| | Gas Kinematics of Nearby Galaxies |
Collaborators: D. R. Andersen, R. Swaters, M. Verheijen, K. Westfall
Face-on systems, as it turns out, are a wonderful laboratory for exploring the intrinsic shapes of disks. Projection effects in the photometric structure are minimized, as too is internal extinction. Historically, the problem with studying such systems has been the inability to determine their precise inclinations and hence true rotation speeds, or higher-order details of their kinematics.
During our previous NSF program we studied a sample of roughly 40 nearby, nearly face-on late-type, luminous spirals using optical and infrared photometry, and echelle-resolution integral-field spectroscopy using DensePak on the WIYN 3.5m telescope. The results of this portion of previous program (AST-9970780; presented here) established the ability to use such data to determine accurate kinematic inclinations below 30 degrees; to probe the intrinsic shape of galaxy disks; and to construct a preliminary face-on Tully-Fisher relation. This formed the starting point for the initial phase of our current program.
In the first year of our current program (AST-0307417) we have extended the H-alpha integral-field spectroscopy survey using SparsePak to 85 additional, nearly-face on galaxies; completed a robust analysis of the "face-on" Tully-Fisher relations to verify that we can use such targets to measure both total mass and disk mass; compared HI to HII line-widths to determine the nature of line-width asymmetries; and continued our kinematic survey of low-surface brightness disks to probe the inner shape of dark-matter haloes.
We have continued a survey to map the Ha line-strength and kinematics of 100 nearly face-on spiral galaxies. The SparsePak IFU on the Bench Spectrograph / WIYN 3.5m Telescope is being used as the measuring engine. Galaxies are selected from the UGC to have apparent inclinations under 30 degrees (based on photometric axis ratios from DSS images), normal morphology (with and without bars), and disk scale-lengths between 8 and 22 arcsec. Over the past year we have had 3 SparsePak runs and two associated runs on the KPNO 2.1m imaging for follow-up optical imaging (UBVRI). We have successfully completed kinematic observations of 85 targets (85% of our survey), and we have been awarded time to complete the survey in Fall 2004.
The purpose of the survey is several fold. First, these targets are to be used as the parent sample for the follow-up stellar kinematic observations with SparsePak and PPAK, as part of the larger Diskmass program. As described in the the link, this program is designed to measure the mass and mass-to-light ratios of these galaxies' disks, and to uniquely decompose rotation curves to establish the dark-matter halo profile and the mass fractions of disk:halo:total. A subset of 40 targets will be selected which have normal Ha kinematics. An example of such a selection is shown above for a subset of our existing data: Galaxies with "regular" Ha kinematics are shown in the left panel, while galaxies with irregular Ha kinematics are shown in the right panel.
The survey will also be used to study the face-on Tully-Fisher relation in statistical detail (the completion of a pilot project is described below), kinematic and photometric asymmetries, and how these asymmetries couple to the global properties of galaxies and their deviations from fundamental scaling laws, such as Tully-Fisher. These "observables" will be used as constraints on the end-points of galaxy-formation models and simulations.
(b) A Face-On Tully-Fisher Relation: Andersen & Bershady (2003)
We have published the first determination of the line-wdith--luminosity relation for face-on galaxies. This is significant for two reasons. First, internal extinction corrections are minimized overall, and the differential systematic errors in these corrections are essentially eliminated. Second, these systems are ideal for measuring the z-component of the stellar disk velocity dispersion. With both rotational and vertical motions in hand, plus an assumption about the relevant disk scale height, the mass distribution of the galaxy can be decomposed between disk and halo. This is the goal of our proposed program with SparsePak described here. The following three figures from Andersen and Bershady (2003) show examples (from left to right) of measured position-velocity diagrams and inclinations for nearly face-on galaxies; the constructed Tully-Fisher relation; and the deviations from the relation, which we find correlates with the kinematic asymmetry (see above).
(c) HI vs HII: The nature of line-width asymmetries.
Andersen and Bershady are nearing completion of an analysis of HI vs HII (Ha) spatially-integrated spectral line profiles for a pilot sample of about 40 face-on galaxies observed in Andersen's Ph.D. thesis with the WIYN 3.5m Telescope, DensePak IFU, and Bench Spectrograph. The survey was the precursor to the current SparsePak Ha survey which we have described. We have consolidated and systematized the various centroid, width, and asymmetry measurements found in the literature, found their correlations and equivalences, and compared optical to radio measurements on a consistent basis. Our findings include the identification of most asymmetry measurements with the 3rd moment of the line-profile, i.e., skew, but that the 4th moment, kurtosis, also has important information for characterizing the line shapes, interpreting their asymmetries, and relating the line-widths to rotation velocities. The lack of correlation we find between HI and HII asymmetries indicates that these asymmetries do not arise because of significant distortions in the mass potential, but instead from asymmetries in the distribution of flux generated from a tracer mass constituent (i.e., the gas, ionized and neutral). Simple models indicate it is very difficult to uniquely determine if kinematic asymmetries exist in the galaxies. Hence the case for resolved, velocity fields to measure asymmetries is clearly made, and indeed, such measurements can (and will) be performed with our existing and recently gathered Ha velocity fields.
(d) Low Surface-Brightness Galaxies:
This program is being led by collaborator R. Swaters. We are using SparsePak data to measure the inner rotation curves of nearby, low surface-brightness (LSB) galaxies to probe the shape of the inner density profile of dark-matter haloes. The assumption is that LSBs area dark-matter dominated at all radii, whereas in normal galaxies, dark-matter only dominates at large radii. While nobody has yet directly checked this assumption, our SparsePak observations were able to overcome two significant observational problems with extant data: Past problems have included poor spatial resolution from HI synthesis maps (i.e., beam smearing artificially flattened the relatively steeply rising inner rotation curve), and uncertain positioning of long-slit optical spectroscopy. What we do find from SparsePak data is the amplitude of non-circular motions -- even in the gas -- become appreciable at small radii. As a result the interpretation of the velocity fields in terms of a circular speed, and hence enclosed mass, is uncertain. On-going is a larger survey where we hope to find several cases where non-circular motions are small. In the absence of such cases, we will develop a suitable statistical model to allow us to interpret ensemble velocity fields in terms of a robust, characteristic density profile.
The Kinematics in the Cores of Low Surface Brightness Galaxies, Swaters, R. A., Verheijen, M. A. W., Bershady, M. A., Andersen, D. R., 2004, to appear in the Proceedings IAU 220 "Dark Matter in Galaxies", eds. S. Ryder et al. (preprint: [astro-ph/0311480]) Funded under AST-0307417
The Kinematics in the Core of the Low Surface Brightness Galaxy DDO 39, Swaters, R. A., Verheijen, M. A. W., Bershady, M. A., and Andersen, D. R., 2003, ApJ Lett, 587, L19 (reprint: [local pdf], [ApJL])
| |
|
|
|