Observations of Galaxy Formation and Evolution

My research interest is multiwavelength (optical, IR, millimeter, radio, and sometimes X-ray) observational studies of galaxy formation and evolution. The main focus has been observations of submillimeter galaxies (SMGs).

SMGs are galaxies actively producing stars. The young stars produce strong UV radiation. The UV photons heat up interstellar dust grains and the dust grains radiate away the heat in the far-IR and submm. A typical SMG forms stars at a rate of approximately a few hundreds to a few thousands of solar mass per year. (For comparison, our entire Milky Way has a star formation rate of only 4 solar mass per year). Such violent star formation is rare in the current universe but is very common in the past. Therefore, understanding the history of formation of stars in our universe requires the understanding of the SMGs.

Faint SMGs and Cosmic Downsizing

Traditionally, studies of SMGs are limited to the very bright ones (with star formation rates greater than several hundreds solar mass per year), because of the low sensitivity of current submm telescopes. However, the number of faint SMGs (star formation rates of roughly a hundred solar mass per year) is much larger than bright SMGs and faint SMGs are the ones responsible for producing most of the stars we see in the current universe. It is thus very important to study these faint SMGs.

With a newly developed statistical method, our group in Hawaii found that the weak submm emission from the faint SMGs can be studied. We found that unlike the bright SMGs, which mainly appear when the age of the universe was 2 to 3 billion years, faint SMGs appear at a much later cosmic time, roughly when the universe was 6 billion year old. Similar phenomena (bright objects tend to appear earlier in the universe and fainter objects appear later) have been observed at other wavelengths as well. This is often called "cosmic downsizing." Downsizing is well known in extragalactic observations but its reason is still unclear. Understanding why the universe behaves this way is an important topic in modern astrophysics.

SMGs in the Cosmic Dawn

The submm wavelength has a very interesting property. Normally, an object appears fainter when its distance is farther away from us. This is not the case in the submm. A very distant SMG and a nearby SMG would appear equally bright in the submm. In other words, seeing SMGs in the very distant universe is as easy as seeing nearby SMGs. Unfortunately, even if we see a very distant SMG in the submm, we usually do not realize the light we see comes from a very distant universe. This is because even if that distant SMG is bright in the submm, it would be extremely faint at all other wavelengths. Without seeing it at other wavelengths, it is very difficult for us to be sure about its distance. Because of this, for a long time, people had not been able to prove that there are SMGs in the very distant universe.

By combining the power of many large telescopes (Subaru, HST, Spitzer, SMA, and VLA), in 2007 we found an SMG that is at a distance of 30 billion light years, or at a cosmic age of 900 million years (7% of the age of the current universe). Our latest HST observations show that this galaxy was likely born when the universe was only 300 million year old. This is to date the most robust evidence for the existence of distant SMGs. The story about this galaxy is reported in an NRAO press release. Our surveys are still ongoing and we believe we will find more examples of distant SMGs.

A distant SMG. The name of the galaxy is GOODS 850-5. The images are, from left to right, from top to bottom: HST optical images; HST and Subaru near-IR images; Spitzer 3.6-8 um images; Spitzer 24 um image; SMA submm image; and VLA radio image.

 

Other Research

My other ongoing research includes:

 

Major Collaborators: L. L. Cowie (UH), A. J. Barger (UW-Madison), E. M. Hu (UH), P. Capak (Caltech), B.-C. Hsieh (ASIAA), J. Lim (ASIAA), N. Kanekar (NRAO), J. Wagg (NRAO), C. Carilli (NRAO), F. Owen (NRAO), and Y. Kakazu (IAP).

Recent Publications