ALMA REVEALS WEAK [N ii] EMISSION IN “TYPICAL” GALAXIES AND INTENSE STARBURSTS AT z = 5–6

Abstract

We report interferometric measurements of [Nii] 205 µm fine-structure line emission from a representative sample of three galaxies at z = 5-6 using the Atacama Large (sub)Millimeter Array (ALMA). These galaxies were previously detected in [Cii] and far-infrared continuum emission and span almost two orders of magnitude in star formation rate (SFR). Our results show at least two different regimes of ionized inter-stellar medium properties for galaxies in the first billion years of cosmic time, separated by theirUV star-forming galaxy, likely directly or indirectly (by its effect on the radiation field) related to low dust abundance and low metallicity. The infrared-luminous modestly star-forming Lyman Break Galaxy (LBG) in our sample is characterized by an ionized-gas fraction (L [CII] /L [NII] 20) typical of local star-forming galaxies and shows evidence for spatial variations in its ionized-gas fraction across an extended gas reservoir. The extreme SFR, warm and compact dusty starburst AzTEC-3 shows an ionized fraction higher than expected given its star-formation rate surface densitydominantly traces a diffuse ionized medium rather than star-forming Hii regions in this type of galaxy. This highest redshift sample of [Nii] detections provides some of the first constraints on ionized and neutral gas modeling attempts and on the structure of the inter-stellar medium at z = 5-6 in "normal" galaxies and starbursts.

Extended ionised and clumpy gas in a normal galaxy at z = 7.1 revealed by ALMA

“…However, IGM absorption and outflows cannot account for velocity shifts relative to other UV transitions (e.g. CIII or UV absorption features, as in the case of Capak et al 2015) and obviously cannot account for velocity shifts between different far-IR transitions, as observed by us and others (Pavesi et al 2016;Decarli et al 2014). In fact, all far-IR and UV emission lines, whose emission is not absorbed by IGM, should have the same velocity offsets if they are emitted from the same region of the galaxy.…”

ALMA Reveals Strong Emission in a Galaxy Embedded in a Giant Lyα Blob at z = 3.1

“…The ratio of LAB1-ALMA3 is shown, compared with those of various galaxies at z∼5 (Decarli et al 2014;Rawle et al 2014;Béthermin et al 2016;Pavesi et al 2016 and references therein) and local (U)LIRGs (Díaz-Santos et al 2013;Zhao et al 2016). ALMA3 shows one of the highest values seen to date, which indicates an enhanced [C II] emission.…”

“…In particular, it is used to evaluate global trend on the fraction of [C II] emission associated with ionized regions (i.e., HII regions; e.g., Oberst et al 2006;Decarli et al 2014;Pavesi et al 2016), mainly because nitrogen's ionization potential (14.5 eV) is higher than that of hydrogen (13.6 eV) so that [N II] arises only from ionized regions. Pavesi et al (2016) reported the expected a line ratio…”

“…ratio is also sensitive to estimate gas metallicity (e.g., Nagao et al 2012;Béthermin et al 2016;Pavesi et al 2016). Nagao et al (2012) suggest that the line ratio increases as metallicity decreases, considering both PDRs and HII regions in their model.…”

“…Gas accretion from the outside of ALMA3 may explain this relatively low metallicity. It is suggested that nitrogen may dominantly be in its doubly ionized state in high ionization conditions with lower dust shielding (e.g., Pavesi et al 2016). This effect is unlikely to be significant in ALMA3 because it is detected in dust continuum.…”