From huchra@fang.harvard.edu Sun Oct 24 21:23:41 1999 Received: from cfa.harvard.edu (cfa.harvard.edu [131.142.10.1]) by abyss.uoregon.edu (8.9.1b+Sun/8.9.1) with ESMTP id VAA01728 for ; Sun, 24 Oct 1999 21:23:41 -0700 (PDT) Received: from fang.harvard.edu (fang [131.142.24.63]) by cfa.harvard.edu (8.9.2/8.9.2/cfunix M-S 0.1) with ESMTP id AAA01828; Mon, 25 Oct 1999 00:23:39 -0400 (EDT) Received: (from huchra@localhost) by fang.harvard.edu (8.9.2/8.9.2/cfunix S 0.5) id AAA03112; Mon, 25 Oct 1999 00:23:40 -0400 (EDT) Date: Mon, 25 Oct 1999 00:23:40 -0400 (EDT) From: John Huchra Message-Id: <199910250423.AAA03112@fang.harvard.edu> To: apj@as.arizona.edu, js@abyss.uoregon.edu Content-Length: 14000 Status: RO October 25, 1999 Prof. James M. Schombert Department of Physics University of Oregon, Eugene OR 97403 Dear Jim, Attached please find the referee's report on your manuscript with Rakos, "Cluster Populations in A115 and A2283." I'm sorry this took so long to get back to you, we had the standard problems of finding a referee then dealing with vacations, etc. The referee recommends publication but after substantial revision and perhaps the inclusion of the Abell 2218 data. You might consider that, but I don't beleive that it is absolutely necessary as long as the connection is made correctly. Please upload your revised manuscript to the ApJ site and send me (huchra@cfa.harvard.edu) a note describing the changes or replies you make in response to the referee's comments. Continue to refer to ApJ MS #50048. Thank you. Sincerely, John Huchra Scientific Editor ======================================================================= Referee's report on "Cluster Populations in A115 and A2283" by Rakos et al. To the Editor: This manuscript describes the photometric analysis of two moderate-redshift galaxy clusters studied using the innovative redshifted uvby photometric system pioneered by Prof. Rakos and his collaborators. The observations described in the article are potentially significant and may be of interest to studies of cluster galaxy evolution. The article is written in an appropriately concise style, accomplished through the use of suitable references to previously published work by these authors. Although the writing is generally clear, numerous grammatical errors are present in the manuscript (at higher frequency than is desirable from a manuscript featuring at least one native English speaker). More significantly, the present study suffers from two major problems which will require substantial revision to the manuscript: (1) the assumption that all the galaxies are cluster members is not justified, and in the case of Abell 115 is clearly false; (2) the methodology used to quantify the blue fraction of galaxies is flawed in several respects. The first and most glaring weakness of the paper is the (unstated) assumption that the two galaxy clusters Abell 115 and Abell 2283 are (a) bona fide clusters of galaxies, and (b) uncontaminated by the presence of fore- and background galaxies. However, a skeptical reader will be concerned that these are not "real" clusters but rather are chance superpositions of smaller groups. In the case of Abell 115, the skepticism is justified by two facts previously published in a highly relevant paper (Beers et al. 1983 ApJ 264 356) of which the present authors are possibly unaware: 1) The X-ray emission from Abell 115 is double rather than single-peaked, suggesting that the purported cluster may actually represent two or more superimposed groups. 2) Of 29 galaxies in the Abell 115 field with measured redshifts, 10 are found to be foreground galaxies projected onto the cluster. This provides conclusive evidence that foreground contamination is a problem in Abell 115, and may force the authors to reconsider their interpretation of Figure 6. As noted by the authors, the dynamical state of Abell 2283 is (apparently) virtually unknown, with only a single redshift measured (Struble & Rood 1991). However, this lack of data does not grant the authors license to assume that this cluster is bona fide and uncontaminated by foreground or background galaxies. In the interest of fairness, the authors must significantly revise their section 2.1 to: (a) discuss the substantial existing evidence that Abell 115 is at best highly contaminated and at worst a chance superposition rather than a cluster, and the apparent lack of any strong dynamical evidence for Abell 2283 being a bona fide cluster; (b) present their arguments as to why these clusters should be considered true galaxy clusters in spite of the evidence against, based on presence of cD galaxies, radio and/or X-ray maps, spatial distribution of galaxies, etc. If the available evidence does not establish that the objects Abell 115 and Abell 2283 are real and uncontaminated, then the manuscript