Evolution of Galaxies & Metallicity

William · #1 (**permalink**) 25-May-2008, 01:55 AM

This thread is a presentation of recent data and analysis concerning the evolution of galaxies and metallicity.

What the data shows is that something limits galaxy growth for spiral galaxies to a maximum of around 10^12 m⊙, the metallicity of large galaxies, greater than 10^10 m⊙, varies little with redshift. There is significantly (almost an order of magnitude) less metals in the early universe (z= 2 to 3) than the cosmological models indicate there should be. There are too few low metallicity dwarf stars in all galaxies. Theories indicate that zero metal population III stars should have formed. There have been no population III stars observed as of yet. As noted before in the forum, quasar spectrum does not vary with redshift except for very high redshift quasars some of which, but not all of which have super solar metallicity.

A) Missing metals problem
B) Dwarf G problem, Population III Star Problem
C) Limit of Spiral Galaxy size, Large galaxies’ metallicity does not evolve with redshift
D) No evolution of quasar metallicity with redshift

The following are links to sources that provide a definition as to what is “metallicity” in astronomy.

http://etacar.umn.edu/~martin/rrlyrae/metals.htm

http://en.wikipedia.org/wiki/Metallicity

“Missing metals problem”

The “missing metals problem”, is the name used for the finding that the average observed metallicity in the early universe (z=2 to 3) is nearly an order of magnitude less than simulations indicate it should be. Hypotheses proposed to resolve the missing metals such as an increase galactic winds due to supernova, which could have blown the metals out of the galaxies have been proposed, however, as noted in the paper below, simulations indicate a higher speed supernova wind would also blow hydrogen and other gases out of the galaxies which would stop star formation.

While the mechanisms have been adjusted to explain the “metallicity problem”, the missing metals is not the only problem.

http://arxiv.org/abs/astro-ph/0310770v1

Damped Lyman- α Absorbers in Cosmological SPH Simulations: the “metallicity problem”

Quote:

We study the distribution of star formation rate (SFR) and metallicity of damped Lyman-α absorbers (DLAs) using cosmological smoothed particle hydrodynamics (SPH) simulations of the lambda cold dark matter (ΛCDM) model.

Quote:

However, we also find that the median metallicity of simulated DLAs is higher than the values typically observed by nearly an order of magnitude. This discrepancy with observations could be due to shortcomings in the treatment of the supernova feedback or the multiphase structure of the gas in our current simulations. Recent observations by Wolfe et al. (2003a,b) seem to point to the same problem; i.e. the observed DLA metallicities are much lower than those expected from the (either observed or simulated) DLA star formation rates, a puzzle that has been known as the “missing metals”-problem for the globally averaged quantities.

Quote:

One potential reason for high metallicity in DLAs is that the feedback by galactic winds is not efficient enough in blowing out metals from DLAs. Clearly, if the feedback by winds were stronger, then star formation and hence metal creation in DLAs would be more strongly suppressed. However, simply making the winds stronger and blowing out more gas will not necessarily decrease the metallicity of DLAs much, because in our current simulation model, the winds transport away metals and gas at the same time; i.e. the wind’s initial metallicity is assumed to be equal to that of the gas of the DLA, leaving the ratio of metal and gas mass in the DLA unchanged.

William · #2 (**permalink**) 25-May-2008, 03:14 AM

The G Dwarfs have a life time of around 12 billion years. Based on simulations that assume a closed box galaxy where the simulation has the highest metallicity stars evolved to near solar metallicity, there are significantly (more than twice as many) more G dwarf low metallicity stars, than in the Milky Way and other galaxies. There have been different hypothesis to explain why observations do not match theory. More detailed analysis of the Milky Way and other galaxies provide data that does not support the initial enrichment hypothesis.

http://arxiv.org/abs/astro-ph/0101376v1

Chemical Evolution of the Galaxy: the G-dwarf problem and radioactive chronology revisited taking account of the Thick Disk by B, Pagel

Quote:

The ‘G-dwarf’ problem has been with us for nearly 40 years and extended to other galaxies. There is still no definitive universally accepted solution; rather too many, the most popular being ‘prompt initial enrichment’ (PIE; Truran & Cameron 1971) and inflow (Larson 1972). Truran & Cameron postulated a first generation of massive stars, sometimes called Population III, collapsing to black holes or ‘collapsars’ in the halo, thereby accounting for its dark mass. Later the idea gained ground that the halo consists mostly of dark matter, wholly or partly non-baryonic, but PIE came in by another route. After discovery of the Thick Disk, Gilmore & Wyse (1986) suggested that (a) the Thick disk was initially enriched by gas shed from the halo as in the model by Hartwick (1976); and (b) the Thin disk was similarly enriched by gas shed from the Thick one.

