RAD-RGB images are false-colored images of astronomical objects.
The astronomical study involves a multi-wavelength analysis of the object from the radio part of the electromagnetic spectrum to X-rays. Astronomers access these data from various archival data services which are sky survey data on a particular wavelength taken by a particular telescope. No telescope can observe in all bands of electromagnetic spectrum. Different telescope images have different sensitivity, resolution (angular/frequency) and many other parameters. Each RGB image is a combination of these imaging survey data in Red, Green, and Blue color channels, with their respective intensities at each pixel, resulting in a false-colored image. Conventionally in RAD@home, the surveys are selected so that the order of wavelength of each data and colour of the channels are in same order i.e survey data in the red channel has a wavelength higher than that in green and green has a wavelength greater than that in blue. This is followed in optical- and IOU-RGB images, and ROR-RGB image has an exception allowed for green channel only.
The two widely used RAD-RGB images in our analysis are:
RAD-RGB Maker Tool is a very simple to use tool. All you need is the name of the astronomical object of interest or its coordinates, the angular radius of the image, and what sort of images you would like to make.
Here is a brief description of each label provided in the form:
Your name will be displayed on the image enabling mentors, professors, fellow students to be able to recognize your work. Credit is important!
The object name or the coordinates of the object in the FK5 (J2000) system. Ex: "14 09 48.86 -03 02 32.6", M87, NGC1243, without quotes.
The size of the image in degrees, this size will be used for the field of view in the resultant image. For reference, in the night sky, the moon is about 0.52 degrees across. Default value is 0.12 degree.
This dropdown gives you a choice of the composite images you want to create.
This option returns four images. Read the top header of each RGB-contour image carefully.
There are two ROR (Radio ( GMRT TGSS ADR1) - Optical (DSS2Red) - Radio ( VLA NVSS)) images. One with TGSS Contours and another with NVSS Contours.
The third image is an IOU (Infrared (WISE 22) - Optical (DSS2 Red) - Ultraviolet (Galex Near UV)) with TGSS Contours.
The final RGB image is an optical image with (DSS2IR - DSS2Red - DSS2Blue) with TGGSS Contours.
You are ready to make your RAD-RGB images! Just press the button, give it a few seconds, and you will have taken your first step in the RGB image analysis. The next and most important step in the analysis. All the best for that!
Once you are good at this analysis (through #DailyGalaxyRGBC in our social media group or #NIUgalaxyRGBCanalysis through a RAD@home Astronomy Workshop Live) you can also try advanced levels of analysis as follows.
This option returns two images.
The first (left panel) is a different kind of ROR Image with TGSS and NVSS source components labelled on an optical (DSS2 R) grey-scale image. The source components are labelled with numbers and have one to one mapping with the details on these sources provided here from the respective survey catalog . Scroll down, click Catalog Table, to see the flux density of source components.
The second image (right panel) is yet another kind of ROR composite image with DSS2Red in greyscale and contours of various radio surveys like TGSS, NVSS, and VLA FIRST (if available), in Magenta, Cyan and Yellow, respectively. Note that each of these radio images has different frequency, sensitivity, angular resolution and angular scale sensitivity (peculiar to radio interferometry images). At the bottom of these images the flux density levels of the contours for each survey has been noted in the unit of Jansky per beam. Beam is similar to the angular resolving power of a particular radio survey imaging.
Spectral Index: Left hand panel has yet another number in black font on a white box which is the spectral index of the source components identified in TGSS/NVSS. The spectral index values for point sources will be correct and for extended sources could be subject to many technical issues.
This dropdown currently offers access to the NVAS image archive (WARNING). Selecting this option will return the top 5 results from NVAS (if exists). These can be downloaded as .imfits files (open with DS9) by using save as an option when right-clicked. Clicking that NVAS image link will lead to opening a large-size binary file on your web-browser, incapable of displaying the image. You may require to rename the file with a suffix .fits to open it in SAO ds9 FITS viewer.
contour maps are lines which joins the pixels with same pixel-value of a given image or lines of equal intensity.
Radio survey data contains a range of values as spectral flux density (unit of the pixel value in radio images is Jansky per beam). These values contain signal or real bright sources in the sky along with background noise data or the empty part. Each sky survey provides a RMS (Root Mean Squared or 1-sigma) value or an accepted signal (3-5 sigma) value for the sky region. Three times the RMS value or a region above 3-sigma is usually taken as detection of the radio source signal and can be studied further. The contour map helps represent a 4th image along with three channels Red-Green-Blue (RGB) image. Its absolutely essential for radio images and if a source is not seen with contours with 3 sigma or above, it is considered unreliable.
Table with details of a few radio surveys :
| Frequency | Survey | 3 sigma Detection | Resolution |
|---|---|---|---|
| - | - | Jy/beam | (arcsec) |
| 1.4 GHz | VLA FIRST | 0.0005 Jy | 5 x 5 |
| 1.4 GHz | VLA NVSS | 0.0015 Jy | 45 x 45 |
| 150 MHz | TGSS ADR1 | 0.015 Jy | 25 x 25 |
| 74 MHz | VLSSr | 0.3 Jy | 75 x 75 |
| 325 MHz | WENSS | 0.018 Jy | 54 x 54 |
| 843 MHz | SUMSS | 0.010 Jy | 45 x 45 |
Please check the RAD-RGB acknowledgment at the bottom of the page for further details : RAD-RGB Maker Tool Acknowledgement