9 research outputs found
The linewidth-size relationship in the dense ISM of the Central Molecular Zone
The linewidth (sigma) - size (R) relationship has been extensively measured
and analysed, in both the local ISM and in nearby normal galaxies. Generally, a
power-law describes the relationship well with an index ranging from 0.2-0.6,
now referred to as one of "Larson's Relationships." The nature of turbulence
and star formation is considered to be intimately related to these
relationships, so evaluating the sigma-R correlations in various environments
is important for developing a comprehensive understanding of the ISM. We
measure the sigma-R relationship in the Central Molecular Zone (CMZ) of the
Galactic Centre using spectral line observations of the high density tracers
N2H+, HCN, H13CN, and HCO+. We use dendrograms, which map the hierarchical
nature of the position-position-velocity (PPV) data, to compute sigma and R of
contiguous structures. The dispersions range from ~2-30 km/s in structures
spanning sizes 2-40 pc, respectively. By performing Bayesian inference, we show
that a power-law with exponent 0.3-1.1 can reasonably describe the sigma-R
trend. We demonstrate that the derived sigma-R relationship is independent of
the locations in the PPV dataset where sigma and R are measured. The uniformity
in the sigma-R relationship suggests turbulence in the CMZ is driven on the
large scales beyond >30 pc. We compare the CMZ sigma-R relationship to that
measured in the Galactic molecular cloud Perseus. The exponents between the two
systems are similar, suggestive of a connection between the turbulent
properties within a cloud to its ambient medium. Yet, the velocity dispersion
in the CMZ is systematically higher, resulting in a coefficient that is nearly
five times larger. The systematic enhancement of turbulent velocities may be
due to the combined effects of increased star formation activity, larger
densities, and higher pressures relative to the local ISM.Comment: 11 pages, 8 figures, Accepted for publication in MNRA
Modeling CO Emission: II. The Physical Characteristics that Determine the X factor in Galactic Molecular Clouds
We investigate how the X factor, the ratio of H_2 column density (NH2) to
velocity-integrated CO intensity (W), is determined by the physical properties
of gas in model molecular clouds (MCs). We perform radiative transfer
calculations on chemical-MHD models to compute X. Using integrated NH2 and W
reproduces the limited range in X found in observations, resulting in a mean
value X=2\times10^20 s/cm^2/K^1/km^1 from the Galactic MC model. However, in
limited velocity intervals, X can take on a much larger range due to CO line
saturation. Thus, X strongly depends on both the range in gas velocities and
volume densities. The temperature (T) variations within individual MCs do not
strongly affect X, as dense gas contributes most to setting X. For fixed
velocity and density structure, gas with higher T has higher W, yielding X ~
T^-1/2 for T~20-100 K. We demonstrate that the linewidth-size scaling relation
does not influence the X factor - only the range in velocities is important.
Clouds with larger linewidths, regardless of the linewidth-size relation, have
a higher W, corresponding to a lower value of X, scaling roughly as X ~
sigma^-1/2. The "mist" model, consisting of optically thick cloudlets with
well-separated velocities, does not accurately reflect the conditions in a
turbulent MC. We propose that the observed cloud-average values of X ~ XGal is
simply a result of the limited range in NH2, temperatures, and velocities found
in Galactic MCs - a ~constant value of X therefore does not require any
linewidth-size relation, or that MCs are virialized objects. Since gas
properties likely differ (slightly) between clouds, masses derived through a
standard X should only be considered as a rough first estimate. For
temperatures T~10-20 K, velocity dispersions ~1-6 km/s, and NH2~2-20\times10^21
cm^-2, we find cloud-averaged X ~ 2-4\times10^20 s/cm^2/K^1/km^1 for
Solar-metallicity models.Comment: 24 pages, including 21 Figures, Accepted to MNRA
Maximally Star-Forming Galactic Disks II. Vertically-Resolved Hydrodynamic Simulations of Starburst Regulation
We explore the self-regulation of star formation using a large suite of high
resolution hydrodynamic simulations, focusing on molecule-dominated regions
(galactic centers and [U]LIRGS) where feedback from star formation drives
highly supersonic turbulence. In equilibrium the total midplane pressure,
dominated by turbulence, must balance the vertical weight of the ISM. Under
self-regulation, the momentum flux injected by feedback evolves until it
matches the vertical weight. We test this flux balance in simulations spanning
a range of parameters, including surface density , momentum injected
per stellar mass formed (), and angular velocity. The simulations are
2D radial-vertical slices, including both self-gravity and an external
potential that confines gas to the disk midplane. After the simulations reach a
steady state in all relevant quantities, including the star formation rate
, there is remarkably good agreement between the vertical weight,
the turbulent pressure, and the momentum injection rate from supernovae. Gas
velocity dispersions and disk thicknesses increase with . The
efficiency of star formation per free-fall time at the mid-plane density is
insensitive to the local conditions and to the star formation prescription in
very dense gas. We measure efficiencies 0.004-0.01, consistent with low
and approximately constant efficiencies inferred from observations. For
(100--1000) \msunpc, we find (0.1--4) \sfrunits,
generally following a relationship. The measured
relationships agree very well with vertical equilibrium and with turbulent
energy replenishment by feedback within a vertical crossing time. These
results, along with the observed relation in high density
environments, provide strong evidence for the self-regulation of star
formation.Comment: 22 pages, 14 figures. Accepted for publication in Ap
The Glucuronyltransferase GlcAT-P Is Required for Stretch Growth of Peripheral Nerves in Drosophila
During development, the growth of the animal body is accompanied by a concomitant elongation of the peripheral nerves, which requires the elongation of integrated nerve fibers and the axons projecting therein. Although this process is of fundamental importance to almost all organisms of the animal kingdom, very little is known about the mechanisms regulating this process. Here, we describe the identification and characterization of novel mutant alleles of GlcAT-P, the Drosophila ortholog of the mammalian glucuronyltransferase b3gat1. GlcAT-P mutants reveal shorter larval peripheral nerves and an elongated ventral nerve cord (VNC). We show that GlcAT-P is expressed in a subset of neurons in the central brain hemispheres, in some motoneurons of the ventral nerve cord as well as in central and peripheral nerve glia. We demonstrate that in GlcAT-P mutants the VNC is under tension of shorter peripheral nerves suggesting that the VNC elongates as a consequence of tension imparted by retarded peripheral nerve growth during larval development. We also provide evidence that for growth of peripheral nerve fibers GlcAT-P is critically required in hemocytes; however, glial cells are also important in this process. The glial specific repo gene acts as a modifier of GlcAT-P and loss or reduction of repo function in a GlcAT-P mutant background enhances VNC elongation. We propose a model in which hemocytes are required for aspects of glial cell biology which in turn affects the elongation of peripheral nerves during larval development. Our data also identifies GlcAT-P as a first candidate gene involved in growth of integrated peripheral nerves and therefore establishes Drosophila as an amenable in-vivo model system to study this process at the cellular and molecular level in more detail