This work presents high-precision measurements of the specific baryon angular momentum j sub(b) contained in stars, atomic gas, and molecular gas, out to gap10 scale radii, in 16 nearby spiral ...galaxies of the THINGS sample. The accuracy of these measurements improves on existing studies by an order of magnitude, leading to the discovery of a strong correlation between the baryon mass M sub(b), j sub(b), and the bulge mass fraction beta , fitted by beta = -(0.34 + or = 0.03)lg(j sub(b)M super(-1)/10 super(-7)kpc km s super(-1) M sub(middot in circle) super(1)) - (0.04 + or = 0.01) on the full sample range of 0 < or = beta lap 0.3 and 10 super(9) M sub(middot in circle) < M sub(b) < 10 super(11) M sub(middot in circle). The corresponding relation for the stellar quantities M sub(*) and j sub(*) is identical within the uncertainties. These M-j- beta relations likely originate from the proportionality between jM super(-1) and the surface density of the disk that dictates its stability against (pseudo-)bulge formation. Using a cold dark matter model, we can approximately explain classical scaling relations, such as the fundamental plane of spiral galaxies, the Tully-Fisher relation, and the mass-size relation, in terms of the M-j(- beta ) relation. These results advocate the use of mass and angular momentum as the most fundamental quantities of spiral galaxies.
ABSTRACT We show that the mass fraction f atm = 1.35 M H I / M of neutral atomic gas (H i and He) in isolated local disk galaxies of baryonic mass M is well described by a straightforward stability ...model for flat exponential disks. In the outer disk parts, where gas at the characteristic dispersion of the warm neutral medium is stable in the sense of Toomre, the disk consists of neutral atomic gas; conversely, the inner part where this medium would be Toomre-unstable, is dominated by stars and molecules. Within this model, f atm only depends on a global stability parameter q j / ( GM ) , where j is the baryonic specific angular momentum of the disk and the velocity dispersion of the atomic gas. The analytically derived first-order solution f atm = min { 1 , 2.5 q 1.12 } provides a good fit to all plausible rotation curves. This model, with no free parameters, agrees remarkably well ( 0.2 dex) with measurements of f atm in isolated local disk galaxies, even with galaxies that are extremely H i-rich or H i-poor for their mass. The finding that f atm increasing monotonically with q for pure stability reasons offers a powerful intuitive explanation for the mean variation of f atm with M : in a cold dark matter universe, galaxies are expected to follow j ∝ M 2 / 3 , which implies the average scaling q ∝ M − 1 / 3 and hence f atm ∝ M − 0.37 , in agreement with the observations.
In this paper we compare the molecular gas depletion times and midplane hydrostatic pressure in turbulent, star-forming disk galaxies to internal properties of these galaxies. For this analysis we ...use 17 galaxies from the DYNAMO sample of nearby (z ∼ 0.1) turbulent disks. We find a strong correlation, such that galaxies with lower molecular gas depletion time (tdep) have higher gas velocity dispersion ( ). Within the scatter of our data, our observations are consistent with the prediction that made in theories of feedback-regulated star formation. We also show a strong, single power-law correlation between midplane pressure (P) and star formation rate surface density ( SFR), which extends for 6 orders of magnitude in pressure. Disk galaxies with lower pressure are found to be roughly in agreement with theoretical predictions. However, in galaxies with high pressure we find P/ SFR values that are significantly larger than theoretical predictions. Our observations could be explained with any of the following: (1) the correlation of SFR−P is significantly sublinear; (2) the momentum injected from star formation feedback (p*/m*) is not a single, universal value; or (3) alternate sources of pressure support are important in gas-rich disk galaxies. Finally, using published survey results, we find that our results are consistent with the cosmic evolution of tdep(z) and (z). Our interpretation of these results is that the cosmic evolution of tdep may be regulated not just by the supply of gas but also by the internal regulation of star formation via feedback.
