Color Gradients in Cooling Flows in Clusters of Galaxies

We have obtained U-, b-, V-, and I-band CCD images of 19 centrally dominant galaxies in cooling-flow clusters (CFDs) and four centrally dominant galaxies in clusters without cooling flows. Using photometry through synthetic elliptical apertures, we find that many CFDs exhibit color-profile anomalies (bluer central colors or positive color gradients) with respect to the control sample and to gE photometry from the recent literature. The anomalies are stronger and extend to larger galactic radii for higher mass accretion rates (m^dot^_CF_) as estimated from X-ray observations. At least eight of eleven CFDs with m^dot^_CF_ >~ 70 M_sun_ yr^-1^ have blue color-profile anomalies that extend radially 5-20 kpc. Some CFDs with smaller m^dot^_CF_s have milder anomalies extending only 55 kpc. The color anomalies are probably due to recent star formation induced by the cooling flows, Only one object (NGC 1275) has nuclear color profiles that unambiguously indicate a nonthermal contribution from an active nucleus. Processes related to nuclear activity are apparently not primarily responsible for the extended color anomalies. The correlation with X-ray properties also argues against an origin fueled by gas-rich galaxies in tidally induced starbursts. The prevalance of color anomalies suggests that star formation which includes massive blue stars occurs continuously for a substantial fraction of the Hubble time, or in frequent episodes. However, in no case are the blue color excesses consistent with star formation with the local initial mass function at the rates inferred from the X-ray observations. In addition, the radial extent of the color anomalies are only ~5%-10% of the cooling radii. If the X-ray inflow rates are correct, then most of the accreting gas must reside in very low mass stars and/or opaque gas. Despite the harsh cluster environment, dust appears to be common in CFDs. At least ~40%-50% of the sample objects show evidence for dust patches or lanes. Seven of the nine objects with dust features also have evidence for star formation. Significant negative color gradients are often observed in the inner ~5 kpc of nonaccreting templates and the CFDs without strong anomalies. These are likely due to metallicity gradients originating from dissipative processes during the early stages of galaxy assembly. Beyond ~5 kpc the color gradients usually flatten to roughly constant color, suggesting that some combinations of dissipationless infall and collisional processes (stripping, mergers) involving other cluster members is important here. High-frequency structure (e.g., ripples) is often found in the radial color profiles. This may be due to dust or to local age/metallicity inhomogeneities. However, we cannot at present rule out the possibility that incomplete removal of interfering images plays a role. Red upturns in the color profiles are present at large radii (r~7-30 kpc) in ~40% of the objects in our sample. These occur at surface brightnesses that are only 10%-30% of the sky background, so they are difficult to establish with certainty. We have been unable to find an instrumental cause for the upturns, and similar effects are present in earlier CCD photometry in the literature. The upturns are often as red as or redder than the metal-rich nuclear stellar populations, so they are unlikely to be the result of stripping or mergers. Their presence in the nonaccreting galaxies suggests that they are not related to the current epoch cooling flows. The flat or redward-rising color gradients in the halos are consistent with high stellar metallicities that extend to large galactic radii. The most intriguing possibility is that the red upturns are related to the dark matter known to he present in clusters from kinematic data.