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Reconstructing the emergence of the Milky Way’s stellar population with Gaia, SDSS-V and JWST

Periodic Reporting for period 2 - EMERGE (Reconstructing the emergence of the Milky Way’s stellar population with Gaia, SDSS-V and JWST)

Reporting period: 2021-04-01 to 2022-09-30

Supernovae (SNe) of all types play critical roles in numerous places in astrophysics: they are the main sources of most of the elements of the periodic table; the kinetic energy released in their explosions are central to the growth and evolution of galaxies; their remnants are the sites where cosmic rays are accelerated to huge energies. However,
our empirical understanding of these events is still quite limited. Little is known about the progenitor stars or stellar systems of the different types of supernovae, or about the distribution of delay times between formation of a stellar population and the explosion of some of its members as supernovae of different types. The detailed element yields of every type are also poorly known, as are the relative numbers and the importance of different SN stypes. A resulting problem is, apart from the fact that we are in the dark regarding all of these aspects of such an important univresal process, is that the other aforementioned processes (chemical evolution of galaxies via SN element production, cosmic ray production,etc.) remain poorly understood, and their modeling can only be based on highly uncertain theoretical constructs. The objective of my EMERGE project is, through astronomical observations and analysis, to put on an empirical basis, as many as possible of the above properties of SNe. This is becoming particularly feasible thanks to the data froma number of massive new observational projects, including Gaia, TESS, SDSS-V, and JWST. Specifically: I am measuring the demographics of binary populations in our Galaxy, from among which come the progenitor systems of many SN types; with mew measurements and analysis, I am refining measurements of the SN delay-time dsitribution; and I am analyzing large photometric and spectroscopic SN datasets to systematically determine the elemnt yields of the various SN types.
1. The demographics of short-period binary populations (work led by ERC-funded postdoc Dr. Matthew Green): we have analyzed the joint TESS-Gaia database to derive a sample of ~20,000 short-period (P~1 day) binary star sytems, revealed via their mutual ellipsoidal tidal distortions. This full sample was incorporated as an Open-Fiber Target program in SDSS-V, and observations have begun, which will provide multi-epoch spectra for most of these systems, revealing their stellar components. A first publication analyzing the characteristics of the close-binary population in this uncharted territory is in preparation and will be submitted in June 2022.

2. The separation and mass-ratio distribution of close double white dwarfs (work led by former student Na'ama Hallakoun): We have obtained multi-epoch radial-velocity data for most of the double-white-dwarf (DWD) candidates (the favored SN Ia progenitors) that we previously found from the SDSS and SPY surveys, using observing time that we succeeded obtaining on a number of large telescopes (ESO-VLT, GTC, LBT, SALT). Analysis is ongoing. One system has turned out to be a remarkable system consisting of a hot WD strongly irradiating a brown-dwarf companion. We have just received followup time to study this system on the Gemini South 8m telescope. A paper analyzing this system has been submitted to Nature.

3. The delay-time distribution of supernovae in galaxy-cluster environments (work led by ERC-funded postdoc Dr. Jonathan Freundlich): we have re-analyzed Hubble Space Telescope data on supernovae in galaxy clusters, taking into account the star-formation present in clusters that are observed at high redshift. We have derived new and more accurate measurements of the Type Ia SN (SN Ia) delay-time distribution, and confirmed the puzzling high efficiency of SN Ia production in clusters compared to field-galaxy environments. A paper with the results was published in 2021 in MNRAS. This work is an important precursor to my planned proposal to discover SNe in clusters at even highr redshifts with JWST, which was successfully launched in December 2021 and is now being commissioned.

4. The SN Ia DTD from IFU data in nearby galaxies (work led by ERC-funded postdocs Drs. Marco Lam and Avishai Gilkis): We have collected several available codes for reconstruction of stellar populations from optical spectra, and are performing tests and simulations to evaluate and compare their performance, systematics and problems. We are then applying the codes to several large integral-field spectroscopy datasets for nearby galaxies that have hosted SNe, to determine the DTD based on spatially resolved stellar populations, as detailed in my original proposal. Such a careful comparison and evaluation of the performaces of the codes, and its application to the assorted and large datasets that have become available (MaNGA, AMUSING) is essential for deriving detailed, reliable, and precise DTDs.

5. The element yields of different SN types (work led by ERC-funded postdoc Dr. Osmar Rodriguez): We have systematically analyzed light curves and spectra for hundreds of SNe, developing new physical tools to determine reliably their element yields. A paper determining the distribution of iron yields of TYpe-II SNe was published in MNRAS in 2021.
A paper on the iron yields of stripped-envelope SNe was submitted to MNRAS in April 2022. We are now beginning work on using available nebular-phase spectra of all types of SNe to determine empirical yields of as many additional elements as possible.

6. The initial mass functions of different stellar populations (work led by former student Na'ama Hallakoun): we have used Gaia data to discover that the so-called "blue-halo" population of Milky Way stars, which is believed to be debris from the merger of the ancient Gaia-Enceladus galaxy with our galaxy 10 billion years ago, has a peculiar "bottom-heavy" initial mass function (IMF), similar to that in the massive elliptical galaxies in clusters. This may be a hint that the high SN Ia production efficiency in clusters (item 3 above) and the "high-alpha" compositions in both ellipticals and in halo stars may be the result of IMF variations and the variations in the SN types and yields that ensue (connected to item 5 above).
The above results already constitute progress toward the stated objective of placing the properties of SNe and their progenitors on an empirical and physical footing. By the end of the project I expect that we will have a much clearer picture than today on the close-binary population, based on the ellipsoidal sample we are about to publish and are beginning to observe with SDSS-V, but also from additional binary samples, e.g. those we will construct using Gaia Data Release 3 in June 2022. We will complete the RV analysis of the DWD data, which will provide the first characterization of this population and a critical test of its viability as the progenitors of SNe Ia.
In the next JWST call for proposals (expected Jan 2023), the first call during a demonstrably working telescope, I will submit a proposal to discover and measure the rates of supernovae in protoclusters at redshifts 2-3, as planned.
I expect to complete the analysis of the DTD based on resolved stellar populations in nearby galaxies, enhancing our knowledge of the DTD and its dependence on galaxy stellar parameters. The current analysis of nebular-phase SN spectra will hopefully lead to the fullest possible empirical inventory of SN element yields.