Abstract
We compare the molecular and ionized gas velocity dispersions of nine nearby turbulent disks, analogs to high-redshift galaxies, from the DYNAMO sample using new Atacama Large ...Millimeter/submillimeter Array and GMOS/Gemini observations. We combine our sample with 12 galaxies at
z
∼ 0.5–2.5 from the literature. We find that the resolved velocity dispersion is systematically lower by a factor 2.45 ± 0.38 for the molecular gas compared to the ionized gas, after correcting for thermal broadening. This offset is constant within the galaxy disks and indicates the coexistence of a thin molecular gas disk and a thick ionized one. This result has a direct impact on the Toomre
Q
and pressure derived in galaxies. We obtain pressures ∼0.22 dex lower on average when using the molecular gas velocity dispersion,
σ
0,mol
. We find that
σ
0,mol
increases with gas fraction and star formation rate. We also obtain an increase with redshift and show that the EAGLE and FIRE simulations overall overestimate
σ
0,mol
at high redshift. Our results suggest that efforts to compare the kinematics of gas using ionized gas as a proxy for the total gas may overestimate the velocity dispersion by a significant amount in galaxies at the peak of cosmic star formation. When using the molecular gas as a tracer, our sample is not consistent with predictions from star formation models with constant efficiency, even when including transport as a source of turbulence. Feedback models with variable star formation efficiency,
ϵ
ff
, and/or feedback efficiency,
p
*
/
m
*
, better predict our observations.
Understanding the H2/H i ratio in galaxies Obreschkow, D.; Rawlings, S.
Monthly Notices of the Royal Astronomical Society,
April 2009, Volume:
394, Issue:
4
Journal Article
Peer reviewed
Open access
We revisit the mass ratio ηgalaxy between molecular hydrogen (H2) and atomic hydrogen (H i) in different galaxies from a phenomenological and theoretical viewpoint. First, the local H2 mass function ...(MF) is estimated from the local CO luminosity function (LF) of the FCRAO Extragalactic CO Survey, adopting a variable CO-to-H2 conversion fitted to nearby observations. This implies an average H2 density
and
in the local Universe. Secondly, we investigate the correlations between ηgalaxy and global galaxy properties in a sample of 245 local galaxies. Based on these correlations we introduce four phenomenological models for ηgalaxy, which we apply to estimate H2 masses for each H i galaxy in the HIPASS catalogue. The resulting H2 MFs (one for each model for ηgalaxy) are compared to the reference H2 MF derived from the CO LF, thus allowing us to determine the Bayesian evidence of each model and to identify a clear best model, in which, for spiral galaxies, ηgalaxy negatively correlates with both galaxy Hubble type and total gas mass. Thirdly, we derive a theoretical model for ηgalaxy for regular galaxies based on an expression for their axially symmetric pressure profile dictating the degree of molecularization. This model is quantitatively similar to the best phenomenological one at redshift z= 0, and hence represents a consistent generalization while providing a physical explanation for the dependence of ηgalaxy on global galaxy properties. Applying the best phenomenological model for ηgalaxy to the HIPASS sample, we derive the first integral cold gas MF (H i+ H2+ helium) of the local universe.
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We present a simulation of the cosmic evolution of the atomic and molecular phases of the cold hydrogen gas in about 3 X 107 galaxies, obtained by postprocessing the virtual galaxy catalog produced ...by De Lucia & Blaizot on the Millennium Simulation of cosmic structure. Our method uses a set of physical prescriptions to assign neutral atomic hydrogen (H I) and molecular hydrogen (H2) to galaxies, based on their total cold gas masses and a few additional galaxy properties. These prescriptions are specially designed for large cosmological simulations, where, given current computational limitations, individual galaxies can only be represented by simplistic model objects with a few global properties. Our recipes allow us to (1) split total cold gas masses between H I, H2, and helium, (2) assign realistic sizes to both the H I and H2 disks, and (3) evaluate the corresponding velocity profiles and shapes of the characteristic radio emission lines. The results presented in this paper include the local H I and H2 mass functions, the CO luminosity function, the cold gas mass-diameter relation, and the Tully-Fisher relation (TFR), which all match recent observational data from the local universe. We also present high-redshift predictions of cold gas diameters and the TFR, both of which appear to evolve markedly with redshift.
