Keck Cosmic Web Imager (KCWI) spectra of globular clusters and ultracompact dwarfs in the halo of M87

Using the Keck Cosmic Web Imager, we obtain spectra of several globular clusters (GCs), ultracompact dwarfs (UCDs), and the inner halo starlight of M87, at a similar projected galactocentric radius of ∼5 kpc. This enables us, for the first time, to apply the same stellar population analysis to the GCs, UCDs, and starlight consistently to derive ages, metallicities, and alpha-element abundances in M87. We find evidence for a dual stellar population in the M87 halo light, i.e. an ∼80 per cent component by mass that is old and metal-rich and a ∼20 per cent component that is old but metal-poor. Two red GCs share similar stellar populations to the halo light suggesting they may have formed contemporaneously with the dominant halo component. Three UCDs, and one blue GC, have similar stellar populations, with younger mean ages, lower metallicities, and near solar alpha-element abundances. Combined with literature data, our findings are consistent with the scenario that UCDs are the remnant nucleus of a stripped galaxy. We further investigate the discrepancy in the literature for M87\u27s kinematics at large radii, favouring a declining velocity dispersion profile. This work has highlighted the need for more self-consistent studies of galaxy haloes

The assembly history of the nearest S0 galaxy NGC 3115 from its kinematics out to six half-light radii

Using new and archival data, we study the kinematic properties of the nearest field S0 galaxy, NGC 3115, out to $\sim6.5$ half-light radii ($R_\mathrm{e}$) from its stars (integrated starlight), globular clusters (GCs) and planetary nebulae (PNe). We find evidence of three kinematic regions with an inner transition at $\sim0.2\ R_\mathrm{e}$ from a dispersion-dominated bulge ($V_\mathrm{rot}/\sigma <1$) to a fast-rotating disk ($V_\mathrm{rot}/\sigma >1$), and then an additional transition from the disk to a slowly rotating spheroid at $\sim2-2.5\, R_\mathrm{e}$, as traced by the red GCs and PNe (and possibly by the blue GCs beyond $\sim5\, R_\mathrm{e}$). From comparison with simulations, we propose an assembly history in which the original progenitor spiral galaxy undergoes a gas-rich minor merger that results in the embedded kinematically cold disk that we see today in NGC 3115. At a later stage, dwarf galaxies, in mini mergers (mass-ratio $<$ 1:10), were accreted building-up the outer slowly rotating spheroid, with the central disk kinematics largely unaltered. Additionally, we report new spectroscopic observations of a sample of ultra-compact dwarfs (UCDs) around NGC 3115 with the Keck/KCWI instrument. We find that five UCDs are inconsistent with the general rotation field of the GCs, suggesting an \textit{ex-situ} origin for these objects, i.e. perhaps the remnants of tidally stripped dwarfs. A further seven UCDs follow the GC rotation pattern, suggesting an \textit{in-situ} origin and, possibly a GC-like nature.Comment: 22 pages (including 3 pages of Appendix material), 14 figures, published in MNRA

NGC 474 as viewed with KCWI: Diagnosing a shell galaxy

We present new spectra obtained using Keck/KCWI and perform kinematics and stellar population analyses of the shell galaxy NGC 474, from both the galaxy centre and a region from the outer shell. We show that both regions have similarly extended star formation histories although with different stellar population properties. The central region of NGC 474 is dominated by intermediate-Aged stars (8.3 ± 0.3 Gyr) with subsolar metallicity ([Z/H] =-0.24 ± 0.07 dex) while the observed shell region, which hosts a substantial population of younger stars, has a mean luminosity-weighted age of 4.0 ± 0.5 Gyr with solar metallicities ([Z/H] =-0.03 ± 0.09 dex). Our results are consistent with a scenario in which NGC 474 experienced a major to intermediate merger with a log$(M_∗/\rm M_\odot) \sim 10$ mass satellite galaxy at least ${\sim}2$ Gyr ago which produced its shell system. This work shows that the direct spectroscopic study of low-surface brightness stellar features, such as shells, is now feasible and opens up a new window to understanding galaxy formation and evolution

