Molecular cloud formation as seen in synthetic H i and molecular gas observations

Abstract

We present synthetic Hi and CO observations of a numerical simulation of decaying turbulence in the thermally bistable neutral medium. We first present the simulation, which produces a clumpy medium, with clouds initially consisting of clustered clumps. Self-gravity causes these clump clusters to merge and form more homogeneous dense clouds. We apply a simple radiative transfer algorithm, throwing rays in many directions from each cell, and defining every cell with A v > 1 as molecular. We then produce maps of Hi, CO-free molecular gas, and CO, and investigate the following aspects: i) The spatial distribution of the warm, cold, and molecular gas, finding the well-known layered structure, with molecular gas being surrounded by cold Hi and this in turn being surrounded by warm Hi. ii) The velocity of the various components, finding that the atomic gas is generally flowing towards the molecular gas, and that this motion is reflected in the frequently observed bimodal shape of the Hi profiles. This conclusion is, however, tentative, because we do not include feedback that may produce Hi gas receding from molecular regions. iii) The production of Hi self-absorption (HISA) profiles, and the correlation of HISA with molecular gas. In particular, we test the suggestion of using the second derivative of the brightness temperature Hi profile to trace HISA and molecular gas, finding significant limitations. On a scale of several parsecs, some agreement is obtained between this technique and actual HISA, as well as a correlation between HISA and the molecular gas column density. This correlation, however, quickly deteriorates towards sub-parsec scales. iv) The column density PDFs of the actual Hi gas and those recovered from the Hi line profiles, finding that the latter have a cutoff at column densities where the gas becomes optically thick, thus missing the contribution from the HISA-producing gas. We also find that the powerlaw tail typical of gravitational contraction is only observed in the molecular gas, and that, before the power-law tail develops in the total gas density PDF, no CO is yet present, reinforcing the notion that gravitational contraction is needed to produce this component.

Synthetic observations of star formation and the interstellar medium

“…Overall, synthetic observations have helped support the identification (and improve the confidence of identification) of a number of candidate collision sites in recent years (e.g. Torii et al, 2015;Fukui et al, 2016;Torii et al, 2017b;Fukui et al, 2017;Torii et al, 2017a;Kohno et al, 2017;Nishimura et al, 2017;Hayashi et al, 2017). With further occurring along the line-of-sight of the reader.…”

The Molecular Cloud Lifecycle

“…However, for most of the assembly time, the clouds may be in a mostly atomic form, and molecule formation may start almost simultaneously with local collapse and star formation (e.g. Hartmann et al 2001;Bergin et al 2004;Clark et al 2012;Heiner et al 2015;, explaining the observation that, in the Solar Neighborhood (i.e. within 1 kpc from the Sun) most of the molecular clouds exhibit signs of (low-mass) star formation (e.g.…”

Energy Budget of Forming Clumps in Numerical Simulations of Collapsing Clouds

“…The simulations used in this work are those presented in Gómez & Vázquez-Semadeni (2014) and Heiner et al (2015). For historical reasons, in this paper we will refer to these simulations as RUN20 and RUN03, respectively.…”