Browsing by Subject "galactic bulges"

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Bulge n And B/T In High-Mass Galaxies: Constraints On The Origin Of Bulges In Hierarchical Models
(2009-05) Weinzirl, Tim; Jogee, Shardha; Khochfar, S.; Burkert, Andreas; Kormendy, John; Weinzirl, Tim; Jogee, Shardha; Kormendy, John
We use the bulge Sersic index n and bulge-to-total mass ratio (B/T) to explore the fundamental question of how bulges form. We perform two-dimensional bulge-disk-bar decomposition on H-band images of 143 bright, high-mass (M(*) >= 1.0 x 10(10) M(circle dot)) low-to-moderately inclined (i < 70 degrees) spirals. Our results are as follows. (1) Our H-band bar fraction (similar to 58%) is consistent with that from ellipse fits. (2) 70% of the stellar mass is in disks, 10% in bars, and 20% in bulges. (3) A large fraction (similar to 69%) of bright spirals have B/T <= 0.2, and similar to 76% have low n <= 2 bulges. These bulges exist in barred and unbarred galaxies across a wide range of Hubble types. (4) About 65% (68%) of bright spirals with n <= 2 (B/T <= 0.2) bulges host bars, suggesting a possible link between bars and bulges. (5) We compare the results with predictions from a set of ACDM models. In the models, a high-mass spiral can have a bulge with a present-day low B/T <= 0.2 only if it did not undergo a major merger since z <= 2. The predicted fraction (similar to 1.6%) of high-mass spirals, which have undergone a major merger since z <= 4 and host a bulge with a present-day low B/T <= 0.2, is a factor of over 30 smaller than the observed fraction (similar to 66%) of high-mass spirals with B/T <= 0.2. Thus, contrary to common perception, bulges built via major mergers since z <= 4 seriously fail to account for the bulges present in similar to 66% of high mass spirals. Most of these present-day low B/T <= 0.2 bulges are likely to have been built by a combination of minor mergers and/or secular processes since z <= 4.
Bulges Of Nearby Galaxies With Spitzer: Scaling Relations In Pseudobulges And Classical Bulges
(2010-06) Fisher, David B.; Drory, Niv; Fisher, David B.
We investigate scaling relations of bulges using bulge-disk decompositions at 3.6 mu m and present bulge classifications for 173 E-Sd galaxies within 20 Mpc. Pseudobulges and classical bulges are identified using Sersic index, Hubble Space Telescope morphology, and star formation activity ( traced by 8 mu m emission). In the near-IR pseudobulges have n(b) < 2 and classical bulges have n(b) > 2, as found in the optical. Sersic index and morphology are essentially equivalent properties for bulge classification purposes. We confirm, using a much more robust sample, that the Sersic index of pseudobulges is uncorrelated with other bulge structural properties, unlike for classical bulges and elliptical galaxies. Also, the half-light radius of pseudobulges is not correlated with any other bulge property. We also find a new correlation between surface brightness and pseudobulge luminosity; pseudobulges become more luminous as they become more dense. Classical bulges follow the well-known scaling relations between surface brightness, luminosity, and half-light radius that are established by elliptical galaxies. We show that those pseudobulges (as indicated by Sersic index and nuclear morphology) that have low specific star formation rates are very similar to models of galaxies in which both a pseudobulge and classical bulge exist. Therefore, pseudobulge identification that relies only on structural indicators is incomplete. Our results, especially those on scaling relations, imply that pseudobulges are very different types of objects than elliptical galaxies.
The Structure Of Classical Bulges And Pseudobulges: The Link Between Pseudobulges And Sersic Index
(2008-08) Fisher, David B.; Drory, Niv; Fisher, David B.
In this paper, we study the properties of pseudobulges (bulges that appear similar to disk galaxies) and classical bulges (bulges which appear similar to E-type galaxies) in bulge-disk decompositions. We show that the distribution of bulge Sersic indices, (n)b, is bimodal, and this bimodality correlates with the morphology of the bulge. Pseudobulges have n(b) less than or similar to 2 and classical bulges have n(b) less than or similar to 2 with little to no overlap. Also, pseudobulges do not follow the correlations of Sersic index with structural parameters or the photometric projections of the fundamental plane in the same way that classical bulges and elliptical galaxies do. We find that pseudobulges are systematically flatter than classical bulges and thus more disk-like in both their morphology and shape. We do not find significant differences between different bulge morphologies which we are collectively calling pseudobulges (nuclear spirals, nuclear rings, nuclear bars, and nuclear patchiness); they appear to behave similarly in all parameter correlations. In the Sersic index, flattening, and bulge-to-total ratio, the distinction appears to be between classical bulges and pseudobulges, not between different pseudobulge morphologies. The Sersic index of the pseudobulges does not correlate with B/T, in contrast to classical bulges. Also, the half-light radius of the pseudobulge correlates with the scale length of the disk; this is not the case for classical bulges. The correlation of Sersic index and scale lengths with bulge morphology suggests that secular evolution is creating pseudobulges with low-Sersic index and that other processes (e. g., major mergers) are responsible for the higher Sersic index in classical bulges and elliptical galaxies.