lit: evolution of dwarf galaxies: a dynamical perspective

I was tasked with presenting Lelli et al. 2013 in our group meeting. Here are the slides [pdf].

lit: The Habitable Epoch of the Early Universe

by A. Loeb.
That's a fantastically crazy paper (by A. Loeb, who's one of the best known names in areas as diverse as gravitational microlensing/black hole evolution/reionisation and 21 cm signal/high-z GRBs/Event Horizon telescope, and many others). He's written several papers about wild ideas before: exploring the (sad and very distant) future of observational cosmology, bio-markers in white dwarf planets' atmospheres, planets of hypervelocity stars, search of artificially-illuminated objects in and beyond the Solar System and cosmology measurements from hypervelocity stars which shows that not all is lost for the future cosmologists.
In this paper he looks at the dawn of the Universe, when it was only ~15 million year old. A. Loeb points out that the temperature of the cosmic microwave background was roughly around 0-30 C then, and therefore liquid water could have existed on any solid surface, meaning that there might have been conditions suitable to life as we know it. In order to form any rocky planet one needs to explode some massive stars before (to get some heavier elements like oxygen, iron, silicium, etc, which are what rocky planets are made of). At that time we're left with the formation of the very first stars in the very first, tiny, dark matter haloes at the far end of the density distribution. Calculations show that the number of such protogalaxies was incredibly small at these redshifts.
However, if we assume that the initial density distribution was not perfectly Gaussian (there are many theories explaining why it might have been the case, although Planck and other observations haven't found any proof of non-gaussianity yet), there might have been some haloes that had formed massive stars by that time.
It's a marvelous article, although AFAIK, it takes much longer than a few Myr for rocky planets to assemble from accretion disks of stars (and cool down due to the decay of radioactive elements..). But think of it: at some time in the history of the Universe the outer space was warm (and at least a million times denser than now).

lit: The baryonic Tully-Fisher relation and its implication for dark matter halos

Trachternach et al 2009: baryonic TFR as constraint of elongation of DM haloes.

lit: TF as a constraint on the temperature of WDM

Constraints on the temperature of Warm Dark Matter from the Galaxy Stellar Mass Function and Tully-Fisher Relation by Kang, Maccio, Dutton. One more reason why TFR is important!

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 #4: Dark halo response and the stellar initial mass function in early-type and late-type galaxies

Our group meeting ate most of my day, so here's a delayed review. Title: Dark halo response and the stellar initial mass function in early-type and late-type galaxies
Authors: Dutton, Conroy, van den Bosch et al., http://arxiv.org/abs/1012.5859
Year: 2011
The authors try to find the origins of the TF and Faber-Jackson relations, using a big sample of observed galaxies from SDSS, a set of observational scaling relations showing the distribution of baryons in galaxies, with constrains on the DM halo structure from simulations.
They claim in the introduction that the origin of TFR and FJR is the relation between the halo virial velocity and its virial mass, which scales as v_vir \propto M^(1/3). The relation between v_opt (*) and v_vir depends on 3 factors:
-- the contribution of baryons to v_opt
-- the structure of pristine (DM-only) haloes. The halo profiles are well-enough defined by the NFW profile.
-- and the halo response to the disk formation (contraction/expansion)
The biggest source of uncertainty for the first factor is the IMF, as most of the baryonic mass in galaxies (except the low-mass star-forming ones) is in stars.
The third factor, the halo response, is quite uncertain. It can be adiabatic or non-adiabatic contraction, or expansion due to a number of different processes.
They use a large number of observed relations (TF and FJ relations too) to constrain or predict unknown quantities in their analytic model, modelling the DM halo, bulges, disks, gas disks, constructing 3D models of their SDSS sample galaxies (~100 000 LTGs, ~160 000 ETGs).
They construct mass models for all those galaxies using various IMF and assumptions about the halo response. Then they compare them with the observed circular velocity -- stellar mass relations (I do not understand it at this point -- they already used the TFR as a constraint while solving for the halo contraction at a given IMF, so it is inbuilt in their model somehow?).
Nevertheless, they do not reproduce the zero-point of the TFR with the standard Chabrier IMF and standard adiabatic halo contraction model, as the velocity is overpredicted. They claim that it is possible to fix that by either assuming a different IMF, or changing the halo response, or having different halo concentrations (set by formation time or \sigma_8, they claim that both ways to do it do not solve the zero-point problem).
However, they find that early-type and late-type galaxies cannot have the same IMF and halo response. For any given halo response model, ETGs and LTGs must have different IMF normalisations to reproduce the TFR. If we assume a universal IMF, early-type galaxies require stronger halo contraction.
They look into the model circular velocity profiles (see attached) for models that reproduce the TF and FJRs, with Chabrier IMF. The velocity profiles look rather constant at higher radii, reaching the maximum at ~0.3 R_vir. (For the LTGs, they convert the velocity dispersion to v_circ picking a rather simple conversion factor between sqrt(2) and sqrt(3).) They claim that the flatness of the rotation curves does not depend strongly on the IMF and the halo response model, and 'is a natural consequence for galaxies embedded in LCDM haloes'.
Then they discuss possible mechanisms that would account for differences in IMF or halo response for ETGs and LTGs. The halo contraction/expansion may depend on galaxy morphology via a number of processes:
-- adiabatic contraction due to smooth accretion -- results in disk growth and SF, important for LTGs
-- wet major mergers -- result in halo contraction because gas settles in the center of a galaxy, transforms LTGs to ellipticals
-- dry major mergers ( for ETGs) -- halo contraction due to violent relaxation which mixes stars and DM
-- clumpy accretion/minor mergers: halo expansion due to dynamical friction. Clumps must be baryonic, so that's likely to occur at high redshifts (A. Dekel was arguing in Jerusalem that it must be the case).
-- feedback causes halo expansion, more important to LTGs and for higher redshifts.
-- bars -- halo expansion or contraction, not clear.

