Daily Paper #6: A physical model of cosmological simulations of galaxy formation

I picked this article for the group meeting presentation, and actually read it, because it is rather relevant.
Title: A physical model of cosmological simulations of galaxy formation
Authors: Vogelsberger, Genel, Sijacki et al.
Year: 2012, http://arxiv.org/pdf/1305.2913v1.pdf
They present galaxy formation simulations using the AREPO code, which is a moving mesh hydro code, claimed to combine the advantages of SPH and semi-analytical models built on top of N-body simulations. What is important, however, is that they include, among other things:
-- BH seeding and growth
-- 3 modes of AGN feedback (thermal, kinetic (radio jets) and EM radiation). The discrepancy between the high-mass end of McCarthy, Schaye et al. TFR and observations I discussed yesterday is probably due to lack of AGN feedback prescriptions.
-- realistic cooling (including metals) with self-shielding
-- realistic enrichment timescales and yields
-- different modes of outflows

The article itself is quite long and rather technical, dealing with the details of the implementation of physics. However, there are some interesting observations. For instance, they discuss the stellar mass function and its relation to feedback, halo mass function and the cosmic SFH. The stellar mass function is 'chewed' by stellar feedback (SN and stellar winds) at the low halo mass end, and by AGN feedback at the higher mass end. It can be viewed as the convolution of the halo mass function with the efficiency with which stars form in these haloes (i.e. feedback mechanisms).

I think this relation is really important -- actually, it shows why the TFR exists, and why there is a knee at the high-mass end. _Why_ the slope, zeropoint and scatter are such are different questions -- but the answer to that should be some combination of the halo concentration (i.e. response to baryons, mostly), details of feedback (which determine the shape of the M* -- Mhalo relation) and the initial halo mass function (i.e. cosmology). The latter also sets the angular momentum of galaxies (Vvir -- Mvir are related, see the description of Dutton 2011).

They reproduce the stellar mass-halo relation, stellar mass function, evolution of the cosmic SFR density, the cosmic stellar mass density, mass-metallicity relation, SDSS luminosity function, M_BH -- M* relation and the Tully-Fisher relation (in the Vcirc -- M* sense, see the other attached plot). They claim (by citing McCarthy & Schaye, and experimenting with different radii) that the precise radius of the velocity measurement is not very important.

They conclude that TFR is not very sensitive to feedback parameters (shown in the plot). However, feedback must be included, otherwise it is not possible to reproduce TFR correctly. They find that the stellar mass in a halo is primarily set by the stellar feedback and radio-mode AGN feedback.

Daily Paper #3: Rotation rates, sizes and star formation efficiencies of a representative population of simulated disc galaxies

I was a bit late today, because I went to see a couple of talks at the HETDEX meeting. A couple of interesting points from there:
-- The LAE people are worried about selection bias arising due to galaxy spins' alignment with the large scale structure (http://arxiv.org/abs/1004.3611)
-- Cluster growth depends not only on the proto-cluster environment (i.e. the immediate neighbourhood), but on super-halo scales as well (it's probably better defined here: http://arxiv.org/abs/1109.6328)

...On to the paper:
Title: Rotation rates, sizes and star formation efficiencies of a representative population of simulated disc galaxies
Authors: McCarthy, Schaye et al.
Year: 2012, http://arxiv.org/abs/1204.5195 They use resimulated (SPH) Millenium sub-regions with selected representative mean densities, including baryonic physics. They include background UV/Xray radiation field that 'reionises' the simulations at z = 9 and the CMB. They model star formation using an effective prescription reproducing the Kennicut's relation and assuming Chabrier IMF, as well as yields and timing of nucleosynthesis and SN feedback.
Then they fit the surface brightness profiles of galaxies in 9 - 11.5 log M* range with a Sersic function and classify their galaxies into disc and spheroid-dominated usin n = 2.5 as the cut.
-- The stellar mass - rotation velocity relation:
They examine this relation for their disk galaxy population (see the attached picture) and compare it with Reyes 2011 observations, claiming that this is the first cosmological hydrodynamical simulation that produces a population of galaxies consistent with the observed TFR. The slope is slightly higher, but in general, the agreement is really good. They use v80 (velocity at 80% light radius) and Vmax as the velocity measures. The velocities at their high-mass range are too big, and they claim that lack of prescription for AGN feedback may explain that.
The authors also discuss the scatter in TF relation, claiming that it is not possible to directly compare observed and simulated TF scatter without careful analysis of selection criteria and observational uncertainties.
Simulated spheroid galaxies follow the TFR well at this mass range, except for the most massive galaxies. The rotation curves of the galaxies with M* > 10.6 are not realistic (due to overcooling, too efficient SF of the highest mass haloes). Their simulated galaxies are also compatible with observed M* - size and star formation efficiency -- M* relations, up to the higher mass range.
One more quantity that simulations are compared with is the ratio of v_circ to v_200. If there is no significant halo expansion/contraction, this ratio should be roughly equal to 1.2 (Duffy 2010 writes more on halo expansion/contraction issue and impact on this ratio). Again, their results agree with this quite well up to log(M*) > 10.5.
They try to reproduce the main galaxy sequence at z = 0, comparing their results with SFR from Galex. Except for the lowest mass galaxies where their resolution is too small, it works well.
The section 4 is an interesting discussion of connection between SF efficiency, stellar mass function and the TF relation. They show that by assuming power-law shapes for halo mass function and stellar mass function in certain stellar mass range where this is valid, it is possible to match haloes (=velocities) and stellar masses using the abundance matching method, and reproduce TFR and SFR efficiency-M200 relations well (and maybe even constrain the faint end of the IMF). The way I understand it, the TFR arises due to the linear (in log-space) match between the halo mass function (set by cosmology) and the stellar mass function (set by IMF, SFR efficiency, thus feedback, etc.). I'll read Mo, van den Bosch and White 2010 for more discussion on this matter.

