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.
References
[1]
McConnachie, A.W., Irwin, M.J., Ferguson, A.M.N., et al. (2005) Distances and Metallicities for 17 Local Group Galaxies. Monthly Notices of the Royal Astronomical Society, 356, 979-997. https://doi.org/10.1111/j.1365-2966.2004.08514.x
[2]
Williams, B.F., Lang, D., Dalcanton, J.J., et al. (2014) The Panchromatic Hubble Andromeda Treasury. X. Ultraviolet to Infrared Photometry of 117 Million Equidistant Stars. The Astrophysical Journal Supplement Series, 215, 9-43. https://doi.org/10.1088/0067-0049/215/1/9
[3]
Martin, D.C., Fanson, J., Schiminovich, D., et al. (2005) The Galaxy Evolution Explorer: A Space Ultraviolet Survey Mission. The Astrophysical Journal Letters, 619, L1-L6. https://doi.org/10.1086/426387
[4]
Singh, K.P., Tandon, S.N., Agrawal, P.C., et al. (2014) ASTROSAT Mission. Proceedings of the SPIE, 9144E, 1S.
[5]
Leahy, D.A., Bianchi, L. and Postma, J.E. (2018) ASTROSAT/UVIT Survey of M31, First Results: UV-Bright Stars in the Bulge. The Astronomical Journal, 156, 269-281. https://doi.org/10.3847/1538-3881/aae9e8
[6]
Leahy, D.A., Postma, J., Buick, M., et al. (2020) New Results from the UVIT Survey of the Andromeda Galaxy. https://arxiv.org/abs/2012.02727
[7]
Leahy, D.A. and Chen, Y. (2020) AstroSat UVIT Detections of Chandra X-Ray Sources in M31. The Astrophysical Journal Supplement Series, 250, 23-37. https://doi.org/10.3847/1538-4365/abadfb
[8]
Leahy, D.A., Postma, J., Chen, Y. and Buick, M. (2020) AstroSat UVIT Survey of M31: Point-Source Catalog. The Astrophysical Journal Supplement Series, 247, 47-63. https://doi.org/10.3847/1538-4365/ab77a9
[9]
Leahy, D., Buick, M., Postma, J. and Morgan, C. (2021) Far-Ultraviolet Variable Sources in M31. The Astronomical Journal, 161, 215-229. https://doi.org/10.3847/1538-3881/abe9b3
[10]
Tandon, S.N., Subramaniam, A., Girish, V., et al. (2017) In-Orbit Calibrations of the Ultraviolet Imaging Telescope. The Astronomical Journal, 154, 128-142. https://doi.org/10.3847/1538-3881/aa8451
[11]
Tandon, S.N., Postma, J., Joseph, P., et al. (2020) Additional Calibration of the Ultraviolet Imaging Telescope on Board AstroSat. The Astronomical Journal, 159, 158-169. https://doi.org/10.3847/1538-3881/ab72a3
[12]
Postma, J.E. and Leahy, D. (2017) CCDLAB: A Graphical User Interface FITS Image Data Reducer, Viewer, and Canadian UVIT Data Pipeline. Publications of the Astronomical Society of the Pacific, 129, 115002. https://doi.org/10.1088/1538-3873/aa8800
[13]
Postma, J.E. and Leahy, D. (2020) An Algorithm for Coordinate Matching in World Coordinate Solutions. Publications of the Astronomical Society of the Pacific, 132, 054503. https://doi.org/10.1088/1538-3873/ab7ee8
[14]
Lockhart, K.E. (2017) A High Resolution View of Galactic Centers: Arp 220 and M31. Ph.D. Thesis, University of Hawai’i, Manoa.
[15]
Sersic, J.L. (1968) Atlas de Galaxias Australes. Observatorio Astronomico, Cordoba.
[16]
Fitzpatrick, E.L. and Massa, D. (2007) An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology. The Astrophysical Journal, 663, 320-341. https://doi.org/10.1086/518158
[17]
Castelli, F. and Kurucz, R.L. (2003) New Grids of ATLAS9 Model Atmospheres. Modelling of Stellar Atmospheres. Proceedings of the 210th Symposium of the IAU, Uppsala, 17-21 June 2002, A20.
