Formation and spatial distribution of hypervelocity stars in AGN outflows

Abstract

We study star formation within outflows driven by active galactic nuclei (AGN) as a new source of hypervelocity stars (HVSs). Recent observations revealed active star formation inside a galactic outflow at a rate of ∼ 15 M ⊙ yr −1 . We verify that the shells swept up by an AGN outflow are capable of cooling and fragmentation into cold clumps embedded in a hot tenuous gas via thermal instabilities. We show that cold clumps of ∼ 10 3 M ⊙ are formed within ∼ 10 5 yrs. As a result, stars are produced along outflow's path, endowed with the outflow speed at their formation site. These HVSs travel through the galactic halo and eventually escape into the intergalactic medium. The expected instantaneous rate of star formation inside the outflow is ∼ 4 − 5 orders of magnitude greater than the average rate associated with previously proposed mechanisms for producing HVSs, such as the Hills mechanism and three-body interaction between a star and a black hole binary. We predict the spatial distribution of HVSs formed in AGN outflows for future observational probe.

Enhanced UV radiation and dense clumps in the molecular outflow of Mrk 231

“…The prominent CN line wings of Mrk 231 may indicate that the outflow is bathed in a strong UV radiation field, either due to star formation in the galaxy disk or to new stars forming within the outflow itself. The latter is a scenario proposed by various recent models and simulations (Zubovas et al 2013;Zubovas & King 2014;Ishibashi & Fabian 2012;Ishibashi et al 2013;El-Badry et al 2016;Wang & Loeb 2018;Decataldo et al 2019). Star formation in the outflow could have major implications, since such stars would have highly radial orbit and could, for instance, contribute to the formation of…”

Widespread star formation inside galactic outflows

“…Ishibashi et al (2013) have even suggested that star formation in AGN-driven outflows can contribute to the observed size evolution of ellitpical galaxies across the cosmic epochs. Wang & Loeb (2018) have also suggested that the double-shells often observed in elliptical galaxies, typically ascribed to merging events, can actually be the result of star formation in outflows.…”

“…Within this context star formation in outflows has also been invoked to explain the population of Hypervelocity stars in the Galactic halo (Wang & Loeb 2018;Silk & Mamon 2012;Zubovas et al 2013c).…”

“…However, some models have also proposed that outflows and jets can also have a positive feedback effect; indeed, compressing gas in the interstellar and circumgalactic galactic outflows can foster the fragmentation and gravitational collapse of clouds, hence boost star formation (Rees 1989;Nayakshin & Zubovas 2012;Bieri et al 2016;Zubovas et al 2013a). However, more recently, models have proposed an even more fascinating scenario, according to which the outflowing gas should cool, fragment and, therefore, form stars inside the outflow (Ishibashi & Fabian 2012;Zubovas et al 2013c;Silk 2013;Zubovas & King 2014;Ishibashi et al 2013;Gaibler et al 2012;Dugan et al 2014;Ishibashi & Fabian 2014;Zubovas et al 2013b;El-Badry et al 2016;Wang & Loeb 2018;Mukherjee et al 2018). This new star formation mode is drastically different from star formation in galactic discs and even from star formation resulting from the "standard" positive feedback scenario discussed above.…”

Self-sustaining Star Formation Fronts in Filaments during the Cosmic Dawn

“…Next, we follow the prescription of our previously derived model for star formation within shells (Wang & Loeb 2018). The swept-up gas cools and fragments into cold clumps embedded in a hot tenuous gas via a thermal instability, which occurs if the heating rate at a constant pressure rises faster than the cooling rate as a function of temperature, consistent with observations (Maiolino et al 2017) and numerical simulations (Ferrara & Scannapieco 2016;Schneider et al 2018).…”