must CLEARLY state, both at the outset and in the conclusion, that the authors' conclusions are based on the ASSUMPTION that the objects are bona fide clusters which lack contamination. A second major weakness of the paper is that the technique used to quantify the fractions of blue galaxies is flawed and certainly not equivalent to the methods of Butcher & Oemler, to whom the paper refers. In their series of important papers on evolution of cluster galaxies, Butcher & Oemler (1984 ApJ 285 426) defined the "blue" galaxies to be those which: 1) lie within the "core" of the cluster (inner 30% of the spatial distribution) 2) exceed the rest-frame absolute magnitude limit M_V=20 3) are at least 0.2 mag bluer in rest-frame color than a present-day E/S0 galaxy of equal absolute magnitude The blue fraction is thus defined as the ratio of galaxies which meet all three criteria to the number of galaxies meeting only the first two criteria, MINUS the fraction of blue galaxies found in suitable background samples measured using control fields in the vicinity of each cluster. This represents a reasonably well-defined quantity. Although the present manuscript also measures a quantity called "f_B", the meaning is, in fact, quite different: 1) no mention is made of having restricted the sample to those galaxies lying in the cluster code; 2) no mention is made of having restricted the sample to those galaxies exceeding M_V=-20; 3) no mention is made of having corrected the derived value of "f_B" for background galaxies measured in control fields. Given these significant differences between the criteria employed in the two studies, it is, at best, misleading to refer to the quantity measured in this study as "f_B". The fact that no background correction has apparently been made also makes the measured blue fraction in the present work virtually useless as a diagnostic of cluster populations. Ideally, the authors should revise their way of measuring f_B to coincide with the definition established by BO84. Otherwise, the manuscript should be revised to (1) abandon the use of the designation "f_B" for what is obviously not a blue fraction in the sense of BO84, (2) clearly state the important differences noted above between the blue fraction measured here and the blue fraction as defined by BO84, (3) measure and apply a background correction to the blue fraction. In addition to the two major items discussed above, several additional items require the attention of the authors. I discuss each section of the paper below. ------------------------------------------------------------------------ Section 1: ------------------------------------------------------------------------ Wording: The opening paragraph reviews of the prominent role played by rich cluster galaxies in the study of galaxy evolution, but suffers two problems. First, the opening line of the article overstates the importance of photometric studies. While it is fair to state that purely photometric investigations have been USEFUL in the study of cluster galaxy, most astronomers would consider morphological investigations completed with HST and fundamental plane work involving spectroscopy to have borne far more fruit. Please revise. References: Secondly, the paragraph alludes to several important areas of study but includes not a single citation to guide the interested reader to such work. And although the manuscript makes numerous references to the work of Butcher & Oemler, nowhere is any of their work cited. Please correct these omissions both here and elsewhere (e.g., discussion of tidal interaction in fourth paragraph). ------------------------------------------------------------------------ Section 2.1: ------------------------------------------------------------------------ Data: Given that the Abell 115 data are of questionable value due to uncertainties about contamination, the inclusion of more data at this point would benefit the study. So why not merge the Abell 2218 data into this paper, particularly since certain data for this cluster are employed in Figure 8? Unless the methodology employed in studying Abell 2218 differs significantly from the present work, it would be preferable to avoid publishing another paper with similar presentation and conclusions. Completeness: The manuscript states that "objects were selected based on detection in all four filters at the 3-sigma level." Although this method ensures the reality of objects, it also risks excluding objects of extreme color from the sample. After they have selected a limiting magnitude for their sample, the authors should determine and report the B-V colors at which they will begin to lose objects from the sample. I suspect that they will find the colors to be extreme, but they should be reported nonetheless in order to convince the skeptic that the