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Useful Astronomy Links:
12/16 --- On Our II Zw40 40 ALMA results here & here
03/15 --- On Our NGC 5253 SMA results here & here
Stars form from dense molecular clouds. However the particulars of how this process is
regulated, especially on galactic scales, remains a field of active research. My
research focuses on the evolution of star formation and its natal fuel across galaxies.
My primarily observational program uses radio/millimeter interferometry and IR
continuum/spectroscopy of nearby star forming galaxies to image, at high spatial resolution, the
physical and chemical properties of star formation's molecular gas fuel. The radio/millimeter
spectroscopy is done using interferometers including, NRAO's
Jansky Very Large Array,
the Atacama Large Millimeter/Submillimeter Array,
the Australia Telescope Compact Array and the
IRAM Plateau de Bure Interferometer.
Such observations illuminate the physical (temperature, density, pressure, amount and location of
heating/cooling sources) and dynamical (gas motions, locations of shocked gas, presence of
outflows, feedback from pre-existing stars or supermassive black holes [AGN]) properties
of a galaxy. The changing gas properties are compared with maps of star formation rate
and efficiency from radio/mm continuum and IR continuum/spectroscopy. The IR observations
are done with both ground and space-based telescopes including, the
Spitzer Space Telescope and the
W. H. Keck Observatories.
Studying spectral lines originating from transitions between quantum mechanical
levels within a wide variety of molecules give us directly the chemical abundances
in the interstellar medium, as well as can be used as 'thermometers' and 'barometers'
of the gas. [For a description of interstellar molecular spectroscopy see the
NRAO Essentials of Radio Astronomy Lecture section.] Moreover, the abundances
and temperatures of the molecular gas tell us about the internal microphysics that
control the cloud's state. Different molecules trace different chemical regimes.
Fortunately the interstellar medium has a rich chemistry of diagnostic molecules
see list; with one of these [Cyanoformaldehyde] being co-discovered
by yours truly), including both common species found on Earth (e.g. water [H2O],
ammonia [NH3], methane [CH4], formaldehyde [H2CO], carbon monoxide [CO])
and more exotic species (e.g. HCO+, N2H+, c-C3H2, HCCNC, HC9N). Example research
highlights are given below:
Recent Research Highlights:
See Publications for an exhaustive list of my publications. If you would like to use the
images below, a citation to the paper is required and I would appreciate an email indicating their use.
The Chemical Structure of Galaxies:
We have been surveying the chemical structure of nearby star forming galaxies at spatial resolutions that approach
individual giant molecular clouds. Different environments and star formation rates are sampled. This work builds
upon the initial discovery work on resolved extragalactic astrochemistry presented in
Meier & Turner 2005. In
Meier & Turner 2012 we study the chemical structure of the very nearby, nuclear barred
starburst, Maffei 2, with the OVRO and BIMA millimeter
interferometers. We discover, for the first time, extremely dramatic variations in the gas chemistry at 50 - 100 pc scales
across this nucleus. A principle-component analysis shows that certain groups of species are very tightly correlated, while
others are not correlated at all. Moreover, the differences in chemistry match the changing structural properties
of the nucleus. HCN, HCO+, HNC, HC3N and C2H are tightly correlated with star formation. Star formation is intimately
related to the abundance of dense gas. C2H shows that this dense gas is beginning to experience radiative feedback from
the current starburst episode. We discover, for the first time, a minor-axis molecular outflow in C2H, which allows us to kinematically
date the age of this burst. SiO, HNCO, and CH3OH are coupled and trace shocked gas associated with colliding molecular clouds
in the barred potential. This study represent some of the clearest evidence to date of the potential of large scale chemistry
to diagnose structural properties in galaxies.
In Meier et al. 2014
we image the nuclear starburst and inner disk of the nearby luminous infrared galaxy (LIRG), IRAS 04296+2923 with the
CARMA interferometer. This work builds on the previous chemical surveys by pushing the frontiers of the extragalatic chemical
surveys to more intense star forming environments and to regions outside the nucleus. These two environments are key to broadening
the range of physical and chemical parameter space covered. Comparisons of the CO isotopologues (13CO and C18O) demonstrate
that the nucleus of this LIRG has a different molecular cloud structure and possible nucleo-synthetic history from the inner disk.
The isotopic abundances suggest, that away from the starburst, a relatively unprocessed disk dominates, consistent with that found in
Meier et al. 2010. We tentatively detect shocked CH3OH emission well outside the nucleus.
The emission is found at the end of the large scale bar, indicating that large scale bars have the same chemical structure
as their smaller nuclear counterparts. Toward the nuclear starburst dense, low opacity molecular gas dominates,
explaining the extreme star formation rate.
