Central molecular outflows in spiral galaxies are assumed to modulate their host galaxy’s star formation rate (SFR) by removing gas from the inner region of the galaxy. Outflows consisting of ...different gas phases appear to be a common feature in local galaxies, yet, little is known about the frequency of molecular outflows in main sequence galaxies in the nearby universe. We develop a rigorous set of selection criteria, which allow the reliable identification of outflows in large samples of galaxies. Our criteria make use of central spectra, position-velocity diagrams and velocity-integrated intensity maps (line-wing maps). We use this method on high-angular resolution CO (2–1) observations from the PHANGS-ALMA survey, which provides observations of the molecular gas for a homogeneous sample of 90 nearby main sequence galaxies at a resolution of ∼100 pc. We find correlations between the assigned outflow confidence and stellar mass or global SFR. We determine the frequency of central molecular outflows to be 25 ± 2% considering all outflow candidates, or 20 ± 2% for secure outflows only. Our resulting outflow candidate sample of 16−20 galaxies shows an overall enhanced fraction of active galactic nuclei (AGN) (50%) and bars (89%) compared to the full sample (galaxies with AGN: 24%, with bar: 61%). We extend the trend between mass outflow rates and SFR known for high outflow rates down to lower values (log
10
Ṁ
out
M
⊙
yr
−1
< 0). Mass loading factors are of order unity, indicating that these outflows are not efficient in quenching the SFR in main sequence galaxies.
ABSTRACT The cloud-scale density, velocity dispersion, and gravitational boundedness of the interstellar medium (ISM) vary within and among galaxies. In turbulent models, these properties play key ...roles in the ability of gas to form stars. New high-fidelity, high-resolution surveys offer the prospect to measure these quantities across galaxies. We present a simple approach to make such measurements and to test hypotheses that link small-scale gas structure to star formation and galactic environment. Our calculations capture the key physics of the Larson scaling relations, and we show good correspondence between our approach and a traditional "cloud properties" treatment. However, we argue that our method is preferable in many cases because of its simple, reproducible characterization of all emission. Using, low-J 12CO data from recent surveys, we characterize the molecular ISM at 60 pc resolution in the Antennae, the Large Magellanic Cloud (LMC), M31, M33, M51, and M74. We report the distributions of surface density, velocity dispersion, and gravitational boundedness at 60 pc scales and show galaxy-to-galaxy and intragalaxy variations in each. The distribution of flux as a function of surface density appears roughly lognormal with a 1 width of ∼0.3 dex, though the center of this distribution varies from galaxy to galaxy. The 60 pc resolution line width and molecular gas surface density correlate well, which is a fundamental behavior expected for virialized or free-falling gas. Varying the measurement scale for the LMC and M31, we show that the molecular ISM has higher surface densities, lower line widths, and more self-gravity at smaller scales.
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.
We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and mass-luminosity relations of GMCs in a variety of ...extragalactic systems. Although the data are heterogeneous, we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. The property that shows the largest variability is their resolved brightness temperature, although even that is similar to the average Galactic value in most sources. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H sub(2) factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the molecular cloud. We also find that the virial CO-to-H sub(2) conversion factor in CO-bright GMCs is very similar to Galactic and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H sub(2) envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of objects that exhibit a remarkably uniform set of properties from galaxy to galaxy.
H i Kinematics along the Minor Axis of M82 Martini, Paul; Leroy, Adam K.; Mangum, Jeffrey G. ...
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.
Abstract
We present PHANGS–ALMA, the first survey to map CO
J
= 2 → 1 line emission at ∼1″ ∼100 pc spatial resolution from a representative sample of 90 nearby (
d
≲ 20 Mpc) galaxies that lie on or ...near the
z
= 0 “main sequence” of star-forming galaxies. CO line emission traces the bulk distribution of molecular gas, which is the cold, star-forming phase of the interstellar medium. At the resolution achieved by PHANGS–ALMA, each beam reaches the size of a typical individual giant molecular cloud, so that these data can be used to measure the demographics, life cycle, and physical state of molecular clouds across the population of galaxies where the majority of stars form at
z
= 0. This paper describes the scientific motivation and background for the survey, sample selection, global properties of the targets, Atacama Large Millimeter/submillimeter Array (ALMA) observations, and characteristics of the delivered data and derived data products. As the ALMA sample serves as the parent sample for parallel surveys with MUSE on the Very Large Telescope, the Hubble Space Telescope, AstroSat, the Very Large Array, and other facilities, we include a detailed discussion of the sample selection. We detail the estimation of galaxy mass, size, star formation rate, CO luminosity, and other properties, compare estimates using different systems and provide best-estimate integrated measurements for each target. We also report the design and execution of the ALMA observations, which combine a Cycle 5 Large Program, a series of smaller programs, and archival observations. Finally, we present the first 1″ resolution atlas of CO emission from nearby galaxies and describe the properties and contents of the first PHANGS–ALMA public data release.
