The 2014 Alma Long Baseline Campaign: An Overview

Abstract

A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

A SEARCH FOR SUBMILLIMETER H2O MASERS IN ACTIVE GALAXIES: THE DETECTION OF 321 GHz H2O MASER EMISSION IN NGC 4945

“…Since the masers detected in this program are unresolved, their spatial structures have yet to be explored for studying the circumnuclear regions of their host galaxy. ALMA has progressively achieved longer baselines and has attained a maximum angular resolution of ∼30 mas (corresponding to 1.5 pc at a distance of 10 Mpc) in Band 7 (e.g., ALMA Partnership et al 2015aPartnership et al , 2015b. Future observations will be able to resolve the circumnuclear gas of AGNs on scales that are becoming comparable to very long baseline interferometry observations at 22 GHz.…”

ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT)

“…1). The existence of disk substructures around a Class I object is intriguing, but it is not a first (e.g., ALMA Partnership et al 2015;Sheehan & Eisner 2017b. Spiral arms in young objects have already been seen in the millimeter (Pérez et al 2016;Andrews et al 2018).…”

“…On the other hand, rings in disks are recurrently observed at any evolutionary stage (e.g., Andrews et al 2016;Sheehan & Eisner 2018). Since the dynamical interaction with (forming) planets is among the most promising explanations for these structures (e.g., Dong et al 2015;Isella et al 2016), their presence so early in the disk lifetime has been used to support the scenario of rapid giant planet formation (ALMA Partnership et al 2015). The timescale for the formation of these planets could play a role in determining the predominance of the two competing processes of dust drift and dust trapping and, in turn, could allow or hinder a rapid (1−2 Myr) evolution of the disk.…”

“…An efficient way to tackle this problem is imaging the less well studied Class I objects, namely protostars younger than 1 Myr that are deeply embedded in their natal envelope and that show a flatter SED between near-and far-IR wavelengths. The few ALMA observations of such objects (e.g., ALMA Partnership et al 2015;Sheehan & Eisner 2017b) reveal that disk substructures are already present in the disk suggesting an early onset of planet formation.…”

The origin of tail-like structures around protoplanetary disks

“…While directly observing planet formation is difficult, several indirect signatures of this process have recently been identified. For instance, observations from the near-infrared to millimeter bands in dust continuum and in scattered light have revealed a variety of disk substructures, such as spiral arcs and arms, clumps, rings, and gaps (e.g., van der Marel et al 2013; ALMA Partnership et al 2015;Pérez et al 2016;Liu et al 2016), indicating that protoplanetary disks often do not have simple monotonically declining surface density profiles. Theoretical interpretations of these substructures imply a presence of already-formed massive planets (e.g., Kley & Nelson 2012) or/and physical phenomena at work, such as gravitational instability or vortices, which can assist planet formation (e.g., Dong et al 2016;Regály et al 2017).…”