The G Dwarf Problem Exists in Other Galaxies by Guy Worthey

http://aps.arxiv.org/abs/astro-ph/9606017v1

Quote:

We therefore conclude that, from small galaxies barely large enough to attain solar metallicity to gigantic elliptical galaxies, and from disks to spheroids regardless of field/cluster environment, and from the nuclei alone to consideration of every star in the galaxies there is no evidence for an abundance distribution as broad as the Simple model predicts. Despite the small number of observations we cite for spiral disks and bulges (two: ours and M31’s), it is quite likely that the G dwarf problem is universal; that nowhere in the universe does chemical enrichment produce as many metal-poor stars as predicted by the Simple model if the enrichment process proceeds to near-solar composition.

Quote:

In addition to the classical G dwarf problem, which is mostly considered to apply to stars in the [Fe/H] = −1.0 to −2.5 range, there is “the flagrant scandal that we so rarely mention in public: where is the First Generation?” (King 1977). Nearly primordial stars with very small metal content ([Fe/H] < −3.5) are difficult to find (Beers, Preston, & Schectman 1992), and a good question is whether they are present in the numbers predicted by the Simple model.

Quote:

About early-type galaxies we learn that they cannot have formed primarily from the merging of purely stellar satellite galaxies like dwarf spheroidals. If this were the case, they would have many more metal poor stars than they do. That pure-stellar mergers are unworkable is known from dynamical simulations (e.g. Villumsen 1983, Hernquist 1993), which show that the merging of purely stellar galaxies results in a large, flat core that is dissimilar from real galaxies.

http://arxiv.org/abs/astro-ph/0602422v1

The G-dwarf problem in the Galactic spheroid by R. Caimmi

Quote:

In addition to the G-dwarf problem, a lack of a well-defined empirical age-metallicity relation (EAMR) seems to be established for both the disk solar neighbourhood (e.g., Meusinger, Reimann & Stecklum 1991; Edvardsson et al. 1993; Rocha-Pinto et al. 2000; Feltzing, Holmberg & Hurley 2001; Nordstr¨om et al. 2004; Karatas, Bilitz & Schuster 2005) and globular clusters (Salaris & Weiss 2002; De Angeli et al. 2005). The large scatter observed in the EAMR is probably universal, at least with regard to massive enough (M >∼10^10m⊙) galaxies, independent of the morphological type.

Quote:

The existence of a G-dwarf problem i.e. the observation of too few metal deficient G dwarfs (or, more generally, of a selected spectral type) with respect to what expected from the Simple model of chemical evolution (e.g., Searle & Sargent 1972; Pagel & Patchett 1975; Haywood 2001) was first established in the solar neighbourhood (van den Bergh 1962; Schmidt 1963). Though in less extreme form, a G-dwarf problem appears to exist in both the halo (e.g., Hartwick 1976; Prantzos 2003) and in the bulge (e.g., Ferreras, Wyse & Silk 2003). In addition, a G-dwarf problem has been detected in both bulgedominated and disk-dominated galaxies (Henry & Worthey 1999), which is consistent with the idea that the G-dwarf problem is universal (Worthey, Dorman & Jones 1996).

Quote:

The deficit of metal-poor stars (with respect to the prediction of the Simple model) may be interpreted in different ways, such as changes in the initial mass function (Schmidt 1963; Adams & Fatuzzo 1996; Bromm 2004; Bromm & Larson 2004; Larson 2005), inflow of unprocessed (Larson 1974) or processed (Thacker, Scannapieco & Davis 2002) material from outside, or evolution with inhomogeneous mixing (Searle 1972; Malinie et al. 1993; Caimmi 2000, 2001b, hereafter quoted as C001 and C01, respectively; Oey 2003; Karlsson 2005). For additional alternatives and further details see e.g., Pagel & Patchett (1975); Pagel (1989).

Kaptain K · #3 (**permalink**) 25-May-2008, 03:20 AM

These are not problems of the universe, merely problems of our incomplete understanding of the universe!

