Optical Color Gradients in Star‐forming Ring Galaxies

Abstract

We compute radial color gradients produced by an outwardly propagating circular wave of star formation and compare our results with color gradients observed in the classical ring galaxy, the "Cartwheel". We invoke two independent models of star formation in the ring galaxies. The first one is the conventional density wave scenario, in which an intruder galaxy creates a radially propagating density wave accompanied by an enhanced star formation following the Schmidt's law. The second scenario is a pure self-propagating star formation model, in which the intruder only sets off the first burst of stars at the point of impact. Both models give essentially the same results. Systematic reddening of B − V , V − K colors towards the center, such as that observed in the Cartwheel, can be obtained only if the abundance of heavy elements in the starforming gas is a few times below solar. The B − V and V − K color gradients observed in the Cartwheel can be explained as a result of mixing of stellar populations born in a star-forming wave propagating through a low-metallicity gaseous disk, and a pre-existing stellar disk of the size of the gaseous disk with color properties typical to those observed in nearby disk galaxies.

Radial $B-V/V-K$ color gradients, extinction-freeQBVKcombined color indices, and the history of star formation of the Cartwheel ring galaxy

“…The internal extinction is only measured towards two bright HII regions in the Cartwheel's outer ring (A V = 1. m 94) and is not applicable to the whole galactic disk. The value of internal extinction interior to the outer ring of A V = 0. m 3 has been estimated from the available HI + H 2 data (Korchagin et al 2001). Clearly, correction for the internal extinction would greatly distort the observed Cartwheel's B−V/V −K radial color pattern, as shown by correction vectors in Fig.…”

“…For a homogeneous mixture of dust and stars the ratio of color excesses E(B − V )/E(V − K) is 10% less than for a "dust shield" approximation (Zasov & Moiseev 1998) and equal to 0.329. As to the Cartwheel galaxy, the internal extinction dominates at the outer ring and is comparable to the Galactic reddening in the inner part (Korchagin et al 2001). Hence, we choose a "homogeneous mixture" approximation rather than a "dust shield" for modelling the Cartwheel's color properties.…”

“…The balance of surface density of stars (Σ s ) at a distance r from the center and time t can be then written as follows (Korchagin et al 2001):…”

“…We mainly confirm their conclusion. However, use of the extinction-free Q BV K indices allows us to notice some interesting features that have been missed by Korchagin et al (2001).…”

The ring galaxy HRG 54 103: a first study

“…Multiple-band observations were performed by Appleton & Struck-Marcell (1997, and references therein). The prototype RG, the Cartwheel, has been tested observationally to study the ring expansion, star formation in the ring, radial color gradient and stellar ages (see Fosbury & Hawarden 1977;Higdon 1995;Marcum et al 1992;Korchagin et al 2001;Vorobyov & Bizyaev 2001). Color gradients were found in some other RGs (Appleton & Marston 1997), and Marston & Appleton (1995) found that the majority of the star-forming regions are located in the ring.…”