19 research outputs found
Starbursts driven by central gas compaction
Starburst (SB) galaxies are a rare population of galaxies with star formation
rates (SFRs) greatly exceeding those of the majority of star-forming galaxies
with similar stellar mass. It is unclear whether these bursts are the result of
either especially large gas reservoirs or enhanced efficiencies in converting
gas into stars. Tidal torques resulting from gas-rich galaxy mergers are known
to enhance the SFR by funneling gas towards the centre. However, recent
theoretical works show that mergers do not always trigger a SB and not all SB
galaxies are interacting systems, raising the question of what drives a SB. We
analyse a large sample of SB galaxies and a mass- and redshift-matched sample
of control galaxies, drawn from the FIREbox cosmological volume at z=0-1. We
find that SB galaxies have both larger molecular gas fractions and shorter
molecular depletion times than control galaxies, but similar total gas masses.
Control galaxies evolve towards the SB regime by gas compaction in their
central regions, over timescales of about 70 Myr, accompanied by an increase in
the fraction of ultra-dense and molecular gas. The driving mechanism behind the
SB varies depending on the mass of the galaxy. Massive (Mstar > 1e10 Msun)
galaxies undergoing intense, long-lasting SBs are mostly driven by galaxy
interactions. Conversely, SBs in non-interacting galaxies are often triggered
by a global gravitational instability, that can result in a breathing mode in
low-mass galaxies.Comment: 21 pages, 18 figures, submitted to MNRA
Realistic H i scale heights of Milky Way-mass galaxies in the FIREbox cosmological volume
Accurately reproducing the thin cold gas discs observed in nearby spiral galaxies has been a long standing issue in cosmological simulations. Here, we present measurements of the radially resolved H i scale height in 22 non-interacting Milky Way-mass galaxies from the FIREbox cosmological volume. We measure the H i scale heights using five different approaches commonly used in the literature: fitting the vertical volume density distribution with a Gaussian, the distance between maximum and half-maximum of the vertical volume density distribution, a semi-empirical description using the velocity dispersion and the galactic gravitational potential, the analytic assumption of hydrostatic equilibrium, and the distance from the midplane which encloses ≳60 per cent of the H i mass. We find median H i scale heights, measured using the vertical volume distribution, that range from ∼100 pc in the galactic centres to ∼800 pc in the outskirts and are in excellent agreement with recent observational results. We speculate that the presence of a realistic multiphase interstellar medium, including cold gas, and realistic stellar feedback are the drivers behind the realistic H i scale heights
WISDOM Project -- XV. Giant Molecular Clouds in the Central Region of the Barred Spiral Galaxy NGC 5806
We present high spatial resolution ( pc) Atacama Large
Millimeter/sub-millimeter Array CO(2-1) observations of the central
region of the nearby barred spiral galaxy NGC 5806. NGC 5806 has a highly
structured molecular gas distribution with a clear nucleus, a nuclear ring and
offset dust lanes. We identify spatially- and spectrally-resolved giant
molecular clouds (GMCs). These clouds have comparable sizes ()
and larger gas masses, observed linewidths () and
gas mass surface densities than those of clouds in the Milky Way disc. The size
-- linewidth relation of the clouds is one of the steepest reported so far
(), the clouds are on
average only marginally bound (with a mean virial parameter
), and high velocity dispersions
are observed in the nuclear ring. These behaviours are likely due to bar-driven
gas shocks and inflows along the offset dust lanes, and we infer an inflow
velocity of kms and a total molecular gas mass inflow rate
of M yr into the nuclear ring. The observed internal
velocity gradients of the clouds are consistent with internal turbulence. The
number of clouds in the nuclear ring decreases with azimuthal angle downstream
from the dust lanes without clear variation of cloud properties. This is likely
due to the estimated short lifetime of the clouds ( Myr), which
appears to be mainly regulated by cloud-cloud collision and/or shear processes.