They sum it all up by saying that the key formation event that distinguishes ETGs from LTGs is a major merger (dry or wet), which causes halo contraction. For LTGs, haloes could expand due to a combination of clumpy cold accretion and intensive feedback.
The IMF may plausibly vary with redshift due to its alleged dependence on ISM temperature via the Jeans mass. Since ETGs form their stars earlier than LTGs, their IMF might be different. They discuss the van Dokkum & Conroy 2008 article on the different IMF at the cores of massive ellipticals, and conclude that this IMF would not fit for the other ETGs well.
All in all, they show that there is a degeneracy between IMF and the halo response to disk formation, and we cannot match the two for ETGs and LTGs if we want to reproduce the TFR. However, I'm puzzled here -- like in the previous Mo, Mao, White article, they do not model galaxy evolution in a self-contained way, and a model cannot show things that are not included in it. Mo, Mao & White include one set of parameters in their article, and get a set dependences on their parameters (like the disk formation time). Here Dutton et al. model rotation curves as functions of other parameters, and get dependence on another set of parameters. I think it's tricky to understand which parameters are really important, because in astrophysics we often have many coupled parameters, and it's not difficult to find correlations between them. It's more difficult to find out which ones are the fundamental ones, which really affect the TFR.
-- a question: what does it mean "TFR is an edge-on projection of the fundamental plane" (p. 323)?
(*) v2.2 for late-type galaxies and some measure of velocity proportional to dispersion at the effective radius for early types

Daily Paper #2: The Tully-Fisher Zero Point Problem

Today's is a short paper (conference proceedings, just 4 or so pages), dealing with the halo contraction/expansion issue.

Title: The Tully-Fisher Zero Point Problem
Authors: Dutton, van den Bosch & Courteau
Year: 2008, http://arxiv.org/abs/0801.1505

The authors claim that CDM-based disk formation models can reproduce the slope/scatter of the TFR, however, its zeropoint has not been matched. This problem gets even worse if additional constraints (disk sizes, number densities) are imposed.
The TFR zeropoint can be matched in models if the halo contraction or disk self-gravity are not taken into account, or, as is shown in the article, halo contraction is counteracted by baryon feedback. They model an exponential disk in a NFW halo, obtain its luminosity from observed TFR and its scale-length from the size-luminosity relation. Assuming WMAP3 cosmology and a concentration parameter value from simulations, they solve for the virial mass and obtain rotation curves with and without the halo contraction (see the attached plot). The halo-contraction case is in conflict with both theory and observations, meaning that the halo is too small and has too high spin parameter.
They suggest 3 ways to solve this problem:

• -- lower the stellar mass-to-light ratio (but there is no known IMF that could provide that). Also, they claim that baryons at the disk and bulge account for at least half of the v2.2 -- I didn't know that (Courteau 1999, Weiner 2001)!
• -- lower the initial concentration (then the virial radius of a halo is increased) -- but that's incompatible with WMAP3 cosmology.
• -- turn off or reverse halo contraction: disk galaxies do not form in isolated haloes by smooth cooling. Halo contraction can be reversed by:
  • a) Feedback during adiabatic disk formation, if a large fraction of its mass is removed. It would also explain why galaxy formation is so inefficient (I think they mean that the gas is not converted into stars rapidly).
  • b) Dynamic friction between baryons and DM due to bars or large baryonic clumps (do they mean violent disk instabilities here?). They do not explain the details of such a process here, but I think what happens is that baryons transfer some of their angular momentum to the halo due to formation of massive, concentrated, rapidly moving structures.

