The JWST Galactic Center Survey -- A White Paper

TL;DR

This white paper advocates for a large JWST/NIRCam Treasury survey of the GC's inner 100 pc to address how the region formed, how stars formed over the last ~30 Myr, and whether the IMF varies with environment. It argues that NIRCam's unparalleled extinction mitigation, angular resolution, and multi-epoch astrometry will yield dereddened color-magnitude diagrams for ~10^7 stars, per-star extinction maps, young stellar object identifications, and precise proper motions that tie the GC to the broader Milky Way. The proposed multi-epoch, multi-wavelength, large-area survey will map the 3D structure of the ISM, measure GC kinematics, and provide a comprehensive census of stellar populations, enabling insights into galactic nuclei physics and informing extragalactic studies. By delivering a long-lasting community data set, this program promises transformative legacy value and strong synergies with ALMA, HST, Roman, Rubin, and future facilities.

Abstract

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.

Figures (9)

• Figure 1: Comparison of the same field in the GC, about $25"$ northeast of Sgr A*, imaged by VVV Minniti:2010fk, GALACTICNUCLEUS Nogueras-Lara:2019yj and JWST (JWS Proposal 1939, PI J. Lu). These images illustrate why angular resolution is of key importance for studying the Galactic Center, where crowding is a serious limitation. The angular resolution increases roughly by a factor of three between the images, meaning that source confusion is reduced by a factor of almost ten between the images obtained by the different instruments.
• Figure 2: Overview of the Galactic Center. Galactic north is up and east to the left. Prominent H II regions are labelled in blue, star clusters in white, and major molecular clouds in green. The large ellipse outlines the region containing most of the stellar mass of the NSD, most of the molecular gas in the CMZ, and the region of active star formation.
• Figure 3: Simulated CMDs and luminosity functions. We used the SPISEA python package to simulate observations of the stellar population at the GC. We assumed a star formation history roughly similar to the one inferred by Nogueras-Lara:2020pp and Schodel:2023ov, that is 1% of the (originally formed) stellar mass forms at 10 Myr, 4% at 200 Myr, 10% at 1 Gyr and 85% at 10 Gyr. We assumed a distance of $8.25$ kpc, a mean extinction of $A_{K}=2$ mag with Gaussian differential reddening $\sigma A_{K}=0.2$ mag Schodel:2014fkNogueras-Lara:2021di, and constant photometric uncertainties of 0.01 mag. We did not include any uncertainty of the distance modulus and assumed a perfectly-known and spatially non-variable extinction curve. Upper left: CMD without differential reddening or photometric uncertainties. Upper right: Observed CMD, after including differential reddening and observational uncertainties. Here, we have included a foreground population from the inner bulge, with the same properties as the 10 Gyr NSD population, but at a distance of 8 kpc, mean reddening of $A_{K}=1.3$ mag, and with Gaussian differential reddening $\sigma A_{K}=0.05$ mag. These values have been derived empirically from GNS data Nogueras-Lara:2018prNogueras-Lara:2022wa. The latter work also demonstrates that we can reliably exclude additional polluting components from the Galactic foreground through color cuts. Lower left: Dereddened CMD. The intrinsic color terms of almost all stars in the simulation are small and vary very little, as shown in Fig. \ref{['fig:stellar_colors']}. By assuming a constant color term ($[F212N - F480M]\approx-0.03$) for all stars, we can therefore deredden the CMD. Lower right: Observed and de-reddened luminosity function. In the absence of accurate short-wavelength observations, we can infer the star formation history from the luminosity function Schodel:2018dbNogueras-Lara:2020ppSchodel:2020qcNogueras-Lara:2022jz. There are several clearly visible markers of different star formation episodes (AGB, RC and RGBB bumps, MS turnoffs for different ages). Using the CMD can provide more information, but requires costly deep observations in the short NIR (F140M).
• Figure 4: Overview of the proposed target fields. Upper panel: Proposed NIRCam pointings superposed on a Spitzer IRAC $4.5\,\mathrm{\mu}$m image of the GC. The white polygon indicates the area of the GALACTICNUCLEUS survey Nogueras-Lara:2019yj. The green rectangles indicate the NIRCam target fields, assuming a FULLBOX 6TIGHT dither pattern, which we consider to provide the best compromise between sampling and speed. Lower panel: Coverage map of parallel MIRI pointings superposed on a Spitzer MPIS $24\,\mathrm{\mu}$m image.
• Figure 5: Cumulative distribution functions of the observed magnitudes of YSOs at the distance of the CMZ based on the Robitaille:2017lf model grid. Each curve represents a luminosity bin as labeled in the legend.