Kristen B. Wingfield McQuinn
JWST Early Release Science program on Resolved Stellar Populations
Our early release science program is focused on imaging resolved stars in the nearby globular cluster M92, the ultra-faint dwarf galaxy Draco II, and gas-rich star-forming galaxy WLM. These systems are all located in our own Local Group of galaxies, in the neighborhood of the Milky Way.
The science goals of our program are to reconstruct star formation histories, measure the sub-Solar mass stellar IMF, create extinction maps, study evolved stars, and measure the proper motions of the systems. We are also developing tools for the community to help with future data reduction and analysis of similar data.
The photo to the left shows me with graduate and undergraduate students in astrophysics at Rutgers looking at the JWST image of WLM projected on the Liberty Science Center's Jennifer Chalsty planetarium dome. It's the largest planetarium dome in the US! The incredible resolution and detail of the JWST images made it feel like we were inside the WLM galaxy looking up the night sky (but with bionic vision)! It was truly a spectacular experience.
WLM Image Credit: NASA/ESA/CDA/Judy Schmidt
The Boundary of Galaxy Formation: Constraints from the Ancient Star Formation of the Isolated, Extremely Low-Mass Galaxy Leo P
In the early universe, at the lowest masses where gravity is in a tug-of-war with the energy injected by internal events (stellar feedback) and outside events (reionization, environmental effects), the dominant factors that govern the growth of the smallest structures are still speculative. Archeological studies of low-mass galaxies in the Local Group (LG) have striven to answer this question through detailed analyses of the oldest stars. Yet, the LG is, by definition, a complicated system and the history of its satellite galaxies are intertwined with their evolution in the shadow of their massive host. The only laboratory to truly measure how galaxies at the limit of structure formation grow is a very low-mass system that is isolated.
Leo P is an isolated, extremely low-mass (M*~10^5 Msun), metal poor (<3% Solar), gas-rich galaxy just outside the LG at 1.6 Mpc. It is the quintessential system to test theories of how the smallest structures in our universe survive and grow. We will image the resolved stars in Leo P to below the old main sequence turn-off, a depth unreachable with HST but achievable with JWST, to derive the ancient star formation history of the galaxy. The star formation history will test if (i) early star formation in Leo P was ``quelled'', as predicted nearly uniformly by reionization models, and then re-ignited, (ii) Leo P has a delayed onset in star formation suggesting the accretion history of baryons may be dependent on environment at low masses, or (iii) Leo P has constant star formation across all epochs which is not predicted by cosmological models in this mass range and which would also be in tension with reionization models.
Securing the TRGB Distance Indicator:
A Pre-Requisite for a JWST Measurement of H0
The tip of the red giant branch (TRGB) is arguably one of the most precise and efficient distance indicators for nearby galaxies. TRGB distances are a key rung in the distance ladder, contributing to a precision measurement of the Hubble constant (H0), independent of Cepheid variable stars. These TRGB distances are measured with HST F814W (i.e., I-band) observations where the luminosity of the TRGB has only a modest and well- characterized dependence on metallicity.
In the NIR, the TRGB is up to 2 mag brighter offering profound observational gains. Simply put, JWST is capable of measuring TRGB distances out to ~50 Mpc over a 125x greater volume than HST, and reaching galaxies with diverse properties across all morphological types. The TRGB with JWST will become the primary distance indicator in the nearby universe. However, the luminosity of the TRGB in the NIR is expected to have a greater dependence on metallicity and possibly stellar age. Thus, for JWST to be used for TRGB distance work, it must be empirically calibrated and robustly tested.
This JWST Cycle 1 program will obtain the data necessary to calibrate the TRGB for distance work in the NIR. It builds on an HST program that is calibrating the NIR filters for TRGB distance work (Max Newman et al. in preparation) and will provide a precise and robust calibration in 6 filters on NIRCam and 2 filters on NIRISS.