Date: Fri, 1 Aug 2008 16:44:56 -0500 (CDT) From: apj@ociw.edu To: sj@astro.as.utexas.edu Cc: ApJ-MS75213@mss.uchicago.edu, apj@ociw.edu Subject: Your ApJ Submission MS# 75213 Dr. Shardha Jogee Department of Astronomy University of Texas at Austin 1 University Station C1400 Austin, TX 78712--0259 USA Dear Dr. Jogee: Enclosed please find the referee's report on your submission to the ApJ entitled "History of Galaxy Interactions and Their Impact on Star Formation Over the Last 7 Gyr from GEMS" by Shardha Jogee, Sarah H. Miller, Kyle Penner, Rosalind E. Skelton, Christopher J. Conselice, Rachel S. Somerville, Eric F. Bell, Xian Zhong Zheng, Hans-Walter Rix, Aday R. Robaina, Fabio D. Barazza, Marco Barden, Andrea Borch, Steven V. W. Beckwith, John A. R.Caldwell, Chien Y. Peng, Catherine Heymans, Daniel H. McIntosh, Boris Haeußler , Knud Jahnke, Klaus Meisenheimer, Sebastian F. Sanchez, Lutz Wisotzki, Christian Wolf, and Casey Papovich ( MS# 75213). The report is largely positive, but the referee has included some suggestions for improvement. When you resubmit the manuscript, please include a detailed cover letter containing the listing of the changes you've made to the text and your responses to the report. Processing of your revised manuscript will be expedited if you make your revisions to the manuscript latex file available for downloading from the ApJ Web Peer Review System (http://mss.uchicago.edu/ApJ/). This version includes your previous submission plus any modifications to latex commands necessary for smooth processing by the ApJ electronic system. The associated PDF file is the version seen by the referee. The Astrophysical Journal has adopted a new policy that manuscript files become inactive, and are considered to have been withdrawn, six months after the most recent referee's report goes to the authors, provided a revised version has not been received by that time. If you have any questions, feel free to contact me. Best regards, Dr. John Mulchaey, Scientific Editor The Astrophysical Journal apj@ociw.edu ************************************************* This paper presents an observational estimate of the frequency of interacting galaxies in the redshift range 0.241/10, so involving 2 (or more) interacting galaxies/components. Reading the paper, and looking at the examples proposed in Fig. 2, I understand that in the majority of the cases the single galaxies/components are not resolved in the multiband photometry used to measure mass, or in l=2800 COMBO-17 band used to measure UV-SFR and in the 24micron photometry. Hence just one mass and one estimate of the SFR for the whole system are available. Is this correct? On the contrary, for how many interacting systems both colliding components are photometrically resolved and have distinct mass measurements (like case 1 in Fig. 2)? How these "resolved" cases are treated in Fig. 1, 12 and 17? Are the different components shown separately? I think that for homogeneity with "unresolved cases" in Fig. 1 they should be represented by a single point, with the mass being the sum of the two masses, the color being the some luminosity-weighted average of the two colors. How the "resolved" cases are treated when counting the interacting systems, deriving the galaxy interaction statistics? For example, I think that the 2 galaxies in panel 1 of figure 2 should count just as one interacting system. Moreover, how these cases are treated when studying the contribution of mergers on the star formation? In my opinion, the sum of the star formation of the two galaxies must be taken, to be consistent to cases in which the two interacting systems are not resolved, and one total star formation is available for the whole system. Do the authors agree on that? What about potential mergers in which the two components have two distinct mass measurements, both below the limit of 2.5x10^10 Msun, but for which M1+M2>2.5x10^10 Msun? They should be included in the sample, I think. Summarizing, it is not clear to me how photometrically "resolved" and "unresolved" cases are treated throughout the paper, and if in that respect the sample of merging candidates is homogeneous. More detail is needed in my opinion about the sed fitting procedure and stellar mass determination, that is a key point for the whole paper. How do the complexity, the distorted morphology and the multiplicity of these galaxies impact the multi-band photometry and finally the mass measurement? Are the star formation histories used for the fitting complex enough to reproduce multi-phase systems like interacting galaxies? In section 4.3 authors claim that on average each massive galaxy has undergone 0.7 mergers of mass ratio > 1/10 over the redshift