On the Origin of Galaxy Morphology in a Hierarchical Universe Gabriella De Lucia INAF – Astronomical Observatory of Trieste

Hierarchical simulations of galaxy formation z=3. z=1. z=0. Hierarchical simulations of galaxy formation Baryonic physical processes modeled using physically and/or observationally motivated prescriptions. Allow a large dynamic range to be accessed, fast exploration of the parameter space, efficient investigation of the influence of different physical prescriptions. No description of the gas dynamics. feedback star formation Hot Gas Cold Gas cooling Ejected Gas Stars re-incorporation - AGN heating

The stage An era of `precision cosmology’. Although dark matter remains to be detected, and the nature of dark energy remains elusive, we do have a standard scenario for the formation of large scale structure. Planck 2015 results Dark matter haloes grow in a bottom up fashion, and build through regular mergers and smooth accretion. Abundances (as well as environmental dependency) of different types driven by structure growth Genel et al. 2010

Disks Steinmetz & Navarro 1999 Forming realistic disk galaxies has been (and largely still is) a long-standing issue in hierarchical scenarios. Effective cooling at high-z leads to the formation of small clumps that merge due transferring angular momentum to the dark matter (the angular momentum `catastrophe’) Crucial the improvement of the sub-grid model(s) adopted for stellar feedback. Scannapieco et al. 2012

Bulges Strong internal dynamical response driving the formation of asymmetries in the disk that leads to the inflow of gas towards the central region and its compression. Toomre & Toomre 1972 M51 Barnes & Hernquist 1996 A bulge or of a lowered disk surface density stabilizes the system against the growth of instabilities. In a sufficiently close encounter, violent relaxation destroys the dynamically cold disk and produces a kinematically hot merger remnant. Presumed connection with AGN activity. Mihos 2004

Galaxy mergers Controlled simulations show that the classical Chandrasekhar formula tends to over-estimate merging times. Boylan-Kolchin et al. 2008 “Scatter” is introduced when considering a more realistic environment Villalobos et al. 2014

disk-like structure – may have rings, bars, spiral structure dynamically cold in types Sbc or later from evolution of disk instabilities (e.g. bars) Pseudo bulges What’s the fraction of pseudo-bulges formed during mergers? High redshift starbursts? When should a disk become unstable? What happens to an unstable disk? Okamoto 2013

From disks to bulges and ellipticals Semi-analytic models typically consider three channels of bulge formation: In `minor’ mergers, the stellar mass of the merged galaxy is transferred to the bulge of the remnant + burst of star formation (a fraction of the combined cold gas is turned into stars). New stars in the disk or in the bulge. A `major’ merger destroys completely the stellar disc + a significant burst of star formation (little or no gas is left over) Disk instability can convert part of the stellar disc (or stellar + gas disk) into a bulge-like component. Details of the modeling (in particular for the disk instability) can vary significantly from a model to another. Prescriptions tuned on results from numerical simulations (but then used at all redshifts). De Lucia et al. 2011, Porter et al. 2014, Fontanot et al. 2015

How and when do bulges form? The contribution from major mergers decreases with increasing mass, while contribution from minor mergers increases. Disk instability represents an important contribution to the bulges of intermediate mass galaxies. Trends are similar in the two models considered but the timings of bulge formation and the contribution from disk instability differ significantly. De Lucia et al. 2011

Where do bulges form? De Lucia et al. 2011 Typical halo mass where different processed contribute to bulge formation increases with increasing stellar mass. Note that there is a lot of bulge formation in haloes that are marginally resolved.

Ellipticals are “transient”? Disk re-growth following cooling or (but this is not significant in these models) mergers with gas rich satellites. Disk re-growth is not significant for the most massive galaxies, but it represents a non-negligible component in the evolution of low and intermediate mass galaxies. Disk re-growth is inhibited by radio-mode feedback and by subsequent mergers. Fraction of galaxies experiencing disk regrowth is larger in MORGANA because of less efficient AGN feedback. De Lucia et al. 2011

Modelling disk instabilities Efstathiou, Negroponte & White (1982) Systematic differences between the quantities entering the equation in the two models, but the total fraction of unstable disks is similar. What happens to an unstable disk?

Too many or too few ellipticals? Porter et al. 2014 Opposite conclusions: Wilman et al. used pure mergers models and found that they predict too many ellipticals. Porter et al. argue that without disk instability they cannot reproduce the observed fraction of ellipticals. Wilman et al. 2013

How well do we measure elliptical fractions? Hyde & Bernardi (2009) Wilman & Erwin (2012) Fisher & Drory (2011) Mendel et al. (2014) Fontanot et al. 2015

The other side: bulgeless galaxies Bulgeless galaxies do not constitute a rare population in these models. There is a sufficient number of DM haloes with merger quiet assembly histories to account for local statistics (see also Fontanot et al. 2011 and Porter et al. 2014). Fontanot et al. 2015

Summary : We are able to make detailed predictions of morphological fractions. The comparisons between data and different models is able, in principle, to constrain processes at play. An important contribution to bulge formation (in particular for intermediate mass galaxies) comes from disk instabilities. Bulge dominated galaxies can grow a new disk if they are fed by appreciable cooling flows. The rates of disk regrowth are negligible for the most massive galaxies, but more important for low-to intermediate massive galaxies, in particular at high redshift. Present uncertainties in the data represent an important limitation to our understanding of spheroid formation. There are enough haloes with quiet merger histories to explain the abundance of bulgeless galaxies measured in the local Universe