[18]
Bressan, A., Marigo, P., Girardi, L., et al. (2012) PARSEC: Stellar Tracks and Isochrones with the PAdova and TRieste Stellar Evolution Code. Monthly Notices of the Royal Astronomical Society, 427, 127-145. https://doi.org/10.1111/j.1365-2966.2012.21948.x
[19]
Pastorelli, G., Marigo, P., Girardi, L., et al. (2020) Constraining the Thermally Pulsing Asymptotic Giant Branch Phase with Resolved Stellar Populations in the Large Magellanic Cloud. Monthly Notices of the Royal Astronomical Society, 498, 3283-3301. https://doi.org/10.1093/mnras/staa2565
[20]
Groenewegen, M.A.T. (2006) The Mid- and Far-Infrared Colours of AGB and Post-AGB Stars. Astronomy & Astrophysics, 448, 181-187. https://doi.org/10.1051/0004-6361:20054163
[21]
Trabucchi, M., Wood, P.R., Montalbán, J., et al. (2019) Modelling Long-Period Variables—I. A New Grid of O-Rich and C-Rich Pulsation Models. Monthly Notices of the Royal Astronomical Society, 482, 929-949. https://doi.org/10.1093/mnras/sty2745
[22]
Kroupa, P. (2001) On the Variation of the Initial Mass Function. Monthly Notices of the Royal Astronomical Society, 322, 231-246. https://doi.org/10.1046/j.1365-8711.2001.04022.x
[23]
Hammer, F., Yang, Y.B., Wang, J.L., et al. (2018) A 2-3 Billion Year Old Major Merger Paradigm for the Andromeda Galaxy and its Outskirts. Monthly Notices of the Royal Astronomical Society, 475, 2754-2767. https://doi.org/10.1093/mnras/stx3343
[24]
McConnachie, A.W., Ibata, R., Martin, N., et al. (2018) The Large-Scale Structure of the Halo of the Andromeda Galaxy. II. Hierarchical Structure in the Pan-Andromeda Archaeological Survey. The Astrophysical Journal, 868, 55-91. https://doi.org/10.3847/1538-4357/aae8e7
[25]
Dong, H., Olsen, K., Lauer, T., et al. (2018) The Star Formation History in the M31 Bulge. Monthly Notices of the Royal Astronomical Society, 478, 5379-5403. https://doi.org/10.1093/mnras/sty1381
[26]
Saglia, R.P., Opitsch, M., Fabricius, M.H., et al. (2018) Stellar Populations of the Central Region of M 31. Astronomy & Astrophysics, 618, A156. https://doi.org/10.1051/0004-6361/201732517
[27]
Stephens, A.W., Frogel, J.A., DePoy, D.L., et al. (2003) The Stellar Content of the Bulge of M31. The Astronomical Journal, 125, 2473-2493. https://doi.org/10.1086/374570
[28]
Courteau, S., Widrow, L.M., McDonald, M., et al. (2011) The Luminosity Profile and Structural Parameters of the Andromeda Galaxy. The Astrophysical Journal, 739, 20-36. https://doi.org/10.1088/0004-637X/739/1/20
[29]
Peng, C.Y., Ho, L.C., Impey, C.D. and Rix, H.-W. (2010) Detailed Decomposition of Galaxy Images. II. Beyond Axisymmetric Models. The Astronomical Journal, 139, 2097-2129. https://doi.org/10.1088/0004-6256/139/6/2097
![\"\"](\"Edit_e39892d7-b9e0-45ce-a6a6-195582fec54a.png\")
, with ![\"\"](\"Edit_f4e60eca-0723-4b08-b4cc-9f711c149e9e.png\")
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, ![\"\"](\"Edit_471d9a72-5049-4771-be8d-964327283de5.png\")
) population. The results are consistent with previous studies of M31 in optical: both reveal an active merger history for M31.
