Guy Worthey

Recent analysis of 2968 MaNGA early-type galaxies has yielded two notable trends with velocity dispersion ( σ ) not previously discussed in the literature. First, Fe abundance rises with σ but only until σ ≈ 100 km s ⁻¹ , after which it falls. This kink is reproduced by TNG100 simulations, implying that hierarchical merger processes might explain it. Second, astrophysical scatter in N is high for galaxies with σ < 100 km s ⁻¹ . Due to the restricted list of nucleosynthetic sources for N, it is likely that asymptotic giant branch stars provide most of this N. A varied star formation history (compared to that of massive galaxies) along with variable retention and recycling of N-enriched gas might explain the fuzz of N abundance in low- σ galaxies. Because a timescale argument seems necessary to explain the nitrogen fuzz, and an initial mass function argument is ruled out, similar timescale arguments for the [Mg/Fe] trend as a function of velocity dispersion are supported.

We derive stellar population parameters and radial gradients within 0.65 R _e for spatially resolved spectra of 2968 early-type galaxies from MaNGA, spanning stellar velocity dispersions ( σ ) of 50–340 km s ⁻¹ . Light-weighted mean age, C, N, Na, Mg, and Fe abundances are obtained by inverting metallicity-composite stellar population models with isochrones that respond in T _eff to individual element abundance changes. Globally, log age increases (∼0.53 dex per decade), [Fe/H] declines slightly (∼–0.06 dex per decade), and [X/Fe] for light elements rises (0.19–0.37 dex per decade for log σ < 2.0 but steepens to nearly double slope for log σ > 2.0) with log σ . [Fe/H] peaks at log σ ≈ 2.0 and falls on either side. Light element [X/Fe] anticorrelates with [Fe/H] (∼–0.1 dex per decade). Astrophysical scatter is largest in low- σ galaxies, especially for Fe and N. Internally, age gradients are nearly flat in low- σ galaxies and slightly negative in high- σ systems (∼–0.04 dex per decade). [Fe/H] radial gradients steepen from −0.06 dex per decade to –0.15 dex per decade across σ , while light elements (except Na) show ∼–0.03 dex per decade gradients. Scatter in gradients peaks in high- σ galaxies, most strongly for Fe (∼0.23 dex), suggesting comparable numbers of inside-out and outside-in formation. A near-zero (∼–0.03) age and light-element gradient plus mild [Fe/H] gradients supports hierarchical merging for ETG evolution. Simulations match the observed age structure, slope change at log σ ≈ 2.0, and flat gradients, though they overpredict absolute abundances.

We analyze Hubble Space Telescope optical imaging of eight low-mass galaxies hosting active galactic nuclei (AGN) identified via their photometric variability in V. F. Baldassare et al. We use GALFIT to model the 2D galaxy light profiles, and find a diversity of morphologies. The galaxies with regular morphologies are best fit with pseudobulges and disks, rather than classical bulges. We estimate black hole masses using scaling relations and find black hole masses of 10^3.7−6.6 M _⊙. We compare this sample to dwarf galaxies with AGN selected via optical spectroscopy. On average, the variable host galaxies have lower-mass black holes. We analyze the brightest point source in each galaxy and find their properties are not entirely consistent with star clusters, indicating that they are likely AGN. These point sources are found to have lower luminosities than spectroscopically selected dwarf AGN, but brighter than the point sources in dwarf galaxies not identified as AGN. Our detailed imaging analysis shows that variability selection has the potential to find lower-mass black holes and lower-luminosity AGN than optical spectroscopy. These active dwarfs may have been missed by spectroscopic searches due to star formation dilution or low gas content.

In this study, we extend the dust-independent Hatzidimitriou relation between cluster age and d _B−R color difference between the red giant branch (RGB) and red clump to younger cluster ages. We perform membership analysis on 14 galactic open clusters using Gaia DR3 astrometry, then compute the difference in color of the RGB and red clump d _B−R using Gaia photometry. We also compute d _B−R for five fields surrounding Small Magellanic Cloud clusters. We find that the trend derived from older clusters does not extrapolate to younger ages and becomes double valued. We confirm that d _B−R is independent of metallicity. Current stellar evolutionary isochrones do not quantitatively reproduce the trend and furthermore predict an increased color gap with a decrease in metallicity that is not echoed in the data. Integrated light models based on current isochrones exaggerate the color change over the ​​​​​​−0.5 < [Fe/H] < 0 interval at the few-percent level.

In order to provide fundamental stellar spectra that extend into the UV, Hubble Space Telescope’s Space Telescope Imaging Spectrograph targeted 556 stars via proposals GO9088, GO9786, GO10222, and GO13776. Exposures through three low-resolution gratings provide wavelength coverage from 0.2 < λ < 1 μm at λ/Δλ ∼ 1000. The UV grating (G230LB) scatters red light that results in unwanted signal, especially in cool stars. We applied scattered-light corrections and flux corrections arising from pointing errors relative to the center of the 0.″2 slit based on Worthey et al. We present 513 fully reduced stellar spectra, fluxed, dereddened, and cross correlated to zero velocity. Because of the broad spectral range, we can simultaneously study Hα and Mg ii λ2800, indicators of chromospheric activity. Their behaviors are decoupled. Besides three cool dwarfs and one giant with mild flares in Hα, only Be stars show strong Hα emission. Mg2800 emission, however, strongly anticorrelates with temperature such that warm stars show absorption and stars cooler than 5000 K universally show chromospheric emission regardless of dwarf/giant status or metallicity. Transformed to Mg2800 flux emerging from the stellar surface, we find a correlation with temperature with approximately symmetric astrophysical scatter. Previous work had indicated a basal level with asymmetric scatter to strong values. The discrepancy is primarily due to our improved treatment of extinction. We confirm statistically significant time variability in Mg2800 strength for one star.

Integrated light models that incorporate common types of chemically peculiar (CP) stars are assembled using synthetic spectra. Selected spectral features encode significant age information for populations with ages ∼50 Myr < age < ∼2 Gyr. Due to the alleviation of template mismatch, the inclusion of CP star features in model spectra improves the accuracy of recovered stellar population parameters, but we are not able to show that new or unique age information can be extracted from the weak CP features compared to continuum fitting and strong-feature strengths, at least at the present state of the art. An age-extraction routine that recovers two- and three-burst age structures is employed to analyse the spectra of local group galaxies. NGC 224 (M 31) has a stellar population too old for the types of CP stars we examine. NGC 221 (M 32) also shows no CP spectral features. It appears to contain a component at age ∼1 Gyr at 1 per cent by mass in addition to its dominant 4.7 Gyr population. Unlike Sloan Digital Sky Survey galaxy spectrum averages, NGC 205 (M 110) contains no features due to mercury–manganese (HgMn) stars. This excludes the age range associated with HgMn production, and its near-nuclear spectrum is best fitted by a 68 ± 2 Myr population superimposed on an older population with a 1.85 ± 0.1 Gyr component. Both NGC 205 and NGC 221 have an ancient component whose mass is not easy to constrain given the overwhelming light dominance of the younger populations.