Research objectives

Previous detections of thick disk stellar light in external galaxies have been originally made for early-type edge-on galaxies. Without the need for a detailed structure decomposition the thick disk component was clearly visible in several studies. In these, the scaleheight ratio was well determined, but this was not the case for the scalelength ratio. However, only recently Pohlen et. al (2004) conducted the first detailed three dimensional decomposition (taking the line-of-sight into account) of the surface brightness profiles of lenticular galaxies.
Since the work of Tsikoudi and Burstein thick disks are considered to be quite common in lenticular and early-type galaxies, but for late-type galaxies it has not been considered an unambiguous feature because structural decomposition has not been done on a large sample of galaxies to provide sufficient proof.
Thick disks could not be easily seen in late-type galaxies. To do this a detailed structure decomposition was necessary. Van der Kruit & Searle (1981a, 1981b) performed the first three dimensional decomposition of late-type galaxies. Since that time detailed one or two dimensional thick/thin disk decompositions have been reported for only a handful of late-type galaxies.
Recently the inconclusive status of the thick disk in late-type galaxies has been challenged. Dalcanton & Bernstein (2002) and Yoachim & Dalcanton (2006) state that a thick disk is also a common component in late-type disk galaxies. In the first paper thick disk components are inferred from colour maps and colour gradients which show a redding at larger scaleheights, which they attribute to the typical stellar populations in thick disks. For this they focus on low-mass late-type disk galaxies, whereas high-mass disk galaxies do not seem to show such behaviour, which they address to a reddening caused by a larger and more extended dustlane. The reddening in the outer part, or blueing in the inner part, could be addressed to other components or stellar populations.
Continuing on the findings of Dalcanton & Bernstein (2002), Yoachim & Dalcanton (2006) progressed by doing two dimensional one and two disk fits on a subsample of their very late-type disk galaxies. They confirmed the existence of the 'colour' thick disks in all the galaxies of their sample. They also state and show that the thick disk component in low-mass galaxies dominates the profile, contributing nearly half the luminosity of the total galaxy. Although the properties of the components of their high-mass galaxies are comparable to literature, the properties of the components of their low-mass galaxies raise questions and doubts, especially since such they give thin and thick disk central surface brightness differences between -0.8 and 2 mag arcsec$^{-2}$, and thin/thick disk scaleheight ratios between 1.4 and 2.0 for 11 of their galaxies, which can be seen as unusual considering the commonly addressed properties of the thick disk.
The survey done by Dalcanton & Bernstein (2002) and Yoachim & Dalcanton (2006) is the first extensive search for thick disk components in late-type disk galaxies in many years, so there are few to put their findings to the test. To test if the disk component parameters they find for late-Type disk galaxies can be confirmed we do an extensive research to find and qualify thick disk components in late-type disk galaxies.
To discover a distinct thick disk component in late type disk galaxies we are using the classic way by doing deep surface photometry and structural decomposition of surface brightness profiles. To do this about six edge-on disk galaxies, as a pilot sample, were observed in the J- and/or K'-band, expanded with a second sample of six similar galaxies. We use near-infrared images because those are much less contaminated by the absorbing dust in the mid-plane when doing structure analysis. We test various methods for fitting vertical surface brightness profiles to tackle the problem whether a vertical structure can (or should) be called a thick disk or not.
To compare our ($J-K$) gradient results we use the results of Dalcanton & Bernstein dalcanton2002. This comparison is limited as they use ($R-K$) profiles and we can only compare to their high-mass galaxy results. We will also compare our results with De Grijs & Peletier grijs2000, who determined vertical ($B-I$) colour gradients for a complete sample of edge-on disk galaxies. Besides this, there has not been determined a ($J-K$) gradient for late-type disk galaxies before with the current observation quality. We use the opportunity to determine the gradients and compare our findings with previous results.