Hintze Centre for Astrophysical Surveys

Abstract:

In order to identify the sources of the observed diffuse high-energy neutrino flux, it is crucial to discover their electromagnetic counterparts. To increase the sensitivity of detecting counterparts of transient or variable sources by telescopes with a limited field of view, IceCube began releasing alerts for single high-energy (Eν >  60 TeV) neutrino detections with sky localisation regions of order 1° radius in 2016. We used Pan-STARRS1 to follow-up five of these alerts during 2016–2017 to search for any optical transients that may be related to the neutrinos. Typically 10–20 faint (miP1 ≲ 22.5 mag) extragalactic transients are found within the Pan-STARRS1 footprints and are generally consistent with being unrelated field supernovae (SNe) and AGN. We looked for unusual properties of the detected transients, such as temporal coincidence of explosion epoch with the IceCube timestamp, or other peculiar light curve and physical properties. We found only one transient that had properties worthy of a specific follow-up. In the Pan-STARRS1 imaging for IceCube-160427A (probability to be of astrophysical origin of ∼50%), we found a SN PS16cgx, located at 10.0′ from the nominal IceCube direction. Spectroscopic observations of PS16cgx showed that it was an H-poor SN at redshift z = 0.2895 ± 0.0001. The spectra and light curve resemble some high-energy Type Ic SNe, raising the possibility of a jet driven SN with an explosion epoch temporally coincident with the neutrino detection. However, distinguishing Type Ia and Type Ic SNe at this redshift is notoriously difficult. Based on all available data we conclude that the transient is more likely to be a Type Ia with relatively weak Si II absorption and a fairly normal rest-frame r-band light curve. If, as predicted, there is no high-energy neutrino emission from Type Ia SNe, then PS16cgx must be a random coincidence, and unrelated to the IceCube-160427A. We find no other plausible optical transient for any of the five IceCube events observed down to a 5σ limiting magnitude of miP1 ≈ 22 mag, between 1 day and 25 days after detection.

Abstract:

We examine the co-evolution of galaxies and supermassive black holes in the simba cosmological hydrodynamic simulation. simba grows black holes via gravitational torque-limited accretion from cold gas and Bondi accretion from hot gas, while feedback from black holes is modelled in radiative and jet modes depending on the Eddington ratio (fEdd). simba shows generally good agreement with local studies of black hole properties, such as the black hole mass-stellar velocity dispersion (MBH-σ) relation, the black hole accretion rate versus star formation rate (BHAR-SFR), and the black hole mass function. MBH-σ evolves such that galaxies at a given MBH have higher σ at higher redshift, consistent with no evolution in MBH-M∗. For MBH ≤ 108 M⊙, fEdd is anticorrelated with MBH since the BHAR is approximately independent of MBH, while at higher masses fEdd-MBH flattens and has a larger scatter. BHAR versus SFR is invariant with redshift, but fEdd drops steadily with time at a given MBH, such that all but the most massive black holes are accreting in a radiatively efficient mode at z ≥ 2. The black hole mass function amplitude decreases with redshift and is locally dominated by quiescent galaxies for MBH > 108 M⊙, but for z≥ 1 star-forming galaxies dominate at all MBH. The z = 0 fEdd distribution is roughly lognormal with a peak at fEdd ≤ 0.01 as observed, shifting to higher fEdd at higher redshifts. Finally, we study the dependence of black hole properties with H i content and find that the correlation between gas content and SFR is modulated by black hole properties, such that higher SFR galaxies at a given gas content have smaller black holes with higher fEdd.