Galaxy colors


The bimodal galaxy distribution in the optical color—magnitude 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 objects, neither can it be used for galaxy classi cation 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 color (GALEX NUV λeff = 227 nm) to the optical (g—r vs Mr) CMD reveals a tight relation in the three-dimensional color—color—magnitude 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, NUV — r) with a standard deviation of 0.03—0.07 mag making it the tightest known galaxy photometric relation given the ~ 0.9 mag range of k-corrected g — r colors. Similar relations exist in other NUV—optical colors. There is a strong correlation between morphological types and integrated NUV — r colors 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. Using stellar population models for galaxies with different star formation histories, we show that (a) the (NUV — r, g — r) distribution at a given luminosity is formed by objects having constant and exponentially declining star formation rates with different characteristic timescales with the red sequence part consistent also with simple stellar population; (b) color evolution for exponentially declining models goes along the relation suggesting a weak evolution of its shape up-to a redshift of 0.9; (c) galaxies with truncated star formation histories have very short transition phase offset from the relation thus explaining the rareness of E+A galaxies. This relation can be used as a powerful galaxy classi cation tool when morphology remains unresolved. Its mathematical consequence is the possibility of precise and simple redshift estimates from only three broad-band photometric points. We show that this simple approach being applied to SDSS and GALEX data works better than most existing photometric redshift techniques applied to multi-color datasets. Therefore, the relation can be used as an efficient search technique for galaxies at intermediate redshifts (0.3 < Z < 0.8) using optical imaging surveys.

Photometric relations

Here we provide links to the FITS files containing the best-fitting surfaces of color-color-magnitude relations in different combinations of colors and absolute magnitudes. Every FITS file contains a primary "image" HDU and a "COLCOLMAG_SURFACE" binary fits extension.

  • The primary HDU header contains the identifiers of colors and magnitudes used to construct the relation (COL_UV, COL_OPT1, COL_OPT2, MAG_BAND), the selection extinction values (A_UV, A_OPT1, A_OPT2), the absolute magnitude zero-point (MAG_OFFS), and the mean dispersion of the residuals of the best-fitting approximation in magnitudes (SIG_RES)
  • The primary HDU contains the coefficients of the best-fitting polynomial surface of an optical color (e.g. g-r) as a 2-dimensional array. Every (x,y) value corresponds to the coefficient of the (Mabs-MAG_OFFS)x(NUV - optical)y term, where x and y are starting from 0 and MAG_OFFS is a value provided in the FITS-header
  • The first extension header duplicates the one from the primary HDU. In addition, it includes the MINFCNT keyword defining the minimum value of the 2D histogram used to perform the polynomial fitting. The first extension contains a binary table of a more complex structure. Its fields are as follows:
    • MAGGRID - a 1D array defining the grid of absolute magnitude values for the photometric relation
    • COL1GRID - a 1D array defining the grid of the color values (e.g. NUV-r) for the photometric relation
    • C2VAL - a 2D array containing the median values of the optical color (e.g. g-r) in every bin computed on the grid defined by MAGGRID and COL1GRID (see Appendix B in the paper)
    • C2STDEV - a 2D array containing standard deviations of the best-fitting residuals in every bin computed on the grid defined by MAGGRID and COL1GRID
    • C2COUNT - a 2D histogram of galaxy counts per bin of the grid defined by MAGGRID and COL1GRID
    • C2FIT - a 2D array containing the best-fitting surface of C2VAL
    • COEFF - the same as the primary HDU

Links to the FITS files:


Chilingarian, I., Zolotukhin, I. 2012: A universal ultraviolet-optical color—color—magnitude relation of galaxies, MNRAS, 419, 1727 [BibTeX entry]   [PDF]