Ultraviolet color-color relation of early-type galaxies at 0.05 < z <
  0.12
  We present the ultraviolet (UV) color-color relation of early-type galaxies
(ETGs) in the nearby universe (0.05 < z < 0.12) to investigate the properties
of hot stellar populations responsible for the UV excess (UVX). The initial
sample of ETGs is selected by the spectroscopic redshift and the morphology
parameter from the Sloan Digital Sky Survey (SDSS) DR7, and then cross-matched
with the Galaxy Evolution Explorer (GALEX) Far-UV (FUV) and Near-UV (NUV) GR6
data. The cross-matched ETG sample is further classified by their emission line
characteristics in the optical spectra into quiescent, star-forming, and AGN
categories. Contaminations from early-type spiral galaxies, mergers, and
morphologically disturbed galaxies are removed by visual inspection. By drawing
the FUV - NUV (as a measure of UV spectral shape) vs. FUV - r (as a measure of
UVX strength) diagram for the final sample of ~3700 quiescent ETGs, we find
that the "old and dead" ETGs consist of a well-defined sequence in UV colors,
the "UV red sequence", so that the stronger UVX galaxies should have a harder
UV spectral shape systematically. However, the observed UV spectral slope is
too steep to be reproduced by the canonical stellar population models in which
the UV flux is mainly controlled by age or metallicity parameters. Moreover, 2
mag of color spreads both in FUV - NUV and FUV - r appear to be ubiquitous
among any subsets in distance or luminosity. This implies that the UVX in ETGs
could be driven by yet another parameter which might be even more influential
than age or metallicity.