The interesting message of the article is that the TFR zeropoint can provide clues into baryon feedback (which should also explain the core-cusp problem of DM haloes, at least that was what A. Dekel and J. Primack maintained at the Jerusalem Winter School). That's important.

Daily Paper: The formation of galactic discs

Me and my supervisor agreed that I would read a paper a day and write it up, so I'll be copying the summaries here.

Title: The formation of galactic discs (http://adsabs.harvard.edu/abs/1998MNRAS.295..319M)
Authors: Mo, Mao, White
Year: 1998
It's actually a really important paper -- I'm glad not to have missed it. They model galactic disk formation (of pure exponential ones, though they address influence of a simple bulge model later) in 3 CDM cosmologies. The authors make 4 important assumptions:
1) The mass of a disk is a fixed fraction of its halo
2) The angular momentum of the disk is also a fixed fraction of that of the DM halo (later they find that the specific angular momenta of the disk and halo are similar, so these two ratios should be equal).
3) The disk is thin, has an exponential density profile and is centrifugally supported
4) Real galaxies are dynamically stable disks, i.e. they reject unstable models.

They calculate the halo mass function using Press-Schechter formalism (see Appendix), assume halo angular momentum distribution determined by N-body simulations and a constant mass-to light ratio, and NFW profile. They calculate the total energy of a truncated (to r_200) NFW halo using the virial theorem, the rotation curve corresponding to it (after accounting for adiabatic halo contraction and gravitational disk influence).
See the attached picture (Mo1998_rotcurves.png) for their results. They claim that the shape of a rotation curve depends on 3 parameters only: the halo concentration parameter c(see *), fraction of mass in the disk m_d, and the spin parameter.

There is a lot more in this paper, but they devote a significant section of it to Tully-Fisher relation, explaining the origins of its slope, zeropoint and intrinsic scatter.

• -- slope: They claim that it is a generic prediction of hierarchical structure formation models with CDM-like fluctuation spectra (which set the halo formation times and thus their concentration c distribution).
• -- zeropoint: mostly set on disk mass fraction, mass-to-light radio and the redshift at which a given disk was assembled. They claim that disks must have effectively formed at redshifts close to zero (?), unless we assume an unphysical mass-to-light ratio.
• -- scatter: they analyse possible sources of intrinsic scatter of the disks, and find that:
  • --- a small amount of scatter comes from the spin parameter distribution
  • --- scatter in concentration c also adds scatter to the TF relation (disks at more concentrated haloes spin faster).
  • --- bulge is not included in their model, so bulge luminosity contribution is not accounted for -- that's a source of scatter as well
  • --- mass-to-light ratio may vary, adding some more scatter
  • --- formation times may vary as well, adding even more scatter.

The intrinsic scatter because of the first two factors cannot be smaller than 0.15 mag.
In conclusion, they show that it is possible to derive a realistic TF relation theoretically, however, I'm not sure if that is the only way it could arise? They do not include bulges, which are important for our sample, too. I am also unsure what do they mean with 'effective disk assembly redshift' at z <= 1.

* c == r_200/r_s, here r_200 -- the limiting radius of a halo, defined so that the mean density inside this radius is 200 times larger than the background density, r_s -- the disk scale length.
There is also a brief explanation of the \sigma_8 parameter -- see attached.

lit: Non-linear structure formation

Slides from F. van den Bosch, from a talk given at Jerusalem Winter School.
A Gaussian random field is completely specified by either the two-point correlation function ξ(r), or, equivalently, the power spectrum P(k)

lit: Halo assembly bias

slides by F. van den Bosch.

lit: J. Primack lectures on semi-analytic galaxy models and feedback

Lecture notes about quenching, feedback, SAMs. Includes a nice image of the 'star forming band'.

lit: Spins and orientations of haloes in cosmic filaments

Zhang et al., many useful references on methods therein.