Pseudobulges in the Disk Galaxies NGC 7690 and NGC 4593

Kormendy, John; Cornell, Mark E.; Block, David L.; Knapen, Johan H.; Allard, Emma L.
Bibliographical reference

The Astrophysical Journal, Volume 642, Issue 2, pp. 765-774.

Advertised on:
5
2006
Number of authors
5
IAC number of authors
0
Citations
30
Refereed citations
25
Description
We present Ks-band surface photometry of NGC 7690 (Hubble type Sab) and NGC 4593 (SBb). We find that, in both galaxies, a major part of the ``bulge'' is as flat as the disk and has approximately the same color as the inner disk. In other words, the ``bulges'' of these galaxies have disklike properties. We conclude that these are examples of ``pseudobulges,'' that is, products of secular dynamical evolution. Nonaxisymmetries such as bars and oval disks transport disk gas toward the center. There star formation builds dense stellar components that look like-and often are mistaken for-merger-built bulges, but that were constructed slowly out of disk material. These pseudobulges can most easily be recognized when, as in the present galaxies, they retain disklike properties. NGC 7690 and NGC 4593 therefore contribute to the growing evidence that secular processes help to shape galaxies. NGC 4593 contains a nuclear ring of dust that is morphologically similar to nuclear rings of star formation that are seen in many barred and oval galaxies. The nuclear dust ring is connected to nearly radial dust lanes in the galaxy's bar. Such dust lanes are a signature of gas inflow. We suggest that gas is currently accumulating in the dust ring and hypothesize that the gas ring will starburst in the future. The observations of NGC 4593 therefore suggest that major starburst events that contribute to pseudobulge growth can be episodic. Based on observations made with the Anglo-Australian Telescope. Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. The observations of NGC 7690 are associated with program IDs 7331 (NICMOS: M. Stiavelli) and 6359 (WFPC2: M. Stiavelli). The observations of NGC 4593 are associated with program IDs 7330 (NICMOS: J. Mulchaey), and 5479 (WFPC2: M. Malkan).