The Complex Structure of the Bulge of M31

TL;DR

This study uses UVIT observations to analyze the complex structure of M31's bulge, revealing a mix of classical and pseudobulge features through multi-component modeling and UV imaging.

Contribution

First detailed UV imaging and multi-component modeling of M31's bulge, showing evidence of both classical and pseudobulge characteristics.

Findings

Bulge has Sersic indices around 2.1-2.5, intermediate between classical and pseudobulges.

Detection of dust lanes and nuclear spiral in UV absorption images.

Bulge exhibits boxy contours consistent across UV bands.

Abstract

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.