Vertical colour profiles & gradients

Another way to gain insight in the structure of galaxies is by studying the stellar populations which build up galaxies and their variation within the galaxy. This can be done by analyzing the colours and colour gradients of galaxies. With our sample containing some galaxies observed in the J- and the K'-band, we are able to create ($J-K$) colour profiles and study their properties and behaviour through the determination of the colour gradients.
We can study the properties and behaviour with stellar population synthesis models, which are tools for interpreting the integrated light (colors, line indices, and mass-to-light ratios) that we observe from galaxies. Ideally, we want to determine what mix of stars give rise to the observations, but as this is very complex, it is needed to make some assumptions about how the number of different types of stars are related. We will make use of the Single Stellar Population (SSP) models with a single age and a single metallicity. In these, all the stars are formed at the same time, with distribution in mass given by the chosen initial mass function (IMF), and with identical chemical composition. More advanced models take evolutionary processes into account, like enrichment of the interstellar medium, differential loss of various elements by galactic winds, and a time-dependent IMF. However, these processes are not well understood, and no consensus has yet been reached on these matters.
James et al. james2006, who performed a study on ($B-K$) and ($J-K$) colours on early-type galaxies, state that the integrated colour ($J-K$) appears to be a good tracer of the initial metallicity of a SSP and is only weakly affected by age, whereas the integrated ($B-K$), which we can compare to the ($B-I$) results of De Grijs & Peletier (2000), who also determined vertical gradients on edge-on galaxies, is a good age indicator, mildly affected by the SSP metallicity. The J and $K$ integrated fluxes are dominated by AGB stars when the SSP age is below $\sim$1 Gyr, and by upper red giant branch (RGB) objects for higher ages. This means that the integrated ($J-K$) colour is mainly determined by the colour of AGB and/or RGB stars, whose evolution is strongly affected by their initial metallicity, whereas ($B-K$) is sensitive to the magnitude and colour of the TO (turn off) stars, hence to the SSP age. However, for ages above $\sim$10 Gyr the ($B-K$) colour tends to lose sensitivity to age. The age sensitive ($B-K$) colours are very sensitive to the presence of young populations, whose main sequence stars are very bright in the $B$ band.
The interpretation of broadband colour gradients relies on a careful mapping of the dust extinction within a galaxy (Knapen et. al. 1995, Huizinga 1998). No attempt was made to correct the surface brightness measurements for internal extinction, since no unique recipe is available to do this. We determine our gradients on the dustfree side of the galaxy and take the galactic extinction into account when determining our ($J-K$) values.
In section 6.1 we will describe how colour profiles are created. Section 6.2 describes how we determined the vertical colour gradient of each galaxy. The results are shown in Section 6.3 and these will also be discussed there.