Letters of Intent received in 2015

LoI 2017-294
The link between dense gas and star formation

Topics

What are reliable dense gas and star formation tracers? (including both observational and theoretical talks)
-- small-scale (sub-pc) regime
-- giant molecular cloud regime
-- kpc-scale to entire galaxies/low to high z
-- extreme environments: Galactic Center, AGNs, low metallicity environments.

What is the link between dense gas and star formation in our Galaxy:
-- small-scale (sub-pc) processes
-- processes/conditions/relations on giant molecular cloud scales
-- extreme environments: Galactic Center, outer disk, diffuse clouds ...

What is the link between dense gas and star formation in nearby galaxies
-- processes/conditions/relations on giant molecular cloud scales
-- galactic scales/importance of galactic environment/stellar mass etc.
-- extreme environments: AGN nuclei, starbursts/mergers, outflows, outer disks, low metallicity objects

High redshift universe
-- galactic scales/importance of galactic environment/stellar mass etc.
-- extreme environments: ULIRGs, AGN, ...

Rationale

Extragalactic research has demonstrated a clear link between molecular gas and star formation in recent years, at scales ranging from observations of clouds in the Milky Way to whole galaxies. Recent technical progress, both in terms of sensitivity and instantaneous bandwidth, has made it possible to observe a broad variety of lines tracing dense gas in the Milky and in nearby galaxies. It is becoming possible to observe galaxies in detail in several lines simultaneously, making the degree of comparison with Galactic observations incomparable to the situation only a few years ago. Much, but not all, of this progress is due to the Atacama Large Millimeter Array (ALMA) in Chile, whose construction is almost finished although observations started several years ago. By 2017, ALMA will have produced a much much larger database of observations of a variety of dense gas tracers in objects near and far.

The star formation process involves conversion of atomic to molecular gas, formation of dense molecular gas (cores/clumps), followed by the onset of star formation in the densest material. While this general picture is accepted, the relation between bulk molecular gas and dense gas and the ensuing star formation is less clear. Significant progress is expected towards a better description of the star formation process and its 'quantification', i.e. star formation law and its variation with environment.

A linear relation is observed between the star formation rate and the mass of dense gas over 10 orders of magnitude, from local clouds to entire galaxies (as measured by Far-IR emission and the HCN(1-0) line). Very recent refinements, however, suggest that the "scatter" around the linear relationship is not scatter but coherent differences due to environment or position within spiral disks (Hao et al 2015 ApJ 810, 140; Usero et al 2015, 2015arXiv150600703U). This is for the HCN(1-0) line and could be linked to variations in the dense gas fraction and/or abundance variations. The proposed symposium should provide a forum for this relatively unexplored subject. Notably, over the next 2 years, a lot of progress will be made interpreting these variations and identifying whether other lines share these variations and, thus, link processes on cloud scales to properties observed on kpc-scales (i.e. independent of stochastic effects).

Many instruments are improving rapidly and ALMA, still ramping up to full sensitivity, will be able to map many transitions of these lines in very short times. The angular resolution of ALMA is such that we should be able to separate the effect of AGN on the surrounding gas, currently hampered by presence of insufficiently resolved AGN. The consistency of the Far-IR to HCN(1-0) luminosity ratio over broadly varying conditions (e.g. clouds to ULIRGs) is striking. Is this telling us that star formation transforms dense gas into stars with a non-varying IMF throughout the universe? Measurements currently underway including higher-J transitions should help provide an explanation and 2017 is a very timely period for this.