ALMA suggests outflows in z ∼ 5.5 galaxies

Abstract

We present the first attempt to detect outflows from galaxies approaching the Epoch of Reionization (EoR) using a sample of 9 star-forming (SFR = 31 ± 20 M yr −1 ) z ∼ 5.5 galaxies for which the [CII]158µm line has been previously obtained with ALMA. We first fit each line with a Gaussian function and compute the residuals by subtracting the best fitting model from the data. We combine the residuals of all sample galaxies and find that the total signal is characterised by a flux excess of ∼ 0.5 mJy extended over ∼ 1000 km s −1 . Although we cannot exclude that part of this signal is due to emission from faint satellite galaxies, we show that the most probable explanation for the detected flux excess is the presence of broad wings in the [CII] lines, signatures of starburst-driven outflows. We infer an average outflow rate oḟ M = 54±23 M yr −1 , providing a loading factor η =Ṁ/SFR = 1.7±1.3 in agreement with observed local starbursts. Our interpretation is consistent with outcomes from zoomed hydro-simulations of Dahlia, a z ∼ 6 galaxy (SFR ∼ 100 M yr −1 ) whose feedback-regulated star formation results into an outflow rateṀ ∼ 30 M yr −1 . The quality of the ALMA data is not sufficient for a detailed analysis of the [CII] line profile in individual galaxies. Nevertheless, our results suggest that starburst-driven outflows are in place in the EoR and provide useful indications for future ALMA campaigns. Deeper observations of the [CII] line in this sample are required to better characterise feedback at high-z and to understand the role of outflows in shaping early galaxy formation.

The ALPINE-ALMA [C II] survey: Star-formation-driven outflows and circumgalactic enrichment in the early Universe

“…On the one hand, the diversity of ALPINE galaxies (almost 2 dex in SFR and M are spanned across the main sequence) and the wealth 1 Following the commonly adopted nomenclature in the literature, we use normal when referring to galaxies on the starforming Main Sequence (e.g., Brinchmann et al 2004;Noeske et al 2007;Daddi et al 2010;Rodighiero et al 2011;Speagle et al 2014). of ancillary multi-wavelength photometric data (from UV to FIR) enable us to investigate primary dependencies of stellar outflows on galaxy physical properties. On the other hand, the large statistics provided (the number of [C II]-detected galaxies used for the stacking is approximately six-fold higher than similar previous studies; see Gallerani et al 2018) yields enough sensitivity to (i) map the spatial extension of the outflowing gas and (ii) constrain the circumgalactic enrichment on scales of a few tens of kiloparsec, providing new critical pieces of information on the baryon cycling physics that drives the evolution of high-z galaxies.…”

“…Unfortunately, even significant investments of ALMA time ( 1 hour; see e.g., Capak et al 2015) do not provide sufficiently good spectra to analyze in detail the weak broad components of FIR lines in individual "normal" 1 star-forming galaxies at z > 4, and stacking of large samples would be needed. Some indications of the discovery potential of the FIR-line stacking analysis come from recent results by Gallerani et al (2018), who found flux excesses at about v ± 500 km s −1 in the stacked residual [C II]-spectrum of a small sample of nine galaxies at z ∼ 5 − 6 (Capak et al 2015), likely ascribed to broad wings tracing star-formation-driven outflows.…”

ALMA Reveals Metals yet No Dust within Multiple Components in CR7

“…Such relatively complex [C II] structures and the observed velocity offsets are also observed in hydrodynamical simulations of galaxies at z 6 7 » -with halo masses of M 10 11  (e.g., Vallini et al 2013;Pallottini et al 2017b). The line profiles do not show evidence for broad-line components due to outflows (e.g., Gallerani et al 2016). Therefore, we are likely observing satellites (B, B-2, C, C-2) falling into a more massive central galaxy.…”

On the discovery of fast molecular gas in the UFO/BAL quasar APM 08279+5255 atz= 3.912

“…Only a couple of high-redshift massive outflows of atomic gas are known, one seen in [CII] in the quasar J1148 at z=6.4 (Maiolino et al 2012, Cicone et al 2015, the other possibly seen in [NII] in the bright submillimetre galaxy HLSJ091828.6+514223 ay z=5.2 (Rawle et al 2014). Recent results suggest that [CII] outflows may also be present in starburst galaxies at z ∼ 5.5 (Gallerani et al 2017). What about molecular outflows?…”