Spheroids are an integral component of practically all disk galaxies. Studies of spheroid formation are complicated by the fact that there is no global theory of star formation even at the semi-empirical level that works moderately well for cold gas disks.
Small spheroids most likely form by secular evolution of bar-unstable disks. However it is not possible to form massive spheroids by secular evolution. Galaxy mergers are undoubtedly the major trigger for formation both of massive spheroids in early-type disk galaxies and of elliptical galaxies. It has been realized from the earliest simulations of mergers that gas-rich precursors are required in order to attain the high central densities of massive spheroids. The gas dissipates, forms stars that in turn self-enrich more gas, ending up with the metal-rich nuclei characteristic of massive ellipticals. This must have happened with high efficiency, since spheroidal stellar populations are characteristically old. The necessarily high star formation rate constitutes a star burst, defined simply by the requirement that the mean star formation rate is inferred to have been much higher, perhaps by two orders of magnitude, than the mass of stars divided by a Hubble time.
Nearby starbursts are well studied. Typically, starbursts are rare in the
nearby universe but increasingly common at high redshift. Star formation
rates of up to 1000
M
yr-1 are inferred. The most extreme star
formation rates are invariably associated with evidence for an ongoing
merger. Most of the star formation generated by gas concentration triggered
in a merger is shrouded by dust, and most of the radiation is absorbed and
reemitted in the far infrared. Simulations demonstrate that the mergers of
gas-rich galaxies efficiently drive the gas into the central kiloparsec of
the merged galaxy by a powerful combination of tidal torquing on the gas due
to the merged transient stellar bar that results from the merger, which
removes angular momentum and compresses the gas, followed by strong
dissipation of energy by radiative cooling of the gas.
Ultraluminous infrared starbursts are likely to be the sites of ongoing spheroid formation. This conjecture is supported by near-infrared observations of post-starburst galaxies, where the characteristic de Vaucouleurs profiles found in spheroids can be recognized in the newly formed stars.
There is an intriguing complication. Spheroids have been found to be
intimately linked with supermassive black holes. The tight correlation
observed
between black hole mass and spheroid velocity dispersion covers the range of
black hole masses 106 to 109
M. and
encompasses stellar
spheroids as small as that of the Milky Way (~ 109
M) to those
as massive as M87 (~ 1012
M). The clear
implication is that
the formation of spheroids and supermassive black holes was
contemporaneous. If the spheroid formed by a merger, the strong central gas
enhancement provides an ideal environment for supermassive black hole
formation. The enormous amount of binding energy released
as the SMBH formed would impact the
protogalactic gaseous environment, as would the
ensuing effects of energy released by
infall into the newly formed black hole. Outflows would be a natural
outcome, and these could help stimulate further star formation.
Observationally, it is unclear what role quasar-like activity plays in the energetics of the far-infrared and submillimeter galaxies that are undergoing luminous starbursts. In the nearby case of Arp 220, it is apparent that star formation and buried quasars play comparable roles in accounting for the observed luminosity.