We compare the observed turbulent pressure in molecular gas, Pturb, to the required pressure for the interstellar gas to stay in equilibrium in the gravitational potential of a galaxy, PDE. To do ...this, we combine arcsecond resolution CO data from PHANGS-ALMA with multiwavelength data that trace the atomic gas, stellar structure, and star formation rate (SFR) for 28 nearby star-forming galaxies. We find that Pturb correlates with-but almost always exceeds-the estimated PDE on kiloparsec scales. This indicates that the molecular gas is overpressurized relative to the large-scale environment. We show that this overpressurization can be explained by the clumpy nature of molecular gas; a revised estimate of PDE on cloud scales, which accounts for molecular gas self-gravity, external gravity, and ambient pressure, agrees well with the observed Pturb in galaxy disks. We also find that molecular gas with cloud-scale in our sample is more likely to be self-gravitating, whereas gas at lower pressure it appears more influenced by ambient pressure and/or external gravity. Furthermore, we show that the ratio between Pturb and the observed SFR surface density, , is compatible with stellar feedback-driven momentum injection in most cases, while a subset of the regions may show evidence of turbulence driven by additional sources. The correlation between and kpc-scale PDE in galaxy disks is consistent with the expectation from self-regulated star formation models. Finally, we confirm the empirical correlation between molecular-to-atomic gas ratio and kpc-scale PDE reported in previous works.
The Spatially Resolved Dust-to-metals Ratio in M101 Chiang, I-Da; Sandstrom, Karin M.; Chastenet, Jérémy ...
Astrophysical journal/The Astrophysical journal,
10/2018, Letnik:
865, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The dust-to-metals ratio describes the fraction of heavy elements contained in dust grains, and its variation provides key insights into the life cycle of dust. We measure the dust-to-metals ratio in ...M101, a nearby galaxy with a radial metallicity (Z) gradient spanning ∼1 dex. We fit the spectral energy distribution of dust from 100 to 500 m with five variants of the modified blackbody dust emission model in which we vary the temperature distribution and how emissivity depends on wavelength. Among them, the model with a single-temperature blackbody modified by a broken power-law emissivity gives the statistically best fit and physically most plausible results. Using these results, we show that the dust-to-gas ratio is proportional to . This implies that the dust-to-metals ratio is not constant in M101, but decreases as a function of radius, which is equivalent to a lower fraction of metals trapped in dust at low metallicity (large radius). The dust-to-metals ratio in M101 remains at or above what would be predicted by the minimum depletion level of metals observed in the Milky Way. Our current knowledge of the metallicity-dependent CO-to-H2 conversion factor suggests that variations in the conversion factor cannot be responsible for the trends in dust-to-metals ratio we observe. This change of dust-to-metals ratio is significantly correlated with the mass fraction of molecular hydrogen, which suggests that the accretion of gas-phase metals onto existing dust grains could contribute to a variable dust-to-metals ratio.
We compare the properties of giant molecular clouds (GMCs) in M51 identified by the Plateau de Bure Interferometer Whirlpool Arcsecond Survey with GMCs identified in wide-field, high-resolution ...surveys of CO emission in M33 and the Large Magellanic Cloud (LMC). We find that GMCs in M51 are larger, brighter, and have higher velocity dispersions relative to their sizes than equivalent structures in M33 and the LMC. These differences imply that there are genuine variations in the average mass surface density left angle bracket capital sigma sub(H) sub(2)rig ht angle bracket) of the different GMC populations. To explain this, we propose that the pressure in the interstellar medium surrounding the GMCs plays a role in regulating their density and velocity dispersion. We find no evidence for a correlation between size and linewidth in M51, M33, or the LMC when the CO emission is decomposed into GMCs, although moderately robust correlations are apparent when regions of contiguous CO emission (with no size limitation) are used. Our work demonstrates that observational bias remains an important obstacle to the identification and study of extragalactic GMC populations using CO emission, especially in molecule-rich galactic environments.
Abstract
Using the PHANGS–ALMA CO(2–1) survey, we characterize molecular gas properties on ∼100 pc scales across 102,778 independent sightlines in 70 nearby galaxies. This yields the best synthetic ...view of molecular gas properties on cloud scales across the local star-forming galaxy population obtained to date. Consistent with previous studies, we observe a wide range of molecular gas surface densities (3.4 dex), velocity dispersions (1.7 dex), and turbulent pressures (6.5 dex) across the galaxies in our sample. Under simplifying assumptions about subresolution gas structure, the inferred virial parameters suggest that the kinetic energy of the molecular gas typically exceeds its self-gravitational binding energy at ∼100 pc scales by a modest factor (1.3 on average). We find that the cloud-scale surface density, velocity dispersion, and turbulent pressure (1) increase toward the inner parts of galaxies, (2) are exceptionally high in the centers of barred galaxies (where the gas also appears less gravitationally bound), and (3) are moderately higher in spiral arms than in inter-arm regions. The galaxy-wide averages of these gas properties also correlate with the integrated stellar mass, star formation rate, and offset from the star-forming main sequence of the host galaxies. These correlations persist even when we exclude regions with extraordinary gas properties in galaxy centers, which contribute significantly to the inter-galaxy variations. Our results provide key empirical constraints on the physical link between molecular cloud populations and their galactic environment.
