This week’s paper: Demographics of Bulge Types within 11 Mpc and Implications for Galaxy Evolution

The Astronomy Journal Club is going to be on Sunday this week at the East-of-GMT friendly time of 10 a.m. UT (11 a.m. UK, more time zones here). We’re going to be discussing Demographics of Bulge Types within 11 Mpc and Implications for Galaxy Evolution (Fisher & Drory, 2011).

The paper looks at local galaxies (within 11 Mpc) and sorts them into ellipticals, classical bulges, pseudobulges and bulgeless. A ‘bulge’ is a spherical concentration of stars seen in a galaxy’s centre; an elliptical galaxy could be thought of as all bulge. A galaxy with a ‘pseudobulge’ has the same stellar enhancement in the centre, but distributed in the plane of the disc. Galaxy formation models assume that galaxy bulges grow through mergers, but these don’t seem to explain how pseudobulges form; some people think that they occur through internal evolution of a galaxy’s disc instead. The results presented in the paper suggest that bright galaxies within the volume they probe are most likely to have pseudobulges, which disagrees with the model predictions.

This is just a brief summary of the paper. For more details see this post over at astrobites, have a look at the paper abstract below, or even read the paper yourself (it’s only 4 pages)!


We present an inventory of galaxy bulge types (elliptical galaxy, classical bulge, pseudobulge, and bulgeless galaxy) in a volume-limited sample within the local 11 Mpc volume using Spitzer 3.6 micron and HST data. We find that whether counting by number, star formation rate, or stellar mass, the dominant galaxy type in the local universe has pure disk characteristics (either hosting a pseudobulge or being bulgeless). Galaxies that contain either a pseudobulge or no bulge combine to account for over 80% of the number of galaxies above a stellar mass of 109 MSun. Classical bulges and elliptical galaxies account for ~1/4, and disks for ~3/4 of the stellar mass in the local 11 Mpc. About 2/3 of all star formation in the local volume takes place in galaxies with pseudobulges. Looking at the fraction of galaxies with different bulge types as a function of stellar mass, we find that the frequency of classical bulges strongly increases with stellar mass, and comes to dominate above 1010.5 MSun. Galaxies with pseudobulges dominate at 109.5 – 1010.5 MSun. Yet lower-mass galaxies are most likely to be bulgeless. If pseudobulges are not a product of mergers, then the frequency of pseudobulges in the local universe poses a challenge for galaxy evolution models.

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