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Insights from JWST observations suggest that active galactic nuclei feedback evolved from a short-lived, high-redshift phase in which radiatively cooled turbulence and/or momentum-conserving outflows stimulated vigorous early star formation (“positive” feedback), to late, energy-conserving outflows that depleted halo gas reservoirs and quenched star formation. The transition between these two regimes occurred at z ∼ 6, independently of galaxy mass, for simple assumptions about the outflows and star formation process. Observational predictions provide circumstantial evidence for the prevalence of massive black holes at the highest redshifts hitherto observed, and we discuss their origins.

We present measurements of the gas-phase oxygen and nitrogen abundances obtained by applying the direct method to JWST NIRspec R ∼ 1000 spectroscopy for six galaxies at redshift greater than 3. Our measurements are based on rest-frame optical nitrogen [N ii]λλ6548,6583 lines and are complemented by six additional objects from the literature at 3 ≤ z ≤ 6. We find that 9 out of 12 objects have values of log(N/O) that are compatible with those found for low-redshift, metal-poor, dwarf galaxies and for H ii regions of more luminous local galaxies. However, 3 out of 12 objects have log(N/O) values that are overabundant compared to what is expected on the basis of their oxygen abundance. We explore a few standard scenarios to explain the observations and conclude that, within the limited statistics available to us, none of them can be definitely excluded even though we prefer dilution by pristine gas infall in between star formation bursts, as this is predicted by simulations to take place as a natural part of bursty star formation.

This paper presents improved constraints on the low-mass stellar initial mass function (IMF) of the Boötes I (Boo I) ultrafaint dwarf galaxy, based on our analysis of recent deep imaging from the Hubble Space Telescope. The identification of candidate stellar members of Boo I in the photometric catalog produced from these data was achieved using a Bayesian approach, informed by complementary archival imaging data for the Hubble Ultra Deep Field. Additionally, the existence of earlier-epoch data for the fields in Boo I allowed us to derive proper motions for a subset of the sources and thus identify and remove likely Milky Way stars. We were also able to determine the absolute proper motion of Boo I, and our result is in agreement with, but completely independent of, the measurement(s) by Gaia. The best-fitting parameter values of three different forms of the low-mass IMF were then obtained through forward modeling of the color–magnitude data for likely Boo I member stars within an approximate Bayesian computation Markov Chain Monte Carlo algorithm. The best-fitting single power-law IMF slope is α=−1.95−0.28+0.32 , while the best-fitting broken power-law slopes are α1=−1.67−0.57+0.48 and α2=−2.57−1.04+0.93 . The best-fitting lognormal characteristic mass and width parameters are Mc=0.17−0.11+0.05⊙ and σ=0.49−0.20+0.13 . These broken power-law and lognormal IMF parameters for Boo I are consistent with published results for the stars within the Milky Way, and thus it is plausible that Boötes I and the Milky Way are populated by the same stellar IMF.

We show with Gaia XP spectroscopy that extremely metal-rich (EMR) stars in the Milky Way ([M/H]XP ≳ 0.5) are largely confined to a tight “knot” at the center of the Galaxy. This EMR knot is round in projection, has a fairly abrupt edge near R GC,proj ∼ 1.5 kpc, and is a dynamically hot system. This central knot also contains very metal-rich (VMR; +0.2 ≤ [M/H]XP ≤ +0.4) stars. However, in contrast to EMR stars, the bulk of VMR stars forms an extended, highly flattened distribution in the inner Galaxy (R GC ≲ 5 kpc). We draw on TNG50 simulations of Milky Way analogs for context and find that compact, metal-rich knots confined to ≲1.5 kpc are a universal feature. In typical simulated analogs, the top 5%–10% most metal-rich stars are confined to a central knot; however, in our Milky Way data this fraction is only 0.1%. Dust-penetrating wide-area near-infrared spectroscopy, such as the fifth Sloan Digital Sky Survey, will be needed for a rigorous estimate of the fraction of stars in the Galactic EMR knot. Why in our Milky Way only EMR giants are confined to such a central knot remains to be explained. Remarkably, the central few kiloparsecs of the Milky Way harbor both the highest concentration of metal-poor stars (the “poor old heart”) and almost all EMR stars. This highlights the stellar population diversity at the bottom of galactic potential wells.

