Jul 23rd: The Missing Satellite Problem

By on July 23, 2019 in

Podcaster: Cosmic

Apogee Podcast

Title: The Apogee Podcast – The Missing Satellite Problem

Links: astroandmusic.blogspot.com
Youtube: https://www.youtube.com/user/cosmiclettuce
Twitter: @AstroAndMusic
Email: cosmiclettuce@gmail.com

Description:  In this Apogee Podcast, Cosmic discusses how we came to the “missing satellite problem” as it pertains to galaxy and galaxy cluster evolution, and how it helps us understand the structure of the local universe.

1934 – ‘The Distribution of Extra-Galactic Nebulae’ by Hubble
1941 – ‘On a Cluster of Nebulae in Hydra’ by Zwicky
1959 – ‘The Hercules Cluster of Nebulae’ by Burbidge & Burbidge
1961 – ‘Evidence Regarding Second-Order Clustering of Galaxies and
Interactions between Clusters of Galaxies’ by Abell
1965 – ‘The Black-Body Radiation Content of the Universe and the
Formation of Galaxies’ by Peebles
1967 – ‘Pertubations of a Cosmological Model and Angular Variations
of the Microwave Background’ by Sachs and Wolfe
1970 – ‘Primeval Adiabatic Pertubation In an Expanding Universe’
by Peebles and Yu
1974 – ‘Formation of Galaxies and Clusters of Galaxies by Self-
Similar Gravitational Condensation’ by Press and Schechter
1978 – ‘Core condensation in heavy halos: a two-stage theory for
galaxy formation and clustering’ by White and Rees
1999 – ‘Dark matter substructure within galactic halos’ by Moore et al
2019 – ‘The dwarf galaxy satellite system of Centaurus A-star’ by
Muller et al

Bio: Cosmic (aka Matt Cheselka) is an independent research astronomer and space musician.

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Hello! This is Cosmic, and welcome to the Apogee podcast! In these podcasts, I chronicle a single astronomical reference thread from the past to the present. Many threads are possible — I’ve chosen just one. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.

The apogee for this podcast took place a couple days ago on 21 July 2019, at
00:02 UTC. The lunar distance at that time was 405,478 km, which is 930 km further away than last apogee on 23 June, and 765 km closer than the next apogee on 17 August.

If you have any suggestions for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT gmail DOT com.

The music you hear in the background are my own compositions. I hope you enjoy listening to space music as much as I do.

The study of the distribution of galaxies and galaxy clusters in the universe has been happening for at least the past 100 years. These observations help us understand how the universe has evolved over time, but they’ve also puzzled us in many ways. In a recent paper by Oliver Muller and colleagues, the Centarus A-star galaxy cluster was studied. It was pointed out in this paper that understanding the number and distribution of dwarf galaxies in this cluster could be used to probe cosmological models on “small scales”. One of the discrepancies between observation and theory is the so-called “missing satellite problem”. There seem to be too few satellites (small, dwarf galaxies) in galaxy clusters. Is the current theory wrong, or are our observations just incomplete?

To see where this problem came from, I now take you all the way back to 1934 and Edwin Hubble’s paper concerning the distribution of ‘extra-galactic nebulae’. Photographing them with the 100-inch and 60-inch telescope on Mt Wilson, no one knew for sure what these faint fuzzy objects were! This paper by Hubble does a great job all by itself tracing the history of this subject, and if you want to know even more about all of this I encourage you to take a look at this amazing paper. In this paper, Hubble attempts to formalize the distribution of galaxies in the sky.

This idea was picked up my Zwicky in 1941 in his work studying a ‘cluster of nebulae’ in the contellation Hydra. Zwicky used Hubble’s distribution formula to find that it closely matched observations of ‘nebulae’ in this cluster. The model fit the data fairly accurately which increased confidence in the model!

These observations and conclusions were mentioned in paper published in 1959 by the husband and wife team of Geoffrey and Margaret Burbidge, who had been studying the Hercules cluster of ‘Nebulae’ with photos taken with the 200-inch Hale telescope at Palomar Observatory. Margaret Burbidge, by the way, is still alive and will turn 100 years old on 12 August. Along with determining the distribution of ‘nebulae’ in this cluster, they also determined the radial velocities of some objects using spectroscopy. From this, the total kinetic and potential energy of the cluster could be estimated. Combining the current distribution of ‘nebulae’ with their motions, it was estimated that the average mass of each object was on the order of 10^12 solar masses (1 trillion solar masses). If this were true, they concluded that there must be a lot of intergalactic matter (stuff we can’t see), and that the cluster is expanding (meaning that this and other galaxy clusters are very young). Note that by this time astronomers were using the term ‘galaxy’ more often.