We present a detailed study of a molecular outflow feature in the nearby starburst galaxy NGC 253 using ALMA. We find that this feature is clearly associated with the edge of NGC 253's prominent ...ionized outflow, has a projected length of ∼300 pc, with a width of ∼50 pc, and a velocity dispersion of ∼40 km s−1, which is consistent with an ejection from the disk about 1 Myr ago. The kinematics of the molecular gas in this feature can be interpreted (albeit not uniquely) as accelerating at a rate of 1 km s−1 pc−1. In this scenario, the gas is approaching an escape velocity at the last measured point. Strikingly, bright tracers of dense molecular gas (HCN, CN, HCO+, CS) are also detected in the molecular outflow: we measure an HCN(1-0)/CO(1-0) line ratio of in the outflow, similar to that in the central starburst region of NGC 253 and other starburst galaxies. By contrast, the HCN/CO line ratio in the NGC 253 disk is significantly lower ( ), similar to other nearby galaxy disks. This strongly suggests that the streamer gas originates from the starburst, and that its physical state does not change significantly over timescales of ∼1 Myr during its entrainment in the outflow. Simple calculations indicate that radiation pressure is not the main mechanism for driving the outflow. The presence of such dense material in molecular outflows needs to be accounted for in simulations of galactic outflows.
We utilize archival far-infrared maps from the Herschel Space Observatory in four Local Group galaxies (Small and Large Magellanic Clouds (SMC and LMC), M31, and M33). We model their spectral energy ...distribution from 100 to 500 m using a single-temperature modified blackbody emission with a fixed emissivity index of β = 1.8. From the best-fit model, we derive the dust temperature, , and the dust mass surface density, , at 13 pc resolution for SMC and LMC, and at 167 pc resolution for all targets. This measurement allows us to build the distribution of dust mass and luminosity as functions of dust temperature and mass surface density. We compare those distribution functions among galaxies and between regions in a galaxy. We find that LMC has the highest mass-weighted average , while M31 and M33 have the lowest mass-weighted average . Within a galaxy, star-forming regions have higher and relative to the overall distribution function, because of more intense heating by young stars and higher gas mass surface density. When we degrade the resolutions to mimic distant galaxies, the mass-weighted mean temperature gets warmer as the resolution gets coarser, meaning that the temperatures derived from unresolved observations are systematically higher than those in highly resolved observations. As an implication, the total dust mass is lower (underestimated) in coarser resolutions. This resolution-dependent effect is more prominent in clumpy star-forming galaxies (SMC, LMC, and M33) and less prominent in a more quiescent massive spiral (M31).
We present a study of the spatially resolved radio continuum-star formation rate (RC-SFR) relation using state-of-the-art star formation tracers in a sample of 17 THINGS galaxies. We use SFR surface ...density ( capital sigma sub(sfr)) maps created by a linear combination of GALEX far-UV (FUV) and Spitzer 24 mu m maps. We find a tight correlation between the radial profiles of the radio and FUV/MIR-based capital sigma sub(sfr) for the entire extent of the disk. The ratio R of the azimuthally averaged radio to FUV/MIR-based capital sigma sub(sfr) agrees with the integrated ratio and has only quasi-random fluctuations with galactocentric radius that are relatively small. We studied the ratio R of radio to FUV/MIR-based integrated SFR as a function of global galaxy parameters and found no clear correlation. We can reconcile our finding of an almost linear RC-S FR relation and sub-linear resolved RC- capital sigma sub(sfr) relation by proposing a non-linear magnetic field-SFR relation, B is proportional to SFR super(0.30 + or - 0.02) sub(hyb), which holds both globally and locally.