William · #4 (**permalink**) 25-May-2008, 04:05 AM

The following is a recent large area survey and analysis of galaxy metallicity, looking for correlation of metallicity, to both galactic mass and redshift. As noted in the first comment in this thread, there appears to be some mechanism that limits spiral galaxies’ to a mass of about 10^12 M⊙. The authors of this paper find that galaxies in excess of 10^10 M⊙ show little evolution of metallicity, with time (See figures 7 and 8 in the attached paper.) The finding that galaxies reach a metallicity plateau might be related to the finding that quasars show almost no evolution of metallicity with redshift, except for very high redshift quasars which sometimes but not allows show super solar metallicity. The authors of this paper also find enhanced metallicity evolution of galaxies in clusters.

http://lanl.arxiv.org/abs/0804.3091v1

The Cosmic Evolution of Metallicity from the SDSS Fossil Record by B. Panter, R. Jimenez, A. Heavens, S. Charlot

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For galaxies with present stellar masses > 10^10 M⊙, the time evolution of stellar metallicity is very weak, with at most 0.2−0.3 dex over a Hubble time- for this reason the mass-metallicity relation evolves little with redshift.

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Finally, we have explored the relationship between cluster environment and metallicity, and find a strong correlation in the sense that galaxies in high density regions have high metallicity.

Comment:
The observation of a metallicity plateau for large galaxies at all redshifts and for small galaxies after sufficient time seems (to me any way) to indicate that there could be a non nucleosynthesis mechanism that is producing metals in a galaxy. (The Galaxy metallicity plateaus because of the limit of that mechanism. The large galaxy, is large because of enhancement of that mechanism. A trigger for the mechanism would be looked for related to close proximity of galaxies in galactic clusters. There is also observed anomalous high temperature gas which is found in the centre of the galactic clusters. An observation that might support close proximity of galaxies hypothesis would be activation of star burst when galaxies approach one another.

I am currently looking at what is known concerning Wolf Rayet stars, to see if there is any observational data that would support a hypothesized non nucleosynthetic source for metals in galaxies in addition to the supernova mechanism. The Wolf Rayet stars eject large amounts of metals via Wolf Rayet winds and have a peculiar broad line spectrum which seems similar to quasars. The WR winds are unusually strong (i.e. It is difficult to come up with a stellar radiation model that can produce the observed wind speed and density.) I am looking for observational data that would support the hypothesis that WR stars have a much longer lifetime than would be expected and looking a different mechanism that could possible explain the observations.

turbo-1 · #5 (**permalink**) 26-May-2008, 01:40 AM

http://www.stsci.edu/institute/itsd/...lStrauss110205

Watch this streaming video and see what Michael Strauss (Science Spokesperson for SDSS) has to say about metalicity and redshift with regard to quasars. Pay special attention to the part where he says that quasars at z~6.5 are not different from local quasars in total or relative metalicity. That pokes a big hole in the argument that the early universe was pristine and metal-poor and our present metal-rich universe is the result of multiple generations of supernovae.

BTW, z~6.5 is way less than a billion years after the purported BB event, and still the universe at that redshift seems to be mature and comprised of objects with super-Solar metalicities.

Cougar · #6 (**permalink**) 26-May-2008, 07:39 PM

Quote:

Originally Posted by turbo-1

BTW, z~6.5 is way less than a billion years after the purported BB event, and still the universe at that redshift seems to be mature and comprised of objects with super-Solar metalicities.

That's not what I hear.

Rather than "mature," I've heard the early universe is observed to be quite a bit different than the 13 billion-year-old universe, particularly with respect to galaxy size and morphology. I expect there are abundant papers investigating the differences. Still, structure formation details remain one of astrophysics' unsolved problems, as far as I know.

Metallicity in the early universe follows mass. For large masses, it follows quickly.

William · #7 (**permalink**) 27-May-2008, 02:56 AM

In reply to Cougar's

Quote:

Metallicity in the early universe follows mass. For large masses, it follows quickly.

Cougar,

To me the quasar metallicity uniformity at any redshift, in addition to the lack of quasar metallicity evolution with redshift does not make sense for a quasar accretion disk mechanism.

The accretion disk processes matter. As a galaxy is not uniformly formed, shouldn’t the quasar metallicity vary depending on the origin of the matter that moves through the disk? For example we find in our own galaxy the metallicity of the halo is different than that of the disk. The Milky Way’s halo has two components that differ in metallicity which is explained as material from different sources making up the halo. That explanation makes sense (for the halo, but does not explain the metallicity variation in the disk.) as a galaxy would be expected to gather material. Why would we not see random metallicity variation in quasars, in addition to metallicity variation based on evolution of the surrounding gas?

