Title: The Connection Between Metal Abundances and the Evolution of Galaxies
Podcaster: Duane Lee from Columbia Astronomy
Organization: Columbia University Astronomy
Description: Metals in astronomy refer to any element heavier than Helium on the periodic table. The abundance of metals, with the exception of a few, tells us the processing steps needed in the past to create the current abundance patterns we see in galaxies today. In both stellar atmospheres and interstellar/intergalactic gas we can probe the nature of galaxy evolution via merger/accretion histories, stellar evolution, and chemical enrichment of galaxian gas. I will share with you some basic observational and theoretical factoids that will illuminate the importance of metals in the study of galaxy evolution.
Bio: Duane Lee is currently a graduate student at Columbia University. He attended Wesleyan University where he obtained a M.A. in Astronomy in 2006. He also attended Williams College where he received a B.A. with Honors in Astrophysics in 2001. Originally from Pittsfield, Massachusetts, he grew to love astronomy from his early encounters with a neighbor’s telescope and the clear skies available to him in Western Massachusetts. His research interests include galaxy evolution and structure, chemical enrichment of galaxies, optical and radio surveys of galaxian structure. Other interests include singing, writing songs, sports, and politics.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by joseph Brimbacombe.
Hello everyone, and welcome to Columbia Mondays! My name is Duane Lee and I’m a 3rd year graduate student at Columbia University in the City of New York. Today the topic of our podcast is going to be on The Connection Between Metal Abundances and the Evolution of Galaxies. This discussion will include tidbits on the history of abundance studies, what astronomers describe as metals and why, what do metals tell us about galaxy evolution and history, and what are the future plans for research on metals in astronomy.
Before I get to the exciting new prospects for metal abundance studies in astronomy, I need to take minute or so to explain a few of the foundational studies that support our current understanding on linking metals to both past internal and external galactic events such as Supernovas (SN), galaxy mergers, and, indeed, the Big Bang itself.
So let’s start with two questions: 1) What are metals and 2) where do they come from?
Well, the first question may seem trivial since we have a basic understanding of what metals are: good conductors of heat and electricity, they’re shiny, durable, malleable, and strong, and some are very valuable – rare and precious commodities that are ultimately foundational to life itself! And this understanding is derived from our historical experience, as in Copper, Bronze, and Iron Ages, and our modern knowledge from chemistry and physics which successfully separates what we normally think of as metals from nonmetals like Carbon (C), Nitrogen (N), and Oxygen (O).
However, we astronomers subscribe to a more basic and all-encompassing definition of metals – any element heavier than H and He is considered a metal. In the scope of astrophysical events and processes, this makes sense since because chemically-defined metallic interactions are scarce due to the rarity of heavy elements and the nature of most astrophysical environments of interest. For instance, in the medium of space between stars and galaxies – aptly known by the names, interstellar and intergalactic mediums, respectively – we find that the environment is either too hot, that is, near stars or accreting black holes, too cold as in not enough energetic interactions between said metals to form meaningful chemical metallic bonds or, and this is probably the main reason metallic bonds are rare in space – these “true” metals are at least 100 to 1000 times more rare than nonmetals, which themselves are only less than 2% of all baryonic, or normal, matter in the Universe. Everything else is either Hydrogen (H) or Helium (He).
This fact of nature leads us to our second question – where do metals come from? Metals come from two types of phenomena in the Universe – Big Bang nucleosynthesis and high mass dying stars. Nucleosynthesis is the process by which heavier elements are created by fusing lighter elements with themselves or with some combination of nucleons like protons and neutrons. This process requires a combination of high density, pressure, and temperature. As the Universe was expanding early on, it was tremendously hot. In fact it was too hot to form stable nucleons. However, as the Universe continued to expand, the Universe also cooled down because all of the energy and matter was being distributed across a larger volume of space. Another way of putting this is to say that the energy density of the Universe is decreasing due to the increasing volume of space. This means that the temperature of the Universe is decreasing since thermodynamics tells us that the energy density of a system like the Universe can be directly related to the temperature of that system.
As the Universe continues to cool, it goes through a phase where both its density and temperature are just right for fusing Hydrogen into Helium, and Helium into heavier elements, and so on. Since this Goldilocks period only lasts for about 17 minutes, the Universe doesn’t have enough time to fuse a large amount of these elements to heavier ones – what we call metals. However, trace amounts of metals are created with most of the metals being that of Lithium, Beryllium, and Boron. These elements are the lightest metals and are the next three elements on the periodic table above He.