The assembly history of the nearest S0 galaxy NGC 3115 from its kinematics out to six half-light radii

Using new and archival data, we study the kinematic properties of the nearest field S0 galaxy, NGC 3115, out to ∼6.5 half-light radii (Re) from its stars (integrated starlight), globular clusters (GCs), and planetary nebulae (PNe). We find evidence of three kinematic regions with an inner transition at ∼0.2 Re from a dispersion-dominated bulge (Vrot/σ \u3c 1) to a fast-rotating disc (Vrot/σ \u3e 1), and then an additional transition from the disc to a slowly rotating spheroid at ∼ 2-2.5Re, as traced by the red GCs and PNe (and possibly by the blue GCs beyond ∼ 5Re). From comparison with simulations, we propose an assembly history in which the original progenitor spiral galaxy undergoes a gas-rich minor merger that results in the embedded kinematically cold disc that we see today in NGC 3115. At a later stage, dwarf galaxies, in mini mergers (mass ratio \u3c 1:10), were accreted building up the outer slowly rotating spheroid, with the central disc kinematics largely unaltered. Additionally, we report new spectroscopic observations of a sample of ultracompact dwarfs (UCDs) around NGC 3115 with the Keck/KCWI instrument.We find that five UCDs are inconsistent with the general rotation field of the GCs, suggesting an ex situ origin for these objects, i.e. perhaps the remnants of tidally stripped dwarfs. A further seven UCDs follow the GC rotation pattern, suggesting an in situ origin and, possibly a GC-like nature

Origins of ultradiffuse galaxies in the Coma cluster - II. Constraints from their stellar populations

In this second paper of the series we study, with new Keck/DEIMOS spectra, the stellar populations of seven spectroscopically confirmed ultradiffuse galaxies (UDGs) in the Coma cluster. We find intermediate to old ages (similar to 7Gyr), low metallicities ([Z/H] similar to -0.7 dex) and mostly supersolar abundance patterns ([Mg/Fe] similar to 0.13 dex). These properties are similar to those of low-luminosity (dwarf) galaxies inhabiting the same area in the cluster and are mostly consistent with being the continuity of the stellar mass scaling relations of more massive galaxies. These UDGs' star formation histories imply a relatively recent infall into the Coma cluster, consistent with the theoretical predictions for a dwarf-like origin. However, considering the scatter in the resulting properties and including other UDGs in Coma, together with the results from the velocity phase-space study of the Paper I in this series, a mixed-bag of origins is needed to explain the nature of all UDGs. Our results thus reinforce a scenario in which many UDGs are field dwarfs that become quenched through their later infall onto cluster environments, whereas some UDGs could be genuine primordial galaxies that failed to develop due to an early quenching phase. The unknown proportion of dwarf-like to primordial-like UDGs leaves the enigma of the nature of UDGs still open

Origins of ultradiffuse galaxies in the Coma cluster - I. Constraints from velocity phase space

We use Keck/DEIMOS spectroscopy to confirm the cluster membership of 16 ultradiffuse galaxies (UDGs) in the Coma cluster, bringing the total number of spectroscopically confirmed UDGs from the Yagi et al. (Y16) catalogue to 25. We also identify a new cluster background UDG, confirming that most (similar to 95 per cent) of the UDGs in the Y16 catalogue belong to the Coma cluster. In this pilot study of Coma UDGs in velocity phase space, we find evidence of a diverse origin for Coma cluster UDGs, similar to normal dwarf galaxies. Some UDGs in our sample are consistent with being late infalls into the cluster environment, while some may have been in the cluster for >= 8 Gyr. The late infallen UDGs have higher absolute relative line-of-sight velocities, bluer optical colours, and within the projected cluster core, are smaller in size, compared to the early infalls. The early infall UDGs, which may also have formed in situ, have been in the cluster environment for as long as the most luminous galaxies in the Coma cluster, and they may be failed galaxies that experienced star formation quenching at earlier epochs

