Superluminous Spiral Galaxies
  We report the discovery of spiral galaxies that are as optically luminous as
elliptical brightest cluster galaxies, with r-band monochromatic luminosity
L_r=8-14L* (4.3-7.5E44 erg/s). These super spiral galaxies are also giant and
massive, with diameter D=57-134 kpc and stellar mass M_stars=0.3-3.4E11 M_sun.
We find 53 super spirals out of a complete sample of 1616 SDSS galaxies with
redshift z<0.3 and L_r>8L*. The closest example is found at z=0.089. We use
existing photometry to estimate their stellar masses and star formation rates
(SFRs). The SDSS and WISE colors are consistent with normal star-forming
spirals on the blue sequence. However, the extreme masses and rapid SFRs of
5-65 M_sun/yr place super spirals in a sparsely populated region of parameter
space, above the star-forming main sequence of disk galaxies. Super spirals
occupy a diverse range of environments, from isolation to cluster centers. We
find four super spiral galaxy systems that are late-stage major mergers--a
possible clue to their formation. We suggest that super spirals are a remnant
population of unquenched, massive disk galaxies. They may eventually become
massive lenticular galaxies after they are cut off from their gas supply and
their disks fade.