Overall, it thus seems that the presence of the large-scale bar and gas inflows
to the centre of NGC 5806 affect cloud properties.Comment: Accepted for publication in MNRAS, 20 pages, 16 figure
The WISDOM of power spectra: how the galactic gravitational potential impacts a galaxy?s central gas reservoir in simulations and observations
Observations indicate that the central gas discs are smoother in early-type galaxies than their late-type counterparts, while recent simulations predict that the dynamical suppression of star formation in spheroid-dominated galaxies is preceded by the suppression of fragmentation of their interstellar media. The mass surface density power spectrum is a powerful tool to constrain the degree of structure within a gas reservoir. Specifically here, we focus on the power spectrum slope and aim to constrain whether the shear induced by a dominant spheroidal potential can induce sufficient turbulence to suppress fragmentation, resulting in the smooth central gas discs observed. We compute surface density power spectra for the nuclear gas reservoirs of fourteen simulated isolated galaxies and twelve galaxies observed as part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM) project. Both simulated and observed galaxies range from disc-dominated galaxies to spheroids, with central stellar mass surface densities, a measure of bulge dominance, varying by more than an order of magnitude. For the simulations, the power spectra steepen with increasing central stellar mass surface density, thereby clearly linking the suppression of fragmentation to the shear-driven turbulence induced by the spheroid. The WISDOM observations show a different (but potentially consistent) picture: while there is no correlation between the power spectrum slopes and the central stellar mass surface densities, the slopes scatter around a value of 2.6. This is similar to the behaviour of the slopes of the simulated galaxies with high central stellar mass surface densities, and could indicate that high shear eventually drives incompressible turbulence
WISDOM Project - XXI. Giant molecular clouds in the central region of the barred spiral galaxy NGC 613: a steep size - linewidth relation
NGC 613 is a nearby barred spiral galaxy with a nuclear ring. Exploiting high spatial resolution (≈20 pc) Atacama Large Millimetre/submillimetre Array 12CO(1–0) observations, we study the giant molecular clouds (GMCs) in the nuclear ring and its vicinity, identifying 158 spatially and spectrally resolved GMCs. The GMC sizes (Rc) are comparable to those of the clouds in the Milky Way (MW) disc, but their gas masses, observed linewidths (σobs,los), and gas mass surface densities are larger. The GMC size–linewidth relation (
) is steeper than that of the clouds of the MW disc and centre, and the GMCs are on average only marginally gravitationally bound (with a mean virial parameter 〈αobs,vir〉 ≈ 1.7). We discuss the possible origins of the steep size–linewidth relation and enhanced observed linewidths of the clouds and suggest that a combination of mechanisms such as stellar feedback, gas accretion, and cloud–cloud collisions, as well as the gas inflows driven by the large-scale bar, may play a role
WISDOM Project - X. The morphology of the molecular ISM in galaxy centres and its dependence on galaxy structure
We use high-resolution maps of the molecular interstellar medium (ISM) in the centres of 86 nearby galaxies from the millimetre-Wave Interferometric Survey of Dark Object Masses (WISDOM) and Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) surveys to investigate the physical mechanisms setting the morphology of the ISM at molecular cloud scales. We show that early-type galaxies tend to have smooth, regular molecular gas morphologies, while the ISM in spiral galaxy bulges is much more asymmetric and clumpy when observed at the same spatial scales. We quantify these differences using non-parametric morphology measures (Asymmetry, Smoothness, and Gini), and compare these measurements with those extracted from idealized galaxy simulations. We show that the morphology of the molecular ISM changes systematically as a function of various large-scale galaxy parameters, including galaxy morphological type, stellar mass, stellar velocity dispersion, effective stellar mass surface density, molecular gas surface density, star