Abstract
We measure the low-
J
CO line ratios
R
21
≡ CO (2–1)/CO (1–0),
R
32
≡ CO (3–2)/CO (2–1), and
R
31
≡CO (3–2)/CO (1–0) using whole-disk CO maps of nearby galaxies. We draw CO (2–1) from ...PHANGS-ALMA, HERACLES, and follow-up IRAM surveys; CO (1–0) from COMING and the Nobeyama CO Atlas of Nearby Spiral Galaxies; and CO (3–2) from the James Clerk Maxwell Telescope Nearby Galaxy Legacy Survey and Atacama Pathfinder Experiment Large APEX Sub-Millimetre Array mapping. All together, this yields 76, 47, and 29 maps of
R
21
,
R
32
, and
R
31
at 20″ ∼ 1.3 kpc resolution, covering 43, 34, and 20 galaxies. Disk galaxies with high stellar mass,
log
(
M
⋆
/
M
⊙
)
=
10.25
–
11
, and star formation rate (SFR) = 1–5
M
⊙
yr
−1
, dominate the sample. We find galaxy-integrated mean values and a 16%–84% range of
R
21
= 0.65 (0.50–0.83),
R
32
= 0.50 (0.23–0.59), and
R
31
= 0.31 (0.20–0.42). We identify weak trends relating galaxy-integrated line ratios to properties expected to correlate with excitation, including SFR/
M
⋆
and SFR/
L
CO
. Within galaxies, we measure central enhancements with respect to the galaxy-averaged value of ∼
0.18
−
0.14
+
0.09
dex for
R
21
,
0.27
−
0.15
+
0.13
dex for
R
31
, and
0.08
−
0.09
+
0.11
dex for
R
32
. All three line ratios anticorrelate with galactocentric radius and positively correlate with the local SFR surface density and specific SFR, and we provide approximate fits to these relations. The observed ratios can be reasonably reproduced by models with low temperature, moderate opacity, and moderate densities, in good agreement with expectations for the cold interstellar medium. Because the line ratios are expected to anticorrelate with the CO (1–0)-to-H
2
conversion factor,
α
CO
1
−
0
, these results have general implications for the interpretation of CO emission from galaxies.
H i Kinematics along the Minor Axis of M82 Martini, Paul; Leroy, Adam K.; Mangum, Jeffrey G. ...
Astrophysical journal/The Astrophysical journal,
03/2018, Letnik:
856, Številka:
1
Journal Article
Recenzirano
Odprti dostop
M82 is one of the best-studied starburst galaxies in the local universe, and is consequently a benchmark for studying star formation feedback at both low and high redshift. We present new VLA H i ...observations that reveal the cold gas kinematics along the minor axis in unprecedented detail. This includes the detection of H i up to 10 kpc along the minor axis toward the south and beyond 5 kpc to the north. A surprising aspect of these observations is that the line-of-sight H i velocity decreases substantially from about 120 to from 1.5 to 10 kpc off the midplane. The velocity profile is not consistent with the H i gas cooling from the hot wind. We demonstrate that the velocity decrease is substantially greater than the deceleration expected from gravitational forces alone. If the H i consists of a continuous population of cold clouds, some additional drag force must be present, and the magnitude of the drag force places a joint constraint on the ratio of the ambient medium to the typical cloud size and density. We also show that the H i kinematics are inconsistent with a simple conical outflow centered on the nucleus, but instead require the more widespread launch of the H i over the ∼1 kpc extent of the starburst region. Regardless of the launch mechanism for the H i gas, the observed velocity decrease along the minor axis is sufficiently great that the H i may not escape the halo of M82. The inferred H i outflow rate at 10 kpc off the midplane is much less than 1 yr−1.
We use new ALMA observations to investigate the connection between dense gas fraction, star formation rate (SFR), and local environment across the inner region of four local galaxies showing a wide ...range of molecular gas depletion times. We map HCN (1-0), HCO+ (1-0), CS (2-1), 13CO (1-0), and C18O (1-0) across the inner few kiloparsecs of each target. We combine these data with short-spacing information from the IRAM large program EMPIRE, archival CO maps, tracers of stellar structure and recent star formation, and recent HCN surveys by Bigiel et al. and Usero et al. We test the degree to which changes in the dense gas fraction drive changes in the SFR. (tracing the dense gas fraction) correlates strongly with ICO (tracing molecular gas surface density), stellar surface density, and dynamical equilibrium pressure, PDE. Therefore, becomes very low and HCN becomes very faint at large galactocentric radii, where ratios as low as become common. The apparent ability of dense gas to form stars, (where dense is traced by the HCN intensity and the star formation rate is traced by a combination of H and 24 m emission), also depends on environment. decreases in regions of high gas surface density, high stellar surface density, and high PDE. Statistically, these correlations between environment and both and are stronger than that between apparent dense gas fraction ( ) and the apparent molecular gas star formation efficiency . We show that these results are not specific to HCN.
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