List of Changes and Response to Referee Report

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1 List of Changes and Response to Referee Report Article ID: ApJ/348130/ART Title: Stacking in COSMOS: A Far-IR Characterization of 24 micron Selected Galaxies at 0 < z < 2.5 using Stacking at 70 microns and 160 microns in the COSMOS Field Authors: Dr Nicholas Lee et al Referee Comment: 92% of all COSMOS 24µm sources have photo-z s (this bring the sample down to from 38679). However, many of the especially highflux/high-redshift bins are still quite small. In particular the highest-flux bins are quite small at all redshifts. Please comment on the distribution in 24 µm fluxes of the sources without phot-z s. Author s Reply: From Figure 7 in LeFloch et al. 2009, the percentage of 24 µm sources with phot-zs ranges from 90% at low S24 to almost 100% at higher S24. Thus, the distribution in 24 µm flux of sources without phot-zs is concentrated in the low flux bins, and would not significantly increase the number of sources in the high-flux bins. A sentence regarding the flux distribution of the sources w/o phot-zs has been added to Section 2.3. Referee Comment: Similarly, you state that only 3% of the galaxies have 24 µm neighbours inside the 160 µm beam. Please comment on any potential trends with 24µm flux. I might expect the 24 µm-bright bins to have a larger fraction of near neighbours. This could have implications for the clustering properties of 24 µm bright sources. Some of the bins do not have that many sources in them, so even a few such neighbours might contribute significantly. Author s Reply: The fraction of galaxies with neighbors within the 50 beam size at 160 µm is actually fairly stable with 24µm flux. In fact, theres a slight, steady decrease in the fraction of sources with neighbors from 3.5% to 2.8% with increasing 24 µm flux (using the same bins as in the stacking analysis). A sentence has been added to Sect Referee Comment: You point out that a minority of the stacks are bad or no-detections. Figure 1 illustrates such a bad detection. Could you specify which 1

2 stacks show these bad detections is there a common trend with flux or redshift? My first guess would be that precisely near neighbours as discussed above are affecting the observed beam; however, the example shown in Figure 1 has 1025 sources inside the bin so that s unlikely. Author s Reply: The bad detections are mostly clustered in the low flux/high redshift regime, although there are a few that are low flux, intermediate/low redshift. It seems like the more important trend is that bins of low flux are more likely to have a bad detection. All of these bins, however, have very high numbers of sources (even the non-detections), so we dont expect that the 3% of nearby neighbors are the cause of this. The poor quality of the stacked images could be simply due to poor signal to noise for these faint objects, just like the non-detections. A couple of sentences have been added to S4.1. Referee Comment: In Figure 2, I find the trend of slightly increasing S70/S24 vs. S24 at the two highest redshift bins to be quite curious. My guess would be that the lowest-flux sources/lower luminosity also have the most prominent PAH features which additionally boost their 24µm. This might be affecting the S160/S24 colors of the two lowest-flux bins. Author s Reply: This very well may explain the trend, although I am hesitant to draw too many conclusions from these bins since the lowest flux/high redshift bins have the worst signal to noise and are mostly non-detections or bad detections. Referee Comment: Figure 3 The Dale models are a one-parameter (alpha) family of models. In the figure caption, please specify the range of alpha shown say top to bottom. Comment on the typical alpha values. Author s Reply: The models shown represent the full Dale library, covering 1 < α < 2.5. This has been added to the text and figure caption. Referee Comment: Figure 4f the highest flux panel seems poorly represented by the range of models used. This bin also has the warmest S70/S24 and S160/S24. The SEDs of the highest flux sources for z 1.4, all seem like they should have considerable AGN fractions (although probably still largely SF-dominated especially for the z 1.8 bin potentially due to the PAH-boost to the 24µm flux?). Author s Reply: The highest flux bin in Figure 4f indeed has the second poorest 2

3 fit out of all the bins. Indeed, many of the highest flux sources do have considerable AGN fractions (see next comment), but the SEDs themselves are not necessarily dominated by AGN. We take this as a source of error, but one that we cannot account for in this paper. Referee Comment: You mention that there are only 1000 X-ray AGN in the field. Please comment on which redshift/flux bins they populate? I would also suggest performing stacks of only the X-ray sources, but there are unlikely to be enough of them to sufficiently populate your bins. Author s Reply: The AGN are spread throughout all the redshift and flux bins, with the number in each bin going roughly with the number of sources in each bin. However, if we look at the fraction of X-ray sources, we see a clear trend of increasing X-ray fraction with increasing S24 (see Figure 1). This is a well documented trend (e.g. Brand et al. 2006, ApJ 644), and we even find evidence for it in our analysis of the 160/24 um colors (Sect 4.2). Although there is a high fraction of X-ray sources in the highest flux bin, it does not imply that they are all AGN (X-rays is also produced by star formation) or that the mid-ir and/or far-ir fluxes of these galaxies is dominated by the AGN itself (many X-ray sources are PAH dominated in IR), so we believe that fitting to models of star forming galaxies is still a reasonable approach. The goal of this section is to fit the 3 MIPS bands as well as we can to get the most reliable estimate of L IR independently of what powers the emission. The fact that our SED fitting shows a reasonable fit up to 160 µm supports this strategy. The small number statistics at the high flux end make a further analysis of AGN effects difficult, and beyond the scope of this paper. I did try stacking only X-ray sources, but your suspicions were correct that we could not derive any meaningful stacked results in most flux bins. I added a paragraph explaining most of this in Sect. 5.2, and added the figure showing XMM fraction. Referee Comment: In table 4, it might be useful to show the uncertainties as well (as determined from the range in Lir given by the various models). The error will clearly be very small for most bins ( 6% as given in the paper), but much worse for the highest redshift bins. Author s Reply: I have adjusted the table as requested. I have also given 3

4 a slightly more detailed account of our error analysis of LIR. Indeed, the highest redshift bins generally have the highest errors. Referee Comment: the second paragraph of S4.2 should say S70/S24 5 at z 2 Author s Reply: Fixed, thank you Referee Comment: about the uncertainty in the absolute calibration of MIPS160 observations, I would cite Stansberry et al Author s Reply: I have fixed this, and added the correct citation for the uncertainty in the MIPS70 observations. Additional Changes to Paper: Changed Figure 5 to show ratio of L IR,24µm /L IR,stack to reduce clutter and make it easier to interpret. Added a few grammatical fixes. 4

5 Fraction of X-Ray sources z=0.20 z=0.50 z=0.70 z=0.90 z=1.10 z=1.40 z=1.80 z= S 24 Figure 1: Fraction of X-ray detected sources in each bin. 5