Update of Individual Sections

Modern Cosmological Observations and Problems

Chapter 2 Section 2.3.4 The Distance to M31:

Stanek and Garnavich have proposed using "red clump" stars as distance indicators. The red clump is a grouping of stars in the HR diagram at the "red" end of the horizontal branch. Such stars are numerous in our galaxy and hence calibration could come from both parallax as well as globular clusters. The Hipparchos sample shows a variation in the mean I-band magnitude of these stars of 0.10 -- 0.15 mag. The stars are sufficiently bright that HST can detect them as individual stars in clusters in M31. Preliminary results give a distance modulus to M31 of (m-M) = 24.47, about 0.15 mag larger than the mean distance found from several other methods detailed in the book.

Prior to the initiation of the CFA redshift survey, there was already a compelling data set that was able to provide a cursory outline of the Stickman and Great Wall structures. It was a serious oversight on my part, not to mention poor scholarship, for not initiating the discussion in 3.5.2 with the Gregory and Thompson (1978) paper which presented the first wide angle redshift survey, done in a SLICE style, of the A1367- Coma region. Their Figure 2, reproduced below,





clearly shows the presence of a void (later known as the CFA bubble) as well as a portion of the Great Wall. Thus, prior to the initiation of the CFA redshift survey, there already was a strong hint of the basic structure which would become more well defined with more redshifts. However, the work of Gregory and Thompson did show the power of sparse sampling coupled with clever survey design in revealing the qualitative features of Large Scale Structure in a hierarchically clustered Universe. In retrospect, historically this is an important paper and it was a serious omission on my part not to have included it in the published book.

The 2MASS project has produced its first important result. An initial analysis of 421 square degrees has detected 20 candidates for stars that are cooler than the coolest hydrogen burning stars on the main sequence. Subsequent spectroscopy with the Keck 10-m shows the candidates to all possess the Lithium doublet feature, indicating that they must be sub-hydrogen burning objects. This is the first successful detection of a new population of brown dwarf stars in the solar neighborhood. These stars define a new spectral type which will likely receive the designation "L".

At the moment, distances to these newly discovered brown dwarfs are not yet known with great reliability so their total contribution to the mass in the Solar Neighborhood can not yet be determined. However, the mere fact that they were detected in the one survey that was specifically tailored to find them, indicates that they are likely much more numerous than previously thought. This has significant implications for the sub-stellar mass function in the Galactic Disk.

Dr. Neil Reid has kindly provided the following: (words to come)

Chapter 5 Section 5.4.1

Clusters of Galaxies: The book discusses how the detection of strongly virialized clusters at relatively high redshift places severe constraints on various structure formation scenarios. With the launch of AXAF in December of 1998 we should be able to detect distant clusters of galaxies if they exist. However, currently there are already three interesting cases:
• Hattori etal Nature, 388, 146-148 (1997) have detected an object at z ~ 0.95 which appears to be an X-ray cluster of galaxies but without much optical emission from the individual galaxies. This may be an example of a "failed cluster" in which only the ICM forms inside the cluster potential and not the individual galaxies. This is an intriguing observation that needs follow-up and confirmation.

• Stanford etal 1997 report the detection, from an IR survey, of a z = 1.27 cluster. This is a prime candidate for AXAF observations to detect the associated X-ray emission, if there is any

• Tucker etal report the discovery of a kT = 17.4 keV cluster. This is extremely hot and indicates an extreme potential well depth. Such objects can only form from very high density peaks in the statistical density field and should be extremely rare. In some cosmological models, the space density of these objects would be zero. This is a very interesting discovery. Most models will have a hard time accommodating these objects, particularly if they exist at relatively high redshift.

• Donahue et al 1998 have discovered a 12.3 +/- 3 keV cluster at a redshift of z = 0.892. This discovery was made in the Einstein Medium Sensitivity Survey. Donahue etal argue that the proabability of discovering this massiv of clsuter in the EMSS is 10^-5 if Gaussian fluctuations in an Omega = 1 Universe made these structures. To quote Donahue etal:
  These observations, along with the fact that these luminosities and temperatures of the high-z clusters all agree with the low-z LX-TX relation, argue strongly that Omega 0 < 1. Otherwise, the initial perturbations must be non-Gaussian, if these clusters' temperatures do indeed reflect their gravitational potentials.

Chapter 6 Section 6.1.3

The UV Background:

Galaxies which emit a substantial amount of UV luminosity (for instance due to a high star formation rate), will show a sharp decrease or cutoff of that luminosity at a wavelength of 912 angstroms. This corresponds to the Lyman limit for the ionization of hydrogen (e.g. 13.6 eV). Photons with energies in excess of that will not reach a distant observer but will instead maintain the ionization of neutral hydrogen. This break in the distribution of UV emission is known as the Lyman break. For a galaxy at z = 3, this break would occur at 3648 Angstroms with respect to an observer at z = 0. Therefore if one did filter photometry and noticed that some galaxies "disappeared" when imaged in the ground-based U-filter, then such galaxies would be good candidates for Lyman break objects at high redshift.

Steidel etal, as documented in the book, have had excellent success using this technique to discover a significant population of star forming galaxies at redshifts z = 2.7 -- 3.4. Recently, Steidel etal (1999) have extended this technique to looking for B-band "dropouts" which would correspond to Lyman break galaxies between z = 3.5 -- 4.5. In detecting this population, Steidel etal can directly compare the UV luminosity densities in the redshift range z = 2.5 -- 3.5 with those in the range z = 3.5 -- 4.5 to search for signs of evolution.

