June 2, 2016 Dr. Igor V Chilingarian Smithsonian Astrophysical Observatory 60 Garden Street MS09 Cambridge, MA 02138 Title: RCSED -- A Value-Added Reference Catalog of Spectral Energy Distributions of 800,299 Galaxies in 11 Ultraviolet, Optical, and Near-Infrared Bands: Morphologies, Colors, Ionized Gas and Stellar Populations Properties Dear Dr. Chilingarian, I have received the referee's report on your above submission to the AAS Journals, and appended it below. As you will see, the referee thinks that your article is interesting and that it will merit publication once you have addressed the issues raised in the report. When you resubmit the manuscript, please include a detailed cover letter containing the (mandatory) listing of the changes you've made to the text and your responses to the report. 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Sincerely, Chris Lintott University of Oxford Department of Physics cjl@astro.ox.ac.uk ---------------------------------------------------------------------- Referee Report Reviewer's Comments: This is a referee report for the paper "RCSED - A value-added reference catalog of spectral energy distributions of 800,299 galaxies in 11 ultraviolet, optical, and near-infrared bands: morphologies, colors, ionized gas and stellar populations properties" submitted for publication on ApJ by I. Chilingarian and colleagues. This paper presents a new value-added catalogue for the Sloan Digital Sky Survey which brings new gas-phase metallicity estimates for star-forming galaxies, provides new measurements for the stellar-population age and metallicity and - most important - adds an impressive suite of total and central broad-band measurements from the UV to the NIR domain to the existing SDSS optical measurements. This catalogue has also the advantage of being easily accessible and Virtual-Observatory complaint. All these aspects are going to be very useful to the astronomical community and thus make this paper certainly worth publishing. There are a number of issues with this catalogue and associated paper that need to be addressed before this can happen, however, and in particular as regards their non-parametric approach for dealing with nebular emission. These are described below in their order of appearance in the paper. Please accept my apologies for the delay in writing this initial report. - Abstract Towards the end of the abstract, the authors indicate that the paper will include a discuss existing and future scientific application of their catalogue. I could not find a section specifically dedicated to this, except for some sparse reference to past work by the authors in the second-last paragraphs of the introduction (where some reference are provided) and the summary (where none are). Please either explicitly add a section (perhaps before the summary) that delivers what promised in the abstract, or remove that sentence from it. - Introduction Here the authors must explicitly acknowledge the existence of the MPA-JHU and the Oh et al. (2011, also known as OSSY) value-added catalogues. Indeed, although based only on SDSS data only, these have provided for several years most of the spectroscopy measurements presented here, including the stellar and gas kinematics, emission-line fluxes (MPA-JHU, OSSY) and the gas-phase metallicity (MPA-JHU), in addition to other quantities not provided here, such as absorption-line strength corrected for emission-infill (OSSY) or stellar mass and star-formation rates (MPA-JHU). In fact, the statement "We aim to provide ... (iii) the first consistent analysis of absorption and emission line in SDSS spectra including parametric and non-parametric emission-line fitting" is not entirely correct in this respect, since both the MPA-JHU and OSSY catalogue analyse the stellar and emission-line spectrum in a similar way as done here, with the exception of the non-parametric emission-line fitting. Please amend. - Sec. 2.1 Please provide the number/fraction of objects with robust Galaxy Zoo classification in your catalogue and the redshift range of the latter, since I do not think that these would have covered all 800,000 of them. - Sec. 2.2.1 Regarding the mismatch between the GALEX and SDSS or UKIDSS resolution, I actually don't think that the corresponding central-magnitude offset for compact sources can be so easily dismissed. A simple calculation assuming typical seeing conditions suggests that for a compact Gaussian source with an intrinsic extension of just 1" the GALEX fluxes within a 3"-wide aperture would be around 75% of that collected with SDSS images, or 0.3 magnitudes. Although this may be comparable to the GALEX uncertainties in the NUV magnitudes, the effect is systematic, and would apply to many compact or distant objects. I suggest that the authors estimates the impact of such a small offset on their stellar ages, in particular for old objects. - Sec. 2.2.2 This section should be re-written to make it clearer, in particular the 2nd paragraph. The reasons for anticipating an offset between SDSS and UKIDSS should be anticipated at the beginning of the section. A figure could also help to better understand the process. - Sec. 2.3 The NBURST procedure adopted here would seem rather similar to the pPXF code of Cappellari & Emsellem (2004), so perhaps the authors may possibly want to start by stating any advantages of their procedure. The authors may also want to provide some motivation for adopting an exponentially declining star-formation history that assume an initial burst of star-formation nearly 13 Gyr ago. For instance they could quote here the work of Chiligarian & Zolotukhin (2012), and specify the limitation of this approach for objects dominated by young stellar populations. More important, it would be important to specify what entails a detection of nebular emission, which where excluded in a second iteration of NBURST. Do the authors stick to a threshold based on the measured line-peak amplitude to residual-noise level ratio? And if so, what is that threshold? I would presume a Gaussian emission-line profile would have been adopted in this first assessment. Similarly, please specify exactly what it is provided in the catalogue from the SSP approach. I would imagine these are luminosity-weighted ages and [Z/H] values. - Sec. 2.4 This is the most problematic section of this paper, for several reasons. First, this is not the first catalogue that carefully subtracts the stellar spectrum before measuring emission lines, and simply stating that this approach is superior to the standard SDSS pipeline is not sufficient. Second, if on one hand the non-parametric profile fitting could capture gas outflows and peculiar motions on the other hand it could lead to misleading results in object with central Type 1 AGNs. Indeed in the latter case it would make no sense to provide a single flux, velocity and velocity dispersion for recombination lines when these originates