In Meier et al. 2015 we use the new ALMA interferometer to map, at giant molecular cloud
scales, the structure of 50 different spectral lines, towards the nearby prototypical starburst galaxy
NGC 253. This is one of the most extensive imaging surveys of
the molecular properties of an external galaxy. A core of intense star formation, traced by radio recombination lines,
is embedded in the center of a rich molecular gas disk. The molecular gas disk is dense, warm and opaque even in HCN and HCO+.
A sheath of UV irradiated gas envelops the central starburst. Shocked gas signatures are seen across the disk reack out to the
base of the minor axis molecular outflow. Large molecules with seven atoms (and possibly more) are clearly seen and extended
across the entirety of the nucleus, demonstrating the rich, complex chemistry supported by the interstellar medium in this
(Left) The molecular morphology of ten species towards the nuclear bar, Maffei 2. Blue contours are CO,
yellow contours are 13CO and the colorscale are the chemical tracers. Peach contours delineate the nuclear starburst
(Meier & Turner 2012). (Right) The molecular morphology taken along a slice through the
major axis of NGC 253 vs. the frequency. Species are labeled. Those in italics are considered tentative indentifications
(Meier et al. 2015).
The Structure, Star Formation and Physical Conditions of Starbursts:
To understand the evolution of molecular gas in galaxies and its impact on future generations of star formation, it is
necessary to determine the structure and physical properties of that gas. To do this we map the structure and excitation of
molecules and correlate them with locations of star formation, shocks and radiative feedback. In
Meier et al. 2011 we present spatially resolved radiative transfer modeling of the
J=5-4, 10-9, and 16-15 transitions of cyanoacetylene (HC3N) in IC 342,
using the VLA, OVRO, and PdB interferometers. This gives a detailed characterization of gas physical conditions of this
Milky Way Galactic center clone, at subarcsecond resolution. Comparisons of HC3N with literature studies of other species
reveal a multicomponent interstellar medium in IC 342. Away from the starburst, densities and temperatures are fairly
uniform, being relatively high and cool. These dense cool cores are the proto-starbursts that are just beginning their
starburst phase. The current starburst, however, is unique. It shows faint, but hot HC3N. Implied thermal pressures argue that
the (small amount of) very dense gas here is in pressure equilibrium with the starburst ionized gas HII region. The
extreme star formation efficiency and pressure equilibrium indicates that the starburst must be more evolved and have
nearly finished consuming / dispersing the natal fuel that drove the burst. This study demonstrates that spatially
resolved physio-chemical modeling can be a powerful tool for studying starburst / galaxy evolution.
In Meier et al. 2010 we execute the first detailed, multiwave-wavelength study
of one of the closest luminous infrared galaxies (LIRGs), IRAS 04296+2923.
LIRGs are rare in the local Universe, so increasing the number of well-studied LIRGs is vital. This LIRG lies (optically)
hidden behind the Taurus Molecular Cloud and hence has remained unstudied. We combine Palomar near infrared, Keck
mid infrared, OVRO millimeter and VLA radio continuum observations to investigate the structure and star formation
properties of this LIRG. The galaxy appears as a 'normal' barred spiral with little indication of interaction, which
is rare for (U)LIRGs. This suggests that slow bar-driven transport drives the (secular) evolution. We determine
that it is one of the most molecular gas-rich objects out to its distance. The nucleus has two very compact
(<50 pc) super star clusters that have a total star formation rate >10x that of the entirety of the Milky Way. Based
on the rate of gas consumption and transport, this LIRG appears to be experiencing one of its first major starburst,
(Left) Shows the HC3N flux vs. rotational quantum number towards a 'proto-starburst' core
(Far Left - Top) and towards the current (mature) starburst (Far Left - Bottom), along with fitted
Large-Velocity-Gradient radiative transfer models. (Middle Left) The HC3N(5-4) nuclear morphology compared
to existing HCN(1-0) observations (Downes et al. 1992) (Meier et al. 2011).
(Right) The optical image of the Taurus Molecular
Cloud Region (courtesy of W.-H. Wang) with the (invisible) location of IRAS 04296+2923 indicated.
(Upper Far Right) The CO(1-0) emission, tracing the total molecular gas distibution, overlaid on the 6 cm
radio continuum which locates the nuclear super star cluster. (Lower Far Right) The Palomar near infrared
image of the disk ( Meier et al. 2010).
Mark McCoy (Ph.D.) (current)
Crystal Anderson (Ph.D.) (Graduated - 2016)
Alexandra Lutz (M.S.) (Graduated - 2013)
Molecular Spectroscopy Notes:
Below are a couple of molecular line primers to aid the extragalactic community is setting up
(The VLA band primer may also be found linked from the
NRAO spectral line intro webpage.)