Band-9 ALMA Observations of the [N II] 122 μm Line and FIR Continuum in Two High-z Galaxies
We present Atacama Large Millimeter Array (ALMA) observations of two high-redshift systems (SMMJ02399-0136 at z_1 ~ 2.8 and the Cloverleaf QSO at z_1 ~ 2.5) in their rest-frame 122 μm continuum (νsky ~ 650 GHz, λsky ~ 450 μm) and [N ii] 122 μm line emission. The continuum observations with a synthesized beam of ~0."25 resolve both sources and recover the expected flux. The Cloverleaf is resolved into a partial Einstein ring, while SMMJ02399-0136 is unambiguously separated into two components: a point source associated with an active galactic nucleus and an extended region at the location of a previously identified dusty starburst. We detect the [N ii] line in both systems, though significantly weaker than our previous detections made with the first generation z (Redshift) and Early Universe Spectrometer. We show that this discrepancy is mostly explained if the line flux is resolved out due to significantly more extended emission and longer ALMA baselines than expected. Based on the ALMA observations we determine that ≥75% of the total [N ii] line flux in each source is produced via star formation. We use the [N ii] line flux that is recovered by ALMA to constrain the N/H abundance, ionized gas mass, hydrogen- ionizing photon rate, and star formation rate. In SMMJ02399-0136 we discover it contains a significant amount (~1000 M_⊙ yr^(−1)) of unobscured star formation in addition to its dusty starburst and argue that SMMJ02399-0136 may be similar to the Antennae Galaxies (Arp 244) locally. In total these observations provide a new look at two well-studied systems while demonstrating the power and challenges of Band-9 ALMA observations of high-z systems.
© 2015 American Astronomical Society. Received 2015 February 18; accepted 2015 April 30; published 2015 June 23. This paper makes use of the following ALMA data: ADS/JAO.ALMA# 2011.0.00747.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (CANADA) and an NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science foundation operated under cooperative agreement by Associated Universities, Inc. This research also made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration 2013). We thank the anonymous referee for helpful feedback. We also would like to thank the staff at the NRAO and North American ALMA Science Center for their support of these observations and their reduction. Lastly, C.F. thanks R. Ivison for providing the continuum data from Ivison et al. (2010) and F. Walter for helpful comments on the draft of this paper.
Published - 0004-637X_806_2_260.pdf