The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars D
Total Page：16
File Type：pdf, Size：1020Kb
The Astrophysical Journal, 629:797–815, 2005 August 20 # 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE VERTICAL STELLAR KINEMATICS IN FACE-ON BARRED GALAXIES: ESTIMATING THE AGES OF BARS D. A. Gadotti1 and R. E. de Souza Departamento de Astronomia, Universidade de Sa˜o Paulo, Rua do Mata˜o, 1226 Cidade Universitaria CEP, 05508-090, Sa˜o Paulo SP, Brazil; [email protected], [email protected] Received 2004 September 17; accepted 2005 April 28 ABSTRACT In order to perform a detailed study of the stellar kinematics in the vertical axis of bars, we obtained high signal- to-noise spectra along the major and minor axes of the bars in a sample of 14 face-on galaxies and used them to determine the line-of-sight stellar velocity distribution, parameterized as a Gauss-Hermite series. With these data, we developed a diagnostic tool that allows one to distinguish between recently formed and evolved bars, as well as to estimate their ages, assuming that bars form in vertically thin disks that are recognizable by low values for the vertical velocity dispersion z. Through N-body realizations of bar unstable disk galaxies we were also able to check the timescales involved in the processes that give bars an important vertical structure. We show that z in evolved bars is roughly 100 km sÀ1, which translates to a height scale of about 1.4 kpc, giving support to scenarios in which bulges form through disk material. Furthermore, the bars in our numerical simulations have values for z generally smaller than 50 km sÀ1, even after evolving for 2 Gyr, suggesting that a slow process is responsible for making bars as vertically thick as we observe. We verify theoretically that the Spitzer-Schwarzschild mechanism is quantitatively able to explain these observations if we assume that giant molecular clouds are twice as concentrated along the bar as in the rest of the disk. Subject headinggs: galaxies: bulges — galaxies: evolution — galaxies: formation — galaxies: kinematics and dynamics — methods: n-body simulations 1. INTRODUCTION physical science is to measure timescales for natural phenom- ena, we are not aware of any directed and systematic study of In the last 10 years or so, bars have gradually become much the ages of bars. Areas in which such work might have a sub- more than just an intriguing dynamical curiosity (as in the pio- stantial impact include the models of Bournaud & Combes neering studies of, e.g., Toomre 1963, 1964; Kalnajs 1972), so (2002), in which bars may be destroyed and rebuilt a few times in that attention has now shifted to the major role they play in the a Hubble period, the polemic and long-sought correlation be- formation and evolution of galaxies. Contributions to this change tween the presence of bars and AGNs in galaxies (e.g., Mulchaey of perspective came from various different kinds of analysis, in- cluding the realization that bars may induce the formation of spiral & Regan 1997; Ho et al. 1997a, 1997b; Knapen et al. 2000; Laine et al. 2002; Crenshaw et al. 2003; Laurikainen et al. 2004), arms and rings (e.g., Schwarz 1981; Combes & Gerin1985; Buta the debated frequency of bars at higher redshifts (van den Bergh 1986; see also Buta & Combes 1996), involving the transfer of angular momentum to the outer parts of the galaxy and resulting in 2002; Sheth et al. 2003, and references therein; see also Elmegreen et al. 2004; Jogee et al. 2004), and obviously the formation his- an accumulation of gas in the central regions (e.g., Athanassoula tory of galactic bulges. 1992a, 1992b; Friedli & Benz 1993, 1995; Sakamoto et al. 1999a, To estimate how long a bar has been evolving in a specific 1999b), sweeping large-scale chemical abundance gradients galaxy, one essentially has to measure its vertical extent. This (Martin & Roy 1994; Zaritsky et al. 1994), possibly building a follows from the fact, first shown by Combes & Sanders (1981), reservoir of active galactic nucleus (AGN) fuel (Shlosman et al. that when bars form in disks, they are vertically thin, while the 1989, 1990) and provoking central bursts of star formation (e.g., onset of vertical resonances rapidly causes them to become Se´rsic & Pastoriza 1965, 1967; Carollo et al. 1997; Gadotti & thicker in this direction. It is also possible that the hose insta- dos Anjos 2001). Moreover, a number of works developed a new bility (Toomre 1966; Merritt & Sellwood 1994), which occurs bulge-building scenario in which bars play a fundamental role (e.g., Combes & Sanders 1981; Kormendy 1982; Kormendy & whenever the velocity dispersion in the vertical direction is 3 times smaller than it is in the plane of the disk, also plays its Illingworth 1983; de Souza & dos Anjos 1987; Pfenniger & role in this context by raising the vertical extent of the stellar Norman 1990; Combes et al. 1990; Balcells & Peletier 