interval z~0.24-0.8. Of these, 0.2 are major mergers. I think that these estimates can be easily used to quantify the contribution of major (an minor) mergers to the evolution of the mass function for massive galaxies between z=0.8 and z=0.24. This is an issue of tremendous interest today, and could be a good complement to the study of the impact of galaxy interactions on the average star formation history of the universe presented in Sect. 4.6. Finally, just a "style" annotation: many phrases are repeated, unchanged, at different positions of the paper. It could be a personal feeling, but I think that this does not facilitate the reading. Minor issues: Section 1. The introduction is not well balanced between the overview of the literature results and the outline of the paper. I suggest to move the discussions in points 4 and 5 before the outline, that possibly has to be shortened. Moreover, I suggest to expand the part presenting results about the evolution of the merging rate. In my opinion, it is interesting to know whether the merging fraction/rate evolves with redshift or is constant, but even more important is to quantify the impact of the merging events on the evolution of massive galaxies. In other words, not only the slope of the evolution is important, but even the zero point. Note that the cited papers do not agree on that: for example, Lin et al. 2004 say that just 9% of the massive ETG experienced a major merger since z=1.2; on the contrary, Bell et al. 2006 claim that ~50% of all galaxies with present-day masses M>5×10^10 Msun have undergone a major merger since z=0.8. Discuss briefly these differences. For the sake of completeness, add perhaps de Ravel et al. 2008, recently appeared on astro-ph. A small inaccuracy: Cassata et al. 2005 actually claimed a mild increase of the merger rate, even within large errors due to the small sample. Section 2: Is the initial sample containing 4740 selected down to R=24(Vega)? I can guess it is, but is not clearly said in Section 2. Section 3. section 3.2: I find a bit misleading that, introducing Int-1&2 cases, authors describe the effect of the merging event just on one of the 2 galaxies involved in the interaction, since the aim of the paper is to identify interacting systems and not single galaxies involved in a merger. I would rather say something like "a SYSTEM (not a galaxy) is assigned to class Int-1 if the interacting galaxies exhibit strong morphological distortion...". And "Int-2 systems involve two galaxies fairly symmetric, with overlapping envelopes of light, and masses satisfying the criterion M1/M2>1/10..." Do the authors agree on that? Section 3.3: Attention: case 6 in Fig. 2 is reported both as an example of ambiguous major/minor merger and as an example of major merger. Again, when describing major mergers, it is not clear to me why authors concentrate on one of the two galaxies interacting rather than on the whole system. Fig.2. What is the size of the postage stamps? I would put the size in arcsec in the caption (if it is the same for all the galaxies, as I guess) and the size in kpc in each panel, together with the redshift. This can help the reader to have an idea about the physical scale of the interaction. Case 1. The two galaxies shown constitute just one merging system, so they must weight as one system in the merging rate statistics. Is this correct? Section 4. Section 4.1 & 4.2: At the moment, authors first discuss fig. 4, then they pass to fig. 5, an finally they come back to fig.4. Moreover, the reader can understand that redshift dependent systematics concern only the visual classification, while in may opinion they affect also CAS measurements. I think that this can be avoided moving the part describing the redshift-dependent systematic effects at the beginning or at the end of section 4, perhaps in a separated subsection. Fig.5 Add at least the size in arcsec in the caption. Say which systems are classified as interacting. Section 4.4 Fig.10. I do not understand how the samples plotted in the figure are chosen. Why do the authors just plot Lotz et al. and Concelice et al. and not the other samples discussed in section 4.4? Section 4.6 & 4.7 Why do the authors here discuss just sample S2 and not even the massive one? Not enough statistics? Larger uncertainties on SFR? Not enough 24 um detections? What about using the 24-micron stacking technique? Authors state that the evolution of the cosmic SFR density over z~0.24-0.8 is predominantly shaped by non-interacting galaxies. Note that a similar result was already found by Lotz et al. 2008: cite it here and in the introduction. [ Part 2, "" Text/PLAIN (Name: "mass.txt") 32 lines. ] [ Unable to print this part. ]