A few StePS forward in unveiling the complexity of galaxy evolution: light-weighted stellar ages of intermediate-redshift galaxies with WEAVE

Context. The upcoming new generation of optical spectrographs on four-meter-class telescopes, with their huge multiplexing capabilities, excellent spectral resolution, and unprecedented wavelength coverage, will provide invaluable information for reconstructing the history of star formation in individual galaxies up to redshifts of about 0.7. Aims. We aim at defining simple but robust and meaningful physical parameters that can be used to trace the coexistence of widely diverse stellar components: younger stellar populations superimposed on the bulk of older ones. Methods. We produced spectra of galaxies closely mimicking data from the forthcoming Stellar Populations at intermediate redshifts Survey (StePS), a survey that uses the WEAVE spectrograph on the William Herschel Telescope. First, we assessed our ability to reliably measure both ultraviolet and optical spectral indices in galaxies of different spectral types for typically expected signal-to-noise ratios. We then analyzed such mock spectra with a Bayesian approach, deriving the probability density function of r- and u-band light-weighted ages as well as of their difference. Results. We find that the ultraviolet indices significantly narrow the uncertainties in estimating the r- and u-band light-weighted ages and their difference in individual galaxies. These diagnostics, robustly retrievable for large galaxy samples even when observed at moderate signal-to-noise ratios, allow us to identify secondary episodes of star formation up to an age of similar to 0.1 Gyr for stellar populations older than similar to 1.5 Gyr, pushing up to an age of similar to 1 Gyr for stellar populations older than similar to 5 Gyr. Conclusions. The difference between r-band and u-band light-weighted ages is shown to be a powerful diagnostic to characterize and constrain extended star-formation histories and the presence of young stellar populations on top of older ones. This parameter can be used to explore the interplay between different galaxy star-formation histories and physical parameters such as galaxy mass, size, morphology, and environment

A few StePS forward in unveiling the complexity of galaxy evolution: light-weighted stellar ages of intermediate-redshift galaxies with WEAVE

Context. The upcoming new generation of optical spectrographs on four-meter-class telescopes, with their huge multiplexing capabilities, excellent spectral resolution, and unprecedented wavelength coverage, will provide invaluable information for reconstructing the history of star formation in individual galaxies up to redshifts of about 0.7. Aims. We aim at defining simple but robust and meaningful physical parameters that can be used to trace the coexistence of widely diverse stellar components: younger stellar populations superimposed on the bulk of older ones. Methods. We produced spectra of galaxies closely mimicking data from the forthcoming Stellar Populations at intermediate redshifts Survey (StePS), a survey that uses the WEAVE spectrograph on the William Herschel Telescope. First, we assessed our ability to reliably measure both ultraviolet and optical spectral indices in galaxies of different spectral types for typically expected signal-to-noise ratios. We then analyzed such mock spectra with a Bayesian approach, deriving the probability density function of r- and u-band light-weighted ages as well as of their difference. Results. We find that the ultraviolet indices significantly narrow the uncertainties in estimating the r- and u-band light-weighted ages and their difference in individual galaxies. These diagnostics, robustly retrievable for large galaxy samples even when observed at moderate signal-to-noise ratios, allow us to identify secondary episodes of star formation up to an age of similar to 0.1 Gyr for stellar populations older than similar to 1.5 Gyr, pushing up to an age of similar to 1 Gyr for stellar populations older than similar to 5 Gyr. Conclusions. The difference between r-band and u-band light-weighted ages is shown to be a powerful diagnostic to characterize and constrain extended star-formation histories and the presence of young stellar populations on top of older ones. This parameter can be used to explore the interplay between different galaxy star-formation histories and physical parameters such as galaxy mass, size, morphology, and environment