Purpose Of This Page
As discussed on the page about Gérard de Vaucouleurs' Atlas of Galaxies
, de Vaucouleurs developed an extension of the Hubble classification of galaxy types involving a "three-dimensional" classification grid extending from ellipticals at one end to irregulars at the other. In 2015, Ron Buta (who maintains the website linked to images on my deVaucouleurs page) and a number of other specialists in the field used the Spitzer space telescope S4
G survey by Salo et al.
to develop a detailed extension of the original deVaucouleurs classification scheme, publishing a paper (A Classical Morphological Analysis of Galaxies in the Spitzer Survey of Stellar Structure in Galaxies
) discussing the extension and numerous examples of its use in detail, based on Salo's paper and Oulo's online database
for the 2352 galaxies in the Survey (there was an implication that other galaxies might eventually be added to the catalog, but six years have passed without any apparent changes). The page you are reading, like the "MAIN" page for that database, will include a list of all those galaxies, but instead of linking to their entry in the Oulo database, the links below are to their entries on this
website, which are in my NGC/IC catalog if they are NGC/IC objects, or in my PGC catalog if they are not NGC/IC objects (to maintain a structure similar to that of the Oulo database, their ESO/PGC/UGC entries will be listed as in that database; but all ESO and UGC entries will link to PGC entries on my site, with appropriate cross-references).
As should be obvious, there are nowhere near 2000+ entries on this page, so it will be quite a while before it is done; but all links posted so far lead somewhere (though for the individual entries shown below, only a basic placeholder for now: designations, magnitude, the infrared CVRHS classification, constellation and position, a link to their S4
G database entry, and a 12 arcmin wide image centered on the galaxy), and I will be adding at least a few more links and entries on a daily basis until the page is complete.
Advantages and Disadvantages of Infrared Classifications
The infrared images taken by the Spitzer space telescope have the advantage that unlike visible light, which is easily blocked by interstellar dust, infrared radiation mostly passes right through the dust, because its longer wavelength makes the scattering and absorption by microscopic dust particles much less likely to occur (this is the same thing that makes the sky blue; shorter wavelengths of light are more easily scattered by air molecules, and since those wavelengths look blue or blue-violet, the scattered light by which we see the "sky" is what we call "blue"). This means that particularly in the case of "edge-on" galaxies, where dust lanes can completely block our view of the central regions, infrared images can make it possible to study those regions in far better detail. However, some things that are easy to see in visible light do not show up as well in the infrared, so classifications of galaxies using only infrared light can be very different from what one would expect when viewing visible-light images of those same galaxies.
For that reason, although the CVRHS classifications in Buta et al.'s paper allow for far more detailed descriptions of what a galaxy looks like, the ones provided by the Spitzer Survey S4
G database have to be compared to visible-light photographs and classifications to give a complete and proper description of galaxy "types". In setting up this page and the thousands of entries that will be linked from it, the problems caused by the possible divergence of visible-light and infrared classifications will mostly be ignored at first, to allow for more timely completion of this part of the project. But particularly for the NGC/IC objects, and eventually for all the objects in this list, I will be using not only the Spitzer Survey classifications, but also classifications obtained from other references and, where adequate images are available, the images themselves, to try to provide a more accurate description not only of the internal structure revealed by the infrared images, but also the structure that seems appropriate when using visible light to study the galaxies.