3.1. The near-IR
Traditionally, almost all nearby galaxy catalogues have been based on flux-limited observations through an optical B or blue bandpass filter (~ 400 - 450 nm), for example: the RC3; the Two-degree Field Galaxy Redshift Survey; the Millennium Galaxy Catalogue etc. This has been driven by technological/commercial necessity with flux detectors typically optimised to the spectral response of the human eye (400 - 800nm). However the most physically meaningful bandpass in which to observe a galaxy is in the near-IR (rest-H band or 1.65 µm). This is mostly because the stellar population that dominates the total stellar mass - and therefore best traces a galaxy's gravitational potential - is the long-lived low-mass population which emits in the near-IR (see for example Gavazzi, Pierrini & Boselli 1996). This is most clearly demonstrated by the obviously smoother appearance of a galaxy in the near-IR than in progressively bluer wavelengths (see upper panel of Fig. 5 showing a montage of images for M51 in a variety of filters - the near-IR images (right most) are significantly smoother. The flux and shape of a galaxy in the near-IR is most dependent on the older relaxed stellar population and therefore a better tracer of the underlying potential. Conversely the flux and shape in the optical is linked to the young stars and therefore dependent on the current and possibly transient star-formation rate. Both optical and near-IR data are important if one wishes to understand galaxy formation and evolution. The near-IR however appears to be the optimal filter for the investigation of the structural properties. The other great advantage is of course the minimisation of the impact of dust obscuration. This is illustrated in the main panel of Fig. 5 which shows the location of the B and H band filters superimposed on the night sky spectrum (dotted line), a galaxy's continuum before and after star-burst (solid lines), and the dust attenuation curve (long dashed line). The impact of star-formation and dust is clearly less in the near-IR. The upcoming near-IR facilities and in particular the UKIRT/WFCAM and VISTA will have the capabilities to provide exactly the kind of wide, deep, high resolution data required for the comprehensive structural analysis of nearby galaxies.
Figure 5. An illustration of the advantages of the near-IR. M51 images in UBVRIJHK respectively are shown along the top. The main panel shows a spectrum of the night sky, a spectrum of a galaxy before and after star-burst and the location of the B and H filters. The extinction curve is also show. At shorter wavelengths one has to contend with the vagaries of dust and star-formation. At longer wavelength images are smoother, less affected by dust and star-formation. For detailed structural analysis the longer wavelengths are clearly optimal.