Results. We probe possible sources of uncertainty in our Q b measurements: the assumed scale height and its radial variation, the influence of the spiral arms torques, the effect of non-stellar emission in the bar region, and the dilution of the bar forces by the dark matter halo (our models imply that only ∼10% of the disks in our sample are maximal). We find that for early-and intermediate-type disks (−3 ≤ T < 5), the relatively modest influence of the dark matter halo leads to a systematic reduction of the mean Q b by about 10−15%, which is of the same order as the uncertainty associated with estimating the vertical scale height. The halo correction on Q b becomes important for later types, implying a reduction of ∼20−25% for T = 7−10. Whether the halo correction is included or not, the mean Q b shows an increasing trend with T . However, the mean A max 2 decreases for lower mass late-type systems. These opposing trends are most likely related to the reduced force dilution by bulges when moving towards later type galaxies. Nevertheless, when treated separately, both the early-and late-type disk galaxies show a strong positive correlation between Q b and A max 2 . For spirals the mean ε ≈ 0.5 is nearly independent of T , but it drops among S0s (≈0.2). The Q b and ε show a relatively tight dependence, with only a slight difference between early and late disks. For spirals, all our bar strength indicators correlate with the bar length (scaled to isophotal size). Late-type bars are longer than previously found in the literature. The bar fraction...

Boxy/Peanut/X-shaped Bulges: Steep Inner Rotation Curve Leads to Barlens Face-on Morphology

“…Our halo model is chosen so that at h 2.2 r , the disk accounts for 65% of the total radial force. This corresponds to the typical value estimated for an MW mass galaxy in Díaz-García et al (2016b), obtained by combining the gravitational field calculated from the 3.6 μm images using the NIRQB code (Salo et al 1999;Laurikainen & Salo 2002), with the rotation amplitudes obtained from the HI-kinematics (Courtois et al 2009 , which correspond to the maximum disk contribution to circular velocity ∼170 km s −1 (total circular velocity ∼215 km s −1 ; see Figure 1). Our standard value for gravity softening is  = h 0.01 r , and the time step is~-10 2 time units; in physical units one time unit corresponds to ∼10 Myr.…”

Kinematic Clues to Bar Evolution for Galaxies in the Local Universe: Why the Fastest Rotating Bars are Rotating Most Slowly

“…When they calculated the bar strength from the torque maps, they found a steady growth along the Hubble sequence. The same method was employed by Díaz-García et al (2016) with a large sample of ∼600 galaxies, finding a similar result. Similarly, Seidel et al (2015) used torques to estimate the bar strength and also found this growth of the bar strength from early to intermediate type of galaxies, with a sample of 16 galaxies.…”

“…This technique was initially applied to real galaxies in Wozniak & Pierce (1991) and in Wozniak et al (1995) based on the technique described by Jedrzejewski (1987). This method has been extensively used on real galaxies (Laine et al 2002;Erwin & Sparke 2003;Sheth et al 2003;Erwin 2004Erwin , 2005 Aguerri et al 2009;Díaz-García et al 2016). A variation of this technique has also been used to determine the bar length in simulated galaxies (Athanassoula et al 1990;Villa-Vargas et al 2009Athanassoula 2014).…”

“…(ii) The bar strength can be defined from the Fourier decompositions of the galaxy image (Ohta et al 1990;Marquez et al 1996;Laurikainen et al , 2005Athanassoula 2003;Athanassoula et al 2013;Díaz-García et al 2016;Martínez-Valpuesta et al 2017). (iii) Finally, the bar strength can be determined from the torque of the bar measured from the gravitational potential inferred from the infrared image (Combes & Sanders 1981;Quillen et al 1994;Buta & Block 2001;Berentzen et al 2007;Tiret & Combes 2008;Salo et al 2010;Díaz-García et al 2016).…”

“…(iii) Finally, the bar strength can be determined from the torque of the bar measured from the gravitational potential inferred from the infrared image (Combes & Sanders 1981;Quillen et al 1994;Buta & Block 2001;Berentzen et al 2007;Tiret & Combes 2008;Salo et al 2010;Díaz-García et al 2016).…”