Stellar kinematic data for the central region of spiral galaxies. II.
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Abstract:
We present a second dataset of absorption spectroscopy on the inner region of spiral galaxies. We have determined the central velocity dispersion for 42 Sa-Sc objects and, for 32 of them, stellar rotation curves and velocity-dispersion profiles. Some of these profiles are limited to the bulge, some others do reach a region dominated by the luminosity of the disk. These data are intended to provide basic material for the study of the mass distribution and dynamical status in the central regions of spiral galaxies. Although no elaborate bulge-and-disk photometric decomposition is performed, we estimate the effects of limited resolution and contamination by disk light on the central velocity dispersion of the bulge. All the material presented in this paper, in particular the spectra, is available on-line.Keywords:
Velocity dispersion
Barred spiral galaxy
A kinematical study of the galaxy NGC 3504 (Hubble type SABab) finds that the velocity field is dominated by circular motions, with no evidence (within the observational error) of noncircular gas streaming motions of the type suggested by barred galaxy models. The rotation curve shows the characteristic spiral galaxy pattern of a rapid central rise to a radius of 6 arc seconds beyond which point the velocity continues to rise but at a smaller rate; these new data do not confirm the form of the rotation curve found in the prior study by Burbidge, Burbidge, and Prendergast (1961).
Circular motion
Barred spiral galaxy
Irregular galaxy
Hubble sequence
Unbarred spiral galaxy
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We present spectral changes measured across the bulge of an edge-on spiral (Sa) galaxy, NGC 3190, and an edge-on lenticular galaxy, NGC 1023, probing spatial metallicity and stellar population changes. We find that gradients in spectral line strengths are measurable in the bulge of the spiral galaxy. The lenticular galaxy shows a sharp increase in strength of spectral absorption features towards the galaxy centre, with more gradual gradients further out. Outside the central 1 or 2 arcsec we find no significant difference from gradients found in many ellipticals, indicating that similar amounts of dissipation could have occurred in the formation of ellipticals and the two bulges studied here.
Barred spiral galaxy
Irregular galaxy
Hubble sequence
Equivalent width
Stellar population
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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' 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.
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We present the results of absorption spectroscopy on the inner region of 34 Sa-Sc galaxies. We have determined the central velocity dispersion and, for 32 of these objects, stellar rotation curves and velocity-dispersion profiles. Some of these profiles are limited to the bulge, some others do reach a region dominated by the luminosity of the disk. These data are intended to provide basic material for the study of the mass distribution and dynamical status in the central regions of spiral galaxies. Although no elaborate bulge-and-disk photometric decomposition is performed, we estimate the effects of limited resolution and contamination by disk light on the central velocity dispersion of the bulge.
Velocity dispersion
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In a recent Letter (astro-ph/9605001), Cowsik et al. claim that the best fit value of the velocity dispersion of the Galactic dark matter is 600 kms. This result is obtained by a numerical method of solving the coupled self-gravitation equations, assuming Maxwellian velocity distributions. The rotation curve presented in their original paper is not the whole story, as it does not match onto the spherical self-gravitating halo with a Maxwellian velocity distribution -- as it must. This was pointed out in the original version of this Comment. In their Reply (astro-ph/9608035), Cowsik et al. provide a rotation curve that does behave with the correct asymptotics. Unfortunately, it violates mass estimates of the Local Group by over an order of magnitude. It is the pressure required to balance the weight of the overlying layers of this phenomenal mass at large radii that is the physical origin of Cowsik et al's anomalously high dark matter velocity dispersion.
Velocity dispersion
Mixed dark matter
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We present first results from a long‐slit spectroscopic survey of bulge kinematics in local spiral galaxies. Our optical spectra were obtained at the Hobby‐Eberly Telescope with the LRS spectrograph and have a velocity resolution of 45 km/s (σ*), which allows us to resolve the velocity dispersions in the bulge regions of most objects in our sample. We find that the velocity dispersion profiles in morphological classical bulge galaxies are always centrally peaked while the velocity dispersion of morphologically disk‐like bulges stays relatively flat towards the center—once strongly barred galaxies are discarded.
Velocity dispersion
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The rotation curve of the small-mass starburst galaxy M82 has a steep nuclear rise, peaking at 200 pc radius, which then declines in a Keplerian fashion. This rotation curve mimics that for a central bulge of spiral galaxies with a high concentration of stellar mass. The declining rotation indicates that its extended disk mass is missing. In order to explain this peculiar rotation characteristic, we propose a hypothesis that M82 is a surviving central bulge of a much larger disk galaxy, whose outer disk was truncated during a close encounter with M81. We simulated a tidal truncation of the disk of a companion galaxy by a tidal penetration through its more massive parent galaxy. The model can well reproduce the observed peculiar feature of M82.
Barred spiral galaxy
Tidal force
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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.
Envelope (radar)
Velocity dispersion
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The apparent galactic rotation curve as inferred from HI and CO terminal velocities shows a narrow peak (|$v_{max}\simeq250-260\text {kms}^{-1}$|) at |$r\simeq500$| pc, and a steep decline for 600 |$\text {pc}\le r \le1.5$| kpc down to a rather broad minimum (|$v_{min}\simeq195\text{kms}^{-1}$|) around 2.8 kpc. In this paper, we show that this morphology can be reconciled with the bulge density distribution determined by infrared observations and the local density of spheroid stars only if the bulge of our Galaxy is non-axisymmetric and the resulting potential triaxial. In order to reproduce the galactic rotation curve satisfactorily, we require that the Sun be within 20° of the short axis of the bulge in the equatorial plane (the long axis of the closed loop orbits), and that the mean axial ratio of the bulge in the disc plane be 0.6 inside the knee in the density distribution at 800 pc. From such a viewing direction this simultaneously moves the peak in the apparent rotation curve inwards to 500 pc. and explains the near-Keplerian fall-off in terms of the rapidly decreasing quadrupole moment of the potential. For the same peak rotation velocity, our best-fitting bulge model has only 2/3 of the mass of a corresponding axisymmetric bulge. In several external galaxies, the inner rotation curves show anomalies in the sense expected if the bulges of these systems were triaxial. In NGC 2708 and NGC 3054 we appear to observe the bulge from a direction near the plane containing the major and minor axes, and in NGC 3200 and UGC 2885 from near the plane containing the intermediate and minor axes. The apparent mass-to-light ratios of the bulge components inferred from the inner rotation curves will depend considerably on the viewing geometry in cases where the bulge is triaxial.
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We present a second dataset of absorption spectroscopy on the inner region of spiral galaxies. We have determined the central velocity dispersion for 42 Sa-Sc objects and, for 32 of them, stellar rotation curves and velocity-dispersion profiles. Some of these profiles are limited to the bulge, some others do reach a region dominated by the luminosity of the disk. These data are intended to provide basic material for the study of the mass distribution and dynamical status in the central regions of spiral galaxies. Although no elaborate bulge-and-disk photometric decomposition is performed, we estimate the effects of limited resolution and contamination by disk light on the central velocity dispersion of the bulge. All the material presented in this paper, in particular the spectra, is available on-line.
Velocity dispersion
Barred spiral galaxy
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Citations (41)