O vi EMISSION IMAGING OF A GALAXY WITH THE HUBBLE SPACE TELESCOPE: A WARM GAS HALO SURROUNDING THE INTENSE STARBURST SDSS J115630.63+500822.1^∗

Abstract

We report results from a new HST campaign that targets the O vi λλ1032, 1038Å doublet in emission around intensely star-forming galaxies. The programme aims to characterize the energy balance in starburst galaxies and gas cooling in the difficult-to-map coronal temperature regime of 2 − 5 × 10 5 Kelvin. We present the first resolved image of gas emission in the O vi line. Our target, SDSS J115630.63+500822.1, is very compact in the continuum but displays O vi emission to radii of 23 kpc. The surface brightness profile is well fit by an exponential with a scale length of 7.5 kpc. This is ten times the size of the photoionized gas, and we estimate that about 1/6 the total O vi luminosity comes from resonantly scattered continuum radiation. Spectroscopy -which closely resembles a stacked sample of archival spectra -confirms the O vi emission, and determines the column density and outflow velocity from blueshifted absorption. The combination of measurements enables a large number of calculations with few assumptions. The O vi regions fill only ∼ 10 −3 of the volume. By comparing the cooling time with the cloud sound-crossing time, the cooling distance with the size, and the pressure in the O vi and nebular gas, we conclude that the O vi-bearing gas cannot have been lifted to the scale height at this temperature, and must be cooling in situ through this coronal temperature regime. The coronal phase contains ∼ 1 % of the ionized mass, and its kinetic energy at a given instant is ∼ 1 % of the budget set by supernova feedback. However a much larger amount of the gas must have cooled through this phase during the star formation episode. The outflow exceeds the escape velocity and the gas may become unbound, but it will recombine before it escapes and become visible to Lyman (and O i) spectroscopy. The mapping of this gas represents a crucial step in further constraining galaxy formation scenarios and guiding the development of future astronomical satellites.

Initial mass function variations cannot explain the ionizing spectrum of low metallicity starbursts

“…Observations at high redshift, where the extragalactic background is softer due to evolution in the AGN luminosity function, again imply additional sources of hard photons or collisional mechanisms (Chisholm et al 2018), while there are also now a handful of examples of intense starbursts which show the ultraviolet O VI lines in emission (e.g. Hayes et al 2016).…”

The Structure of Multiphase Galactic Winds

“…Despite the wealth of observational data, a precise determination of the properties of winds and what drives them remains elusive because of inherent difficulties in disentangling the location and physical nature of the observed gas. Upcoming emission observations with KCWI and MUSE provide a promising path forward to greatly improve our understanding galactic wind as has been recently demonstrated observationally (Hayes et al 2016;Rupke et al 2019;Burchett et al 2021) and theoretically (e.g., Nelson et al 2021).…”

ESA Voyage 2050 white paper: A complete census of the gas phases in and around galaxies, far-UV spectropolarimetry as a prime tool for understanding galaxy evolution and star formation

“…We are only starting to understand empirically how outflows from galaxies might be able to heat the halo gas. Hayes et al (2016) have, for instance, undertaken a comprehensive study of halos of star forming galaxies at z ∼ 0.2 with HST/ACS and COS. In this first study of a single star-forming galaxy, they have detected a halo of O vi 1032, 1038 Å emission.…”

“…About 1/6 of the total O vi luminosity was estimated to come from resonantly scattered continuum radiation. From the spectra, Hayes et al (2016) derive a large column of O vi absorption which is outflowing at several hundred km s −1 . Since the UV spectrum of this galaxy resembles a stack of star-forming galaxies in the HST archive, this strongly suggests that this is a common phenomenology in star-forming galaxies.…”