Secular evolution and the assembly of bulges

Bulges are of different types, morphologies and kinematics, from pseudo-bulges, close to disk properties (Sersic index, rotation fraction, flatenning), to classical de Vaucouleurs bulges, close to elliptical galaxies. Secular evolution and bar development can give rise to pseudo-bulges. To ensure prolonged secular evolution, gas flows are required along the galaxy life-time. There is growing evidence for cold gas accretion around spiral galaxies. This can explain the bar cycle of destruction and reformation, together with pseudo-bulge formation. However, bulges can also be formed through major mergers, minor mergers, and massive clumps early in the galaxy evolution. Bulge formation is so efficient that it is difficult to explain the presence of bulgeless galaxies today. 1. Secular evolution and bulges, gas flows There are excellent recent reviews on secular evolution (Kormendy & Kennicutt 2004, Jogee 2006), and in particular the formation of bulges has been debated in detail last year in Oxford, with the IAU Symposium 245 on ”Formation and Evolution of Bulges”. Only more recent work will then be reviewed here. A clear distinction is now well established between classical bulges and pseudo-bulges, from the luminosity distribution (Sersic index, flattening, color) and the kinematics. On the color-magnitude diagram, the pseudo-bulges, similar in properties to disks, are clearly on the blue cloud, while the classical bulges sit on the red sequence (Drory & Fisher 2007). There is a clear bimodality in Sersic index, with the pseudo-bulge peak at n=1-2, and the classical bulge peak at n=4 (Fisher & Drory 2008). Gaseous haloes around galaxies The secular evolution is fueled by external gas accretion, and there is now growing evidence of gas infalling on nearby spiral galaxies, although this gas is quite diffuse. One of the best example is the edge-on galaxy NGC891 (Fraternali et al 2007). HI gas is observed up to 20kpc above the plane, with its rotation decreasing with the altitude. Part of this gas could come from galactic fountain, but not all, since the angular momentum should then be conserved. Moreover, modelisation of the fountain effect predicts gas outflow (cf the non edge-on galaxy NGC 2403), while mostly inflow is observed, like for high velocity clouds in the Milky Way. Gaseous haloes require accretion of external gas (Fraternali & Binney 2006). A recent review by Sancisi et al (2008) gathers many examples of extraplanar gas. Part of it is due to dwarf companions or tidal streams, but evidence is mounting for extragalactic inflow of gas due to cosmic accretion. This external