Bulges or disks in the centers of late-type spirals?

Details Bulges or disks in the centers of late-type spirals? Bulges or disks in the centers of late-type spirals?

Jul 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10157c&link_type=abstract

HST Proposal ID #10157

Physics

1

Hst Proposal Id #10157 Galaxies

Scientific paper

In a simple view of the Hubble sequence, smaller bulges should be rescaled versions of bigger bulges. Bulges however have been found to change their structural properties with decreasing luminosity, approaching at the faintest end a disk-like, exponential light profile. This indicates a complex mass-dependent bulge formation history. Particularly, the intermediate-to-small size bulges have been suggested to form due to secular evolution processes within their host disks. However, the alleged small bulges may even be 'simply' denser inner regions of the disks. Two major ingredients are missing in order to {a} understand the nature of bulges in the disk-dominated galaxies, {b} establish whether and which secular evolution processes actually occur, and {c} in which mass range they are preferentially active: {1} High-resolution numerical simulations of disk secular evolution, to provide a quantitative basis for interpreting real data; {2} Observational diagnostics which can break the degeneracy between very cold, dense disks and relatively hot bulges, and to compare with the simulations. We are carrying out a large N-body + SPH simulations campaign to settle the first issue. Stellar kinematics are the ideal observational diagnostics. We have acquired ground kinematic data for the medium-sized bulges. However, both the spectroscopic and the spatial resolution requirements necessary to trace the relative contributions of cold and hot motions become very stringent at the faint-end of the bulge sequence: only the HST can provide radially-extended kinematics for the smallest bulges. We therefore ask for STIS/G750M spectroscopy to measure internal resolved stellar kinematics for two small bulges selected from our previous HST imaging program. Even just these two "data-points" in this unexplored mass-regime will allow significant progress in the understanding of the origin of the Hubble sequence: Complemented by our ground-study of the medium-sized bulges, they will allow us to establish whether the ratio of cold-to-hot motions in bulges in the intermediate-to-small mass regime depends on the bulge mass, and, by comparing with our simulations, to constrain the initial conditions and physical parameters that allow disk secular evolution processes to grow central bulges similar to those that are observed along the entire Hubble sequence.

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