from two physically distinct emission-line region (i.e. narrow- and broad-line regions). In this respect, the choice of a Seyfert nuclei to illustrate the advantages of their non-parametric approach is most unfortunate, since this is exactly the kind of objects where the addition of a BLR could help just as well in improving the fit to the observed profile. In fact, authors should at least caution the users of their catalogue when using their non-parametric results for objects with central AGN. The recent paper by Oh et al. (2015) vividly illustrates how relatively common these objects are in the SDSS, and provides a catalogue of such objects. Third, even putting broad-line regions aside, the use of a more sophisticated non-parametric fitting of the emission-lines contrasts with the relatively cruder interactive approach whereby emission-lines are measured from the residuals of a stellar-continuum fit performed while masking regions affected by emission, which could include many age-sensitive features, in particular in heavily star-forming galaxies. Comparing this methodology to the approach used in the OSSY catalogue, whereby emission-lines are fitting with Gaussian profiles but where the nebular and stellar component of the spectrum are simultaneously matched, it would not be clear to a careful reader which catalogue would provide the most reliable emission-line measurements. As a sanity check, the authors should start by comparing their results based on Gaussian profiles with the ones provided by the OSSY catalogue, in particular for star-forming objects where the recovery of the underlying stellar continuum may suffer from the fact that most Balmer absorption lines would have been masked during the NBURST fits. In this respect, I note that the fraction of objects with Hb emission with a S/N > 3 (48%, with Gaussian fit) is nearly the same as the fraction of objects with Ha emission above 2.86 times this threshold, that is S/N > 8.6 (estimated to be 55% from Tab. 2), whereas in the presence of dust absorption one would expect that the Hb fluxes corresponding to these Ha emitting objects would be generally smaller than just Ha/2.86. Perhaps this means that in general there is little reddening across the whole population of emission-line galaxies, but it would be worth checking that the Hb fluxes are not systematically biased as an outcome of the NBURST fitting procedure. Fourth, coming back to the non-parametric fit, I suspect this would be prone to biases in the case of weak emission either in objects with noisy data or in the presence of substantial template-mismatch. How robust is the non-parametric fit in these cases? And what defines a detection? Test are needed to reassure the reader against these effects. For instance, the author could produce artificial spectra with Gaussian emission lines and see how well these profiles are recovered non-parametrically as the lines becomes progressively immersed in the noise level associated to the stellar continuum, for lines with different equivalent widths. Similarly, they should compare their Gaussian and non-parametric results for massive early-type galaxies, where the Gaussian fits suggest the presence of weak but yet detected emission (i.e. most of the objects with LINER-like emission). For instance, the authors could check for systematic shifts or broadening in the emission-line position (a tell-tale behaviour of emission-line fits when adjusting to template-mismatch) as a function of the mismatch between the data and models in the Na D (e.g. Fig 3) or Mgb region. This would assume that weak LINER-like emission has intrinsically Gaussian line profiles, which should be the case considering that this emission most-likely originates from diffuse ionised-gas emission and that gas in early-type galaxies is generally relaxed. Finally, but this is a minor quibble, in Fig. 4 you may want to use different colors in the central panel, for the stellar-continuum and nebular fits - Sec. 4.1 I was a bit puzzled by Fig. 8 here, and in particular by the absence of bright objects among the color-coded symbols in the four panels corresponding to z < 0.25. I can see why at higher redshift faint objects would be absent (either due to the SDSS magnitude cut but also to the difficulty in visually classifying distant objects), but I don't understand why bright objects are absent at lower redshift. Please clarify. - Sec. 4.2 This section is a quit confusing. The authors want to prove here when comparing the results from SSP or exp-SFH fits? The authors are trying blame the fragmented nature of the SSP results on the way the SSP models are built, but I frankly do not fully understand their arguments for this, in particular when they point to the - obviously - smoother results from the exp-SFH fits. To be honest, I think most likely the SSP fits are based on solutions that are non-unique, similar to the case of orbit-superposition models for the stellar kinematics of galaxies. Also, it is not clear what it is that is plotted in the top panel of Fig. 9. Does each point represent the luminosity-weighted age and metalliticy? Maybe here the author should instead convey the weight given to each of the template used in their NBURST fit. For instance instead of plotting one point per object, they could plot 100 points, e.g. for a NBURST fit returning 70% weight to one template and 30% to another, they could plot 70 times the point corresponding to the age and [Z/H] of the most important template, and 30 times the age and [Z/H] of the least important one. Maybe this would remove the "spotty" appearance? As regards the velocity-dispersion degeneracy, does the 15% systematic errors on sigma depend on the velocity dispersion itself? Does this hold also for large velocity dispersions? This part would be worth expanding and explaining better in my opinion. - Sec. 4.3.1 Given my previous comments, Fig. 13 should really be purged of those objects potentially containing a broad-line region. Care should also be take to plot here only objects where a non-parametric fit would make sense and would not be biased in the presence of weak lines in noisy spectra or by template mismatch. After this is taken care of, histograms would help in both axis to really appreciated the fraction of objects where a non-parametric fit really makes a difference in the Ha flux estimates. On the Chi^2 axis, it is more difficult to form a judgment without knowing the number of free involved in the non-parametric fit. I presume these correspond to the velocity sampling of the gas line-of-sight velocity distribution, so it's probably a considerable number (this should in fact be specified above). - Sec. 5 and catalogue Maybe it would be clearer to split table 4 in two tables, thus isolating the stellar-population fitting results. - Summary See my previous comments on the lack of a comprehensive discussion of present and future applications for this catalogue.