An Improved Bulge Model for M31
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New mass models are derived with emphasis on determining the bulge mass/light ratio. Inside 6 kpc, the gas rotation curve is complex and probably does not measure the true circular velocity. Therefore, the bulge M/L ratio is constrained primarily by the bulge stellar velocity dispersion. The bulge kinematics inside 1 arcmin are consistent with the bulge being an oblate isotropic rotator. Overall, the best-fitting model has an apparent bulge M/L = 5 and disk M/L = 10. The rotation curve predicted by this model rises smoothly from the center to 1 kpc, and then is nearly flat out to 30 kpc. The predicted inner rotation curve forms an upper envelope to the observed complex gas kinematics. 27 refs.Keywords:
Envelope (radar)
Velocity dispersion
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We explore the kinematics (both the radial velocity and the proper motion) of the vertical X-shaped feature in the Milky Way with an N-body bar/bulge model. From the solar perspective, the distance distribution of particles is double-peaked in fields passing through the X-shape. The separation and amplitude ratio between the two peaks qualitatively match the observed trends towards the Galactic bulge. We confirm clear signatures of cylindrical rotation in the pattern of mean radial velocity across the bar/bulge region. We also find possible imprints of coherent orbital motion inside the bar structure in the radial velocity distribution along l=0 degree, where the near and far sides of the bar/bulge show excesses of approaching and receding particles. The coherent orbital motion is also reflected in the slight displacement of the zero-velocity-line in the mean radial velocity, and the displacement of the maximum/minimum in the mean longitudinal proper motion across the bulge region. We find some degree of anisotropy in the stellar velocity within the X-shape, but the underlying orbital family of the X-shape cannot be clearly distinguished. Two potential applications of the X-shape in previous literature are tested, i.e., bulge rotation and Galactic center measurements. We find that the proper motion difference between the two sides of the X-shape can be used to estimate the mean azimuthal streaming motion of the bulge, but not the pattern speed of the bar. We also demonstrate that the Galactic center can be located with the X-shape, but the accuracy depends on the fitting scheme, the number of fields, and their latitudinal coverage.
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Analyzing high-resolution longitude-velocity (LV) diagrams of the Galactic Center observed with the Nobeyama 45-m telescope in the CO and CS line emissions, we obtain a central rotation curve of the Milky Way. We combine it with the data for the outer disk, and construct a logarithmic rotation curve of the entire Galaxy. The new rotation curve covers a wide range of radius from r ~ 1 pc to several hundred kpc without a gap of data points. It links, for the first time, the kinematical characteristics of the Galaxy from the central black hole to the bulge, disk and dark halo. Using this grand rotation curve, we calculate the radial distribution of surface mass density in the entire Galaxy, where the radius and derived mass densities vary over a dynamical range with several orders of magnitudes. We show that the galactic bulge is deconvolved into two components: the inner (core) and main bulges. Both the two bulge components are represented by exponential density profiles, but the de Vaucouleurs law was found to fail in representing the mass profile of the galactic bulge.
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Black hole (networking)
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Surface photometry in I, J, K of the oval disk galaxy M 94 (NGC 4736) reveal a weak central stellar bar of 0.7 kpc semi-major axis length, comprising ≈ 14% of the total light within 20″. By stellar kinematics the existence of a small spheroidal bulge with v / à ≈ 0.8 was discovered. The ionized gas ( H α ) in this region shows global and local deviations from the stellar kinematics. Model calculations of closed orbits for the cold gas in the combined potential of bar, disk, and bulge predict large non-circular motions in equilibrium flow. However, these do not fit the observed gas kinematics; obviously hydrodynamical forces play a role in the central region of M 94.
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We present radial velocities for a sample of 723 planetary nebulae (PNe) in the disk and bulge of M31, measured using the WYFFOS fibre spectrograph on the William Herschel telescope. Velocities are determined using the [OIII] 5007 Angstrom emission line. Rotation and velocity dispersion are measured to a radius of 50 arcminutes (11.5 kpc), the first stellar rotation curve and velocity dispersion profile for M31 to such a radius. Our kinematics are consistent with rotational support at radii well beyond the bulge effective radius of 1.4kpc, although our data beyond a radius of 5kpc are limited. We present tentative evidence for kinematic substructure in the bulge of M31 to be studied fully in a later work. This paper is part of an ongoing project to constrain the total mass, mass distribution and velocity anisotropy of the disk, bulge and halo of M31.
