{"action":"created","issue":{"url":"https://api.github.com/repos/LSSTDESC/DC2-production/issues/266","repository_url":"https://api.github.com/repos/LSSTDESC/DC2-production","labels_url":"https://api.github.com/repos/LSSTDESC/DC2-production/issues/266/labels{/name}","comments_url":"https://api.github.com/repos/LSSTDESC/DC2-production/issues/266/comments","events_url":"https://api.github.com/repos/LSSTDESC/DC2-production/issues/266/events","html_url":"https://github.com/LSSTDESC/DC2-production/issues/266","id":360844363,"node_id":"MDU6SXNzdWUzNjA4NDQzNjM=","number":266,"title":"Run 2.0i: galaxy saturation due to internal galaxy extinction","user":{"login":"rmandelb","id":1454906,"node_id":"MDQ6VXNlcjE0NTQ5MDY=","avatar_url":"https://avatars3.githubusercontent.com/u/1454906?v=4","gravatar_id":"","url":"https://api.github.com/users/rmandelb","html_url":"https://github.com/rmandelb","followers_url":"https://api.github.com/users/rmandelb/followers","following_url":"https://api.github.com/users/rmandelb/following{/other_user}","gists_url":"https://api.github.com/users/rmandelb/gists{/gist_id}","starred_url":"https://api.github.com/users/rmandelb/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/rmandelb/subscriptions","organizations_url":"https://api.github.com/users/rmandelb/orgs","repos_url":"https://api.github.com/users/rmandelb/repos","events_url":"https://api.github.com/users/rmandelb/events{/privacy}","received_events_url":"https://api.github.com/users/rmandelb/received_events","type":"User","site_admin":false},"labels":[{"id":845356970,"node_id":"MDU6TGFiZWw4NDUzNTY5NzA=","url":"https://api.github.com/repos/LSSTDESC/DC2-production/labels/Run%202.0","name":"Run 2.0","color":"d619c3","default":false},{"id":731041964,"node_id":"MDU6TGFiZWw3MzEwNDE5NjQ=","url":"https://api.github.com/repos/LSSTDESC/DC2-production/labels/validation","name":"validation","color":"b60205","default":false}],"state":"open","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":76,"created_at":"2018-09-17T12:25:07Z","updated_at":"2018-09-26T15:39:19Z","closed_at":null,"author_association":"CONTRIBUTOR","body":"This weekend some test images that were produced in preparation for Run 2.0i revealed some problems - see y-band image below:\r\n\r\n![v12365_r22_s10_y_knots1_gal2](https://user-images.githubusercontent.com/1454906/45622515-4a36f380-ba52-11e8-969a-d89377a8d117.png)\r\n\r\nThis issue is about the galaxy that I’ve circled here, which as you can see is saturated.  The magnitudes in the instance catalogs indicate that none of the galaxy components should saturate; there is no AGN or overlapping star either, and no issue with the knots model.  Eventually we traced this problem to a combination of two factors: \r\n\r\n1. this galaxy has Rv=0.49 (note that for Rv<1, the internal dust extinction law brightens the SED at some wavelengths)\r\n2. the galaxy has an extremely large value of Av=48, which **greatly** enhances effect (1).\r\n\r\nFurthermore, Scott @danielsf has found that with Rv<0.1, the internal dust law can correspond to brightening by a very substantial factor for some wavelengths, while for Rv in the range [0.1,1], it’s a problem only if Av is large and positive.  Av<0 also has the potential to become somewhat of a problem depending on the Rv value..\r\n\r\nUnfortunately, the range of Rv in the catalogs is quite broad, with the majority of objects in that [0.1,1] range and a non-negligible fraction below that; see this histogram of Rv for disks for one instance catalog:\r\n\r\n![image 1](https://user-images.githubusercontent.com/1454906/45622596-89fddb00-ba52-11e8-9dd0-ee5fb3b3568f.png)\r\n\r\nBased on some discussion with @egawiser and @danielsf , we agreed to make the following changes while post processing of the instance catalogs (using the same script used for issue #265):\r\n\r\n* Clip bulge and disk Av values at a minimum of zero (i.e., set the negative values of Av to 0).