The puzzling bulge to disk nova ratio in the Andromeda galaxy (M31).

摘要

Novae in M31 are often associated with the bulge component of the light from this galaxy, i.e., more novae are assumed to be produced in the bulge of M31. But examining this from a population synthesis approach, one expects that evolved binaries in the disk should produce more novae. We strive to understand this bulge to disk nova ratio puzzle in M31 by exploring two scenarios. The nova rate normalized to the K-band luminosity for different galaxy Hubble types is approximately constant. We utilize this observed correlation to model the bulge and disk nova distribution. However, the decomposition of K light into bulge and disk does not yield a good match to the observed spatial distribution of novae in M31. Therefore, we conclude that the assumption that the nova rate follows total K light is too simple to explain the actual distribution of novae in this galaxy.;Second, we examine the role of dust in the disk of M31 in extinction novae, possibly more so in the disk, which would increase the relative number of observed bulge novae compared to those in the disk. We construct a three dimensional multi-component dust model (uniform background, 10 kpc ring, 2 logarithmic spirals) and apply it to novae in the bulge and the disk of M31. With model parameters calibrated from infrared emission models, this results in hiding only ~ 1 % of the novae in the disk and 0.3--0.4 % in the bulge. We, therefore conclude that dust in M31 does not play a significant role in shrouding novae in the disk. In fact, the effect of the dust is not much higher for disk novae in comparison to bulge novae. Therefore, we conclude that the common assumption that "novae trace the K-band light" is not supported by the detailed spatial models of novae in M31, and that extinction by dust is insufficient to resolve the puzzle of the relative scarcity of disk novae in M31.