detection criteria are not biasing the sample. ------------------------------------------------------------------------ Section 2.2 ------------------------------------------------------------------------ The good discussion of photometric classifications is appreciated. Classes: although the manuscript emphasizes that the classification scheme "is not morphological", it then confuses the issue by using the morphological designations "E/S0" and "Sp/Irr" to denote two of the four classes. Instead, the names of these classes should be changed to something which more accurately reflects their status as photometric classes (e.g., "passively evolving" and "normal star forming") as was previously done by Couch et al. (1994). ------------------------------------------------------------------------ Section 2.3: ------------------------------------------------------------------------ Redshifts: the authors will certainly want to revise this section to discuss the fate of the 10 galaxies known from their redshifts to be non-cluster members. Whether the remaining data will be reliable enough to prove useful must still be determined. External errors: The authors report "typical errors" of 0.02-0.08 mag, but do not state whether these estimates are "internal" or "external" errors. Given that the authors have previously studied Abell 115, they should (1) present a plot of the color difference vs. magnitude for all objects in common between the two studies in order to demonstrate to the reader that the two photometric measurements are consistent, and (2) make external estimates of the errors based on these duplicate objects. Reddening vector: the reddening vector referred to in the second paragraph should be shown on the relevant figures to aid the reader. Redshift vectors: given the likelihood of significant contamination of the galaxy sample by foreground and background objects, the two-color and C-M diagrams should indicate the effect on the color of changes in redshift. In other words, the plots should bear vectors indicating the magnitude and direction of shift in color and magnitude for a similar object at a redshift 0.05 less than the cluster redshift. This will indicate to the reader the likely position of contaminant galaxies. ------------------------------------------------------------------------ Section 2.4: ------------------------------------------------------------------------ Analysis: the second paragraph states that "many of those spirals lie blueward in bz-yz for their vz-yz colors". Please quantify this effect: what is the mean deviation in bz-yz and is the deviation statistically significant? ------------------------------------------------------------------------ Section 3.2 ------------------------------------------------------------------------ Analysis: The discussion of Figure 6 compares the slope of the C-M magnitude diagrams for various clusters on a purely qualitative basis. This does not provide any information on whether the disparities are statistically significant. Instead, the manuscript should include measurements of the slopes (plus uncertainty) and use this as a basis for interpreting the differences between the clusters. Suitable methods for measuring the slope are presented in Feigelson & Babu (1992 ApJ 397 55). Homogeneity: the authors claim that "the color-magnitude relation of the red population varies dramatically at intermediate redshifts" seems to fly in the face of certain other recent studies of cluster galaxies showing remarkable homogeneity of the red E/S0 populations out to z=1 and beyond (Schade et al. 1997, Ellis et al. 1997). The authors must (a) cite these other works and (b) attempt to explain the discrepancy. ------------------------------------------------------------------------ Section 3.3 ------------------------------------------------------------------------ C-M interpretation: As already explained, that the blue galaxies dominate the brightest and faintest ends of the luminosity function can also be interpreted as evidence for foreground and background contamination of the sample. Please consider this possiblity when contemplating the meaning of the data. ------------------------------------------------------------------------ Section 3.4 ------------------------------------------------------------------------ Undefined acronyms: In several places the manuscript uses acronyms without first defining them. Please explain the meaning of "SN", "RG", and particularly "WR" when these terms are introduced. I hesitate to critique the analysis section in greater detail at present since the authors may desire to revise it significantly in light of the major points described above. I thank both the editor and the authors for their patience in awaiting this report, and look forward to reviewing a revised copy of the manuscript.