Velocity dispersion
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Stellar kinematics
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We analyze the three-dimensional kinematics of a sample of ∼4400 red clump stars ranging between 5 and 10 kpc from the Galactic center and up to 3 kpc from the Galactic plane. This sample is representative for the metal-rich ([Fe/H] = −0.6 − +0.5) thick disk. Absolute proper motions are from the fourth release of the Southern Proper Motion Program and radial velocities from the second release of the Radial Velocity Experiment. The derived kinematical properties of the thick disk include the rotational velocity gradient ∂Vθ/∂z = −25.2 ± 2.1 km s−1 kpc−1, velocity dispersions km s−1, and velocity-ellipsoid tilt angle αRz = 86 ± 18. Our dynamical estimate of the thin-disk scale length is Rthin = 2.0 ± 0.4 kpc and of the thick-disk scale height is zthick = 0.7 ± 0.1 kpc. The observed orbital eccentricity distribution compared with those from four different models of the formation of the thick disk from Sales et al. favors the gas-rich merger model and the minor merger heating model. Interestingly, when referred to the currently accepted value of the LSR, stars more distant than 0.7 kpc from the Sun show a net average radial velocity of 13 ± 3 km s−1. This result is seen in previous kinematical studies using other tracers at distances larger than ∼1 kpc. We suggest this motion reflects an inward perturbation of the locally defined LSR induced by the spiral density wave.
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view Abstract Citations (144) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Most Completely Sampled Rotation Curves for Galaxies Sofue, Yoshiaki Abstract We have compiled high-resolution position-velocity diagrams observed along the major axes of nearby spiral galaxies in the CO line emission, and derived rotation curves for the inner regions of the galaxies. We have combined the inner rotation curves with the outer H I and optical rotation curves to obtain the total rotation curves. The inner rotation curves are characterized by a steep increase within a radius of a few hundred parsecs, indicating a compact massive concentration near the nucleus. We fit the obtained rotation curves for individual galaxies by a modified Miyamoto-Nagai potential by assuming existence of four mass components: a nuclear mass component with a scale radius of 100-150 pc and a mass of ∼3-5 × 109 Msun a central bulge of 0.5-1 kpc radius of mass ∼1010 Msun; a disk with scale radius 5-7 kpc and thickness 0.5 kpc of mass ∼1-2 × 1011 Msun and a massive halo of scale radius 15-20 kpc with a mass ∼2-3 × 1011 Msun. We discuss the implication of the nuclear compact mass component for the formation mechanism of multiple structures within the central bulge of a galaxy during its formation. Publication: The Astrophysical Journal Pub Date: February 1996 DOI: 10.1086/176796 arXiv: arXiv:astro-ph/9507098 Bibcode: 1996ApJ...458..120S Keywords: GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: ISM; Astrophysics E-Print: Plain TEX, 16 pages, Figures on request, to appear in ApJ full text sources arXiv | ADS | data products SIMBAD (9) NED (9)
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Abstract Gas morphology and kinematics in the Milky Way contain key information for understanding the formation and evolution of our Galaxy. We present hydrodynamical simulations based on realistic barred Milky Way potentials constrained by recent observations. Our model can reproduce most features in the observed longitude–velocity diagram, including the Central Molecular Zone, the Near and Far 3 kpc arms, the Molecular Ring, and the spiral arm tangents. It can also explain the noncircular motions of masers from the recent BeSSeL2 survey. The central gas kinematics are consistent with a mass of 6.9 × 10 8 M ⊙ in the Nuclear Stellar Disk. Our model predicts the formation of an elliptical gaseous ring surrounding the bar, which is composed of the 3 kpc arms, the Norma arm, and the bar-spiral interfaces. This ring is similar to those “inner” rings in some Milky Way analogs with a boxy/peanut-shaped bulge (e.g., NGC 4565 and NGC 5746). The kinematics of gas near the solar neighborhood are governed by the Local arm. The bar pattern speed constrained by our gas model is 37.5–40 km s −1 kpc −1 , corresponding to a corotation radius of R CR = 6.0–6.4 kpc. The rotation curve of our model rises gently within the central ∼ 5 kpc, significantly less steep than those predicted by some recent zoom-in cosmological simulations.