Jerusalem WS lecture notes: 11. The physics of stellar feedback

By M. Krumholz, the slides.

• 'conservative weed'
• Bate 2009 simulation -- SC formation. SF too efficient and fast -- SF efficiency close to 100%
• what inhibits SF? feedback:
• hot gas or photons push material away from the star, kinetic energy in the material shell = star energy output | radius set by momentum conservation (energy or momentum driven cases, radiation or winds). Mass in the shell way larger than the wind mass
• feedback budgets:
  • Q -- radiant energy, wind energy, number of ionising photons
  • IMF-averaged production rate: luminosity per unit mass (~M/L ratio)
  • lifetime-weighted production rate -- energy out of unit mass (e.g. ergs/g)
  • stochastic IMF sampling in dwarfs -- SLUG code
  • galactic wind: at least as much mass as went into stars
  • what feedbacks are interesting? those that can cause velocities higher than escape velocities --> lower limt
  • losses: gravity, collisions (loss of momentum)
• ISM feedback taxonomy:
  • ionising radiation: not important for galactic winds formation (sound speed ~10 km/s, so can influence in smaller MCs, Krumholz 2006, 2009, Dale 2012), probably the most important SF regulator today
  • radiation pressure (photon momentum, Thompson scattering) -- ~200 km/s -- cannot be responsible for galactic winds, unless radiation enhancing fraction f_{trap} >> 1. Can be important for subgalactic objects, dwarf galaxies, can blow up gas clouds.
  • the important question: what is the f_{trap}?:
  • 30 Dor: dust grain temperature can help infer the IR radiation field, Lopez 2011
  • simulations: 2D, high resoluton: RT instability -- similar to oil floating on water, right panel: no gravity, 2 different optical depths[surface densities]: RP may affect sub-galactic objects, but cannot produce galactic winds, Krumholz & Thompson 2013
  • stellar winds: Solar wind is a wimpy old thing, O stars. Momentum driven
  • 30 Dor -- most massive binary star system, each ~83 M_{\odot}, still on the MS
  • supernovae: energy budget in stars of 8-10 M_{\odot}
    • N_{SN}/M = 0.01 M_{odot}
    • less energy and momentum than radiation feedback
    • more energy, less momentum than winds
    • SN are most important because they are much closer to energy conserving feedback -- large velocities, post-schock ejecta temperatures are ~10^{10} K --> cooling time is ~ 60 Myr, whereas time required to escape the galaxy is << 1 Myr, so gas cannot cool
    • Sedov-Taylor similarity solution -- first developed for nuclear tests, open literature only in 1995 --> energy of Trinity blast from Time pictures (R_blast as a fn of time, Sedov), http://www.seas.harvard.edu/brenner/taylor/handouts/bomb/node1.html
    • trapping factor ~30-40, momentum goes up by this factor during the energy conserving phase, density dependent: SNe explode in low density environments due to star radiation --> f_{trap} is elevated
    • SNs can dominate momentum budget --> proper simulation should take other feedbacks into accounts
  • metallicity feedback
  • metallicity changes SF law (makes difference in dwarfs, high z galaxies) --> metallicity regulated SF (Kuhlen 2012 simulation), interactions wih other feedbacks