This past Friday, recent graduate of the Columbia Astronomy doctoral program, Stephanie Tonnesen, gave a talk entitled, "Hubble Deep Field: Looking back in Time." In her talk, Stephanie gave us a detailed background on the nature of the Hubble Deep (HDF) and Ultra Deep fields (UHDF). She also gave us a sense of just how difficult an exposure of 150+ hours is in low Earth orbit due to the glare of the Sun. However, the biggest problem in terms of getting a great picture of the past was finding a patch of sky where few Milky Way stars existed. Too many foreground stars would actually outshine the very distant, and hence, faint galaxies that were the precise interest of that survey.

After showing us great pictures of these galaxies, Stephanie went on to explain the why astronomers are so interested in objects whose features we can barely make out. One main question addressed in the HDF & UHDF pictures is, "How do galaxies form?" Stephanie pointed that there are two main theories for galaxy formation: monolithic collapse and hierarchical merging. In monolith collaspe, different- sized gas clouds collapse due to self-gravity and form tight clumps of stars and/or stellar disks that constitute galaxies. In hierarchical merging smaller galaxies merger together to form larger galaxies that then feed off of remaining smaller satellite galaxies to grow. Occassionally these galaxies would run into another massive galaxy to form elliptical galaxies.

One observational fact that Stephanie pointed out that may favor monolithic collapse (at least in the early universe) is that the light Hubble observes in the visible here was redshifted from the ultraviolet (UV) light emitted from the HDF/UHDF galaxies. She showed us a UV picture of a nearby galaxy and we found that the picture looks clumpy due to the clusters of young star formation in the galaxy. Since massive blue stars in young star clusters emit predominately in the UV, could it be that the "blue dots" that we originally claimed to be merging proto-galaxies are actually starbursts in a single galaxy?

The audience asked many questions about galaxy formation and about the prospect of sorting the "blue dots" issue out. Stephanie explained that telescopes like James Webb and other projects will target visible light from these galaxies in the infrared (IR) to possibly solve this issue. This issue also highlighted why astronomers try to view objects by collecting all wavelengths from the electromagnetic spectrum. Once again, our speaker fielded a myriad of good questions from our audience and we ended our Q&A session only in the interest of observing time.

Thanks to the 7 volunteers and the 85+ people who attended our lecture and observing night!