An earlier study of the Hubble Deep Field (HDF) had suggested that the UV luminsoity density of the Universe peaked at z ~ 3. However, the HDF is just one field (of angular size 2.7x2.7 arc-minutes) and hence is subject to statistical fluctuations as well as just low numbers of objects. The Steidel etal (1999) study has sampled 828 square arc minutes (about 100 times larger than the HDF) and therefore it has greater statistical reliability. The principal result of the Steidel etal work is that the UV luminsoity density shows no sign of a downturn all the way out to z = 4.5 . While this result is completely at odds with respect to the results for the HDF, the Steidel etal sample is much better and is likely a better reflection of the intrinsic state of star forming galaxies at these redshifts. Furthermore, Steidel etal have good spectroscopy which helps to model the reddening in these systems. They find that a typical Lyman break galaxy has about 1.6 magnitudes of reddening at 1500 Angstroms. There is a significant variation around this mean. hence, the reddening of these galaxies is reasonably high indicating that there has been significant dust production, even by redshift 4, in these objects. Because the UV luminosity is so strongly effected by dust, its imperative that accurate corrections for this effect be done if one is to gauge the UV luminsoity density, due to star forming galaxies, at these redshifts.

Altogether, the results of Steidel etal 1999 strongly suggest that there may well be no peak in the star formation activity of galaxies as a function of redshift and that galaxies which have substantial amount of star formation may well be located at redshifts greater than z=4.5.

To quote Steidel etal:

It is not clear that the beginning of the epoch of star formation (in galaxies) has yet been identified

Gamma Ray Bursters:

In 1998 considerable work went into identifying the hosts of gamma ray bursters. In almost all cases, the properties of the host are consistent with a galaxy in the redshift range z: 1-3. The case of GRB 970228, studied by Fructher etal, is a good example. This GRB was observed by HST in the optical and infrared at times 6 months and one year after the original outburst. In both cases the optical/infrared counterpart was detected and it showed spatially extended emission. This "galaxy" is also quit high in surface brightness and has very blue colors. Taken in concert with other recent data, there is now a strong suggestion that GRBs preferentially occur in high star formation rate galaxies located at high redshift. The physics that is operative in such an environment that would favor the production of GRBs however remains elusive. Possible, the presence of GRBs in these kinds of galaxies is a statistical effect. That is, these hosts are engaged in the production of lots of massive stars and hence massive star remnants. This would increase the probability of neutron star-neutron star collisions compared to more modestly star forming galaxies located at these redshifts.

Section 6.2.2

Faint Blue Galaxies:

While this problem, as detailed in the book, remains unresolved, there is a considerably amount of activity in this field. If this book ever sees a second edition, it will be expanded to include more results and information on galaxy evolution. There are several groups world wide now engaged in this kind of research. One of the more productive groups involves astronomers in the US and Europe who have formed the "MORPHS" group. They are studying the detailed properties of galaxies in and out of the cluster environments at intermediate redshifts (z: 0.3 -- 1.0). In a recent paper (Poggianti etal) they have summarized their results obtained to date:

• They find that the cluster environment greatly suppresses the star formation activity of the members. This is accomplished through ram pressure stripping of the available fuel supply. In turn this requires a significant ICM density and such clusters are usually X-ray luminous. This means that selection effects may still be quite significant in constructing samples of cluster and non-cluster galaxies.

• They find that much of the "blue-excess" population in clusters is related to recent infall of actively star forming galaxies. They argue that this effect is more pronounced at z ~ 0.4 than at z =0 because the general star formation rate of galaxies is higher at these earlier terms.

• The SO population that dominates z =0 clusters likely has arisen due mostly to passive evolution and fading of the light from the disk components of spiral galaxies.

The above results are not necessarily striking (much of this was discussed a dozen years ago by Bothun and Dressler (1986) - hey, its my update!) - but the data set the MORPHS group has acquired is absolutely superb and as such removes much of the ambiguity of previous data sets. Still once must be concerned about selection effects in all of this and what might drive the infall around certain clusters at intermediate redshift compared to others.

Chapter 7 Section 7.3

The 2MASS Project

Shortly after this book was published, this survey commenced in both the Northern and Southern Hemisphere (The Northern Hemisphere survey started in June 1997; the Southern Hemisphere in March 1998).

2MASS is a digital sky survey at near-IR wavelengths. Three primary sources of interest are likely to be detected in this survey:

• Very cool stars and possibly brown dwarf candidates. In fact these have now been detected .
• Highly obscured active galactic nuclei, some of which could be at relatively high redshift.
• Possibly new members of the Local Group of galaxies that have remained undetected due to their low galactic latitude. This has important implications regarding the dynamics of the Local Group.



The Parkes 21-cm Multibeam Project

This new 21-cm survey has begun its initial phases of data acquisition. The goal of the survey is to perform a nearly complete survey at 21-cm of the nearby sky in hopes of providing an improved estimation of the neutral hydrogen gas mass function in galaxies. Part of this survey is comparable to the 2MASS survey in that 21-cm provides another means of detecting galaxies at low galactic latitude. Indeed some have already been detected.

Another possibility is that the Parkes survey will discover more examples like Malin 1 or other forms of gas-rich but optically low surface brightness galaxies as discussed in Chapter 6.

Potentially this survey can discover possible examples of dark galaxies, those in which the baryonic gas never was converted to stars. Come back later to this page to see if any such galaxies have been discovered!