Shapley Optical Survey - I. Luminosity functions in the supercluster environment

Abstract

We present the Shapley Optical Survey, a photometric study covering a ∼2-deg 2 region of the Shapley supercluster core at z ∼ 0.05 in two bands (B and R). The galaxy sample is complete to B = 22.5 (>M * + 6, N gal = 16 588) and R = 22.0 (>M * + 7, N gal = 28 008). The galaxy luminosity function (LF) cannot be described by a single Schechter function due to dips apparent at B ∼ 17.5 (M B ∼ −19.3) and R ∼ 17.0 (M R ∼ −19.8) and the clear upturn in the counts for galaxies fainter than B and R ∼ 18 mag. We find, instead, that the sum of a Gaussian and a Schechter function, for bright and faint galaxies, respectively, is a suitable representation of the data. We study the effects of the environment on the photometric properties of galaxies, deriving the galaxy LFs in three regions selected according to the local galaxy density, and find a marked luminosity segregation, in the sense that the LF faint end is different at more than 3σ confidence level in regions with different densities. In addition, the LFs of red and blue galaxy populations show very different behaviours: while red sequence counts are very similar to those obtained for the global galaxy population, the blue galaxy LFs are well described by a single Schechter function and do not vary with the density. Such large environmentally dependent deviations from a single Schechter function are difficult to produce solely within galaxy merging or suffocation scenarios. Instead the data support the idea that mechanisms related to the cluster environment, such as galaxy harassment or ram-pressure stripping, shape the galaxy LFs by terminating star formation and producing mass-loss in galaxies at ∼M * + 2, a magnitude range where blue late-type spirals used to dominate cluster populations, but are now absent.

Anatomy of luminosity functions: the 2dFGRS example

“…They also showed that the Schechter function does not fit the LF of extremely low luminosity galaxies. In a Shapley Supercluster region Mercurio et al (2006) conclude that the bright end of the Schechter function is not sufficient to fit the data. Yang et al (2008a) also use different analytical expressions of LFs for different populations: a log-normal distribution for first-ranked galaxies, and a modified Schechter function for satellite galaxies.…”

“…Our study does not include very faint galaxies, as, for example, the study of the core region of the Shapley supercluster (Mercurio et al 2006). Thus future work is needed to understand the difference between the faint ends of the LFs in our and Mercurio et al study. One difference between the earlier studies and our work lies in the use of the analytical LFs.…”

Recent arrival of faint cluster galaxies on the red sequence: luminosity functions from 119���deg2of CFHTLS

“…Although the existence of dips in the red-sequence/early-type LFs has been established in many studies (e.g. Secker & Harris 1996;Mercurio et al 2006;Popesso et al 2006; Barkhouse et al 2007), the depth of the dip and the faint-end slope of the LFs are still not well constrained, varying significantly in different studies. We examine this issue more closely in Section 8.1.…”

Galaxy morphology, luminosity, and environment in the SDSS DR7

“…A similar double-power law was also used by Vale & Ostriker (2004) to fit the mass-luminosity relation in their subhalo model and by Cooray & Milosavljević (2005) to fit the LF of central galaxies. Since several papers have shown that the Schechter function is not a best fit for the LF (Blanton et al 2005;Mercurio et al 2006;Yang et al 2008;Tempel et al 2009), especially at the bright end, we shall use the double-power law for analytical approximation. Parameters for the LF approximations are given in Table 3.…”

“…In Tempel et al (2009) A&A 529, A53 (2011) we have found that the global environment has an important role in determining galaxy properties. Some studies have been dedicated only to special regions: e.g., Mercurio et al (2006) investigate the Shapley supercluster. The dependence on the environment has been also studied numerically (Mo et al 2004) and using semi-analytical models (Benson et al 2003;Khochfar et al 2007).…”