This mass problem was noted by Abell in his 1961 paper on the distrubition of galaxy clusters in the sky. Abell claims that observed masses in galaxy clusters is “improbably high” and that other “second-order” effects might be playing a role to confuse the models. These effects, Abell argues, are possibly due to larger structures called ‘superclusters’ having an influence on nearby clusters. Abell felt confident that the superclusters were a good enough explaination to remove the discrepancy between measurements of the masses of galaxies within clusters and individual galaxies not in clusters. He pointed out, however, that there are other possiblities: “dark galactic matter”, incorrect distance scales, and expanding clusters.

Abell’s work was noted by a paper written in 1965 by Peebles discussing the relationship between the radiation content of the universe and how that influences the formation of galaxies and galaxy clusters. In this paper, Peebles argues that black-body radiation prevents “the formation of gravitationally bound systems” until the universe expands to a certain size. This radiation, he says, might be able to be directly observed.

By this time, it was generally accepted that the universe was lumpy, and that models that assume a uniform distrubution of matter could not be correct. We must now consider more complex models that take into account non-linear pertubations. This is what Sachs and Wolfe set out to do with their Astrophysical Journal paper in 1967. They attempted to not only map the distrubution of galaxies in the sky, but also show that there might be variations of up to 1% in the recently discovered microwave background radiation depending on where you look.

By 1970 astronomers like Peebles had realized that they’d stumblied up the realization that the cosmic microwave backround (CMB) radiation might actually be the light from the primeval fireball itself — light from the so-called ‘Big Bang’. In their paper published in 1970, Peebles and Yu theorized that there might be a strong relationship between purtubations observed in the CMB and the distribution of galaxies observed in our current epoch. It might be possible, therefore, to trace the evolution of these objects.

Citing the work of Peebles and Yu, and others, the question of how the galaxies we observe got distributed the way they are was taken up once again in 1974 by Press and Schechter. In this paper, the authors come up with a “self- similar condensation model” to explain not only the existence of galaxies but also the existence of galaxy clusters. This model matches observations very well, but they point out that their work is far from complete and that much more needs to be done.

The models developed by Press and Schecter are used in an incredible paper written in 1978 by White and Rees. They argue that modeling purely gravitational clustering isn’t enough to explain all observations, and that another agent — namely ‘dark matter’ may play a major role in galaxy and galaxy cluster formation. They go on to say that this ‘dark matter’ provides greater than 80% of the mass in clusters.

Fast forward twenty-one years to 1999, when Moore et al write a paper to discuss the dark matter substructure of galactic halos. These halos supposedly surround and penetrate galaxy clusters, effecting the clusters’ shape and evolution. In this paper, they acknowledge and expand upon the “attractive and well motivated cosmological model” developed by White and Rees. In this new model, Moore et all predict that the halo of the Milky Way galaxy should contain “about 500 satellites” with masses of about 100 million solar masses and quite small (on the order of a kilo-parsec). But we don’t observe this many Milky Way satellites, so what is causing this discrepancy? They also point out that this many satellites and their underlying dark matter substructure could prevent spiral galaxies from forming. They do point out some specific observations that could be made to overcome this problem, or at least provide some clarity.

And it’s this paper that is cited in the 2019 Muller et al Centaurus A-star paper, which discusses observations made of this cluster using the European Southern Observatory’s Very Large Telescope (VLT). They conclude that due to inadequte observations, they have insufficient data to map the overall distribution of satellites in this galaxy group. The work will continue to solve this curious problem of galaxy and galaxy cluster formation!

Thanks for listening! Until the next apogee, I bid you Peace.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by Planetary Science Institute. Audio post-production by Richard Drumm. Bandwidth donated by libsyn.com and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. This year we will celebrates the Year of Everyday Astronomers as we embrace Amateur Astronomer contributions and the importance of citizen science. Join us and share your story. Until tomorrow! Goodbye!

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