The Disks of Galaxies with Seyfert and Starburst Nuclei. I. Near‐Infrared Colors and Color Gradients

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Abstract

We present near-infrared (NIR) broadband and color images of 26 galaxies that host Seyfert 1, Seyfert 2, or starburst nuclei (SBNs). The study is focused on properties of the host galaxies rather than their nuclei, and to this end, careful attention is paid to photometric accuracy and to reliable measurements of the low surface brightness outer disk. Inspection of the elliptically averaged radial brightness and color profiles reveals that (1) the NIR mean colors of the inner and outer disks of Seyfert and starburst galaxies are consistent with those of a normal late-type stellar population and do not differ significantly with activity class; (2) the color gradients in the outer disks are similar both in sign and in magnitude to those observed in normal spirals; (3) red "ridges" in the inner parts of the J - H profile are evident in the majority of SBNs, but only in a few type 1 Seyferts and in no type 2's; (4) circumnuclear blue "dips" in the J - H profile are seen only in type 2 Seyferts. We then construct color images and find ridges, rings, and filaments, not evident in the broadband images, in the inner disks of SBNs and in NGC 7469, a Seyfert 1. The application of a simple model to these features yields evidence of both dust extinction and excess 2 μm emission. Color-color diagrams of individual pixels confirm these results and also show that the stellar mix in most of the Seyfert 2's comprises a conspicuous contribution from an intermediate-age [(3-5) × 108 yr] population. It appears that ongoing star formation in the inner disks of SBNs is signaled by the presence of dust (and gas); the absence of such features in both Seyfert types implies that star formation episodes are either absent or very old. However, while the blue colors of Seyfert 2's suggest that a burst of star formation did, in fact, occur not more than 109 yr ago, the normal colors of Seyfert 1's imply that any star-forming episodes must be significantly older.

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