The assembly of "normal" galaxies at z=7 probed by ALMA (1502.06634v2)

Abstract: We report new deep ALMA observations aimed at investigating the [CII]158um line and continuum emission in three spectroscopically confirmed Lyman Break Galaxies at 6.8<z<7.1, i.e. well within the re-ionization epoch. With Star Formation Rates of SFR ~ 5-15 Msun/yr these systems are much more representative of the high-z galaxy population than other systems targeted in the past by millimeter observations. For the galaxy with the deepest observation we detect [CII] emission at redshift z=7.107, fully consistent with the Lyalpha redshift, but spatially offset by 0.7" (4 kpc) from the optical emission. At the location of the optical emission, tracing both the Lyalpha line and the far-UV continuum, no [CII] emission is detected in any of the three galaxies, with 3sigma upper limits significantly lower than the [CII] emission observed in lower reshift galaxies. These results suggest that molecular clouds in the central parts of primordial galaxies are rapidly disrupted by stellar feedback. As a result, [CII] emission mostly arises from more external accreting/satellite clumps of neutral gas. These findings are in agreement with recent models of galaxy formation. Thermal far-infrared continuum is not detected in any of the three galaxies. However, the upper limits on the infrared-to-UV emission ratio do not exceed those derived in metal- and dust-poor galaxies.

Overview of ALMA Observations at z≈7

The paper by Maiolino et al. presents an analysis utilizing the Atacama Large Millimeter/submillimeter Array (ALMA) to probe the [CII]158μm line and continuum emission in three spectroscopically confirmed Lyman Break Galaxies situated in the high-redshift universe (6.8 < z ≤ 7.1). The research focuses on galaxies with moderate star formation rates (SFRs), ranging from 5 to 15 M☉ yr⁻¹, representing a more typical population of early galaxies compared to those previously targeted in millimeter observations, which often emphasized extreme systems like quasar hosts.

The paper successfully detects [CII] emission in one of the galaxies, BDF3299, at z = 7.107, showing a notable spatial offset of 0.7'' (approximately 4 kpc) from its optical centroid. This detection supports theoretical models suggesting that stellar feedback in primordial galaxies rapidly disrupts central molecular clouds, leading to [CII] emission predominantly from external, neutral gas clumps surrounding the galaxy. Furthermore, the absence of thermal far-infrared continuum detection in these galaxies aligns with expectations for systems having low dust and metal content.

Key Results and Claims

1. [CII] Emission Detection: Out of the three galaxies, meaningful [CII] emission was detected only in BDF3299. The emission showed a 7σ significance and a velocity offset fully consistent with the systemic velocity adjusted for expected IGM absorption of Lyα. The detection emphasizes the capability of ALMA in detecting such emissions even in high-redshift galaxies with low SFRs.
2. Spatial Offset of Emission: The spatial disparity between [CII] and optical emissions suggests a model where molecular clouds in the galaxy's inner regions are quickly disrupted by stellar feedback. Consequently, [CII] emits predominantly from accreting or satellite clumps of neutral gas.
3. Implications for Galaxy Formation Models: The findings align with current models predicting strong stellar feedback effects in early galaxies, supporting scenarios where [CII] emission is derived mainly from external clumps rather than the central galaxy.
4. Constraints on Dust and Metallic Content: Non-detection of thermal far-infrared continuum suggests these galaxies are characterized by low dust and metallic content. The upper limits on the IR-to-UV emission ratios conform with those observed in local low-metallicity galaxies, reinforcing the view that early galaxies lack significant dust.

Implications and Future Directions

The results provide significant contributions to understanding the ISM of early galaxies and reinforce existing galaxy formation models, validating theories of powerful feedback mechanisms disrupting gas within these galaxies. The spatial offset indicative of [CII] emissions from accreting neutral gas clumps presents an intriguing area for further paper, potentially offering insight into gas accretion processes at cosmic dawn.

Further investigations can broaden the scope by employing deeper observations and improving spatial resolutions to explore additional high-redshift systems. The possibility of gravitational lensing by foreground galaxies boosting detectability of faint [CII] clumps could be an efficient approach for future observations. Additionally, constraints on dust content suggest the necessity for refined models on dust production timescales and mechanisms, enhancing our understanding of dust evolution in the early universe.

Overall, the paper lays a foundational framework for interpreting millimeter observations of early galaxies and provides a template for future research efforts aimed at deciphering the nature and conditions of galaxies during the epoch of re-ionization.