ALBERT

Description: We report the results of optical follow-up observations of 29 gravitational-wave (GW) triggers during the first half of the LIGO–Virgo Collaboration (LVC) O3 run with the Gravitational-wave Optical Transient Observer (GOTO) in its prototype 4-telescope configuration (GOTO-4). While no viable electromagnetic (EM) counterpart candidate was identified, we estimate our 3D (volumetric) coverage using test light curves of on- and off-axis gamma-ray bursts and kilonovae. In cases where the source region was observable immediately, GOTO-4 was able to respond to a GW alert in less than a minute. The average time of first observation was 8.79 h after receiving an alert (9.90 h after trigger). A mean of 732.3 square degrees were tiled per event, representing on average 45.3 per cent of the LVC probability map, or 70.3 per cent of the observable probability. This coverage will further improve as the facility scales up alongside the localization performance of the evolving GW detector network. Even in its 4-telescope prototype configuration, GOTO is capable of detecting AT2017gfo-like kilonovae beyond 200 Mpc in favourable observing conditions. We cannot currently place meaningful EM limits on the population of distant ($hat{D}_L = 1.3$ Gpc) binary black hole mergers because our test models are too faint to recover at this distance. However, as GOTO is upgraded towards its full 32-telescope, 2 node (La Palma & Australia) configuration, it is expected to be sufficiently sensitive to cover the predicted O4 binary neutron star merger volume, and will be able to respond to both northern and southern triggers.

Description: We present observations of Swift J1112.2–8238, and identify it as a candidate relativistic tidal disruption flare. The outburst was first detected by Swift /Burst Alert Telescope (BAT) in 2011 June as an unknown, long-lived (order of days) gamma-ray transient source. We show that its position is consistent with the nucleus of a faint galaxy for which we establish a likely redshift of z  = 0.89 based on a single emission line that we interpret as the blended [O  ii ] 3727 doublet. At this redshift, the peak X-ray/gamma-ray luminosity exceeded 10 47 erg s –1 , while a spatially coincident optical transient source had i '  ~ 22 ( M g  ~ –21.4 at z  = 0.89) during early observations, ~20 d after the Swift trigger. These properties place Swift J1112.2–8238 in a very similar region of parameter space to the two previously identified members of this class, Swift J1644+57 and Swift J2058+0516. As with those events the high-energy emission shows evidence for variability over the first few days, while late-time observations, almost 3 yr post-outburst, demonstrate that it has now switched off. Swift J1112.2–8238 brings the total number of such events observed by Swift to three, interestingly all detected by Swift over a ~3 month period (〈3 per cent of its total lifetime as of 2015 March). While this suggests the possibility that further examples may be uncovered by detailed searches of the BAT archives, the lack of any prime candidates in the years since 2011 means these events are undoubtedly rare.

Description: We use the large cosmological hydro-dynamic simulation B lue T ides to predict the photometric properties of galaxies during the epoch of reionization ( z = 8–15). These properties include the rest-frame UV to near-IR broad-band spectral energy distributions, the Lyman continuum (LyC) photon production, the UV star formation rate calibration, and intrinsic UV continuum slope. In particular we focus on exploring the effect of various modelling assumptions, including the assumed choice of stellar population synthesis (SPS) model, initial mass function, and the escape fraction of LyC photons, upon these quantities. We find that these modelling assumptions can have a dramatic effect on photometric properties leading to consequences for the accurate determination of physical properties from observations. For example, at z = 8 we predict that nebular emission can account for up to 50 per cent of the rest-frame R -band luminosity, while the choice of SPS model can change the LyC production rate up to a factor of x 2.

