548 research outputs found
The Kinematic Properties of Double-Barred Galaxies: Simulations Vs. Integral-Field Observations
Using high resolution N -body simulations, we recently reported that a dynamically cool inner disk embedded in a hotter outer disk can naturally generate a steady double-barred (S2B) structure. Here we study the kinematics of these S2B simulations, and compare them to integral-field observations from ATLAS3D and SAURON. We show that S2B galaxies exhibit several distinct kinematic features, namely:
(1) significantly distorted isovelocity contours at the transition region between the two bars, (2) peaks in ÏLOS along the minor axis of inner bars, which we term âÏ-humpsâ, that are often accompanied by
ring/spiral-like features of increased ÏLOS, (3) h3 â vÂŻ anti-correlations in the region of the inner bar for
certain orientations, and (4) rings of positive h4 when viewed at low inclinations. The most impressive of these features are the Ï-humps; these evolve with the inner bar, oscillating in strength just as the inner bar does as it rotates relative to the outer bar. We show that, in cylindrical coordinates, the inner bar has similar streaming motions and velocity dispersion properties as normal large-scale bars, except for Ïz , which exhibits peaks on the minor axis, i.e., humps. These Ïz humps are responsible for producing the Ï-humps. For three well-resolved early-type S2Bs (NGC 2859, NGC 2950, and NGC 3941) and a potential S2B candidate (NGC 3384), the S2B model qualitatively matches the integral-field data well, including the âÏ-hollowsâ previously identified. We also discuss the kinematic effect of a nuclear disk in S2Bs
Constraints from Dynamical Friction on the Dark Matter Content of Barred Galaxies
We show that bars in galaxy models having halos of moderate density and a
variety of velocity distributions all experience a strong drag from dynamical
friction unless the halo has large angular momentum in the same sense as the
disk. The frictional drag decreases the bar pattern speed, driving the
corotation point out to distances well in excess of those estimated in barred
galaxies. The halo angular momentum required to avoid strong braking is
unrealistically large, even when rotation is confined to the inner halo only.
We conclude, therefore, that bars are able to maintain their observed high
pattern speeds only if the halo has a central density low enough for the disk
to provide most of the central attraction in the inner galaxy. We present
evidence that this conclusion holds for all bright galaxies.Comment: 46 pages, 18 figures, to appear in ApJ. Uses aaspp4.st
Anomalously Weak Dynamical Friction in Halos
A bar rotating in a pressure-supported halo generally loses angular momentum
and slows down due to dynamical friction. Valenzuela & Klypin report a
counter-example of a bar that rotates in a dense halo with little friction for
several Gyr, and argue that their result invalidates the claim by Debattista &
Sellwood that fast bars in real galaxies require a low halo density. We show
that it is possible for friction to cease for a while should the pattern speed
of the bar fluctuate upward. The reduced friction is due to an anomalous
gradient in the phase-space density of particles at the principal resonance
created by the earlier evolution. The result obtained by Valenzuela & Klypin is
probably an artifact of their adaptive mesh refinement method, but anyway could
not persist in a real galaxy. The conclusion by Debattista & Sellwood still
stands.Comment: To appear in "Island Universes - Structure and Evolution of Disk
Galaxies" ed. R. S. de Jong, 8 pages, 4 figures, .cls and .sty files include
Reinforcing the link between the double red clump and the X-shaped bulge of the Milky Way
The finding of a double red clump in the luminosity function of the Milky Way bulge has been interpreted as evidence for an X-shaped structure. Recently, an alternative explanation has been suggested, where the double red clump is an effect of multiple stellar populations in a classical spheroid. In this Letter we provide an observational assessment of this scenario and show that it is not consistent with the behaviour of the red clump across different lines of sight, particularly at high distances from the Galactic plane. Instead, we confirm that the shape of the red clump magnitude distribution closely follows the distance distribution expected for an X-shaped bulge at critical Galactic latitudes. We also emphasize some key observational properties of the bulge red clump that should not be neglected in the search for alternative scenarios
Peanut-shaped bulges in face-on disk galaxies
We present high resolution absorption-line spectroscopy of 3 face-on
galaxies, NGC 98, NGC 600, and NGC 1703 with the aim of searching for
box/peanut (B/P)-shaped bulges. These observations test and confirm the
prediction of Debattista et al. (2005) that face-on B/P-shaped bulges can be
recognized by a double minimum in the profile of the fourth-order Gauss-Hermite
moment h_4. In NGC 1703, which is an unbarred control galaxy, we found no
evidence of a B/P bulge. In NGC 98, a clear double minimum in h_4 is present
along the major axis of the bar and before the end of the bar, as predicted. In
contrast, in NGC 600, which is also a barred galaxy but lacks a substantial
bulge, we do not find a significant B/P shape.Comment: 4 pages, 1 figure. To appear in "Tumbling, twisting, and winding
galaxies: Pattern speeds along the Hubble sequence", E. M. Corsini and V. P.
Debattista (eds.), Memorie della Societa` Astronomica Italian
Forming double-barred galaxies from dynamically cool inner disks
About one-third of early-type barred galaxies host small-scale secondary bars. The formation and evolution of such double-barred (S2B) galaxies remain far from being well understood. In order to understand the formation of such systems, we explore a large parameter space of isolated pure-disk simulations. We show that a dynamically cool inner disk embedded in a hotter outer disk can naturally generate a steady secondary bar while the outer disk forms a large-scale primary bar. The independent bar instabilities of inner and outer disks result in long-lived double-barred structures whose dynamical properties are comparable to those in observations. This formation scenario indicates that the secondary bar might form from the general bar instability, the same as the primary bar. Under some circumstances, the interaction of the bars and the disk leads to the two bars aligning or single, nuclear, bars only. Simulations that are cool enough of the center to experience clump instabilities may also generate steady S2B galaxies. In this case, the secondary bars are âfast,â i.e., the bar length is close to the co-rotation radius. This is the first time that S2B galaxies containing a fast secondary bar are reported. Previous orbit-based studies had suggested that fast secondary bars were not dynamically possibl
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