Yuvan Sharma
01 Aug 2019: Scanning WISE Data for Brown Dwarfs and ULIRGs
01.08.20192 Min Read — In Coding

Introduction

As part of my ATDP 2019 Astronomy and Astrophysics Course, we used data from NASA’s WISE telescope to identify brown dwarfs and ULIRGs (Ultra Luminous Infrared Galaxies). This assignment involved using SalsaJ, a software to study stars in images taken by telescopes. Me and my coursemates had to utilise algorithms and this software effectively to obtain the required data and complete this assignment.

What are brown dwarfs and ULIRGs?

A brown dwarf is an astronomical object that didn’t have enough mass to become a proper star when it formed. Instead of the regular H-H nuclear fusion that occurs and releases energy in stars, brown dwarfs primarily have lithium and deuterium fusion since they don’t quite have the temperature and density required for regular nuclear fusion. Brown dwarfs are very cool and hence dim. So, searching for them is difficult, even though they are thought to be more abundant than stars. The WISE telescope searched for IR radiation to identify brown dwarfs since most of the light they emit is infrared.

A ULIRG is a galaxy which primarily emits infrared light. The primary source of this IR light is dust, which is observed wherever stars are forming. Thus, ULIRGs may be found in galaxies which are merging and leading to bursts of star formation, which leads to collection of dust. ULIRGs emit even more light than brown dwarfs at the lower IR wavelengths.

Approach

We started this assignment by downloading the required images from the WISE Image Service. We downloaded the same image in three different wavelength bands, w1 (3.4 µm), w2 (4.6 µm) and w3 (12 µm). We then used a finder chart to locate 12 different stars on all the 3 wavelengths. This was particularly difficult since some stars were harder to locate on specific wavelengths due to the change in their brightness and intensity as the wavelength changed. It was also very confusing in the beginning.

The WISE Image

The WISE Image

Using each star’s coordinates and its intensity, we calculated the ratio of the intensities for the different wavelengths. We used the ratios w3:w2 and w2:w1. I wrote a Java program for calculating these values. We then plotted the colour-colour diagram(a kind of graph) for the twelve stars. We identified the brown dwarf by studying the position the stars occupied on the colour-colour diagram.

Color Color Diagram

The Color Color Diagram

Then, we repeated the same for the ULIRGs and identified the ULIRG using another colour- colour diagram. However, we then realised that the object we thought was a ULIRG could just be an extreme case of a brown dwarf. So, we used images of the same region from the Hubble Space Telescope(HST) to look at the object, since HST images are much more detailed. Upon intensive examination, we noticed the faint trace of a spiral arm in the HST image, and this confirmed it was indeed an Ultra Luminous Infrared Galaxy.

Conclusion

This was one of my favourite assignments in this course since I applied Computer Science to Astronomy while learning new things in both fields. I feel this was also a very rewarding assignment. It took me a total of 6 hours to plot the stars, collect the data and make the colour-colour diagram. Then, as you plot the graph, it all becomes worth the wait. Confirming our hypothesis using HST images was a great way to end this unique assignment.