formation efficiency, and the presence of a bar. We perform a statistical analysis to determine which of these correlated parameters best predicts the morphology of the ISM. We find the effective stellar mass surface (or volume) density to be the strongest predictor of the morphology of the molecular gas, while star formation and bars maybe be important secondary drivers. We find that gas self-gravity is not the dominant process shaping the morphology of the molecular gas in galaxy centres. Instead effects caused by the depth of the potential well, such as shear, suppression of stellar spiral density waves, and/or inflow, affect the ability of the gas to fragment
WISDOM project XX. - Strong shear tearing molecular clouds apart in NGC 524
Early-type galaxies (ETGs) are known to harbour dense spheroids of stars but scarce star formation (SF). Approximately a quarter of these galaxies have rich molecular gas reservoirs yet do not form stars efficiently. We study here the ETG NGC 524, with strong shear suspected to result in a smooth molecular gas disc and low star-formation efficiency (SFE). We present new spatially resolved observations of the 12CO(2-1)-emitting cold molecular gas from the Atacama Large Millimeter/sub-millimeter Array (ALMA) and of the warm ionized-gas emission lines from SITELLE at the Canada–France–Hawaii Telescope. Although constrained by the resolution of the ALMA observations (≈37 pc), we identify only 52 GMCs with radii ranging from 30 to 140 pc, a low mean molecular gas mass surface density 〈Σgas〉 ≈ 125 M⊙ pc−2 and a high mean virial parameter 〈αobs, vir〉 ≈ 5.3. We measure spatially resolved molecular gas depletion times (τdep ≡ 1/SFE) with a spatial resolution of ≈100 pc within a galactocentric distance of 1.5 kpc. The global depletion time is ≈2.0 Gyr but τdep increases towards the galaxy centre, with a maximum τdep, max ≈ 5.2 Gyr. However, no pure H II region is identified in NGC 524 using ionized-gas emission-line ratio diagnostics, so the τdep inferred are in fact lower limits. Measuring the GMC properties and dynamical states, we conclude that shear is the dominant mechanism shaping the molecular gas properties and regulating SF in NGC 524. This is supported by analogous analyses of the GMCs in a simulated ETG similar to NGC 524
Star Formation Laws and Efficiencies across 80 Nearby Galaxies
We measure empirical relationships between the local star formation rate
(SFR) and properties of the star-forming molecular gas on 1.5 kpc scales across
80 nearby galaxies. These relationships, commonly referred to as "star
formation laws," aim at predicting the local SFR surface density from various
combinations of molecular gas surface density, galactic orbital time, molecular
cloud free-fall time, and the interstellar medium dynamical equilibrium
pressure. Leveraging a multiwavelength database built for the PHANGS survey, we
measure these quantities consistently across all galaxies and quantify
systematic uncertainties stemming from choices of SFR calibrations and the
CO-to-H conversion factors. The star formation laws we examine show 0.3-0.4
dex of intrinsic scatter, among which the molecular Kennicutt-Schmidt relation
shows a 10% larger scatter than the other three. The slope of this
relation ranges , implying that the molecular gas
depletion time remains roughly constant across the environments probed in our
sample. The other relations have shallower slopes (),
suggesting that the star formation efficiency (SFE) per orbital time, the SFE
per free-fall time, and the pressure-to-SFR surface density ratio (i.e., the
feedback yield) may vary systematically with local molecular gas and SFR
surface densities. Last but not least, the shapes of the star formation laws
depend sensitively on methodological choices. Different choices of SFR
calibrations can introduce systematic uncertainties of at least 10-15% in the
star formation law slopes and 0.15-0.25 dex in their normalization, while the
CO-to-H conversion factors can additionally produce uncertainties of 20-25%
for the slope and 0.10-0.20 dex for the normalization.Comment: 10 pages main text + 2 appendices. ApJL in press. Data products
available at
https://www.canfar.net/storage/list/phangs/RELEASES/Sun_etal_2023 . Slides
summarizing key results can be found at
https://www.dropbox.com/s/5gsegexeo9n0t05/Sun_et_PHANGS_2023.pptx?dl=