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Register a new userAccurate determination of the mass distribution in spiral galaxies III. Fabry-Perot imaging spectroscopy of 6 spiral galaxies
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Added by
Sebastien Blais-Ouellette
Publication date
2004
fields
Physics
and research's language is
English
Authors
S. Blais-Ouellette
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High resolution Fabry-Perot data of six spiral galaxies are presented. Those data extend the previous sample of spiral galaxies studied with high resolution 3D spectroscopy to earlier morphological types. All the galaxies in the sample have available HI data at 21 cm from the VLA or Westerbork. Velocity fields are analyzed and Halpha rotation curves are computed and compared to HI curves. The kinematics of NGC 5055 central regions are looked at more closely. Its peculiar kinematics can be interpreted either as a bipolar outflow or as a counter-rotating disk, possibly hosting a 9 pm 2 10^8 Msol compact object. Most of the Halpha rotation curves present a significantly steeper inner slope than their HI counterparts. The 21 cm data thus seems affected by moderate to strong beam smearing. The beam smearing has an effect at higher scale-length/beam-width than previously thought (up to 20 km/s at a ratio of 8.5).
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Using the example of the Sd galaxy NGC 5585, it is shown that high resolution 2-D HII kinematical data are necessary to determine accurately the parameters of the mass distribution in spirals. New CFHT Fabry-Perot Halpha observations are combined with low resolution (20) Westerbork HI data to study its mass distribution. Using the combined rotation curve and best fit models, it can be seen that M/L of the luminous disk goes from 0.3 using only the HI rotation curve, to 0.8 using both the optical and the radio data. This reduces the dark-to-luminous mass ratio in NGC 5585 by ~30% through increasing the dark matter halo core radius by nearly the same amount. This shows the importance of the inner, rising part of the rotation curve for the accurate determination of the parameters of the global mass (luminous & dark) distribution and suggests that such a fine tuning of the rotation velocities using high resolution 2-D HII kinematics is necessary to look at correlations between the parameters of the dark matter component and other properties of galaxies.
New high resolution CFHT Fabry-Perot data, combined with published VLA 21 cm observations are used to determine the mass distribution of NGC 3109 and IC 2574. The multi-wavelength rotation curves allow to test with confidence different dark halo functional forms from the pseudo-isothermal sphere to some popular halo distributions motivated by N-body simulations. It appears that density distribution with an inner logarithmic slope <= -1 are very hard to reconcile with rotation curves of late type spirals. Modified Newtonian Dynamics (MOND) is also considered as a potential solution to missing mass and tested the same way. The new higher resolution data show that MOND can reproduce in details the rotation curve of IC 2574 but confirm its difficulty to fit the kinematics of NGC 3109.
We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the UV to the FIR/submm and can also account for the surface brightness distribution in both the optical/NIR and FIR/submm. The calculations are based on the model of Popescu et al. (2000), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912 AA and 2.2 micron, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the Halpha/Hbeta line ratio.
Recently, it has been shown that a correlation exists between the rate of shear and the spiral arm pitch angle in disk galaxies. The rate of shear depends upon the shape of the rotation curve, which is dependent upon the mass distribution in spiral galaxies. Here, we present an imporoved correlation between shear rate and spiral arm pitch angle, by increasing the sample size. We also use an adiabatic infall code to show that the rate of shear is most strongly correlated with the central mass concentration, c_m. The spin parameter, $lambda$, and the fraction of baryons that cool, F, cause scatter in this correlation. Limiting this scatter, such that it is equal to that in the correlation between shear rate and pitch angle, and using a value of F=0.1 to 0.2, the spin parameter must be in the range 0.03<lambda<0.09 for spiral galaxies. We also derive an equation which links spiral arm pitch angle directly to c_m.
We analyse a high-resolution, fully cosmological, hydrodynamical disc galaxy simulation, to study the source of the double-exponential light profiles seen in many stellar discs, and the effects of stellar radial migration upon the spatio-temporal evolution of both the disc age and metallicity distributions. We find a break in the pure exponential stellar surface brightness profile, and trace its origin to a sharp decrease in the star formation per unit surface area, itself produced by a decrease in the gas volume density due to a warping of the gas disc. Star formation in the disc continues well beyond the break. We find that the break is more pronounced in bluer wavebands. By contrast, we find little or no break in the mass density profile. This is, in part, due to the net radial migration of stars towards the external parts of the disc. Beyond the break radius, we find that ~60% of the resident stars migrated from the inner disc, while ~25% formed in situ. Our simulated galaxy also has a minimum in the age profile at the break radius but, in disagreement with some previous studies, migration is not the main mechanism producing this shape. In our simulation, the disc metallicity gradient flattens with time, consistent with an inside-out formation scenario. We do not find any difference in the intensity or the position of the break with inclination, suggesting that perhaps the differences found in empirical studies are driven by dust extinction.
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