Philosophical Society of Washington

Minutes of the 2300th Meeting

President John Ingersoll called the 2,300th meeting to order at 8:?? pm April 20, 2012 in the Powell Auditorium of the Cosmos Club. Mr. Ingersoll announced the order of business and introduced five new members of the Society, including the speaker of the evening.

The minutes of the 2,299th meeting were read by Corresponding Secretary Robin Taylor and approved.

Mr. Ingersoll then introduced the speaker, Mr. Massimo Stiavelli of the Space Telescope Science Institute. Mr. Stiavelli spoke on the "Cosmic Dawn: The First Stars and Galaxies."

Mr. Stiavelli began by explaining that the development of galaxies and the Universe is analogous to the development of a human being in that the rate of change is fastest during the first ten percent of its lifetime. The early Universe, about 300,000 years after the Big Bang, was generally homogeneous and isotropic, but contained tiny variations in density and temperature. The particles in question are hydrogen, protons, electrons, and molecular hydrogen, but molecular hydrogen was the most common molecule in this early time.

As the Universe cooled, it was not able to remain ionized since protons and electrons recombine to form neutral Hydrogen. Modeling this process shows a very small fraction of the Universe remain ionized, which is very important. Mr. Stiavelli explained that the Hubble Space Telescope (HST) can observe light from the distant Universe and we've determined that most hydrogen is ionized by observing and comparing the spectra of nearby and distant quasars. If Hydrogen was originally neutral, some process must have reionized the Hydrogen, Mr. Stiavelli said.

Mr. Stiavelli then explained how do the first objects in the Universe formed from the small but significant density perturbations. The overdense perturbations collapsed due to gravity, growing and becoming filamentary, as underdense areas become great voids. As an overdense perturbation collapses, it reaches an equilibrium point where its density is about 178 times the average density of the Universe at the time and it has acquired some kinetic energy to balance the gravitational attraction. However, the Sun is thirty orders of magnitude denser than the Universe so a perturbation 178 times denser forming at redshift 30, for example, would be only five million times denser today. Something else must bridge the twenty four order of magnitude gap between the initial collapse and the density of the Sun, Mr. Stiavelli said.

The additional density occurs due to cooling due to collisions, excitation, and photo emission. However, Hydrogen is inefficient at cooling objects below 10,000K so below this temperature we need molecular hydrogen's lower energy levels. The small fraction of the Universe still ionization acted as catalyzers to form molecular hydrogen, enough that the very first stars began to form. These "Population III" stars began in the "dark age" of the Universe but were the first sources of light and began the reionization of Hydrogen.

Mr. Stiavelli explained that a star will be begin forming when the Jeans mass of a gas cloud decreases below the total gas content, at about 200,000 solar masses, at which point the cloud collapses into a single very massive star, one per cloud. These stars were very bright in the ultraviolet and this radiation photo-dissociates the molecular hydrogen needed for their very formation. This process destroyed most of the other stars around them, Mr. Stiavelli said.

Mr. Stiavelli stated that modeling of this process have shown the first stars alone are not sufficient to completely reionize Hydrogen and it is suspected that the first galaxies are responsible. Observing distant quasars and comparing to simple models suggests the reionization process occurred during a very high redshift such as 10 and continued until redshift 6. However, our most powerful computers still cannot fully simulate even a dwarf galaxy, so we must find evidence of these first galaxies. Early galaxies are very faint objects and it is difficult to do spectroscopy above redshift 6, which takes at least a year of observation with the HST to see even the brightest objects, Mr. Stiavelli said.

The James Webb Space Telescope is designed to be a workhouse in this area of research, Mr. Stiavelli said. The JWST contains a much larger mirror than the HST, with cameras and spectrographs designed for both infrared and visible light, which should enable the detection of the first galaxies while effectively replicating the Hubble Deep Field collection at higher redshift values. The HST has collected data from galaxies at redshift 7 to 8, but we have observed an epoch of rapid change in the luminosity density close to the limit of the HST, Mr. Stiavelli explained. The JWST will be better able to study the nature of reionization sources and the luminosity function of the first galaxies, and may even be able to detect supernovae from Population III stars.

With that, he closed his talk and Mr. Ingersoll invited questions.

Someone wondered about the possibility of a defect similar to Hubble Space Telescope's mirror, Mr. Stiavelli stated that the reasons for Hubble's defect are well understood and won't be repeated. Further, he explained that James Webb's mirror is much more deformable, enabling the telescope to compensate as necessary after launch.

In response to a question about the current and future funding status of the James Webb Space Telescope, Mr. Stiavelli commented that, despite concern last year, the project is fully funded for the current fiscal year with an anticipated launch in 2018.

After the question and answer period, Mr. Ingersoll thanked the speaker, made the usual housekeeping announcements, and invited guests to apply for membership. At ?:?? pm, President John Ingersoll adjourned the 2,300th meeting to the social hour.

Attendance: ??
The weather: Scattered Clouds
The temperature: 20°C

Respectfully submitted,

Justin Stimatze,
Recording secretary

Abstract & Speaker Biography
Semester Index - Home