The Shellless Supernova Remnant B0532–67.5 in the Large Magellanic Cloud

The supernova remnant (SNR) B0532−67.5 in the Large Magellanic Cloud (LMC) was first diagnosed by its nonthermal radio emission, and its SNR nature was confirmed by the observation of diffuse X-ray emission; however, no optical SNR shell is detected. The OB association LH75, or NGC 2011, is projected within the boundary of this SNR. We have analyzed the massive star population in and around SNR B0532−67.5 using optical photometric data to construct color–magnitude diagrams, using stellar evolutionary tracks to estimate stellar masses, and using isochrones to assess the stellar ages. From these analyses, we find a 20–25 Myr population in LH75 and a younger population less than 10 Myr old to the southwest of LH75. The center of SNR B0532−67.5 is located closer to the core of LH75 than to the massive stars to its southwest. We conclude that the supernova progenitor was probably a member of LH75 with an initial mass of ∼15 M ⊙. The supernova exploded in an H i cavity excavated by the energy feedback of LH75. The low density of the ambient medium prohibits the formation of a visible nebular shell. Despite the low density in the ambient medium, physical properties of the hot gas within the SNR interior do not differ from SNRs with a visible shell by more than a factor of 2–3. The large-scale H i map shows that SNR B0532−67.5 is projected in a cavity that appears to be connected with the much larger cavity of the supergiant shell LMC-4.

Comparison of Composite and Star-forming Early-type Galaxies

I assemble 4684 star-forming early-type galaxies (ETGs) and 2011 composite ETGs (located in the composite region on the BPT diagram) from the catalog of the Sloan Digital Sky Survey Data Release 7 MPA-JHU emission-line measurements. I compare the properties of both ETG samples and investigate their compositions, stellar masses, specific star formation rates (sSFRs), and excitation mechanisms. Compared with star-forming ETGs, composite ETGs have higher stellar mass and lower sSFR. In the stellar mass and u − r color diagram, more than 60% of star-forming ETGs and composite ETGs are located in the green valley, showing that the two ETG samples may have experienced star formation and that ∼17% of star-forming ETGs lie in the blue cloud, while ∼30% of composite ETGs lie in the red sequence. In the [N II]/Hα versus EWHα (the Hα equivalent width) diagram, all star-forming ETGs and most of the composite ETGs are located in the star-forming galaxy region, and composite ETGs have lower EWHα than their counterparts. We show the relations between 12+log(O/H) and log(N/O) for both ETG samples, and suggest that nitrogen production of some star-forming ETGs can be explained by the evolution scheme of Coziol et al., while the prodution of composite ETGs may be a consequence of the inflowing of metal-poor gas and these more evolved massive galaxies.

Spectroscopy of Candidate Members of the Sco-Cen Complex*

We present spectroscopy of 285 previously identified candidate members of populations in the Sco-Cen complex, primarily Ophiuchus, Upper Sco, and Lupus. The spectra are used to measure spectral types and diagnostics of youth. We find that 269 candidates exhibit signatures of youth in our spectra or previous data, which is consistent with their membership in Sco-Cen. We have constructed compilations of candidate members of Ophiuchus, Upper Sco, and Lupus that have spectral classifications and evidence of youth, which contain a total of 2274 objects. In addition, we have used spectra from previous studies to classify three sources in Ophiuchus that have been proposed to be protostellar brown dwarfs: ISO Oph 70, 200, and 203. We measure spectral types of early M from those data, which are earlier than expected for young brown dwarfs based on evolutionary models (≳M6.5) and instead are indicative of stellar masses (∼0.6 M ⊙).

L-GALAXIES 2020: The formation and chemical evolution of stellar haloes in Milky Way Analogues and galaxy clusters

We present an analysis of the formation and chemical evolution of stellar haloes around (a) Milky Way Analogue (MWA) galaxies and (b) galaxy clusters in the L-Galaxies 2020 semi-analytic model of galaxy evolution. Observed stellar halo properties are better reproduced when assuming a gradual stripping model for the removal of cold gas and stars from satellites, compared to an instantaneous stripping model. The slope of the stellar mass – metallicity relation for MWA stellar haloes is in good agreement with that observed in the local Universe. This extends the good agreement between L-Galaxies 2020 and metallicity observations from the gas and stars inside galaxies to those outside. Halo stars contribute on average only ∼0.1 per cent of the total circumgalactic medium (CGM) enrichment by z = 0 in MWAs, ejecting predominantly carbon produced by AGB stars. Around a quarter of MWAs have a single ‘significant progenitor’ with a mean mass of ∼ 2.3 × 109M⊙, similar to that measured for Gaia Enceladus. For galaxy clusters, L-Galaxies 2020 shows good correspondence with observations of stellar halo mass fractions, but slightly over-predicts stellar masses. Assuming a Navarro-Frenk-White profile for the stellar halo mass distribution provides the best agreement. On average, the intracluster stellar component (ICS) is responsible for 5.4 per cent of the total intracluster medium (ICM) iron enrichment, exceeding the contribution from the brightest cluster galaxy (BCG) by z = 0. We find that considering gradual stripping of satellites and realistic radial profiles is crucial for accurately modelling stellar halo formation on all scales in semi-analytic models.

