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For almost 30 years the Schechter luminosity function (LF; Schechter 1976) has been the standard tool for quantifying the galaxy population 1. The LF is loosely based on the Press-Schechter formalisation for the primordial halo distribution (Press & Schechter 1974). Moreover the LF consistently provides a good formal fit to the observed luminosity distribution (LD; see for example Norberg et al. 2002). This consistency, between the LD and LF, appears to hold regardless of environment (De Propris et al. 2003; Driver & De Propris 2003). The only departure from a pure Schechter function appears to be in the central cores of rich clusters, where the galaxy LD is often seen to show a marked upturn at the giant-dwarf boundary (MB approx - 16 mag). Perhaps the most well known example is the central LD of the Coma cluster (e.g., Trentham 1998; Beijersbergen et al. 2002; Andreon & Culliandre 2002 and references therein). The most plausible explanation is that the core contains an overdensity of giant and dwarf ellipticals bolstering both the bright and faint-end of the core cluster LF. For example the more extensive Coma survey by Mobasher et al. 2004 recovers a flat and invariant LD/LF (alpha = - 1) to MB approx - 14 mag. The phenomena of an upturn in the LD, has also been seen in Virgo (Impey & Bothun1988; Trentham & Hodgkin 2002), A963 (Driver et al. 1994), A868 (Driver et al. 2003), A2554 (Smith et al. 1997) and A2218 (Pracey et al. 2004) for example. However, apart from these "active" core environments, the overall LDs from the field, to the local group, to the local sphere, and near & far rich clusters all consistently follow a smooth LF within the luminosity ranges probed. Fig. 1 shows an (incomplete) summary of b, B, V or g-band field and cluster LFs colour corrected to the Johnson B filter.

Figure 1

Figure 1. Various luminosity functions as measured for the global environment (upper) and cluster environment (lower). Data are taken from Table. 3 of Liske et al. (2003), Table. 1 of Driver & De Propris (2003), Table. 2 & 3 from Blanton et al. (2003) and Table. 2 from Driver et al. (2004). The solid lines show the regions over which the luminosity functions have been fitted and the dotted lines the extrapolations. The cluster luminosity functions have all been arbitrarily normalised to phi* = 0.0161 galaxies per h3 Mpc-3. The main point is that until the systematics are resolved one cannot draw any reasonable inference other than the field and cluster LFs are broadly consistent.

The main point to take from Fig. 1 is that the global and cluster LFs each show a broad but overlapping range of distributions. Clearly one cannot reasonably argue for any significant variation between the global and overall cluster environment on the basis of these data. Studies based within the same survey data, for example the two-degree field galaxy redshift survey study by Croton et al. 200u, generally find fairly subtle changes with environment. Hence it seems that the variations seen in Fig. 1 indicates an unspecified systematic error in the various studies. The most lauded of these is the unsavoury topic of surface brightness selection effects (Disney 1976, Impey & Bothun 1997). The concern is that the galaxy population at each luminosity interval occupies a range in surface brightness (or size). Surveys with shallow detection isophotes may miss both light from a galaxy's halo, as well as entire galaxies (see for example Sprayberry et al. 1997 and Dalcanton 1998). Cross & Driver 2002 explored this possibility in detail and demonstrated that indeed surface brightness selection effects can play havoc with the recovered Schechter function parameters and reproduce exactly the kind of variation seen in both the global and cluster LFs of Fig. 1.

More recently a number of papers have identified a clear luminosity-surface brightness (or size) 2 relation for field galaxies based on diverse datasets including: the Hubble Deep Field Driver 1999; the two-degree Field Galaxy Redshift Survey Cross et al. 2001; the Sloan Digital Sky Survey (Blanton et al. 2001; Shen et al. 2003); and a very local inclination and dust corrected sample of late-type disks de Jong & Lacey 2000. These studies consistently show that low surface brightness is synonymous with low luminosity - with a few notable exceptions as typified by Malin 1 Bothun et al. 1988 and the faint second disk surrounding NGC5084.

To fully resolve the potential impact of surface brightness selection effects one must consider the joint luminosity-surface brightness distribution. This has been advocated in the past, not so much to compensate for selection bias, but to preserve the size (or surface brightness) information which may be of interest in its own right (see Choloniewski 1985 and Sodré & Lahav 1993 for instance). This latter point is illustrated in Plate 1, where I show an example LF for a nearby volume limited sample and images of the actual galaxies contributing to the LF. Clearly much information is lost when one replaces these images with three simple numbers. It is for these reasons -- the need to accommodate selection bias and the desire to explore additional parameter space - coupled with the abundance of data that now moves us beyond the simple LD/LF to start exploring multivariate distributions. Here I introduce two such distributions, the luminosity-surface brightness plane (for the reasons stated above) and the colour-luminosity plane which is also of topical interest (e.g., Baldry et al. 2003; Hogg et al. 2004 and references therein).

Plate 1

Plate 1. The global Galaxy Luminosity Function (red line) condenses the available information of galaxies (images) into three crucial numbers: the characteristc luminosity (M*); the absolute normalisation (phi*); and the faint-end slope (alpha). Although the schechter parameterisation is more often that not a remarkably good fit, one cannot help but feel that too much important infomation may have been lost, for instance the sizes and bulge-to-total parameters.

1 The Schechter function: d (phi) = phi*(L / L*)alpha e(-L / L*)d (L / L*) has three key parameters, L* the characteristic luminosity where the exponential cutoff cuts in, phi*, the normalisation at this characteristic luminosity, and alpha, the faint-end slope parameter. A value of alpha = - 1 implies equal numbers of galaxies in magnitude intervals, a more negative (or steep) value implies numerous dwarf systems. Back.

2 Luminosity, size and surface brightness are related by µeHLR = M + 2.5 log10 [2pi RHLR2] + 36.57 where µe is the effective surface brightness, M the absolute magnitude, and RHLR the semi-major axis half-light radius in kpc, hence the luminosity-surface brightness relation can be readily transformed to a luminosity-size distribution and we use the acronym LSP to indicate either. Back.

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