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Discoveries, UCO Spotlight

Following the recent news story around the origins of elusive ultradiffuse galaxies we spoke with UCO affiliate astronomer Laura Sales, Associate Professor at UC Riverside and co-leader on this project about what direction this research will take next.

Q: Can you give us the big picture – what is your research into ultradiffuse galaxies trying to explain and how does using our Keck instruments help?

Right now, the formation scenario for ultradiffuse galaxies is uncertain: why does the universe decide to form stars in these galaxies so sparsely? Several models have been proposed but we don’t know which one is correct, or maybe it is a combination of several mechanisms. One of the first models to explain them was the idea that they are `failed Milky Way like galaxies’, which means they were destined to be larger galaxies, but somehow stopped forming stars and ended up with a dark matter halo that is more massive than bonafide dwarfs. On the other hand, there are a few ultradiffuse galaxies that seem to have no dark matter at all. In order to understand this large spread of properties –and through that come up with a model for their formation– we need more and more accurate measurements of their dark matter halos. 

Instruments at Keck such as DEIMOS* (control software created by UCO Labs for the DEep Imaging Multi-Object Spectrograph installed on the Keck II telescope on Mauna Kea in 2002) and KCWI** are the best positioned in the world to get kinematical measurements to derive their dark matter content as well as to learn about the age and metallicity of their stars. For instance, in the campaign that led us to the Toloba et al. 2018 paper, we managed to measure the dark matter content of 3 ultradiffuse galaxies in Virgo, finding quite a wide range among only these 3 objects. Most importantly, one of our targets showed clear signs of tidal disruption, providing a very important validation to theoretical models that suggest that some ultradiffuse galaxies might be undergoing tidal disruption, in particular those with low velocities and seemingly dark-matter free. 

*DEIMOS: The DEep Imaging Multi-Object Spectrograph provides slit spectroscopy over a field of view up to 16’x4’.   It was developed at UCO technical labs and installed on the Keck II telescope on Maunakea in 2002.

** KCWI: The Keck Cosmic Web Imager is an integral field spectrograph which provides blue-optimized seeing-limited spectra  from 350-560 nm with configurable spectral resolution from 1000 – 20000 for everything in a field of view up to 20″x33″.  It was installed on the Keck II telescope on Maunakea in 2017.

Q: We understand you have a proposal for using the Keck telescope next semester, you’ll be using KCWI to further your research, can you explain more about that.

There are two different programs that we are working on that will help us get answers. One is exactly what you mentioned, using KCWI, and a second is a program for which we already obtained the data, using DEIMOS (a project that got re-submitted some 4(!) times due to weather, like storms, dome failure and pandemic, but we finally got the data). For the program with DEIMOS, we obtained the velocity of globular clusters around ultradiffuse dwarfs in the Virgo cluster in order to estimate their dark matter content. Partial results have been published sometime ago (Toloba et al. 2018) and the rest of the sample will be published in a paper coming up soon. As for the second program with KCWI, we will target dwarf-elliptical galaxies in Virgo (no ultradiffuse) to try to understand when and how fast they formed their stars. These results will be of course be compared to what we know of ultradiffuse dwarfs and it will serve as a baseline for comparison of properties and star formation history of normal dwarfs (dwarf ellipticals) and this special kind of dwarfs, or “ultradiffuse”. 

Q: How “realistic” are the simulations, ie. what scales do they work on and what scales they don’t? I understand getting to even smaller scales is the goal.

In terms of numerical resolution, we are always looking to push the models to fainter and fainter objects. This is because dwarf galaxies are progressively more dark-matter dominated objects as we move towards fainter objects. Therefore, the fainter we go, the more fundamental probes of the cosmological models they become. The simulations we are using right now resolve dwarf galaxies with stellar masses of approximately 10 million solar masses (M* > 10^7 Msun) and above relatively well, with more than several hundred stellar particles and tens of thousand particles when counting gas and dark matter. However, we would like to reach even fainter dwarfs, with masses 10^6 Msun and below, that are not only more abundant in the universe but also the kind of dwarfs that orbit around the Milky Way (and for which we have the highest quality data due to their proximity).

You can read more about the research project here:  https://news.ucr.edu/articles/2021/09/06/astronomers-explain-origin-elusive-ultradiffuse-galaxies

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