We have developed a semi-empirical simulation of the extragalactic radio continuum sky suitable for aiding the design of next generation radio interferometers such as the Square Kilometre Array ...(SKA). The emphasis is on modelling the large-scale cosmological distribution of radio sources rather than the internal structure of individual galaxies. Here we provide a description of the simulation to accompany the online release of a catalogue of ≃320 million simulated radio sources. The simulation covers a sky area of 20 × 20 deg2- a plausible upper limit to the instantaneous field of view attainable with future (e.g. SKA) aperture array technologies - out to a cosmological redshift of z= 20, and down to flux density limits of 10 nJy at 151, 610 MHz, 1.4, 4.86 and 18 GHz. Five distinct source types are included: radio-quiet active galactic nuclei (AGN), radio-loud AGN of the Fanaroff-Riley type I (FR I) and FR II structural classes, and star-forming galaxies, the latter split into populations of quiescent and starbursting galaxies.
In our semi-empirical approach, the simulated sources are drawn from observed (or extrapolated) luminosity functions and grafted on to an underlying dark matter density field with biases which reflect their measured large-scale clustering. A numerical Press-Schechter style filtering of the density field is used to identify and populate clusters of galaxies. For economy of output, radio source structures are constructed from point source and elliptical subcomponents, and for FR I and FR II sources an orientation-based unification and beaming model is used to partition flux between the core and extended lobes and hotspots. The extensive simulation output gives users the flexibility to post-process the catalogues to achieve more complete agreement with observational data in the years ahead. The ultimate aim is for the 'idealized skies' generated by this simulation and associated post-processing to be fed to telescope simulators to optimize the design of the SKA itself.
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Abstract
We study the relation between the metallicities of ionized and atomic gas in star-forming galaxies at
z
= 0–3 using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) ...cosmological, hydrodynamical simulations. This is done by constructing a dense grid of sight lines through the simulated galaxies and obtaining the star formation rate- and H
i
column density-weighted metallicities,
Z
SFR
and
Z
H I
, for each sightline as proxies for the metallicities of ionized and atomic gas, respectively. We find
Z
SFR
≳
Z
H I
for almost all sight lines, with their difference generally increasing with decreasing metallicity. The stellar masses of galaxies do not have a significant effect on this trend, but the positions of the sight lines with respect to the galaxy centers play an important role: the difference between the two metallicities decreases when moving toward the galaxy centers, and saturates to a minimum value in the central regions of galaxies, irrespective of redshift and stellar mass. This implies that the mixing of the two gas phases is most efficient in the central regions of galaxies where sight lines generally have high column densities of H
i
. However, a high H
i
column density alone does not guarantee a small difference between the two metallicities. In galaxy outskirts, the inefficiency of the mixing of star-forming gas with H
i
seems to dominate over the dilution of heavy elements in H
i
through mixing with the pristine gas. We find good agreement between the available observational data and the
Z
SFR
–
Z
H I
relation predicted by the EAGLE simulations. Though, observed regions with a nuclear starburst mode of star formation appear not to follow the same relation.
Cavitation bubbles collapsing and rebounding in a pressure gradient ∇p form a "microjet" enveloped by a "vapor jet." This Letter presents unprecedented observations of the vapor jets formed in a ...uniform gravity-induced ∇p, modulated aboard parabolic flights. The data uncover that the normalized jet volume is independent of the liquid density and viscosity and proportional to ζ ≡ |∇p|R(0)/Δp, where R(0) the maximal bubble radius and Δp is the driving pressure. A derivation inspired by "Kelvin-Blake" considerations confirms this law and reveals its negligible dependence of surface tension. We further conjecture that the jet only pierces the bubble boundary if ζ ≳ 4 × 10(-4).
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