Dwarf Galaxies Near Us

In the May 2021 AAA lecture, “Getting to Know the Neighbors: Discovering New Galaxies in the Nearby Universe” Katherine Rhode of Indiana University discussed her research. Her main interest is to characterize galaxy formation by in-depth study of stellar populations. The goals are to understand how galaxies form and evolve through star formation and production of heavier elements which enrich the interstellar medium, gas outflows. All these change their size, classification, and clustering with nearby galaxies to make a group. A key observation is that the physical properties of a galaxy correlate to its morphological type.

She reviewed the history of galaxy studies since 1924 when Hubble redefined the universe as being far vaster and primarily consisting of galaxies… right up to the Hubble Ultra Deep Field images. Galaxies are broadly defined as gravitationally bound stars, from 1 million to 1 trillion. These “cities of stars” also contain vast clouds of hydrogen gas, dust from the sooty remains of past stars, and dark matter. Her team’s specific study is dwarf galaxies, containing only ten thousand to a billion stars. These are now understood to be the most common type, 30 times more than ordinary types. Even Edwin Hubble was not aware of how dominant they are. Images of several dwarfs were presented.

Dr. Rhode sees dwarfs as “simple laboratories” for understanding larger, more complex galaxies. It’s become clear that larger galaxies form from the merging of dwarf-sized objects. Her interest is also in how these larger galaxies then form clusters, superclusters, and empty voids between them. The Local Group that our Milky Way Galaxy is a part of contains 3 very large galaxies (we are #2) and perhaps +70 dwarfs. From the data predictive theories emerge, which are used to create computer simulations for how galaxy clusters develop. The data begins with galaxy surveys such as the Sloan Digital Survey. The census indicates that the number of verified dwarfs in our local group rose from a previously steady 10 to nearly 60 in the space of just 20 years. Taking a survey of dwarf galaxies is hampered by their small sizes and faintness. A single large star in the Milky Way can outshine an entire dwarf galaxy! One work-around for this is to look for “stellar over-densities”, i.e., examine images of stars to look for areas of greater concentration which may outline a dwarf galaxy in front of another galaxy. Sometimes a vague outline of one can be seen… perhaps.

Dr. Rhode and her collaborators employ another approach: take radio surveys of the area under investigation to locate concentrations of fast-moving clouds of hydrogen gas. The project that was based at Arecibo Radio Telescope before it collapsed was called ALFALFA. Neutral hydrogen molecules [H2] are known to emit a specific 21-cm radio wave caused by spin transitions of an outer electron. It’s a marker for hydrogen gas. If an area shows hydrogen gas clouds it is examined for star counts from visual images on file at observatories or astronomical libraries, if available and useful. Often the area is star-poor. Therefore, the team uses the WIYN optical telescope on Kitt Peak to examine for stars in that area. It gives better sightings of stars and allows distances to be obtained. One such finding showed the obvious existence of a dwarf named “Leo P”, verified to have low mass, the first such confirmation to use the ALFALFA and WIYN procedures.

Dwarf Galaxy Leo P; Credit: Hubble Space Telescope

From a Hubble telescope examination of Leo P, its stars appear to be as old as those in old galaxies. Leo P is low in heavy elements, i.e., it’s especially rich in hydrogen gas and much like the earliest variety of Pop-III stars in 56 dwarf candidates were obtained by these methods and re-examined. To date, 8 are candidates to be galaxies. An interesting problem arises: distinguishing a dwarf galax from star clusters in the foreground or background. One technique is to measure their star rotation rates to see they indicate the presence of dark matter – which can be in dwarfs but not in star clusters.

Dr. Rhode concluded with a report on “NEID”, an $11M spectrometer added onto the WIYN telescope at the Kitt Peak Observatory. It will have the capability to obtain spectra from exoplanets once they are detected and  measure red-shift readings to obtain their velocities.

In the Q-and-A that followed Dr. Rhode revealed her three prized astronomy hopes for the future: to understand the formation and evolution of our galaxy and have breakthrough understanding of dark matter and dark energy. Curiously, her team and many others have already applied for research time on the Webb telescope which has not been launched and is significantly delayed. Dwarf galaxies have been verified within the 1 Mpc (3 Mly) diameter of our local group. Some have been observed in the M-81 galaxy group about 4-5 Mpc away. Some have even been detected in nearby galaxy clusters, like the Virgo Cluster.

Her challenge is first to find dwarfs, examine them for processes of galaxy evolution, and apply the results to improve galaxy theory and simulations.