Day 1447

Tuesday.

1808.00969
The fast, luminous ultraviolet transient AT2018cow: Extreme supernova, or disruption of a star by a n intermediate-mass black hole?
Perley, et al

Wide-field optical surveys have begun to uncover large samples of fast (t_rise<5d), luminous (M_peak<-18), blue transients.  While commonly attributed to the breakout of supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered a detailed investigation of their properties until now.  Present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the first fast-luminous optical transient to be found in real time at low z.  The first spectra (< 2days after discovery) are entirely featureless.  A very broad absorption feature suggestive of near-relativistic velocities develops between 3-8 days, then disappears.  Broad emission features of H and He develop after >10 days.  The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R<1e14 cm after 1 month).  This behavior does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features.  Alternatively, the transient could originate from the disruption of a star by an intermediate-mass BH, although this would require long-lasting emission of highly super-Eddington thermal radiation.  In either case, AT 2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event.  Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins.


1808.01753
GAMA/G10-COSMOS/3D-HST: evolution of the galaxy stellar mass function over 12.5 Gyrs
Wright, Driver, Robotham

Using a combined and consistently analysed GAMA, G10-COSMOS, ad 3D-HST dataset, explore the evolution of the galaxy stellar-mass function over lookback times t_L in [0.2,12.5] h^{-1}_70 Gyr.  Use a series of volume limited samples to fit Schechter functions in bins of ~constant lookback time and explore the evolution of the best-fit parameters in both single and two-component cases.  In all cases, employ a fitting procedure that is robust to the effects of Eddington bias and sample variance.  Surprisingly, when fitting a two-component Schechter function, find essentially no evidence of temporal evolution in M_star, the two alpha slope parameters, or the normalization of the low-mass component.  Instead, the fits suggest that the various shape parameters have been exceptionally stable over cosmic time, as has the normalization of the low-mass component, and that the evolution of the stellar-mass function is well described by a simple build up of the high-mass component over time.  When fitting a single component Schechter function, there is an observed evolution in both M_star and alpha, however this is interpreted as being an artifact.  Finally, find that the evolution of the stellar-mass function, and the observed stellar mass density, can be well described by a simple model of constant growth in the high-mass source density over the last 11 h^{-1}_70 Gyrs.