Cougar are you saying for large galaxies there is no stellar evolution? Are you saying there is no evolution of galaxies?

Also, there is no explanation as to why the only evolution in quasar metallicity is at the highest redshift and is an increase to super solar metallicity rather than a decrease in metallicity.

To me a different mechanism is required, to explain the uniformity in metallicity and the lack of metallicity evolution.

"Evolution of high-redshift quasars" by Xiaohui Fan

Quote:

...The sample of quasars at z > 5.7 from the SDSS provides the first opportunity to study the evolution of quasar spectral properties at z ≈ 6, less than 1 Gyr after the Big Bang and only 700 million years from the first star formation in the Universe. Optical and infrared spectroscopy of some z ≈ 6 quasars already indicates a lack of evolution in the spectral properties of these luminous quasars: Pentericci et al. (2002) show that the CIV/NV ratio in two z≈ 6 quasars are indicative of supersolar metallicity in these systems. ...

Quote:

... Freudling et al.(2003) and Barth et al. (2003). detected strong FeII emission in the spectra of four z≈ 6 SDSS quasars. In addition, the optical-to-X-ray flux ratios and X-ray continuum shapes show at most mild evolution from low redshift (e.g. Brandt et al. 2002, Vignali et al. 2003). Figure 5 shows the composite of our z≈6 quasar spectra: it is almost identical to the low-z composite, both in term of the spectral slope, and emission line strength. The only differences are that the Lyα forest is much stronger at z≈6 and the blue wing of Lyα is also affected. ...

In reply turbo-1's comment

Quote:

BTW, z~6.5 is way less than a billion years after the purported BB event, and still the universe at that redshift seems to be mature and comprised of objects with super-Solar metalicities.

I agree, additional mechanisms are required to explain the observations. I enjoyed Michael Strauss' lecture. There was an interesting comment that noted based on CMB data re-ionization should occur at z=15 not z=6.5.

There is a thread in the forum that is discussing Lyman Alpha forest. Strauss also noted that the z=6.5 quasar showed less absorption than the z=6.29 quasar. To me the high redshift quasars are showing evidence of the Lyman Alpha forest because they are producing prodigious amounts of gas and dust and are weak emitters and hence cannot ionize all of the emitted gas. I am thinking of the quasar as akin to a Wolf Rayet star which also has a broad emission line spectrum.

William · #8 (**permalink**) 27-May-2008, 03:13 AM

This might finding might support Cougar's comment that large galaxies stop evolving. I would still think, however, if these massive galaxies are formed from standard theory stars that the stars should evolve and if the galaxy is assumed to be close box, the galaxy and its stars should gradually increase in metallicity.

As noted in the paper some mechanism is required to expand the young massive overly dense galaxies which are roughly 40 times smaller than a similar mass nearby galaxy. As a comparison the mass of the Milky Way is 5.8 x 10^11 solar masses and the Milky Way disk has a diameter of around 45 kpc.

http://arxiv.org/abs/0802.4094v1

“Confirmation of the remarkable compactness of massive quiescent galaxies at z~2.3: early-type galaxies did not form in a simple monolithic collapse” by P. G. Van Dokkum , M. Franx, M. Kriek, & et al.

Quote:

Using deep near-infrared spectroscopy Kriek et al. (2006) found that ~45% of massive galaxies at z~2.3 have evolved stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star-assisted Keck/AO. Considering that their median stellar mass is 1.7x10^11 Solar masses the galaxies are remarkably small, with a median effective radius of 0.9 kpc. Galaxies of similar mass in the nearby Universe have sizes of ~5 kpc and average stellar densities which are two orders of magnitude lower than the z~2.3 galaxies. These results extend earlier work at z~1.5 and confirm previous studies at z>2 which lacked spectroscopic redshifts and imaging of sufficient resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most ~10% of the population of K-selected quiescent galaxies at z~2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z~2.3, through dry mergers or other processes, consistent with predictions from hierarchical models.

