The aim of this work is to characterize the stellar population between Earth and the Orion A molecular cloud where the well-known star formation benchmark Orion nebula cluster (ONC) is embedded. We used the denser regions the Orion A cloud to block optical background light, TEST isolating the stellar population in front of it. We then used a multi-wavelength observational approach to characterize the cloud’s foreground stellar population. We find that there is a rich stellar population in front of the Orion A cloud, from B-stars to M-stars, with a distinct 1) spatial distribution, 2) luminosity function, and 3) velocity dispersion from the reddened population inside the Orion A cloud. The spatial distribution of this population peaks strongly around NGC 1980 (iota Ori) and is, in all likelihood, the extended stellar content of this poorly studied cluster. We infer an age of ∼4 − 5 Myr for NGC 1980 and estimate a cluster population of about 2000 stars, which makes it one of the most massive clusters in the entire Orion complex. This newly found population overlaps significantly with what is currently assumed to be the ONC and the L1641N populations, and can make up for more than 10-20% of the ONC population (30-60% if the Trapezium cluster is excluded from consideration). What is currently taken in the literature as the ONC is then a mix of several intrinsically different populations, namely 1) the youngest population, including the Trapezium cluster and ongoing star formation in the dense gas inside the nebula, 2) the foreground population, dominated by the NGC 1980 cluster, and 3) the poorly constrained population of foreground and background Galactic field stars. Our results support a scenario where the ONC and L1641N are not directly associated with NGC 1980, i.e., they are not the same population emerging from its parental cloud, but are instead distinct overlapping populations. The nearest massive star formation region and the template for massive star- and cluster formation models is then substantially contaminated by the foreground stellar population of the massive NGC 1980 cluster, formed about 4–5 Myr ago in a different, but perhaps related, event in the larger Orion star formation complex. This result calls for a revision of most of the observables in the benchmark ONC region (e.g., ages, age spread, cluster size, mass function, disk frequency, etc.).

ABSTRACT The very long, thin infrared dark cloud Nessie is even longer than had been previously claimed, and an analysis of its Galactic location suggests that it lies directly in the Milky Way’s mid-plane, tracing out a highly elongated bone-like feature within the prominent Scutum-Centaurus spiral arm. Re-analysis of mid-infrared imagery from the Spitzer Space Telescope shows that this IRDC is at least 2, and possibly as many as 8 times longer than had originally been claimed by Nessie’s discoverers, ; its aspect ratio is therefore at least 150:1, and possibly as large as 800:1. A careful accounting for both the Sun’s offset from the Galactic plane (∼25 pc) and the Galactic center’s offset from the (lII, bII)=(0, 0) position defined by the IAU in 1959 shows that the latitude of the true Galactic mid-plane at the 3.1 kpc distance to the Scutum-Centaurus Arm is not b = 0, but instead closer to b = −0.5, which is the latitude of Nessie to within a few pc. Apparently, Nessie lies _in_ the Galactic mid-plane. An analysis of the radial velocities of low-density (CO) and high-density (${\rm NH}_3$) gas associated with the Nessie dust feature suggests that Nessie runs along the Scutum-Centaurus Arm in position-position-velocity space, which means it likely forms a dense ‘spine’ of the arm in real space as well. No galaxy-scale simulation to date has the spatial resolution to predict a Nessie-like feature, but extant simulations do suggest that highly elongated over-dense filaments should be associated with a galaxy’s spiral arms. Nessie is situated in the closest major spiral arm to the Sun toward the inner Galaxy, and appears almost perpendicular to our line of sight, making it the easiest feature of its kind to detect from our location (a shadow of an Arm’s bone, illuminated by the Galaxy beyond). Although the Sun’s (∼25 pc) offset from the Galactic plane is not large in comparison with the half-thickness of the plane as traced by Population I objects such as GMCs and HII regions (∼200 pc; ), it may be significant compared with an extremely thin layer that might be traced out by Nessie-like “bones” of the Milky Way. Future high-resolution extinction and molecular line data may therefore allow us to exploit the Sun’s position above the plane to gain a (very foreshortened) view “from above" of dense gas in Milky Way’s disk and its structure.

Instructions for Co-Authors The full file repository for this paper is at a shared Google Drive directory, https://drive.google.com/#folders/0BxIRxiTe1u6BcGlnUGt2ckU1Vms, shared with all co-authors. NOTE: THE “AAS” (PRESS CONFERENCE) SLIDES AT HTTPS://DRIVE.GOOGLE.COM/#FOLDERS/0BXIRXITE1U6BRKLQRZLUAUNUUUU GIVE A BETTER IDEA OF WHERE THIS DRAFT IS GOING THAN THE TEXT/FIGURES HERE AS OF NOW... AG WILL UPDATE ALL BY C.1/1/13! The Mendeley Library “Nessie and Friends” used to house references used in this work, at: http://www.mendeley.com/groups/2505711/nessie-and-friends/, but since Authorea works more directly with ADS links, we’ll use the ADS Private Library at http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?library&libname=Nessie+and+Friends&libid=488e32b08b instead. The Mendeley library is the source of the nessie.bib file in the “Bibliography” folder here on Authorea, but I am not sure how to get the ADS references out as a .bib file. xxAlberto?xx The Glue software used to intercompare data sets used in this work is online through: http://glue-viz.readthedocs.org/en/latest/ We are using Authorea.com as an experimental platform to compile this paper. The manual steps we will need to take before submission include: - download LaTeX file - modify LaTeX file to use aas macros - insert needed information (e.g. about authors, running header) into was version of LaTeX manuscript - extract needed figures from relevant folders here & bundle them with LaTeX manuscript & macros - create .bib file from ADS Private Library - add .bib file to folder with manuscript & figures - fix in-line referencing so that $\citet$ and $\citep$ commands work