\r\n* Values of bulge or disk Rv below 0.1 get set to 0.1, while clipping Av to a maximum value of 1 for those galaxies (set Av values > 1 to 1 for galaxies in this Rv range).\r\n* For bulge and disk Rv values in the range [0.1,1], leave Rv as-is and clip Av to a maximum value of 1.\r\n\r\nWe do not modify galaxies with Av>0 and (Rv>1 OR (Rv in [0.1,1] and Av<1)).\r\n\r\nIn practice, about 20% of disks and 10% of bulges are modified by these cuts. For Av, most objects are near the cut values (e.g., negative Av values tend to be around -0.005).  For the problem case in the image above, the fact that our clipping process changes Av=48 to Av=1 will avoid this saturation issue.\r\n\r\nWhile this change clearly fixes some of the problems that can arise due to (Av, Rv) combinations that result in unphysical amplification of the galaxy fluxes, it was not possible on short notice to investigate the result in terms of color changes in the overall galaxy population.  A subset of the bulges/disks we modified are below the detection limit, but still, the impact on the observed galaxy colors more broadly should be checked.\r\n\r\nI believe @danielsf had some suggestions for more tests we could do today. I hope he will explain them here. Input from others is welcome!  I would be interested in hearing from @aphearin about the impact on observed galaxy colors as well.\r\n"},"comment":{"url":"https://api.github.com/repos/LSSTDESC/DC2-production/issues/comments/424763355","html_url":"https://github.com/LSSTDESC/DC2-production/issues/266#issuecomment-424763355","issue_url":"https://api.github.com/repos/LSSTDESC/DC2-production/issues/266","id":424763355,"node_id":"MDEyOklzc3VlQ29tbWVudDQyNDc2MzM1NQ==","user":{"login":"abensonca","id":7468651,"node_id":"MDQ6VXNlcjc0Njg2NTE=","avatar_url":"https://avatars0.githubusercontent.com/u/7468651?v=4","gravatar_id":"","url":"https://api.github.com/users/abensonca","html_url":"https://github.com/abensonca","followers_url":"https://api.github.com/users/abensonca/followers","following_url":"https://api.github.com/users/abensonca/following{/other_user}","gists_url":"https://api.github.com/users/abensonca/gists{/gist_id}","starred_url":"https://api.github.com/users/abensonca/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/abensonca/subscriptions","organizations_url":"https://api.github.com/users/abensonca/orgs","repos_url":"https://api.github.com/users/abensonca/repos","events_url":"https://api.github.com/users/abensonca/events{/privacy}","received_events_url":"https://api.github.com/users/abensonca/received_events","type":"User","site_admin":false},"created_at":"2018-09-26T15:39:19Z","updated_at":"2018-09-26T15:39:19Z","author_association":"NONE","body":"> @cwwalter is right that we should consider an option (4) in which cosmoDC2 is modified to provide something besides a tabulation of the restframe SED in 40 top-hat filters. This is not at all a necessary feature of Galacticus, as @abensonca described [here](https://github.com/LSSTDESC/imSim/issues/170#issuecomment-424466268), and there are a host of other choices we could make for what we provide. Chris' suggestion to use PCA is one, another would be finer binning and/or tabulation in the observer frame. The downside of (4) is it comes with a significant delay in turnaround time relative to the other solutions.\r\n\r\nOne variation on the idea of PCA would be to use the actual simple stellar population SEDs that are used to build Galacticus SEDs as a template set to fit the coarse-grained \"40 top-hat filters\" SED that we output from Galacticus. Then (like with PCA) just retain the N templates with the largest coefficients for each galaxy. The advantage of this approach is that a) the templates are physically-motivated, b) they're guaranteed to fit the coarse-grained SED (providing N is sufficiently large). "}}
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