Quote:

Also, the smallest galaxies are likely to merge with larger galaxies, and even a merger between two small galaxies will (obviously) reduce their number. Each of these mechanisms could plausibly alter the size – mass relation by a factor of 1.5–2, but not a factor of ∼ 6. This means that some combination of effects is required to bring the compact z ∼ 2.3 galaxies to the local relations— or that we have not yet identified the main mechanism.

http://www.sciencedaily.com/releases...0429095054.htm

parejkoj · #9 (**permalink**) 27-May-2008, 08:48 PM

Quote:

Originally Posted by William

that the stars should evolve and if the galaxy is assumed to be close box, the galaxy and its stars should gradually increase in metallicity.

No, they shouldn't. If there is no gas to produce new stars, there will be no new supernovae to add metallicity to the galaxy.

I'm still waiting for you to propose your grand plan that "fixes" all these "problems."

William · #10 (**permalink**) 28-May-2008, 04:13 AM

In reply to parejkoj’s comments:

Quote:

Originally Posted by William
that the stars should evolve and if the galaxy is assumed to be close box, the galaxy and its stars should gradually increase in metallicity.

Originally Posted by parejkoj:
No, they shouldn't. If there is no gas to produce new stars, there will be no new supernovae to add metallicity to the galaxy.

But there is at least in spiral galaxies, something that is creating pockets of gas. The following is from the Wikipedia article on H II regions. What is the source for the 1000's of pockets of gas in the spiral arms? Primordial unprocessed gas in the galaxy? That does not seem possible, based on the age of the galaxy and the time gas clouds take to cool and collapse.

New extragalactic gas that enters the galaxy? If it is new extragalactic gas, why wouldn't the new gas also enter the elliptical?

Why would the elliptical galaxies not have the gas clouds? Interesting, the finding of an unusual number of large Wolf Rayet stars in the gas clouds.

Quote:

An H II region is a cloud of glowing gas and plasma, sometimes several hundred light-years across, in which star formation is taking place. Young, hot, blue stars which have formed from the gas emit copious amounts of ultraviolet light, ionising the nebula surrounding them.

Quote:

H II regions are found only in spiral galaxies like our own and irregular galaxies. They are never seen in elliptical galaxies. In irregular galaxies, they may be found throughout the galaxy, but in spirals they are almost invariably found with the spiral arms. A large spiral galaxy may contain thousands of H II regions.

Quote:

Originally Posted by parejkoj:
I'm still waiting for you to propose your grand plan that "fixes" all these "problems."

Parejkoj, there does appear to be a solution. I am finding new data and new papers to be helpful, if your are interested in a solution.

This is a link to the news release concerning the finding of ultra compact massive galaxies. As the authors note, some mechanism is required to expand the ultra compact massive galaxy. There is a picture in the news release that compares the ultra compact massive galaxy to the Milky Way.

I believe this is the paper the news release is based on.

“Confirmation of the remarkable compactness of massive quiescent galaxies at z~2.3: early-type galaxies did not form in a simple monolithic collapse” by P. G. Van Dokkum , M. Franx, M. Kriek, & et al. (See above comment above for a link to the paper.)

http://www.sciencedaily.com/releases...0429095054.htm

Excerpt from the new release.

Quote:

"Seeing the compact sizes of these galaxies is a puzzle," said Pieter G. van Dokkum of Yale, who led the study. "No massive galaxy at this distance has ever been observed to be so compact, and it is not yet clear how one of these would build itself up to be the size of the galaxies we see today." The findings appeared in the April 10 issue of The Astrophysical Journal Letters.

Quote:

The galaxies, each only 5,000 light-years across, are a fraction of the size of today's "grownup" galaxies but contain approximately the same number of stars. Each could fit inside the central hub of the Milky Way. "These ultra-dense galaxies, forming the building blocks of today's largest galaxies, might comprise half of all galaxies of that mass at this early time," van Dokkum said.

Cougar · #11 (**permalink**) 28-May-2008, 04:32 PM

Quote:

Originally Posted by William

Cougar are you saying for large galaxies there is no stellar evolution? Are you saying there is no evolution of galaxies?

No.

Quote:

Originally Posted by William

To me a different mechanism is required, to explain the uniformity in metallicity and the lack of metallicity evolution.... "Evolution of high-redshift quasars" by Xiaohui Fan....

Fan doesn't seem to think so. He rather matter-of-factly says in the linked paper....

