Pseudobulges in the Disk Galaxies NGC 7690 and NGC 4593

Abstract

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 disk-like 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 disk-like 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.Comment: 10 pages, 3 Postscript figures; requires emulateapj.cls, apjfonts.sty, and psfig.sty; accepted for publication in ApJ; for a version with full resolution figures, see http://chandra.as.utexas.edu/~kormendy/n7690.pd

A minor merger origin for stellar inner discs and rings in spiral galaxies

“…They have instead formed by the coupling of the satellite disruption to the resonances induced in the primary disc by the encounter. Cases of IRs and sets of concentric rings not associated with bars are also found in real galaxies (Buta & Combes 1996;Muñoz-Tuñón et al 2004;Kormendy et al 2006). In addition, our models point to the resonance-related origin of IRs, independently of whether these resonances are linked to either strong bars or weak non-axisymmetric perturbations as in the ovals found in some of our models (see Sect.…”

Evolution of structure in late-type spiral galaxies

“…Our photometric study of the archival data for NGC 628 suggests the presence of a central structure, which together with our kinematic analysis confirm that this central structure is disc-like and could be built up by inflow from large galactocentric distances. Similar cases, such as NGC 4593 and NGC 7690 (Kormendy et al 2006), suggest that NGC 628 could be in the process of forming a pseudo-bulge.…”

Galaxy And Mass Assembly (GAMA): understanding the wavelength dependence of galaxy structure with bulge-disc decompositions

“…They generally have younger stellar populations than classical bulges, and are likely formed by secular processes. Due to their flattened light profiles they are often difficult to detect at high inclination (Carollo et al 1997;Kormendy et al 2006;Drory & Fisher 2007;Gadotti 2009). Conversely, early-type spiral galaxies appear to have significantly bigger and brighter bulges than late-type spiral galaxies, which tend to have small, faint bulges (Graham & Prieto 2001;Möllenhoff 2004).…”