1994; Kuijken & Merrifield 1995; Norman et al. 1996; Courteau et al. orbits in the bar region. It is now generally agreed that these processes are likely responsible for the existence of the so called 1996; Peletier & Balcells 1996; Berentzen et al. 1998; Merrifield boxy-peanut bulges (see Bureau & Freeman 1999). The age sig- & Kuijken 1999; Bureau & Athanassoula1999, 2005; Athanassoula nature of bars can thus be acquired from studying the kinemat- & Bureau 1999; see also the recent review by Kormendy & ics along their vertical axis, since it is by elevating the velocity Kennicutt 2004). dispersion in this direction that bars evolve and grow away from In spite of the undisputed relevance of bars in the evolution of the plane of the disk. galaxies and even though one of the major concerns of any Another issue that must be discussed in this context regards the difficulties encountered by what until recently has been the 1 Current address: Laboratoire d’Astrophysique de Marseille, 2 Place Le standard scenario for bar formation in trying to explain the exis- Verrier, 13248 Marseille Cedex 4, France. tence of bars in galaxies of such early type as lenticulars (see, 797 798 GADOTTI & DE SOUZA Vol. 629 TABLE 1 Basic Data for All Galaxies in Our Sample Name Type D25 log R25 mB cz AGN Companion (1) (2) (3) (4) (5) (6) (7) (8) NGC 1302.............................. SB0(r) 3.89 0.02 11.40 1730 ... N NGC 1317.............................. SABa(r) 2.75 0.06 11.85 1941 ... Y NGC 1326.............................. SB0(r) 3.89 0.13 11.42 1365 LINER N NGC 1387.............................. SAB0(s) 2.82 0.00 11.82 1328 ... Y NGC 1440.............................. SB0(rs) 2.14 0.12 12.58 1504 ... N NGC 2665.............................. SBa(rs) 2.04 0.13 12.96 1740 ... N NGC 4314.............................. SBa(rs) 4.17 0.05 11.22 963 LINER N NGC 4394.............................. SBb(r) 3.63 0.05 11.53 772 LINER Y NGC 4579.............................. SABb(rs) 5.89 0.10 10.68 1627 LINER/Sey1.9 N NGC 4608.............................. SB0(r) 3.24 0.08 11.96 1823 ... N NGC 4984.............................. SAB0(rs) 2.75 0.10 11.80 1243 ... N NGC 5383.............................. SBb(rs) 3.16 0.07 12.18 2226 ... Y NGC 5701.............................. SB0/a(rs) 4.26 0.02 11.82 1556 LINER N NGC 5850.............................. SBb(r) 4.26 0.06 12.04 2483 ... N Notes.—Cols. (1) and (2) show, respectively, the name and the morphological type of the galaxy, while col. (3) shows its diameter in arcminutes at the 25 B magnitude isophotal level, and col. (4) shows the decimal logarithm of its major to minor axes ratio at the same level. Cols. (5) and (6) show, respectively, the apparent B magnitude and the radial velocity in km sÀ1. All these data were taken from de Vaucouleurs et al. (1991, hereafter RC3). Col. (7) presents an AGN classification according to the NASA Extragalactic Database ( NED). In col. (8), ‘‘Y’’ means that there is a companion galaxy similar in size physically interacting within 100, while ‘‘N’’ means that there are no companion galaxies. To make this analysis we used RC3 and the Lyon Extragalactic Data Archive (LEDA). e.g., Gadotti & de Souza 2003 and references therein). As the sample contains 4 galaxies with an identified companion that velocity dispersion of the stars in the disk rises, the galaxy be- may be gravitationally interacting and 5 with nonstellar nuclear comes more and more stable against bar formation through disk activity. This variety is helpful in trying to evaluate clues related, instability. Moreover, the presence of a conspicuous bulge also for instance, to the prominence of the bulge, bar strength, and the inhibits this instability (Toomre 1981; Sellwood & Moore 1999). gravitational perturbation of a companion. The presence of gal- However, Athanassoula (2003) shows that the unresponsive rigid axies with active nuclei might also be relevant to understanding halo used in older numerical experiments led to a false conclu- the role played by bars in the fueling of this phenomenon. sion, namely, that a dark matter halo prevents the onset of the bar In Figure 1 we show images of all the galaxies in our sample. instability in the interior disk. In reality, the opposite is true: the Lines displayed horizontally show the extent to which our spec- exchange of angular momentum between disk and halo particles tra were taken along the bars’ major and minor axes. For instance, in fact produces even stronger bars and might even be a neces- one can see that in the case of the SB0 galaxy NGC 4608, the sary ingredient to account for the existence of barred lenticulars. spectra along the bar minor axis are all within the bar, while Other authors have introduced additional critical details that were those from the SBb galaxy NGC 5850 extend into the region not considered previously and these are briefly discussed in later outside the bar, in the disk.