Abstract
AstroSat has surveyed M31 with the UVIT telescope from 2017 to 2019. The central bulge of M31 was observed in 2750 - 2850 A, 2000 - 2400 A, 1600 - 1850 A, 1450 - 1750 A, and 1200 - 1800 A filters. A radial profile analysis, averaged along elliptical contours which approximate the bulge shape, was carried out in each filter. The profiles follow a Sersic function with an excess for the inner ~8" in all filters, or can be fitted with two Sersic functions (including the excess). The ultraviolet colours of the bulge are found to change systematically with radius, with the center of the bulge bluer (hotter). We fit the UVIT spectral energy distributions (SEDs) for the whole bulge and for 10 elliptical annuli with single stellar population (SSP) models. A combination of two SSPs fits the UVIT SEDs much better than one SSP, and three SSPs fit the data best. The properties of the three SSPs are age, metallicity (Z) and mass of each SSP. The best fit model is a dominant old, metal-poor (1010 yr, , with the solar metallicity) population plus a 15% contribution from an intermediate (109.5 yr, ) population plus a small contribution (~2%) from a young high-metallicity (108.5 yr, ) population. The results are consistent with previous studies of M31 in optical: both reveal an active merger history for M31.
Highlights
The spiral galaxy Andromeda, known as M31, is the nearest such galaxy to our own
The N219M filter observed surface brightness looks different than the other filters: it is faintest in the inner bulge and approximately matches the shorter wavelength profiles for the outer bulge3
The semi-major axis, a, is encoded in a standard RGB colour scheme, with bluest for a = 0 gradually changing to green at a 50", green changing to orange at a 100" and orange changing to red at the outermost analyzed ellipse, with a = 167"
Summary
The spiral galaxy Andromeda, known as M31, is the nearest such galaxy to our own. Studies of large numbers of stars have been done in great detail for the Milky Way, but distances often possess high uncertainty, due in part to extinction. An advantage of studying objects in M31 is that it is at a well-known distance (785 ± 25 kpc) [1]. The highest resolution observations were carried out with the Hubble Space Telescope (HST), including the Pan-chromatic Hubble Andromeda Treasury (PHAT) survey [2]. In near and far ultraviolet (NUV and FUV), the GALEX instrument has surveyed M31 [3]
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