Browsing by Subject "classical bulges"
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Item Bulges Of Nearby Galaxies With Spitzer: The Growth Of Pseudobulges In Disk Galaxies And Its Connection To Outer Disks(2009-05) Fisher, David B.; Drory, Niv; Fabricius, Maximilian H.; Fisher, David B.We study star formation rates (SFRs) and stellar masses in bulges of nearby disk galaxies. For this we construct a new SFR indicator that linearly combines data from the Spitzer Space Telescope and the Galaxy Evolution Explorer. All bulges are found to be forming stars irrespective of bulge type (pseudobulge or classical bulge). At present-day SFR the median pseudobulge could have grown the present-day stellar mass in 8 Gyr. Classical bulges have the lowest specific SFR implying a growth times that are longer than a Hubble time, and thus the present-day SFR does not likely play a major role in the evolution of classical bulges. In almost all galaxies in our sample the specific SFR (SFR per unit stellar mass) of the bulge is higher than that of the outer disk. This suggests that almost all galaxies are increasing their B/T through internal star formation. The SFR in pseudobulges correlates with their structure. More massive pseudobulges have higher SFR density, this is consistent with that stellar mass being formed by moderate, extended star formation. Bulges in late-type galaxies have similar SFRs as pseudobulges in intermediate-type galaxies, and are similar in radial size. However, they are deficient in mass; thus, they have much shorter growth times, similar to 2 Gyr. We identify a class of bulges that have nuclear morphology similar to pseudobulges, significantly lower specific SFR than pseudobulges, and are closer to classical bulges in structural parameter correlations. These are possibly composite objects, evolved pseudobulges or classical bulges experiencing transient, enhanced nuclear star formation. Our results are consistent with a scenario in which bulge growth via internal star formation is a natural, and near ubiquitous phenomenon in disk galaxies. Those galaxies with large classical bulges are not affected by the in situ bulge growth, likely because the majority of their stellar mass comes from some other phenomenon. Yet, those galaxies without a classical bulge, over long periods of extended star formation are able to growth a pseudobulge. Though cold accretion is not ruled out, for pseudobulge galaxies an addition of stellar mass from mergers or accretion is not required to explain the bulge mass. In this sense, galaxies with pseudobulges may very well be bulgeless (or "quasi-bulgeless") galaxies, and galaxies with classical bulges are galaxies in which both internal evolution and hierarchical merging are responsible for the bulge mass by fractions that vary from galaxy to galaxy.Item Demographics of Bulge Types Within 11 Mpc and Implications for Galaxy Evolution(2011-06) Fisher, David B.; Drory, Niv; Fisher, David B.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 sphere using Spitzer 3.6 mu m and Hubble Space Telescope 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 10(9) M-circle dot. Classical bulges and elliptical galaxies account for similar to 1/4, and disks for similar to 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 stellarmass, we find that the frequency of classical bulges strongly increases with stellar mass, and comes to dominate above 10(10.5) M-circle dot. Galaxies with pseudobulges dominate at 10(9.5)-10(10.5) M-circle dot. 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.