Description: We present Hubble Space Telescope ( HST ) Wide Field Camera 3 UV and near-IR (nIR) imaging of 21 Superluminous Supernovae (SLSNe) host galaxies, providing a sensitive probe of star formation and stellar mass within the hosts. Comparing the photometric and morphological properties of these host galaxies with those of core-collapse supernovae (CCSNe) and long-duration gamma-ray bursts (LGRBs), we find SLSN hosts are fainter and more compact at both UV and nIR wavelengths, in some cases we barely recover hosts with absolute magnitude around M V –14. With the addition of ground based optical observations and archival results, we produce spectral energy distribution fits to these hosts, and show that SLSN hosts possess lower stellar mass and star formation rates. This is most pronounced for the hydrogen deficient Type-I SLSN hosts, although Type-II H-rich SLSN host galaxies remain distinct from the bulk of CCSNe, spanning a remarkably broad range of absolute magnitudes, with ~30 per cent of SLSNe-II arising from galaxies fainter than M nIR ~ –14. The detection of our faintest SLSN hosts increases the confidence that SLSNe-I hosts are distinct from those of LGRBs in star formation rate and stellar mass, and suggests that apparent similarities in metallicity may be due to the limited fraction of hosts for which emission line metallicity measurements are feasible. The broad range of luminosities of SLSN-II hosts is difficult to describe by metallicity cuts, and does not match the expectations of any reasonable UV-weighted luminosity function, suggesting additional environmental constraints are likely necessary to yield hydrogen rich SLSNe.

Description: Using the Binary Population and Spectral Synthesis code, bpass , we have calculated the rates, time-scales and mass distributions for binary black hole (BH) mergers as a function of metallicity. We consider these in the context of the recently reported first Laser Interferometer Gravitational-Wave Observatory (LIGO) event detection. We find that the event has a very low probability of arising from a stellar population with initial metallicity mass fraction above Z = 0.010 ( Z 0.5 Z ). Binary BH merger events with the reported masses are most likely in populations below 0.008 ( Z 0.4 Z ). Events of this kind can occur at all stellar population ages from 3 Myr up to the age of the Universe, but constitute only 0.1–0.4 per cent of binary BH mergers between metallicities of Z = 0.001 and 0.008. However at metallicity Z = 10 –4 , 26 per cent of binary BH mergers would be expected to have the reported masses. At this metallicity, the progenitor merger times can be close to 10 Gyr and rotationally mixed stars evolving through quasi-homogeneous evolution, due to mass transfer in a binary, dominate the rate. The masses inferred for the BHs in the binary progenitor of GW 150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.

Description: The relationship between stellar populations and the ionizing flux with which they irradiate their surroundings has profound implications for the evolution of the intergalactic medium (IGM). We quantify the ionizing flux arising from synthetic stellar populations which incorporate the evolution of interacting binary stars. We determine that these show ionizing flux boosted by 60 per cent at 0.05 ≤ Z ≤ 0.3 Z and a more modest 10–20 per cent at near-solar metallicities relative to star-forming populations in which stars evolve in isolation. The relation of ionizing flux to observables such as 1500 Å continuum and ultraviolet spectral slope is sensitive to attributes of the stellar population including age, star formation history and initial mass function (IMF). For a galaxy forming 1 M  yr –1 , observed at 〉100 Myr after the onset of star formation, we predict a production rate of photons capable of ionizing hydrogen, N ion = 1.4 x 10 53  s –1 at Z = Z and 3.5 x 10 53  s –1 at 0.1 Z , assuming a Salpeter-like IMF. We evaluate the impact of these issues on the ionization of the IGM, finding that the known galaxy populations can maintain the ionization state of the Universe back to z ~ 9, assuming that their luminosity functions continue to M UV = –10, and that constraints on the IGM at z ~ 2–5 can be satisfied with modest Lyman-continuum photon escape fractions of 4–24 per cent depending on assumed metallicity.

Description: Star-forming galaxies at high redshift show ubiquitously high-ionization parameters, as measured by the ratio of optical emission lines. We demonstrate that local ( z  〈 0.2) sources selected as Lyman break analogues also manifest high line ratios with a typical [O iii ]/H $\beta =3.36^{+0.14}_{-0.04}$ – comparable to all but the highest ratios seen in star-forming galaxies at z  ~ 2–4. We argue that the stellar population synthesis code bpass can explain the high-ionization parameters required through the ageing of rapidly formed star populations, without invoking any AGN contribution. Binary stellar evolution pathways prolong the age interval over which a starburst is likely to show elevated line ratios, relative to those predicted by single stellar evolution codes. As a result, model galaxies at near-solar metallicities and with ages of up to ~100 Myr after a starburst typically have a line ratio [O iii ]/Hβ ~ 3, consistent with those seen in Lyman break galaxies and local sources with similar star formation densities. This emphasises the importance of including binary evolution pathways when simulating the nebular line emission of young or bursty stellar populations.

