UV Luminosity Function

The ultraviolet (∼2000Å) provides a measure of the recent SFR over the last 108 yr in galaxies, being dominated by emission from young stars of intermediate masses (2-5 M; Boselli et al. 2001), and hence the ultraviolet LF represents a useful tool to quantify the effects of the cluster environment on star formation.

We estimated the UV LFs of the SSC using two different methods for the background subtraction. The FUV (blue) and NUV (red) galaxy luminosity functions obtained using these methods are shown in Fig.1. We find that the Schechter function well describes the FUV and NUV data.

 

Figure 1.The FUV (blue) and NUV (red) LFs for the whole 2.8334 deg2 region covered by both GALEX and WFCAM K-band imaging obtained using the two different methods for background subtraction: (i) the statistical subtraction of field galaxies method (open circles) and (ii) UV-optical colour selection (solid circles). The dashed and solid lines indicate the best-fitting Schechter functions to the data using the two methods. The error bars include Poisson uncertainties as well as uncertainties due to selection effects and completeness correction (see Haines et al. 2010a for details).

Figure 2 examines the relative contributions of passive (NUV-R≥4.5) and star-forming (NUV-R≤ 4.5) galaxies to the far-UV luminosity function.

 

Figure 2. The FUV LFs for the passive (NUV-R>4.5; red symbols) and star-forming (NUV-R<4.5; blue symbols) supercluster galaxy sub-populations. The solid lines indicate the best-fitting Schechter functions to the data.

We obtain best-fit Schechter functions for the star-forming supercluster galaxy populations consistent (within the 1σ errors) with those obtained by Wyder et al. (2005) and Budavari et al. (2005) for local field galaxies The contribution of the passive galaxies to the far-ultraviolet LF is negligible at the bright end (MFUV<-17), but becomes increasingly important at fainter magnitudes.

The pile-up of quiescent galaxies in the last three magnitude bins (MFUV>-15) appears the primary cause of the discrepancy in the faint-end slopes between cluster and field galaxies. This effect is seen also in the Coma cluster ultraviolet LF (Cortese et al. 2008), and also at optical wavelengths (Mercurio et al. 2006). A second contributing factor to the steeper observed cluster UV LFs could be simply the steeper stellar mass functions (or K-band LFs) of cluster galaxies, as observed by Merluzzi et al. (2010) for this same Shapley supercluster core region.

Given that the FUV emission from the ``passive'' galaxies is unlikely to be due to star-formation (O'Coconnell 1999; Dorman et al. 2001), the relative consistency between the FUV LFs of cluster star-forming galaxies and the field suggests little environmental dependence in the unobscured SFRs of star-forming galaxies.

Go to the paper Haines et al. 2010a.