Vertical Structures
of
Late-Type Edge-On Disk Galaxies

Olof A. van den Berg¹

5 September 2006

Master Thesis

 

Abstract:

The study of the vertical structures of the disks of late-type spiral galaxies is very important as it holds the key to unravelling the formation and evolution of disk galaxies. For example, the uncertainty if all late-type galaxies possess a thick disk, a faint extended disk component containing old stars, plays an important role in this discussion.
To address this question, we collected a pilot sample of 11 late-type disk galaxies, observed in the near-infrared J- and or K'-filter, creating images with high S/N and very flat backgrounds. Various kinds of structure decomposition was done on vertical surface brightness profiles to discover if the galaxies contained thick disks or not and what their parameters would be.
We create vertical colour profile sets for six galaxies available in the J- and K'-band to determine their colour gradients and hence insight in their stellar population.
Our 2D two disk (thin+thick) fits confirm the results of Yoachim & Dalcanton (2006) as we find clear thick disk components in all our sample galaxies except for one case, where the possible existence of a thick disk cannot be excluded. However, our results show clear differences. We only find thick disk components in our high-mass galaxies ( $v_{rot}\ >\ 120$ km s$^{-1}$). For two low-mass galaxies added to our sample we confirm a distinct vertical structure but those could not be fitted as a superposition of thin+thick disk. Our thick disk parameter values show also fainter and flatter thick disks than their high-mass galaxies, with an average difference between $\mu_n$ and $\mu_k$ (the central surface brightness for the thin and thick disk) of 4.5 $\pm$ 0.7 mag arcsec$^{-2}$ and an average scaleheight ratio of 5.6 $\pm$ 1.8. Tests of our fitting methods on artificial galaxies showed that truncated galaxies, a commonly observed phenomenon, will show various deviations from the input parameters, making it difficult to assess the quality of the individual results.
Our ($J-K$) colour gradients show distinct blueing at larger distance from the plane. This is in contrast to the ($B-I$) results of De Grijs & Peletier (2000) who find no clear gradient and even a slight reddening at larger scaleheight. Their result is probably affected by star formation near the plane. Our results stay within the Bruzual-Charlot models, showing dust has little influence even close to the galactic plane on our colour profiles. This in contrast to the ($R-K$) results by Dalcanton & Bernstein (2002) who address their inner reddening to dust influence.