Description: We present 5.5 and 9.0 GHz observations of a sample of 17 GRB host galaxies at 0.5 〈  z  〈 1.4, using the radio continuum to explore their star formation properties in the context of the small but growing sample of galaxies with similar observations. Four sources are detected, one of those (GRB 100418A) likely due to lingering afterglow emission. We suggest that the previously reported radio afterglow of GRB 100621A may instead be due to host galaxy flux. We see no strong evidence for redshift evolution in the typical star formation rate of GRB hosts, but note that the fraction of ‘dark’ bursts with detections is higher than would be expected given constraints on the more typical long GRB population. We also determine the average radio-derived star formation rates of core collapse supernovae at comparable redshift, and show that these are still well below the limits obtained for GRB hosts, and show evidence for a rise in typical star formation rate with redshift in supernova hosts.

Description: We present an analysis of the photometry and spectroscopy of the host galaxy of Swift -detected GRB 080517. From our optical spectroscopy, we identify a redshift of z  = 0.089 ± 0.003, based on strong emission lines, making this a rare example of a very local, low-luminosity, long gamma-ray burst. The galaxy is detected in the radio with a flux density of S 4.5 GHz  = 0.22 ± 0.04 mJy – one of relatively few known gamma-ray bursts hosts with a securely measured radio flux. Both optical emission lines and a strong detection at 22 μm suggest that the host galaxy is forming stars rapidly, with an inferred star formation rate ~16 M  yr –1 and a high dust obscuration ( E ( B – V ) 〉 1, based on sightlines to the nebular emission regions). The presence of a companion galaxy within a projected distance of 25 kpc, and almost identical in redshift, suggests that star formation may have been triggered by galaxy–galaxy interaction. However, fitting of the remarkably flat spectral energy distribution from the ultraviolet through to the infrared suggests that an older, 500 Myr post-starburst stellar population is present along with the ongoing star formation. We conclude that the host galaxy of GRB 080517 is a valuable addition to the still very small sample of well-studied local gamma-ray burst hosts.

Description: Identifying galaxy clustering at high redshift (i.e. z  〉 1) is essential to our understanding of the current cosmological model. However, at increasing redshift, clusters evolve considerably in star formation activity and so are less likely to be identified using the widely used red-sequence method. Here we assess the viability of instead identifying high-redshift clustering using actively star-forming galaxies (submillimetre galaxies, SMGs, associated with overdensities of BzKs /LBGs). We perform both a 2D and 3D clustering analysis to determine whether or not true (3D) clustering can be identified where only 2D data are available. As expected, we find that 2D clustering signals are weak at best and inferred results are method dependent. In our 3D analysis, we identify 12 SMGs associated with an overdensity of galaxies coincident both spatially and in redshift – just 8 per cent of SMGs with known redshifts in our sample. Where an SMG in our target fields lacks a known redshift, their sight line is no more likely to display clustering than blank sky fields; prior redshift information for the SMG is required to identify a true clustering signal. We find that the strength of clustering in the volume around typical SMGs, while identifiable, is not exceptional. However, we identify a small number of highly clustered regions, all associated with an SMG. The most notable of these, surrounding LESS J033336.8–274401, potentially contains an SMG, a quasi stellar object (QSO) and 36 star-forming galaxies (a 〉20 overdensity) all at z  ~ 1.8. This region is highly likely to represent an actively star-forming cluster and illustrates the success of using star-forming galaxies to select sites of early clustering. Given the increasing number of deep fields with large volumes of spectroscopy, or high quality and reliable photometric redshifts, this opens a new avenue for cluster identification in the young Universe.