We analyze the outer regions of M33, beyond 15 kpc in projected distance from its center, using Subaru/Hyper Suprime-Cam multicolor imaging. We identify red giant branch (RGB) stars and red clump (RC) stars using the surface-gravity-sensitive NB515 filter for the RGB sample and a multicolor selection for both samples. We construct the radial surface density profiles of these RGB and RC stars and find that M33 has an extended stellar population with a shallow power-law index of α > −3, depending on the intensity of the contamination. This result represents a flatter profile than the stellar halo that was detected by the previous study focusing on the central region, suggesting that M33 may have a double-structured halo component, i.e., inner/outer halos or a very extended disk. Also, the slope of this extended component is shallower than those typically found for halos in large galaxies, implying intermediate-mass galaxies may have different formation mechanisms (e.g., tidal interaction) from large spirals. We also analyze the radial color profiles of RC/RGB stars and detect a radial gradient, consistent with the presence of an old and/or metal-poor population in the outer region of M33, thereby supporting our proposal that the stellar halo extends beyond 15 kpc. Finally, we estimate that the surface brightness of this extended component is μ V = 35.72 ± 0.08 mag arcsec−2. If our detected component is the stellar halo, this estimated value is consistent with the detection limit of previous observations.

The stellar initial mass function (IMF) describes the distribution of stellar masses that form in a given star formation event. The long main-sequence lifetimes of low-mass stars mean that the IMF in this regime (below ∼ 1 M ⊙) can be investigated through star counts. Ultrafaint dwarf galaxies are low-luminosity systems with ancient, metal-poor stellar populations. We investigate the low-mass IMF in four such systems (Reticulum II, Ursa Major II, Triangulum II, and Segue 1), using Hubble Space Telescope imaging data that reaches to ≲ 0.2 M ⊙ in each galaxy. The analysis techniques that we adopt depend on the number of low-mass stars in each sample. We use Kolmogorov–Smirnov tests for all four galaxies to determine whether their observed apparent magnitude distributions can reject a given combination of IMF parameters and binary fraction for the underlying population. We forward model 1000 synthetic populations for each combination of parameters, and reject those parameters only if each of the 1000 realizations reject the null hypothesis. We find that all four galaxies reject a variety of IMFs, and the IMFs that they cannot reject include those that are identical, or similar, to that of the stellar populations of the Milky Way. We determine the best-fit parameter values for the IMF in Reticulum II and Ursa Major II and find that the IMF in Reticulum II is generally consistent with that of the Milky Way, while the IMF in Ursa Major II is more bottom heavy. The interpretation of the results for Ursa Major II is complicated by possible contamination from two known background galaxy clusters.

Massively multiplexed spectrographs will soon gather large statistical samples of stellar spectra. The accurate estimation of uncertainties on derived parameters, such as the line-of-sight velocity v los, especially for spectra with low signal-to-noise ratios (S/Ns), is paramount. We generated an ensemble of simulated optical spectra of stars as if they were observed with low- and medium-resolution fiber-fed instruments on an 8 m class telescope, similar to the Subaru Prime Focus Spectrograph, and determined v los by fitting stellar templates to the simulated spectra. We compared the empirical errors of the derived parameters—calculated from an ensemble of simulations—to the asymptotic errors determined from the Fisher matrix, as well as from Monte Carlo sampling of the posterior probability. We confirm that the uncertainty of v los scales with the inverse square root of the S/N, but also show how this scaling breaks down at low S/N and analyze the error and bias caused by template mismatch. We outline a computationally optimized algorithm to fit multiexposure data and provide a mathematical model of stellar spectrum fitting that maximizes the so called significance, which allows for calculating the error from the Fisher matrix analytically. We also introduce the effective line count, and provide a scaling relation to estimate the errors of v los measurements based on stellar type. Our analysis covers a range of stellar types with parameters that are typical of the Galactic outer disk and halo, together with analogs of stars in M31 and in satellite dwarf spheroidal galaxies around the Milky Way.

We present an overview of our Galactic Archaeology (GA) survey program with the Prime Focus Spectrograph (PFS) for Subaru. Following successful design reviews, the instrument is now under construction with first light anticipated in 2018. Main characteristics of PFS and the science goals in our PFS/GA program are described.

The conference poster includes a very apt phrase that describes a primary motivation for this conference: Time discovers truth. This aphorism, attributed to Seneca, was certainly affirmed by the many exciting talks and discussions at this conference, in both formal and informal settings.