… and when observations were still done with photographic plates!
Today’s Astrobite looks at the work of Barbara A. Williams, the first African-American woman to achieve a PhD in Astronomy.
Barbara A. Williams started her studies at the University of North Carolina. She then moved to the University of Maryland for her masters degree in Radio Astronomy before also completing her PhD there in 1981 as the first Black American woman to acquire a PhD in astronomy. After her PhD, Williams then moved to the National Radio Astronomy Observatory in Charlottesville, VA where she continued her focus on radio observations of compact groups of galaxies.
Title: MKW 10: A Group of Galaxies with a Compact Core
Author: Barbara A. Williams
Author’s Institute (at the time of publication): National Radio Astronomy Observatory, Charlottesville, VA
Status: Published in Astrophysical Journal [open access]
Today’s Astrobite focusses on a single-author paper written by Williams in 1984. Much of Williams’ research focusses on compact groups of galaxies. Today’s paper looks at the galaxy group MKW10, shown in Figure 1.
The MKW10 Cluster
MKW10 is a small collection, or ‘poor’ cluster of galaxies (with ‘rich’ clusters defined as containing more than a thousand galaxies). Originally identified in a previous study in 1975, the system contains 9 bright galaxies (labelled 2-10), with 5 bright members (labelled 3-7) concentrated towards the centre forming a subsystem known as a ‘compact group’ of galaxies. Many of these compact groups have been identified and were first catalogued by Paul Hickson in 1982, leading to the name ‘Hickson compact groups‘, which have continued to be studied in detail, including in this astrobite.
The radio observations were made using the 305 m Arecibo telescope during June 1982 and October 1983. The Arecibo Observatory located in Puerto Rico and completed in 1963 was, until 2016, the world’s largest single-aperture telescope. Some eagle-eyed readers may also recognise it from the movie Contact (one of my all time favourites!), as well as James Bond’s GoldenEye.
Eight spiral members were detected in the 21cm line of neutral hydrogen, as well as observations of galaxies surrounding the group in order to construct (for the first time) a complete redshift sample of all spiral galaxies within this 10.2 square degree region. The 21cm line, also known as the HI line sits within the radio wavelengths and corresponds to an emission line of neutral hydrogen. When a hydrogen atom transitions from the excited state into the ground state, a photon is emitted at a wavelength of 21cm. This line is very useful in determining the velocity of the source due to doppler shifts in this line. Wilson finds that the spiral galaxies in the compact group show strong signs of tidal interaction due to the strange shapes of their HI profiles. This provides evidence for the fact that the compact group is indeed compact, rather than just being an observational effect.
Williams then looks at the mass-to-light ratios of the galaxies, that is the mass measured within the galaxy as a ratio of the light emitted by the stars. Mass-to-light ratios gives information about the types and ages of a galaxy’s stars. Spiral galaxies have a large percentage of young stars and therefore have relatively high mass-to-light ratios, whereas elliptical galaxies with mostly older stars have lower values. By studying all of the surrounding galaxies as well as the compact group, Wilson is able to build up a picture of the larger scale environment which may be affecting the evolution of the galaxies. Using these mass-to-light ratios along with the morphological type of the galaxies, she finds that the fraction of early-type (Sc) spiral galaxies is higher in the compact group than in the rest of the group, indicating these galaxies have been more disturbed in some way as the rotation of the stars is not as ordered as late-type spiral galaxies. The dynamical friction timescale is also calculated to be short, meaning that these galaxies are interacting with each other. From this, as well as the density of galaxies, the collision rate of galaxies is calculated to be high in this group – a rate of 22 per galaxy per Hubble time (~14 billion years). For this reason, Wilson finds that it is unlikely that the compact group has been compact for a long time. This compact group is likely a bound configuration of galaxies which will coalesce in around a billion years.
Advancing on these findings
The compact central group in the centre of MKW10, known as the Hickson compact group 58 studied in this paper by Williams continues to be studied to this day, as shown in Figure 4 in a more recent observation by SDSS. If you take a look back at the panels of Figure 3, you’ll notice the top right panel corresponds to the bottom three galaxies in Figure 4 (NGC3852, NGC3848 and NGC 3817) and the bottom left panel of Figure 3 corresponds to the top two galaxies of Figure 4 (NGC 3819 and NGC3820). The difference in resolution is astonishing, and makes the findings from the original studies back in the 80s all the more impressive!
The formation and mere existence of these groups is still a mystery as the lifetimes of these groups is predicted to be quite short, meaning that observing these groups should be rare. However, hundreds of these groups have been observed so far in the local Universe. One significant result that Williams found with this study, which has proven to be true of all similar compact groups was that the observed HI abundance was lower than expected. A likely suggestion for this is that as these galaxies are interacting, their gas is being tidally stripped, but the jury is still out and research is still ongoing. These observations as well as the observational techniques pioneered by Williams paved the way for future deeper understanding of the effect of environment on galaxy evolution.
Following her time at National Radio Astronomy Observatory, she returned to the University of North Carolina as a Research Associate, where she had completed her undergraduate studies, before moving to the Department of Physics and Astronomy at the University of Delaware as an Associate Professor, where she also served as Acting Associate Chair. Along with many other awards, Williams was named as the Outstanding Young Woman of America in 1986. As well as her significant contribution to the field of radio observations of groups of galaxies, Williams later studied educational research, with a focus on methods to prevent women from leaving academia.
Williams paved the way for Black American women to follow in her footsteps. As far as I can tell from the information online, Williams is now retired. Wherever you are Barbara, I hope you are enjoying your retirement and that telling your story will continue to inspire generations to come.