The rest of the metals in the Universe come from stars. However, these stars must either be massive enough to cause nucleosynthetic winds, or core-collapse SN in death, or be in binary systems where mass is transferred from a giant companion star to a white dwarf star. When enough mass is transferred, a violent restart to nucleosynthesis occurs in the White Dwarf causing a mass accretion Supernova. The metal yields from these different types of processes will enrich the surrounding gas and eventually end up in the next generation of stars. We detect the build-up of these metals in stars’ atmospheres. Understanding the different supernova types and yields of metals from them and the timescales on which these supernovae take place allows us to characterize different galaxy components, structures, and evolutionary steps in both the Milky Way and her nearby galaxies based on the current metals we see.
Now that we have established what metals are and where they come from, I want to get to the main point on how they can be connected to galactic evolution and what future missions can do with more detailed data on metal abundances.
To illustrate this point, I will explain a scheme that is commonly used to roughly age and distinguish the different stellar components of the Milky Way which are the halo, the bulge, and the disk, by measuring their metallicities. Metallicity is a term that describes the collection of metals that include Iron (Fe) and the daughter products from the fusing two or more Helium nuclei – otherwise called alpha particles in nuclear physics – which are not heavier than Iron. Alpha elements include Oxygen (O), Silicon (Si), Calcium (Ca), and Magnesium (Mg), to name a few, and come from core-collapse supernovae. Fe and Fe-peak elements come from both core-collapse supernovae and mass accretion supernovae, otherwise known as Type II and Type Ia supernovae, respectively. Due to the lack of Alpha-element production in Type Ia supernovae, we can easily distinguish between the effects of Type II and Type Ia’s on their environments.
Now the scheme used to distinguish between these three components of the galaxy relies on the duration of star formation in each one. Based on the theory how each one is formed, astronomers conclude that new star formation ends very early on for the halo, later on for most of the mass of stars in the bulge, and hasn’t yet ended for the disk since it still contains a significant amount of gas required for star formation. To make the same age distinctions using metal signatures, astronomers use metallicity to describe the different chemical signatures we should see in the galaxy components of different ages.
Since both types of supernovae yield Iron, we use this fact to construct a crude chronometer, using the ratio of Iron over Hydrogen, called Fe/H. Since H should be roughly constant or slowly decreasing over time and Fe should be increasing over time due to new stars, the increase in the ratio is used as a clock. For Type II SN, SN events can happen immediately since their high mass progenitors take only a few million years to become supernovae and these supernovae will produce both Alpha-elements and Iron. However, for Type Ia supernovae, it takes about a billion years for the first White Dwarves to be created from stellar evolution, allowing for the first and subsequent Type Ia’s to enrich their environments with Fe-peak elements. Our metallicity amount over time is traced by another ratio, Alpha-elements over Iron called Alpha/Fe, and this is where the scheme proves its mettle! If we plot Alpha/Fe vs. Fe/H, we find that the halo lies in the high Alpha/Fe and low Fe/H part of the plot since it is old and stopped producing new stars and, hence, metals a long time ago. For the bulge, we see it lies in the middle of our plot with lower Alpha/Fe and higher Fe/H than the halo since the bulge would have seen Type Ia’s kick in after a billion years and lower the Alpha/Fe ratio from that of the halo. Since this happens later on, our Fe/H clock would have progressed as well putting the bulge in the middle of our plot. So the bulge is at least a billion years younger than the halo. Finally, the disk, which is still producing metals, will have both the lowest Alpha/Fe ratio and the highest Fe/H ratio which states chemically that it has continued to produce metals for the longest time and thus, has the highest metallicity.
This crude scheme has worked very successfully and can easily be extended to include accreting or accreted galaxies and satellite galaxies. Future missions will actually aim to get even more detailed data which includes metal yields from certain stellar winds that have different signatures from the two supernovae mentioned earlier. With this data, we hope to further our understanding of galactic dynamics and evolution by tracking and chemically tagging most of the stars in our galaxy to assess where they came from – setting up a kind of stellar DNA analysis for a galactic stellar genealogy. These stellar and galactic histories will be expanded greatly in the next decade and I encourage everyone to stay tuned to astronomy for all of the exciting discoveries to come.
This has been a podcast of Columbia University here in the City of New York. For more information about public events of Columbia Astronomy visit outreach.astro.columbia.edu. Our next Columbia Monday podcast will be by Marcel Agueros & Bobbito Garcia on Monday, October 19th entitled ‘I Keep Planets in Orbit: Astronomy in Hip Hop‘. Have a great day and keep listening.
End of podcast:
365 Days of Astronomy
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