The Complex Structure of the Bulge of M31 by Denis Leahy et al. on Wednesday 30 November

The bulge of M31 is of interest in the context of the nature of galactic

bulges and how their structure relates to bulge formation mechanisms and their

subsequent evolution. With the UVIT instrument on AstroSat, we have observed

the bulge of M31 in five far ultraviolet (FUV) and near ultraviolet (NUV)

filters at 1" spatial resolution. Models for the luminosity distribution of the

bulge are constructed using the UVIT data and the galaxy image-fitting

algorithm GALFIT. We fit the bulge without nuclear region with a Sersic

function the five images and find Sersic indices ($\simeq2.1$ to 2.5) similar

to previous studies but smaller $R_e$ values ($\simeq0.5$ to 0.6 kpc). When the

images include the nuclear region, a multicomponent model is used to find the

best-fit. We use an 8-component fit for the FUV 148nm image, which has the

highest sensitivity. The other images (169 to 279 nm) are fit with 4-component

models. The dust lanes in the bulge region are recovered in the residual image

after subtraction of the bright bulge light using the multicomponent model. The

dust lanes show that M31's nuclear spiral is visible in absorption at NUV and

FUV wavelengths. The bulge images show boxy contours in all five UVIT

wavebands, which is confirmed by fitting using GALFIT. The Sersic indices of

$\sim$2.1-2.5 are intermediate between expected values for a classical bulge

and for a pseudobulge. The boxiness of the bulge provides further evidence that

M31's bulge has contributions from a classical bulge and a pseudobulge.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.16311v1