Probing Galactic Disks with Planetary Nebulae

Open Access
Author:
Herrmann, Kimberly A.
Graduate Program:
Astronomy and Astrophysics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 08, 2008
Committee Members:
  • Robin Bruce Ciardullo, Committee Chair
  • Jane Camilla Charlton, Committee Member
  • Steinn Sigurdsson, Committee Member
  • Richard Alan Wade, Committee Member
  • Stephen Wade Schaeffer, Committee Member
Keywords:
  • extragalactic planetary nebulae
  • galactic dynamics
  • disk mass
Abstract:
Our understanding of galaxy formation and evolution 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 rotation curves alone cannot decouple the mass contribution of the dark halo from that of the disk. Thus astronomers typically assume a constant disk mass-to-light ratio (<i>M/L</i>) in models. While studies indicate that <i>M/L</i> is constant in the inner regions of spirals, nothing is known about the <i>M/L</i> of outer disks. To determine this quantity, one must measure the surface mass of a disk directly from the <i>z</i>-motions of its old disk stars. Planetary nebulae (PNe) are ideal test particles because they are: bright and abundant to > 5 scale lengths (<i>h<sub>R</sub></i>), representative of the old disk, relatively easy to distinguish from H II regions, and their velocities are measurable to ~2 km s<sup>-1</sup> with fiber-fed spectrographs. The first step, then, is to use narrow-band imaging to identify large (~100) samples of PNe in face-on spirals. The magnitudes of the PN samples also provide reliable distances to the galaxies themselves via the Planetary Nebula Luminosity Function (PNLF). I discovered 165, 153, 241, 150, 19, and 71 PNe in IC 342, M74 (NGC 628), M83 (NGC 5236), M94 (NGC 4736), NGC 5068, and NGC 6946, respectively, and determined distances of 3.5 ± 0.3 Mpc, 8.6 ± 0.3 Mpc, 4.8 ± 0.1 Mpc, 4.4 <sup>+0.1</sup> <sub>-0.2</sub> Mpc, 5.4 <sup>+0.2</sup> <sub>-0.4</sub> Mpc and 6.1 ± 0.6 Mpc, which agree well with values in the literature. I also explored minor fluctuations in the PNLFs as a function of position in the galaxies. The next step is to perform follow-up spectroscopy to measure as many velocities as possible and yet also obtain a high precision. I used the Hydra multi-fiber spectrographs on the WIYN 3.5-m and CTIO Blanco 4-m telescopes, as well as the Hobby-Eberly Telescope's Medium Resolution Spectrograph, to measure velocities of 99, 102, 162, 127, and 48 PNe in IC 342, M74, M83, M94, and M101, respectively, to better than 15 km s<sup>-1</sup> precision. I performed rigorous tests to determine my velocity uncertainties and I examined the line ratios of the spectra as well. Using these data, I have determined disk mass surface density directly by analyzing the vertical velocity dispersion (&#963;<sub><i>z</i></sub>) of 550 planetaries in the previously mentioned five nearby low inclination spirals. I removed galactic rotation using H I rotation curves and isolated &#963;<sub><i>z</i></sub> from the velocity ellipsoid by using the epicyclic approximation, a maximum likelihood analysis, and stability arguments. Results from edge-on spirals helped me to estimate the vertical scale height, <i>h<sub>z</sub></i>, initially assumed to be constant with radius. My results are interesting. In most cases, within ~3 <i>h<sub>R</sub></i>, the exponential decrease of &#963;<sub><i>z</i></sub> generally follows that of the light, indicating that <i>M/L</i> is indeed constant in the inner regions of my galaxies. These results agree with absorption line studies out to 1.5 <i>h<sub>R</sub></i>. However, in the two galaxies with significant data beyond 4 <i>h<sub>R</sub></i>, &#963;<sub><i>z</i></sub> stops declining and drastically flattens out. Possible physical explanations for my results include (1) variations in the light profile (i.e., a broken exponential) and flaring, possibly indicative of a transition between the thin and thick disks and (2) heating of the disk by halo substructure.