Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...211.6903h&link_type=abstract
American Astronomical Society, AAS Meeting #211, #69.03; Bulletin of the American Astronomical Society, Vol. 39, p.859
Other
Scientific paper
Our understanding of galaxy formation is severely limited by poorly known galaxy mass profiles. Flat rotation curves indicate the presence of dark matter in the outer regions of spirals and determine total galactic mass, but cannot decouple the contribution of the dark halo from that of the disk. Thus astronomers typically assume a constant disk mass-to-light ratio (M/L). While studies indicate M/L is constant in the inner regions of spirals, nothing is known about the M/L of outer disks.
We have determined disk mass surface density directly by measuring the vertical velocity dispersion (σz) of planetary nebulae (PNe) in M33 and five other nearby but more face-on spirals: M74, M83, M94, M101, and IC 342. PNe are ideal test particles because they are: bright and abundant to >5; scale lengths (hR), representative of the old disk, relatively easy to distinguish from H II regions, and measurable to 2 km s-1 with fiber-fed spectrographs. We used narrow-band imaging to find large ( 100) samples of PNe then follow-up spectroscopy to measure their velocities. We removed galactic rotation using H I and CO rotation curves and isolated σz from the velocity ellipsoid by using the epicyclic approximation, a maximum likelihood analysis, and stability arguments. Results from edge-on spirals helped us estimate the vertical scale height, hz, assumed to be constant with radius.
Our results are interesting. Within 3hR, the exponential decrease of σz closely follows that of the light, indicating that M/L is indeed constant in the inner regions. These results agree with absorption line studies out to 1.5hR. However, in the two galaxies with data significantly beyond 4hR, σz stops declining and drastically flattens out. We explore possible causes of this flattening (and consequential rising M/L) and discuss the implications of our findings.
This work is supported by NSF and NASA.
Ciardullo Robin
Herrmann Kimberly A.
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