We study the spectroscopic properties and environments of red ( or passive ) spiral galaxies found by the Galaxy Zoo project .
By carefully selecting face-on , disk dominated spirals we construct a sample of truly passive disks ( i.e . they are not dust reddened spirals , nor are they dominated by old stellar populations in a bulge ) .
As such , our red spirals represent an interesting set of possible transition objects between normal blue spiral galaxies and red early types , making up \sim 6 \% of late-type spirals .
We use optical images and spectra from SDSS to investigate the physical processes which could have turned these objects red without disturbing their morphology .
We find red spirals preferentially in intermediate density regimes .
However there are no obvious correlations between red spiral properties and environment suggesting that environment alone is not sufficient to determine if a galaxy will become a red spiral .
Red spirals are a very small fraction of all spirals at low masses ( M _ { \star } < 10 ^ { 10 } M _ { \odot } ) , but are a significant fraction of the spiral population at large stellar masses showing that massive galaxies are red independent of morphology .
We confirm that as expected , red spirals have older stellar populations and less recent star formation than the main spiral population .
While the presence of spiral arms suggests that major star formation can not have ceased long ago ( not more than a few Gyrs ) , we show that these are also not recent post-starburst objects ( having had no significant star formation in the last Gyr ) , so star formation must have ceased gradually .
Intriguingly , red spirals are roughly four times as likely than the normal spiral population to host optically identified Seyfert/LINER ( at a given stellar mass and even accounting for low luminosity lines hidden by star formation ) , with most of the difference coming from objects with LINER-like emission .
We also find a curiously large optical bar fraction in the red spirals ( 70 \pm 5 \% verses 27 \pm 5 \% in blue spirals ) suggesting that the cessation of star formation and bar instabilities in spirals are strongly correlated .
We conclude by discussing the possible origins of these red spirals .
We suggest they may represent the very oldest spiral galaxies which have already used up their reserves of gas - probably aided by strangulation or starvation , and perhaps also by the effect of bar instabilities moving material around in the disk .
We provide an online table listing our full sample of red spirals along with the normal/blue spirals used for comparison .