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Dynamics
Barred spiral galaxy
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In the fundamental quest of the rotation curve of the Milky Way, the tangent-point method has long been the simplest way to infer velocities for the inner low-latitude regions of the Galactic disk from observations of the gas component. In this article, we test the validity of the method on a realistic gas distribution and kinematics of the Milky Way, using a numerical simulation of the Galaxy. We show that the resulting velocity profile strongly deviates from the true rotation curve of the simulation because it overstimates it in the central regions and underestimates it around the bar corotation. In addition, its shape is strongly dependent on the orientation of the stellar bar with respect to the observer. The discrepancies are caused by the highly nonuniform nature of the azimuthal velocity field and by the systematic selection by the tangent-point method of high-velocity gas along the bar and spiral arms, or low-velocity gas in less dense regions. The velocity profile only agrees well with the rotation curve beyond corotation, far from massive asymmetric structures. Therefore the observed velocity profile of the Milky Way inferred by the tangent-point method is expected to be very close to the true Galactic rotation curve for 4.5 ≲ R ≤ 8 kpc. The gaseous curve is flat and consistent with rotation velocities of masers, red clump, and red giants stars measured with VLBI astrometry and infrared spectroscopy for R ≥ 6 kpc. Another consequence is that the Galactic velocity profile for R< 4 − 4.5 kpc is very likely flawed by the nonuniform azimuthal velocities and does not represent the true Galactic rotation curve, but instead local motions. The real shape of the innermost rotation curve is probably shallower than previously thought. Using an incorrect rotation curve has a dramatic effect on the modeling of the mass distribution, in particular for the bulge component, whose derived enclosed mass within the central kpc and scale radius are, respectively, twice and half of the actual values. We therefore strongly argue against using terminal velocities or the velocity curve from the tangent-point method to model the mass distribution of the Milky Way. The quest to determine the innermost rotation curve of the Galaxy remains open.
Astrometry
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We present a comparison between ionized gas and stellar kinematics for a sample of 5 early-to-intermediate disc galaxies. We measured the major axis V and sigma radial profiles for both gas and stars, and the h3 and h4 radial profiles of the stars. We also derived from the R-band surface photometry of each galaxy the light contribution of their bulges and discs. In order to investigate the differences between the velocity fields of the sample galaxies we adopted the self-consistent dynamical model by Pignatelli & Galletta (1999), which takes into account the asymmetric drift effects, the projection effects along the line-of-sight and the non-Gaussian shape of the line profiles due to the presence of different components with distinct dynamical behavior. We find for the stellar component a sizeable asymmetric drift effect in the inner regions of all the sample galaxies, as it results by comparing their stellar rotation curves with the circular velocity predicted by the models. The galaxy sample is not wide enough to draw general conclusions. However, we have found a possible correlation between the presence of slowly-rising gas rotation curves and the ratio of the bulge/disc half luminosity radii, while there is no obvious correlation with the key parameter represented by the morphological classification, namely the bulge/disc luminosity ratio. Systems with a diffuse dynamically hot component (bulge or lens) with a scale length comparable to that of the disc are characterized by slowly-rising gas rotation curves. On the other hand, in systems with a small bulge the gas follows almost circular motions, regardless of the luminosity of the bulge itself.
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Peculiar galaxy
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I present sensitive high-resolution VLA B, C, and D array observations of the almost edge-on Scd galaxy NGC 4244 in the 21 -cm spectral line of neutral atomic hydrogen. The gas layer of NGC 4244 is rather symmetric in all respects, i.e. the surface density distribution, flaring and warping. This symmetry allows for a reliable determination of the rotation curve, despite the fact that the galaxy is close to edge-on. The rotation curve rises slowly in the inner 6 kpc, is roughly constant at 100 km s^-1^ out to 10 kpc, and decreases in Keplerian fashion by 15% at the last measured point at 14 kpc. The rotation curve constrains the stellar mass-to-light ratio to lie between 50% and 100% of the "maximum-disk" value. A new technique is presented to determine simultaneously the inclination and the thickness of the gas layer from high-resolution H I observations. This procedure uses the apparent widths at many azimuths (many channels) and can be used at inclinations as low as 60^deg^. Kinematic information is used to separate flaring from warping. The inclination of the unwarped disk is about 84.5, while the small warp coincides with a decreasing inclination (to 82.5^deg^+/-1^deg^). The data indicate that at large radii the disk warps back to the plane defined by the inner disk. The measured gaseous velocity dispersion is roughly constant within the optical disk (8.5 +/- 1 km s^-1^) and increases slightly beyond. On both sides of the galaxy the thickness of the gas layer increases gradually from ~400 pc at 5 kpc to ~1.5 kpc at the last measured point (at 13 kpc). The strong gradients in the inferred thickness which bracket the spiral arms probably result from streaming motions associated with the arms and are not intrinsic to the galaxy. In an accompanying paper (AJ, 1996,) I use the measurements presented in this paper to infer that the dark matter halo of NGC 4244 is highly flattened.
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Line (geometry)
Flattening
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