Positive or Negative? The Impact of X-ray Feedback on the Formation of Direct Collapse Black Hole Seeds

TL;DR

This study investigates how X-ray feedback from nearby sources influences the formation of direct collapse black hole seeds, revealing a distance-dependent effect where X-rays can suppress or have minimal impact on gas collapse.

Contribution

It introduces a detailed multi-frequency radiative transfer model including X-ray spectra from high-mass X-ray binaries to assess their feedback on black hole seed formation.

Findings

X-rays have a strongly negative feedback effect at separations >1 kpc.

At <1 kpc, X-ray feedback becomes neutral due to enhanced LW dissociation.

A 'Goldilocks zone' exists where X-ray feedback minimally hinders black hole seed formation.

Abstract

A nearby source of Lyman-Werner (LW) photons is thought to be a central component in dissociating H$_2$ and allowing for the formation of a direct collapse black hole seed. Nearby sources are also expected to produce copious amounts of hydrogen ionising photons and X-ray photons. We study here the feedback effects of the X-ray photons by including a spectrum due to high-mass X-ray binaries on top of a galaxy with a stellar spectrum. We explicitly trace photon packages emerging from the nearby source and track the radiative and chemical effects of the multi-frequency source $(E_{\rm photon} = \rm{0.76\ eV \rightarrow 7500\ eV}$). We find that X-rays have a strongly negative feedback effect, compared to a stellar only source, when the radiative source is placed at a separation greater than $\gtrsim 1 \ \rm kpc$. The X-rays heat the low and medium density gas in the envelope surrounding the collapsing halo suppressing the mass inflow. The result is a smaller enclosed mass compared to the stellar only case. However, for separations of $\lesssim 1 \ \rm kpc$, the feedback effects of the X-rays becomes somewhat neutral. The enhanced LW intensity at close separations dissociates more H$_2$ and this gas is heated due to stellar photons alone, the addition of X-rays is then not significant. This distance dependence of X-ray feedback suggests that a Goldilocks zone exists close to a forming galaxy where X-ray photons have a much smaller negative feedback effect and ideal conditions exist for creating massive black hole seeds.