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Large reservoirs of cold molecular gas at high-redshift

3 septembre 2017

August 24th, 2017 — Galaxies in the early time had not yet formed a lot of stars, but were essentially made of gas. This gas can now be observed with ALMA at high-redshif. The CH+ cation in particular is very useful since it cannot form in cold gas without dissipation of mechanical energy or strong ultraviolet irradiation. In this article to Nature, astronomers from Paris Observatory and collaborators report the detection of CH+ (J = 1–0) emission and absorption lines in the spectra of six lensed starburst galaxies at redshifts near 2.5. The CH+ emission lines, broader than 1 000 km/s originate in dense shock waves powered by hot galactic winds. The CH+ absorption lines reveal highly turbulent reservoirs of cool (T=100 K), low-density gas, extending far (more than 10 kpc) outside the starburst galaxies (size =1kpc).

Most of the stars and galaxies we see today formed when the universe was still very young. Cosmologists use this observational constraint to model galaxy evolution with two key ingredients : the inflow of cold gas in the dark matter haloes and powerful outflows to mitigate star formation. Yet, the former have never been detected and the physics of the latter remain elusive.

With ALMA, Edith Falgarone and collaborators have discovered CH+ lines in all the starburst galaxies targeted so far at that critical epoch. CH+ is a special molecule : it
needs a lot of energy to form and is very reactive. Its lifetime is therefore very short and it highlights the location of energy dissipation in shocks.

Figure 1 : Les images en continu des 6 galaxies amplifiées. Les objets sont tous situés à z 2.3, où 1 seconde d’arc = 8.4kpc. L’émission continue est observée près de 1 mm en longueur d’onde et correspond à l’émission de la poussière chauffée à 360 microns dans le référentiel au repos des galaxies.

With CH+, they learn that, instead of being lost by radiation, energy is stored into the turbulent motions of fast galaxy-scale winds and previously unseen large reservoirs of cold gas. By driving turbulence in these reservoirs, galactic winds make outflowing matter partly recaptured by galaxies, extending the starburst phase instead of quenching it. They also learn that these gas reservoirs, drained by star formation, must be replenished possibly by the long-sought cold streams. These results challenge the theory of galaxy evolution, in which negative stellar feedback plays a critical role in quenching star formation.

Figure 2 : Spectres observés de CH+(1-0) pour les 6 galaxies. L’émission continue a été soustraite. Les spectres sont modélisés par une absorption et une émission gaussiennnes (en bleu), qui donnent le spectre résultant en rouge.

See also the ESO press release.

Reference

  • Falgarone, E., Zwaan, M. A., Godard, B., Bergin, E., Ivison, R. J., Andreani, P. M., Bournaud, F., Bussmann, R. S., Elbaz, D., Omont, A., Oteo, I., Walter, F.., 2017, Nature, 24 August, 548, 431

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