Cosmic rays: the secret agent of galaxy evolution

Cosmic rays are rays of energetic particles and radiation, with their composition in our own Galaxy being dominated by protons. Ellis Owen, a final-year PhD student working on cosmic rays, star-formation and galaxy evolution, tells us about his research on them.

Cosmic ray particles are accelerated to high energies in violent astrophysical environments — in particular, those associated with the end-products of dying stars (for example, supernova remnants) — where they can reach energies as high as 1 PeV = 1015eV = 1 000 000 000 000 000 eV. Cosmic rays are observed to be distributed across a large energy range according to a distinctive power-law spectrum (see also the earlier post, here). 

cr_spectrum
The Cosmic Ray spectrum (Image source: http://www.iexp.uni-hamburg.de/groups/astroparticle/score/en/physics/)

The association of cosmic rays with stellar end-products makes them inextricably linked to the formation and evolution of stellar populations within galaxies. Their injection into an environment is strongly governed by the rate at which nearby stars die — and this, in turn, tracks the rate at which stars form.  Environments which are able to accelerate substantial numbers of cosmic rays tend to develop from the end-products of the most massive type O and type B stars, for which the progenitors have relatively short lifetimes. These cosmic ray source environments would, therefore, emerge shortly after the onset of star-formation in a galaxy. It follows that the galaxies and protogalaxies of the early Universe, as well as nearby rapidly star-forming galaxies (for example M82, Arp 220 and NGC 253), must be rich in cosmic rays.

The recent MSSL-led paper, ‘Interactions between ultra-high-energy particles and protogalactic environments’, explores the role that energetic hadronic particles play in star-forming galaxies. It explores the co-evolution of the magnetic field with cosmic ray injection and shows how a galactic magnetic field can operate to entrap charged cosmic ray particles to internalise their effects. Interstellar gas is heated by cosmic rays as they interact with baryons and radiation fields. In particular, the hadronic interactions they undergo deposit pions into the interstellar medium, which can decay into charged particles that lead to the thermalisation of cosmic ray energy (to drive interstellar gas heating), or into gamma-rays, which may be used as an observable tracer for interacting energetic cosmic rays. Cosmic rays can, therefore, deliver an important heating mechanism in star-forming galaxies — in fact, in the early Universe when conditions were different to local starburst galaxies, such a heating effect can actually dominate over more conventional photon-driven mechanisms.

Many actively star-forming galaxies in the local Universe are known to exhibit a distinctive galactic-scale outflow wind. These can be driven by various mechanisms, all of which are essentially due to the intense star-formation activity arising within a galaxy’s core. An example of this is seen emerging from the nearby starburst galaxy, M82, but such features would presumably also arise in the actively star-forming galaxies of the young Universe.

The magnificent starburst galaxy Messier 82
Optical/IR image of the nearby star-forming galaxy M82, with a clearly visible outflow wind. Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (NSF).

Cosmic rays may be transported by these outflows through advection, which can act to re-direct their heating power from within an internal galaxy environment towards the surrounding circumgalactic medium instead. The paper ‘Hadronic interactions of energetic charged particles in protogalactic outflow environments and implications for the early evolution of galaxies’ begins to explore this process and considers the action of advected cosmic rays in the early Universe. Earlier in cosmic time, intense star-forming activity in galaxies is thought to be fueled by inflowing filaments of cold gas. If cosmic rays can escape from their source galaxy to interact with these inflows, they may start to heat or evaporate them to halt their inflowing supply of cold gas. This would eventually curtail star-formation in the host galaxy (this process is called strangulation).

Together, these studies establish cosmic rays as an agent by which energy emitted by stars and their end-products can be delivered back into the environment of star-forming galaxies, yielding implications for the regulation of future star-forming activity in that system (a process called feedback). This may account for the unusual behavior in the recently detected galaxy (in Hashimoto et al. 2018), located in the very early Universe (at a redshift of 9.11, corresponding to a time 550 million years after the Big Bang). This object, MACS1149-JD1, appears to be forming stars at the point it was observed at a rate of around 4.2 solar masses per year. However, by studying its spectra, researchers were able to show that the existing stars within MACS1149-JD1 could be split into a young stellar population (presumably associated with the ongoing star-formation activity), and a much older stellar population, indicating a burst of activity around 100 million years before the time at which the galaxy was observed, and occurring at a much more intense rate. Much of the stellar mass of the system was built up in this earlier burst, which could have begun as early as 250 million years after the Big Bang.

Hubble and ALMA image of MACS1149-JD1, seen as it was 13.3 billion years ago. Image credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, W. Zheng (JHU), M. Postman (STScI), the CLASH Team, Hashimoto et al.

This early episode of star-formation seems to have occurred in an intense burst, providing the ideal conditions for the injection of cosmic rays. If they are able to be contained by the growing magnetic field of the galaxy, they would deliver a heating effect which could progressively quench star-formation. However, if the star-formation in MACS1149-JD1 is actually fuelled by inflows, the action of any outflow advecting cosmic rays into the circumgalactic environment could lead to strangulation. For star-formation to later be re-established in the second burst, sufficient time must pass to enable either the circumgalactic gases to cool, or for inflows beyond the surrounding heated region to return. The inflow velocities estimated from Lyman alpha velocity offset measurements for this system would suggest that inflows could return from beyond a heated/ionised region after around 100 million years — a time that would be consistent with the estimated length of the quiescent period inferred for this galaxy.

MACS1149_JD1_evo
Co-evolution of the star-formation rate and cosmic ray containment over time in MACS1149-JD1, according to three star-formation models proposed by Hashimoto et al 2018. Figure reproduced from Owen et al. 2019. The solid lines correspond to the star-formation rate of the galaxy (left axis), while the dashed-dotted lines show the magnetic field growth (as driven by the supernova activity). The shaded regions indicate the times during which cosmic ray heating mechanisms operate at maximum intensity under each star-formation model.

This research demonstrates that cosmic rays are an important agent in the regulation of the early star-formation activities of primordial galaxies, and establishes the importance of “multi-messenger” astrophysics in the field of galaxy evolution.

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