Summary
• Quasar Luminosity Function
– Strong evolution from z~3 to 6
– Luminosity-dependent density evolution: early growth of the most luminous
quasars
• Quasar spectral evolution
– Quasar environment matured very early, with rapid chemical enrichment
– 10^9-10 M_sun BH existed at z>6
– Dust properties might be different at z>6

Do we know that the "10^9-10 M_sun BH [that] existed at z>6" are not the near-direct remains of the (must-have-been-monstrous) Pop III "stars"?

William · #12 (**permalink**) 28-May-2008, 11:56 PM

In reply to Cougar's comments.

Galaxy's Evolution of Metallicity with Redshift

Quote:

Originally Posted by William View Post
Cougar are you saying for large galaxies there is no stellar evolution? Are you saying there is no evolution of galaxies?

Originally Posted by Cougar View Post: No.

The finding was that large galaxies show very little evolution of metallicity with redshift. Small galaxy show evolution of metallicity with redshift but then reach a metallicity plateau that matches the large galaxies. To me, that does not makes sense.

Quasar's evolution with Redshift

Quote:

Originally Posted by William View Post
To me a different mechanism is required, to explain the uniformity in metallicity and the lack of metallicity evolution.... "Evolution of high-redshift quasars" by Xiaohui Fan....

Originally Posted by Cougar View Post:

Fan doesn't seem to think so. He rather matter-of-factly says in the linked paper....

"Summary
• Quasar Luminosity Function
– Strong evolution from z~3 to 6
– Luminosity-dependent density evolution: early growth of the most luminous
quasars
• Quasar spectral evolution
– Quasar environment matured very early, with rapid chemical enrichment
– 109-10 M_sun BH existed at z>6
– Dust properties might be different at z>6"

Do we know that the "109-10 M_sun BH [that] existed at z>6" are not the near-direct remains of the (must-have-been-monstrous) Pop III "stars"?

There are two quasar metallicity evolution observations to explain:

1) Why for redshift z<6 do quasars show no evolution with redshift? We know galaxies show evolution with redshift (at least small galaxies.) Based on the super nova enrichment mechanism, galaxies including quasar host galaxies should increase in metallicity with time.

2) Why for redshift z>6 do quasars show a range of metallicity, solar 1 to 10? Note the quasars metallicity is not consistent. If the metallicity was produced by Population III stars, then there would be large clouds of have metallicity gas which are not observed. The population II stars are not high metallicity. Only a subset of the high redshifted quasars.

Fan's hypothesis for how very large black holes formed at high redshift is continuous accretion at the Eddington limit.

Cougar · #13 (**permalink**) 29-May-2008, 12:21 AM

Quote:

Originally Posted by William

Fan's hypothesis for how very large black holes formed at high redshift is continuous accretion at the Eddington limit.

I think you better modify your understanding here.

William · #14 (**permalink**) 30-May-2008, 04:15 AM

Quote:

Originally Posted by William View Post
Fan's hypothesis for how very large black holes formed at high redshift is continuous accretion at the Eddington limit.

Originally Posted by Cougar View Post
I think you better modify your understanding here.

The issue is it is almost not possible for such a large black hole to have formed, in the time allowed by the standard cosmological theory. Other authors have made the same comment concerning the finding of very large black holes in the early universe. Where it is assumed as per the standard theory that quasars (any redshift, including high redshift) cannot have an intrinsic redshift component.

The following is from Fan's paper Evolution of high-redshift quasars

Quote:

... The black hole mass estimates of the z ≈ 6 SDSS quasars ranging from several times 10^8 m⊙ to several times 10^9 m⊙. Assuming continuous Eddington accretion from a seed black hole of 100 m⊙, the formation redshift for seed black holes must be at z > 10. Even with continuous accretion, black holes in the most luminous quasars barely had enough time growing. While there are various ways of accreting faster than Eddington, the fact that the highest redshift quasars sit right at the threshold of the reionization epoch simply indicate that the initial growth of those BHs have to be very efficient and very early on....

Comment:
You did not comment on the finding of dense, ultra compact (1/20 of the size of current galaxies) high redshift galaxies (45% of the observed large galaxies were of that curious ultra compact form.), at z≈2.3. The paper I quoted is the second paper written on that subject. The authors found additional data and if I remember correctly added a different analysis technique to confirm their discovery. As the authors noted, some mechanism is required to expand the galaxy.

I thought the finding concerning HII gas clouds in spiral galaxies but not in elliptical galaxies was also interesting. I am currently looking at what is known theoretically and observationally concerning star formation.