Stellar Kinematics of Boxy Bulges: Large-Scale Bars and Inner Disks

Abstract

Long-slit stellar kinematic observations were obtained along the major axis of 30 edge-on spiral galaxies, 24 with a boxy or peanut-shaped (B/PS) bulge and six with other bulge types for comparison. Such B/ PS bulges are identified in at least 45% of highly inclined systems, and a growing body of theoretical and observational work suggests that they are the edge-on projection of thickened bars. Profiles of the mean stellar velocity V, the velocity dispersion , as well as the asymmetric (h 3 ) and symmetric (h 4 ) deviations from a pure Gaussian are presented for all objects. Comparing these profiles with stellar kinematic bar diagnostics developed from N-body simulations, we find bar signatures in 24 of our sample galaxies (80%). Galaxies with a B/ PS bulge typically show a double-humped rotation curve with an intermediate dip or plateau. They also frequently show a rather flat central velocity dispersion profile accompanied by a secondary peak or plateau, and numerous galaxies have a local central minimum (k40%). The h 3 profiles display up to three slope reversals. Most importantly, h 3 is normally correlated with V over the presumed bar length, contrary to expectations from axisymmetric disks. These characteristic bar signatures strengthen the case for a close relationship between B/ PS bulges and bars and leave little room for other explanations of the bulges' shape. We also find that h 3 is anticorrelated with V in the very center of most galaxies (k60%), indicating that these objects additionally harbor cold and dense decoupled (quasi-) axisymmetric central stellar disks, which may be related to the central light peaks. These central disks coincide with previously identified star-forming ionized-gas disks (nuclear spirals) in gas-rich systems, and we argue that they formed out of gas accumulated by the bar at its center through inflow. As suggested by N-body models, the asymmetry of the velocity profile (h 3 ) appears to be a reliable tracer of asymmetries in disks, allowing us to discriminate between axisymmetric and barred disks seen in projection. B/ PS bulges (and thus a large fraction of all bulges) appear to be made up mostly of disk material, which has acquired a large vertical extent through bar-driven vertical instabilities. Their formation is thus probably dominated by secular evolution processes rather than merging.

Measuring the X-shaped structures in edge-on galaxies

“…These findings were confirmed by Chung & Bureau (2004), who considered 24 edge-on galaxies with X-shaped structures, and demonstrated that 22 of them show clear kinematic bar signatures. Kinematic identification of X-shaped structures for face-on galaxies was performed in Méndez-Abreu et al (2008).…”

On the morphology of sigma-drop galaxies

“…As the angular resolution of spectroscopic data has improved, detailed measurements on nearby galaxies have shown in some galaxies an unexpected drop in the stellar velocity dispersion as the nucleus is approached, a phenomenon referred to as a dispersion drop or σ-drop (Emsellem et al 2001;Emsellem 2004). One of the first known examples of this phenomenon can be found in Jarvis et al (1988), later results include those reported by Jarvis & Peletier (1991), Bottema (1993), van der Marel (1994), Bertola et al (1996), Bottema & Gerritsen (1997), Fisher (1997), Simien & Prugniel (1997a,b,c), Héraudeau & Simien (1998), Simien & Prugniel (1998), Héraudeau et al (1999), Simien & Prugniel (2000), Emsellem et al (2001), Simien & Prugniel (2002), de Zeeuw et al (2002, Aguerri et al (2003), Márquez et al (2003), Shapiro et al (2003), Chung & Bureau (2004), Falcón-Barroso et al (2006), Ganda et al (2006), Emsellem et al (2008), and Dumas et al (2007).…”

Does the stellar disc flattening depend on the galaxy type?

“…It is obviously difficult to unambiguously identify the presence of a bar in nearly edgeon systems. However, about half of disc galaxies demon-strates boxy/peanut-shaped structures in the central region (Lütticke et al (2000); Laurikainen et al (2011)), which is tagged by some authors as projection of the vertical structure of bars (Bureau & Freeman 1999;Chung & Bureau 2004).…”