The Big Bulge

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Science  03 Jun 2011:
Vol. 332, Issue 6034, pp. 1128
DOI: 10.1126/science.332.6034.1128-a

A classical spiral galaxy has three stellar components: a central, roughly spherical bulge; a thin disk around this bulge; and a halo. Typically, the bulge resembles an elliptical galaxy. However, not all bulges follow this classical picture; some, which astronomers call pseudobulges, are disk-like. Fisher and Drory made an inventory of galaxy bulge types within 11 million parsecs from Earth (1 parsec corresponds to ∼3.26 light-years). They counted the number of elliptical galaxies (pure bulges) and spiral galaxies with classical bulges, with no bulges, or with pseudobulges by star formation rate and stellar mass. Galaxies without a bulge (pure disk galaxies) or with a pseudobulge are the most common type in the local region analyzed. In this region, two-thirds of new stars are formed in galaxies with pseudobulges and three-quarters of the stellar mass is in disks. It is not clear how pseudobulges form—whether by merging of galaxies or by slow, steady evolution of the disk. The frequency of enhanced star formation in the central regions of galaxies with bulges indicates that long-term, non-episodic processes are at play in bulge growth. If pseudobulges are not the result of mergers, then it is difficult to reconcile the numbers found in this study with those predicted by models of galaxy evolution.

Astrophys. J. 733, L47 (2011).

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