Rapid build-up of the stellar content in the protocluster core SPT2349−56 at z = 4.3

The protocluster SPT2349−56 at z = 4.3 contains one of the most actively star-forming cores known, yet constraints on the total stellar mass of this system are highly uncertain. We have therefore carried out deep optical and infrared observations of this system, probing rest-frame ultraviolet to infrared wavelengths. Using the positions of the spectroscopically-confirmed protocluster members, we identify counterparts and perform detailed source deblending, allowing us to fit spectral energy distributions in order to estimate stellar masses. We show that the galaxies in SPT2349−56 have stellar masses proportional to their high star-formation rates, consistent with other protocluster galaxies and field submillimetre galaxies (SMGs) around redshift 4. The galaxies in SPT2349−56 have on average lower molecular gas-to-stellar mass fractions and depletion timescales than field SMGs, although with considerable scatter. We construct the stellar-mass function for SPT2349−56 and compare it to the stellar-mass function of z = 1 galaxy clusters, finding consistent shapes between the two. We measure rest-frame galaxy ultraviolet half-light radii from our HST-F160W imaging, finding that on average the galaxies in our sample are similar in size to typical star-forming galaxies at these redshifts. However, the brightest HST-detected galaxy in our sample, found near the luminosity-weighted centre of the protocluster core, remains unresolved at this wavelength. Hydrodynamical simulations predict that the core galaxies will quickly merge into a brightest cluster galaxy, thus our observations provide a direct view of the early formation mechanisms of this class of object.

How Identifying Circumgalactic Gas by Line-of-sight Velocity instead of the Location in 3D Space Affects O vi Measurements

The high incidence rate of the O vi λλ1032, 1038 absorption around low-redshift, ∼L * star-forming galaxies has generated interest in studies of the circumgalactic medium. We use the high-resolution EAGLE cosmological simulation to analyze the circumgalactic O vi gas around z ≈ 0.3 star-forming galaxies. Motivated by the limitation that observations do not reveal where the gas lies along the line of sight, we compare the O vi measurements produced by gas within fixed distances around galaxies and by gas selected using line-of-sight velocity cuts commonly adopted by observers. We show that gas selected by a velocity cut of ±300 km s−1 or ±500 km s−1 produces a higher O vi column density, a flatter column density profile, and a higher covering fraction compared to gas within 1, 2, or 3 times the virial radius (r vir) of galaxies. The discrepancy increases with impact parameter and worsens for lower-mass galaxies. For example, compared to the gas within 2 r vir, identifying the gas using velocity cuts of 200–500 km s−1 increases the O vi column density by 0.2 dex (0.1 dex) at 1 r vir to over 0.75 dex (0.7 dex) at ≈ 2 r vir for galaxies with stellar masses of 109–109.5 M ⊙ (1010–1010.5 M ⊙). We furthermore estimate that excluding O vi outside r vir decreases the circumgalactic oxygen mass measured by Tumlinson et al. (2011) by over 50%. Our results demonstrate that gas at large line-of-sight separations but selected by conventional velocity windows has significant effects on the O vi measurements and may not be observationally distinguishable from gas near the galaxies.

Testing the Relationship between Bursty Star Formation and Size Fluctuations of Local Dwarf Galaxies

Stellar feedback in dwarf galaxies plays a critical role in regulating star formation via galaxy-scale winds. Recent hydrodynamical zoom-in simulations of dwarf galaxies predict that the periodic outward flow of gas can change the gravitational potential sufficiently to cause radial migration of stars. To test the effect of bursty star formation on stellar migration, we examine star formation observables and sizes of 86 local dwarf galaxies. We find a correlation between the R-band half-light radius (R e ) and far-UV luminosity (L FUV) for stellar masses below 108 M ⊙ and a weak correlation between the R e and Hα luminosity (L Hα ). We produce mock observations of eight low-mass galaxies from the FIRE-2 cosmological simulations and measure the similarity of the time sequences of R e and a number of star formation indicators with different timescales. Major episodes of R e time sequence align very well with the major episodes of star formation, with a delay of ∼50 Myr. This correlation decreases toward star formation rate indicators of shorter timescales such that R e is weakly correlated with L FUV (10–100 Myr timescale) and is completely uncorrelated with L Hα (a few Myr timescale), in agreement with the observations. Our findings based on FIRE-2 suggest that the R-band size of a galaxy reacts to star formation variations on a ∼50 Myr timescale. With the advent of a new generation of large space telescopes (e.g., JWST), this effect can be examined explicitly in galaxies at higher redshifts, where bursty star formation is more prominent.

Ubiquitous [O ii] Emission in Quiescent Galaxies at z ≈ 0.85 from the LEGA-C Survey*

Using deep rest-frame optical spectroscopy from the Large Early Galaxy Astrophysical Census (LEGA-C) survey, conducted using VIMOS on the ESO Very Large Telescope, we search for low-ionization [O ii] λ λ 3726,3729 emission in the spectra of a mass-complete sample of z ≈ 0.85 galaxies. We find that 59% of UVJ-quiescent (i.e., non-star-forming) galaxies in the sample have [O ii] emission detected above our completeness limit of 1.5 Å, and the median-stacked spectrum of the remaining sample also shows [O ii] emission. The overall fraction of sources with [O ii] above our equivalent width limit is comparable to what we find in the low-redshift universe from GAMA and MASSIVE, except perhaps at the highest stellar masses (>1011.5 M ⊙). However, stacked spectra for the individual low-equivalent-width systems uniquely indicates ubiquitous [O ii] emission in the higher-z LEGA-C sample, with typical [O ii] luminosities per unit stellar mass that are a factor of ×3 larger than the lower-z GAMA sample. Star formation at higher-z could play a role in producing the [O ii] emission, although it is unlikely to provide the bulk of the ionizing photons. More work is required to fully quantify the contributions of evolved stellar populations or active galactic nuclei to the observed spectra.