We explore the use of mm-wave emission line ratios to trace molecular gas density when observations integrate over a wide range of volume densities within a single telescope beam. For observations ...targeting external galaxies, this case is unavoidable. Using a framework similar to that of Krumholz & Thompson, we model emission for a set of common extragalactic lines from lognormal and power law density distributions. We consider the median density of gas that produces emission and the ability to predict density variations from observed line ratios. We emphasize line ratio variations because these do not require us to know the absolute abundance of our tracers. Patterns of line ratio variations have the potential to illuminate the high-end shape of the density distribution, and to capture changes in the dense gas fraction and median volume density. Our results with and without a high-density power law tail differ appreciably; we highlight better knowledge of the probability density function (PDF) shape as an important area. We also show the implications of sub-beam density distributions for isotopologue studies targeting dense gas tracers. Differential excitation often implies a significant correction to the naive case. We provide tabulated versions of many of our results, which can be used to interpret changes in mm-wave line ratios in terms of adjustments to the underlying density distributions.
We present the first spatially and spectrally resolved image of the molecular outflow in the western nucleus of Arp 220. The outflow, seen in HCN (1-0) by the Atacama Large millimeter/sub-millimeter ...Array, is compact and collimated, with an extension 120 pc. Bipolar morphology emerges along the minor axis of the disk, with redshifted and blueshifted components reaching a maximum inclination-corrected velocity of km s−1. The outflow is also seen in CO and continuum emission, the latter implying that it carries significant dust. We estimate a total mass in the outflow of , a dynamical time of ∼105 yr, and mass outflow rates of yr−1 and yr−1 for the northern and southern lobes, respectively. Possible driving mechanisms include supernovae energy and momentum transfer, radiation pressure feedback, and a central AGN. The latter could explain the collimated morphology of the HCN outflow; however, we need more complex theoretical models, including contributions from supernovae and AGN, to pinpoint the driving mechanism of this outflow.
We estimate the conversion factor relating CO emission to H2 mass, Delta *aCO, in five Local Group galaxies that span approximately an order of magnitude in metallicity--M 31, M 33, the Large ...Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the Delta *aCO that best allows a single gas-to-dust ratio ( Delta *dGDR) to describe each system. This approach remains sensitive to CO-dark envelopes H2 surrounding molecular clouds. In M 31, M 33, and the LMC we find Delta *aCO 3-9 M pc--2 (K km s--1)--1, consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log (O/H) 8.2 and 8.0), exhibit a much higher Delta *aCO. Our best estimates are Delta *aNGC6822 CO 30 M pc--2 (K km s--1)--1 and Delta *aSMC CO 70 M pc--2 (K km s--1)--1. These results are consistent with the conversion factor becoming a strong function of metallicity around 12 + log (O/H) ~ 8.4-8.2. We favor an interpretation where decreased dust shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity.
The nearby (3.8 Mpc) galaxy NGC 4945 hosts a nuclear starburst and Seyfert type 2 active galactic nucleus (AGN). We use the Atacama Large Millimeter/submillimeter Array (ALMA) to image the 93 GHz ...(3.2 mm) free-free continuum and hydrogen recombination line emission (H40 and H42 ) at 2.2 pc (0 12) resolution. Our observations reveal 27 bright, compact sources with FWHM sizes of 1.4-4.0 pc, which we identify as candidate super star clusters. Recombination line emission, tracing the ionizing photon rate of the candidate clusters, is detected in 15 sources, six of which have a significant synchrotron component to the 93 GHz continuum. Adopting an age of ∼5 Myr, the stellar masses implied by the ionizing photon luminosities are (M /M ) 4.7-6.1. We fit a slope to the cluster mass distribution and find β = −1.8 0.4. The gas masses associated with these clusters, derived from the dust continuum at 350 GHz, are typically an order of magnitude lower than the stellar mass. These candidate clusters appear to have already converted a large fraction of their dense natal material into stars and, given their small freefall times of ∼0.05 Myr, are surviving an early volatile phase. We identify a pointlike source in 93 GHz continuum emission that is presumed to be the AGN. We do not detect recombination line emission from the AGN and place an upper limit on the ionizing photons that leak into the starburst region of Q0 < 1052 s−1.
PDF
