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When and How did Galactic Spheroids Form?
| Content Provider | Semantic Scholar |
|---|---|
| Author | Masci, Frank |
| Copyright Year | 2007 |
| Abstract | How elliptical galaxies and the bulges of spirals formed is one of the key questions of modern cosmology. Galactic bulges are centrally concentrated, high surface brightness systems which have undergone more collapse than galactic disks. Their high density indicates either significant gaseous dissipation occured during their formation or, they formed at very high redshift z > 10 (Peebles 1989). Studies of stellar populations in the bulges of field galaxies at intermediate redshifts 0.4 < z < 1 are consistent with them being ‘old’, forming ≈ 10 Gyr ago (Abraham et al. 1999). Recent advances in the understanding of star formation and feedback mechanisms through simulations of hierarchical clustering predict spheroids to form their stars at z ≃ 1-2 (Baugh et al 1998; Carlberg 1999), and that a majority should be obscured by dust at optical/nearInfrared wavelengths (Franceschini et al. 1998; Kauffmann & Charlot 1998). Such epochs are only now starting to be explored with far-IR/sub-mm instrumentation. Our proposed survey in particular will play a key role at probing redshifts z< ∼ 1, or at least the tail of the spheroid formation epoch. A number of theories exist for the formation of galactic spheroids and ellipticals: primordial ‘monolithic’ collapse of individual gas clumps (Eggen et al. 1962; Arimoto & Yoshii 1987) and variations thereof (Peacock et al. 1998; Jiminez et al. 1998), hierarchical merging of pre-formed galaxies (Toomre & Toomre 1972; Kauffmann et al. 1994; Baugh et al. 1996), infall of gas-rich satellites onto pre-existing dark matter disk halos (Cole et al. 1994; Carlberg 1999), and ‘secular’ evolution where bulges form relatively late by gas-inflow from their pre-existing gas-rich outer disk (Norman et al. 1996). Of the above models, ‘monolithic’ and ‘merger’ scenarios are the main competitors. Although no conclusion is yet firmly established, merger models have become increasingly popular since the deep optical/near-IR surveys from HST. Widespread observational evidence for a paucity of evolved (red) spheroids at z> ∼ 1 in optical/nearIR surveys has placed monolithic ‘rapid formation’ at zF ∼ 2 − 5 in somewhat of a dilemma (eg. Zepf 1997; Barger et al. 1999; Menanteau et al. 1999). Some authors however (eg. McCracken et al. 2000) claim that possible uncertainties in morphological identification and field-to-field variations in these studies could still make such models a serious contender. On the other hand, the observed properties of ellipticals and bulges: dynamical disturbances such as shells/ripples, transient dust lanes, multiple and counterrotating cores, globular cluster distributions (Kormendy & Djorgovski 1989; Schweizer & Seitzer 1992) all support a merger (or sattellite accretion) hypothesis for spheroid formation. Furthermore, the formation of structure through hierarchical clustering is a picture predicted by the standard cold dark matter (CDM) cosmology. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://web.ipac.caltech.edu/staff/fmasci/home/miscscience/spheroids.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
