Characteristic Scale and Bimodality in Galaxies: Cold Streams, Shock Heating, Feedback and Clustering

Details Characteristic Scale and Bimodality in Galaxies: Cold Streams, Shock Heating, Feedback and Clustering Characteristic Scale and Bimodality in Galaxies: Cold Streams, Shock Heating, Feedback and Clustering

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aipc..743..162d&link_type=abstract

THE NEW COSMOLOGY: Conference on Strings and Cosmology; The Mitchell Symposium on Observational Cosmology. AIP Conference Proce

Computer Science

3

Origin, Formation, Evolution, Age, And Star Formation, Stellar Content And Populations, Radii, Morphology And Overall Structure, Magnitudes And Colors, Luminosities

Scientific paper

We address the origin of the robustly observed bi-modality in galaxy properties at a characteristic stellar mass of ~ 3 × 10 10Msolar. As seen in large surveys at low redshift and indicated at z ~ 1, less massive galaxies tend to be star-forming blue (some very blue) discs in the ``field'', correlated along a ``fundamental line'' of L/M, surface brightness, internal velocity and metallicity rising with mass. More massive galaxies are mostly spheroids of red (some very red) old stars in groups or clusters, with surface brightness and metallicity ~constant and halo M/L rising with mass. The spheroids tend to host AGNs.
We propose that the bi-modality is the combined effect of the thermal history of the infalling gas and several feedback processes, aided by the gravitational growth of fluctuations into groups of galaxies. In haloes below a critical mass ~ 10 12Msolar, single discs are built by cold streams, not heated by a shock in the extended dark halo, yielding efficient early star formation. It is regulated by supernova and radiative feedback into a long sequence of bursts, giving rise to the blue galaxies along the fundamental line. Further grwoth along the blue sequence into L*-size galaxies is allowed above the threshold mass by mergers and by cold streams, especially in low density environments. This phase of star formation is possibly observed as LIRGs at z <~ 1 and as luminous dusty objects at z ~ 2. Only above the critical mass is the infalling gas shock-heated to near the virial temperature. This hot, dilute gas is vulnerable to feedback from an energetic source such as an AGN, which shuts off the cold gas supply and prevents further disc growth and star formation, especially in clustered galaxies. Subsequent passive evolution, accompanied by gas-poor mergeres, lead to ``red-and-dead'' massive spheroids sharing a common halo in a group, some existing already at z ~ 1. The critical mass is predicted to slightly rise with z. When the detailed models of galaxy formation are revised accordingly, they should recover the bi-modality features including the extreme red and blue objects at z ~ 1 - 2 and the luminous dusty star formers at high z.

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