Download Seminar 2

Stockholm, October 23, 2007-10-25
Martin Lindman
Galaxies course Seminar 2, October 3, 2007
The topic of this seminar was the evolution of galaxies. Five different papers based on
observations of galaxies at different redshifts were discussed. Important discussion topics
were the changes in morphology and star formation rate of galaxies at different redshifts
and how these changes vary with the masses of the systems. We also discussed how these
findings fit with the different models (hierarchical or downsizing) of galaxy formation.
The most important points that were discussed are presented for each paper below.
Paper 1: Abraham, R. G. and van den Bergh, S
The morphological evolution of galaxies
The main point of this paper is that galaxies have different morphologies at different
redshifts. This indicates that they have gone through some sort of evolution in the past.
The redshift range discussed in detail in this paper is 0 < z < 1. At higher redshifts it
becomes very difficult to determine the morphology of galaxies. Observational results
indicate that the galaxies have only taken on their familiar appearance relatively recently.
The galaxy population starts to deviate significantly at z as low as 0.3 and at z ~1 the
morphology is so peculiar that about 30 % of the galaxies cannot be fitted into the
classical Hubble tuning fork system. Although the internal structure and morphology
show significant changes with redshift, the overall space density of galaxies is constant
up to z = 1, indicating that the number of galaxies seem to be constant in this redshift
range. The nature of the large fraction of peculiar galaxies at higher redshifts remains a
mystery, but it seems conceivable that in some of the distant peculiar objects we are
seeing the early stages of present day luminous galaxies where most of the baryons in the
galaxy are locked up in stars. This classic description of a protogalaxy would support the
hierarchical model stating that smaller structure form first.
A short summary of the key developments in galaxy morphology:
At z < 0.3 grand design spiral galaxies exists and the Hubble scheme applies in full detail.
At z ~ 0.5 barred spirals become rare and spiral arms are underdeveloped.
At z > 0.6 the fraction of mergers and peculiar galaxies increases rapidly. By z = 1
around 30 % of luminous galaxies are off the Hubble scheme.
Paper 2: Bell, E. F. et al.
Toward an understanding of the rapid decline of the cosmic star formation rate
This paper is based on observations of nearly 1500 galaxies in the redshift range 0.65< z
< 0.75. The authors use this sample to compare the star formation rate (SFR) of the past
Stockholm, October 23, 2007-10-25
Martin Lindman
to what we see in the local universe. There is an evidence of a significant decline in SFR
of intermediate and high mass galaxies from. At z ~ 7 about 40 % of the high mass and
intermediate mass galaxies are undergoing an intense star formation and in the local
universe less than 1 % of the galaxies have the same SFR. Since only a fraction of the
observed galaxies at 0.65 < z < 0.75 show signs of strong interactions the conclusion is
that a decline in major merger since this redshift cannot be the cause of the decline in the
SFR.
Since the galaxies in the sample have an undisturbed morphology the cause of the decline
in SFR must be changes in the physical properties that do not affect the morphology of
the galaxy. Examples of such explanations are gas consumption and weak interactions
with smaller galaxies.
The contribution of Active Galactic Nuclei was also discussed but more work is needed
in this area. It seems however that the dust heating by AGNi is not the explanation of the
decline in SFR.
Paper3: Juneau, S. et al.
Cosmic star formation history and its dependence on galaxy stellar mass
This paper examines hoe the cosmic star formation rate (SFR) depends on the mass of a
galaxy in the redshift range 0.8 < z < 2. The SFR density of the most massive galaxies
show a significant decline and is about 6 times less today than it is at z = 2. The SFR in
the intermediate mass galaxies however decline more slowly and the low mass systems
show an intense star formation throughout the redshift range of this study. The conclusion
from these observational results is that the era of star formation was extended and
proceeds from high mass systems to lower mass systems. This indicates that the higher
mass systems formed first and the smaller systems later and would thus support the
downsizing theory of galaxy formation. This would contradict the hierarchical model. For
the hierarchical model to be true the star formation must have been more efficient in the
past in the massive galaxies. It could be that different processes dominate at different
redshifts.
Paper 4: Labbé, I. et al.
IRAC mid-infrared imaging of the Hubble deep field-south: Star formation histories
and stellar masses of red galaxies at z > 2
This paper study distant red galaxies (DRGs) at z > 2 and compare them with Lyman
break galaxies (LBGs) at 2 < z < 3. The DRGs can be divided into two groups; galaxies
that are red due to dust reddening (about 70 %) or old and “dead” galaxies (about 30 %).
One important conclusion from the observations is that it is impossible to obtain massselected samples photometrically. The mass to light ratio of the DRGs and LBGs in the
Stockholm, October 23, 2007-10-25
Martin Lindman
sample varies by a factor of 6. These variations could possibly be explained by a relation
between total stellar mass and mass to light ratio and thus the differences in stellar mass
would give these variations.
Another important conclusion is that at high redshifts the red and dead galaxies are the
more massive galaxies. This supports the idea that the more massive systems are the
oldest and have the highest mass to light ratios.
Paper 5: Bouwens, R. & Illingworth G.
Rapid evolution of the most luminous galaxies during the first 900 million years.
This paper presents results from a search for very high redshift galaxies at z ~ 7 – 8. In
addition to these results the authors use simulations to find the expected number of
galaxies at these redshifts and compare with the observational results. In the observations
only one candidate galaxy was found at this high redshift under conservative selection
criteria. The expected number of galaxies under these criteria is ten. Using less
conservative criteria they found four candidates where 17 would be expected. Both of
these simulations were made under the assumption that there was no galaxy evolution.
These results show that the most luminous galaxies are very rare at z ~7 compared to z ~
6.
Other papers conclude that the high mass galaxies have higher star formation rates at
higher redshifts. If the SFR is even higher at z ~7 than at z ~6 we would expect to see
more galaxies at higher redshifts than would be the case if there was no evolution
between these redshifts. The fact that less galaxies than expected are observes indicates
that we have an upper limit on the SFR peak.
The simplest explanation for the lack of luminous galaxies at z ~7 is that these systems
simply have not yet had the time to form. This is in support of the hierarchical model of
galaxy evolution.