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Mon. Not. R. Astron. Soc. 000, 1­13 (2010)

Printed 21 September 2011

A (MN L TEX style file v2.2)

A universal ultraviolet-optical colour­colour­magnitude relation of galaxies
I1gor V. Chilingarian1,2, and Ivan Yu. Zolotukhin
3 ,4 ,2
Centre de Donn´es astronomiques de Strasbourg, Observatoire astronomique de Strasbourg, Universit´ de Strasbourg, e e CNRS UMR 7550, 11 rue de l'Universit´, 67000 Strasbourg, France e 2 Sternberg Astronomical Institute, Moscow State University, 13 Universitetsky prospect, Moscow, 119992, Russia 3 Observatoire de Paris, LERMA, UMR 8112, 61 Av. de l'Observatoire, 75014 Paris, France 4 Observatoire de Paris, VO Paris Data Centre, 61 Av. de l'Observatoire, 75014 Paris, France

arXiv:1102.1159v2 [astro-ph.CO] 19 Sep 2011

Accepted 2011 Sep 15. Received 2011 Sep 15; in original form 2011 Feb 6

ABSTRACT

The bimodal galaxy distribution in the optical colour­magnitude diagram (CMD) comprises a narrow "red sequence" populated mostly by early-type galaxies and a broad "blue cloud" dominated by star-forming systems. Although the optical CMD allows one to select red sequence ob jects, neither can it be used for galaxy classification without additional observational data such as spectra or high-resolution images, nor to identify blue galaxies at unknown redshifts. We show that adding the near ultraviolet colour (GALEX NUV eff = 227 nm) to the optical (g - r vs Mr ) CMD reveals a tight relation in the three-dimensional colour­colour­magnitude space smoothly continuing from the "blue cloud" to the "red sequence". We found that 98 per cent of 225 000 low-redshift (Z < 0.27) galaxies follow a smooth surface g - r = F (Mr , N U V - r) with a standard deviation of 0.03­0.07 mag making it the tightest known galaxy photometric relation given the 0.9 mag range of k -corrected g - r colours. Similar relations exist in other NUV­optical colours. There is a strong correlation between morphological types and integrated N U V - r colours of galaxies, while the connection with g - r is ambiguous. Rare galaxy classes such as E+A or tidally stripped systems become outliers that occupy distinct regions in the 3D parameter space. U