Tools and methods in A.R.I.

Now that I have made some posts about individual discordant redshift systems, I thought it might be good to explain some things related to the contents of those posts. So here goes:


My DRS posts usually contain following sections:

Main article section, where I introduce the previously published results and arguments on the system in question. This involves me searching the papers, reading them (see below for the instructions on how to find and access the papers) and trying to extract the relevant things from them. Here you have to remember that I’m a layman in astronomy, so sometimes I might not understand the papers fully and sometimes I might even get them wrong. In this section I give references so that there’s the name(s) of the author(s) and the publication year of the paper, like this for example: Author & Astronomer (2009). The paper (and links to it) of the given reference can then be found from the reference section, which is sorted by the author name (see below).

Notes section, where I point out some things I have found out of the system, or other things needed to said. Sometimes I make some simple calculations here. Later I’ll try to make a post describing the usual calculation methods I use (which are just basic astronomy calculations), in addition to the redshift component calculations which I already described.

The image of the system, where I have marked the locations of the most interesting objects. Marked objects usually are all objects with available redshifts within the pictured field, but sometimes I might mark some other interesting objects as well (especially if they have been mentioned in the discussions of above mentioned sections). Images I fetch from DSS website, see below. When I have fetched the image, I take it to an image processing software and invert the colors (just so that there wouldn’t be large black areas if someone wishes to print the article), bring out the faint features by adjusting first contrast (I usually adjust contrast to the max) and then brightness (usually to such level that objects are clearly visible but there’s not much noise showing). Next I get the object list from NED (more of this below), and object by object locate them in the DSS image, and then I mark them using same numbers as in object list. Locating can sometimes be difficult. I start with the finder image NED offers for each object. If that doesn’t give me enough clues where the object is in the DSS image, I might look at the coordinates, or if the object is within SDSS coverage then NED has link to SDSS SkyServer page of the object, and it’s quite easy to locate object in SkyServer because it has zoom out feature in the image tools (when you start with a closeup image, you need to zoom out in order to get a big picture of where the object lies).

Object data section, where I give a table of discussed objects, and some of their parameters. For this I use NED (see below for instructions of usage). For each object I give their name, type, redshift (sometimes I additionally give the redshift velocity, cz, in parentheses), apparent magnitude (sometimes with letter indicating the spectral band for the given magnitude), and separation (in minutes of an arc, 60 arcminutes is one degree). Additionally, I usually give some links relating to the objects (NED’s near name search results, SDSS images, etc.).

Reference section, where I give a list of papers I have referenced in the article. Almost always papers are linked to their ADS abstract page (where you usually can also download the paper for free, see below). For each paper, I give the author(s), publication year, journal reference, and the name of the paper.

Main tools and resources

The SAO/NASA Astrophysics Data System (ADS). I use this resource to find the papers relating to the systems I’m discussing. This query page is the one that I have in my bookmarks, and where I have spent many happy hours. However, for my DRS posts, I usually start in NED, which gives reference lists for each objects, and those references then are linked to ADS abstract pages of individual papers. But that search page is really helpful generally. You can search papers by author, by celestial object, by title, by keywords… Anyway, when a paper has been found, the abstract page in ADS contains several interesting things. First is of course the fact that almost all papers are available in some format for anybody to read it for free. For older papers, you need to click the “Full Refereed Journal Article (PDF/Postscript)” link to access the paper. For newer papers (usually 3 years old, but it varies by journal) you need to click “arXiv e-print” link, which directs you to a resource called arXiv (see below), where you the can access the paper. Unfortunately, some papers don’t have any means of access for free. ADS abstract page also has links to “References in the article” and “Citations to the Article”. These are very good for finding out what other papers have discussed the subject you are interested in. Note also that ADS abstract page also offers you a possibility to access NED or SIMBAD objects discussed in the paper in question.

Digitized Sky Survey (DSS). All my images are from this resource because it covers the whole sky, so you can have an image of anything (that is permanently) on the sky, and even the quality is sufficient. Just insert the object name (“NGC 7603” for example) and click “GET COORDINATES”. Then select image height and width. I generally use 5 x 5, 7 x 7, 10 x 10, or 15 x 15 arcmin. Sometimes I need even bigger images, especially if the target galaxy is one of the biggest ones. Remember also to change the image file type to GIF, if you don’t want to use the FITS files that are offered as the default option.

NASA Extragalactic Database (NED). I use the “near name” search so that I insert the name of the main galaxy, select the search radius as 10 arcmin, and choose the “selection in redshift” as “available”. This will return a list of all the objects that have available redshifts within 10 arcmin from the main galaxy. The list contains links to the pages of individual objects (click the number in “Row no.” column). Each object has a page that contains the data, images, and links for it. Look around there, there are quite a lot of interesting stuff hidden behind all those given links (list of papers referencing the object, list of redshift measurements, notes,…). And NED also has plenty of other interesting features than just the near name search, so look around.

– Image processing software. I use whatever I have in my PC for contrast and brightness adjustments, and for object marking I use simply the Windows Paint.

Other useful resources

I won’t give detailed instructions for the usage of these, but if someone needs some help on any of these, just ask.

arXiv. A preprint server, which holds PDF’s of many new papers that aren’t yet freely accessible otherwise. I go here almost every day and click the “new” link in the “Astrophysics” row.
Google Scholar. Very good place to find scientific papers on just about anything.

HyperLeda. Provides data on extragalactic objects similar to NED, byt HyperLeda is (mostly) limited to galaxies only. Gives better estimates for parameters than NED (NED only selects one measurement and gives that, but HyperLeda gives a composite of many measurements, so sometimes NED can be far off from the real value but HyperLeda usually is at least quite close). Excellent features in HyperLeda are the “SQL search” and “Define a sample”. In latter case you can upload your own list of objects and select which parameters you want to be fetched for them. HyperLeda then returns a table containing the parameters for your objects. Very handy and easy to use feature which I have used quite a lot.
SIMBAD. Another place that provides the data for objects. I haven’t used this as much as HyperLeda and NED, but SIMBAD also has some excellent features related to mapping of the fields near objects. SIMBAD links to Aladin which is a great resource for making celestial maps, and you can even make some rough measurements there (of angular distance or position angle for example). Aladin takes a bit learning, but I think it’s worth investing a bit of time on that.
VizieR. A resource for browsing and downloading whole catalogs of objects. Sometimes very useful.
SDSS SkyServer. Another place for object data and images, but is limited only to SDSS objects and data, but as SDSS has quite a large coverage, it is not very badly limiting factor except if you are looking for a certain object that happens to be outside of SDSS coverage. Lot of nice tools here.

– Almost all above mentioned data resources have some images to offer on the systems.
IRSA. This place has some infrared data also, but I have used it for getting DSS images (click the “Finder Charts” link), because IRSA offers some handy options for the fetched images (brightness enhancements for example). Use “reproject” feature for better outcome. It is very slow though.
CADC. Another place I have used for DSS image fetching. Has good batch job features. Also getting the coordinate markings at the side of the images is a nice feature, it makes finding the specific objects much easier.

Well, that’s it I think, but I’m sure I forgot something.

One Response

  1. interesting blog… I hope you keep it running 😉

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