Abell 2744 and it's Y Dropout Galaxies Shown Through Different Filters

I am sorry I have not been updating my blog lately, I have been caught up with other tasks. However, I have been able to save a image of galaxy cluster Abell 2744 including it's distant galaxies. The reason that the distant galaxies are named YD is because the galaxies are so distant and dim, that the wavelengths we receive are literally dropping out of the Y passband on the\electromagnetic spectrum. This is because since the Universe is expanding, it causes the wavelengths to stretch along with the "fabric" of space. This causes the wavelengths we receive from these galaxies to become redshifted.

Galaxy Cluster Abell 2744 in the F160W filter 

Galaxy Cluster Abell 2744 in the F814 filter

One of the coolest things is that these distant galaxies are so distant, that they only appear in the near-infrared wavelength filter, or the F160W. This filter only allows wavelengths at 1.4 to 1.7 Microns to pass through. These wavelengths are found within the near infrared section of the electromagnetic spectrum. F814W however, is a filter allowing wavelengths of .7 to 1.0 Microns to pass through. These wavelengths are found within the optical section of the electromagnetic spectrum.

These distant galaxies are known as Lyman-break galaxies, or LBGs. Lyman-break galaxies are star-forming galaxies at high redshift (Abell 2744 has LBGs at 7 and greater). These galaxies are chosen through a technique called the Lyman-break technique. The Lyman-break technique is used by observing a galaxy's Lyman limit. This represents the energy required to remove an electron completely from a hydrogen atom, starting from it's lowest energy level. Since galaxies consist of a lot of stars, and stars consist mostly of hydrogen gas, it would be appropriate to study and observe the hydrogen atom (simply because there is an abundance of hydrogen in the universe). Because of the photons emitted from the stars within the galaxies, these photons can ionize neutral hydrogen atoms and will be absorbed by interstellar and/or intergalactic clouds of hydrogen. Any photon more energetic than the Lyman limit (912 - 1215 Angstroms) is most likely to be absorbed quickly by a hydrogen atom, because it can completely eject the only electron. This is represented by the sharp break.

Lyman Breaks (straight vertical lines)  can be seen around 1.0 Microns 

A regular galaxy will have it's Lyman break at the 912-1215 angstroms. However, a galaxy such as YD9 with a red shift of 8.1 would have it's Lyman break red shifted all the way into 1.0 Microns rather than having it around 912 - 1215 Angstroms. The only way to observe these galaxies are through observing them through different filters. The Lyman-break can literally be seen through shifting through the F814W and the F160W. This is so, because the wavelengths are so redshifted, no photons can be observed through the regular optical filters. Filters that allow the near infrared will allow the redshifted wavelengths to pass through. 

(Above) YD1 can be barely seen through the F160W filter
(Below) YD1 cannot be seen through the F814W filter. This is so because the Lyman Break is so redshifted, that it is in the near infrared part of the electromagnetic spectrum

(Above) YD9 is clearly shown in the F160W filter, while it is barely observable in the F814W filter (Below)

My biggest question as of right now, is how do Astronomers simply "pick" these distant Galaxies? Do they just know that these are good candidates for LBGs? Or is there much more of a process to finding these LBGs?

More soon!