The CO-to-H2 Conversion Factor Bolatto, Alberto D; Wolfire, Mark; Leroy, Adam K
Annual review of astronomy and astrophysics,
08/2013, Letnik:
51
Journal Article
Recenzirano
CO line emission represents the most accessible and widely used tracer of the molecular ISM. This renders the translation of observed CO intensity into total H
2
gas mass critical to understanding ...star formation and the ISM in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H
2
"conversion factor,"
X
CO
, in different environments. In the Milky Way disk, we recommend a conversion factor of
cm
−2
(K km s
−1
)
−1
with ±30% uncertainty. Studies of other "normal galaxies" return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that
X
CO
increases with decreasing metallicity, turning up sharply below metallicity ∼1 3-1 2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations,
X
CO
appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower
X
CO
is also present in the overwhelmingly molecular ISM of starburst galaxies, where several lines of evidence point to a lower CO-to-H
2
conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments.
Cool outflows in galaxies and their implications Veilleux, Sylvain; Maiolino, Roberto; Bolatto, Alberto D. ...
The Astronomy and astrophysics review,
12/2020, Letnik:
28, Številka:
1
Journal Article
Recenzirano
Neutral-atomic and molecular outflows are a common occurrence in galaxies, near and far. They operate over the full extent of their galaxy hosts, from the innermost regions of galactic nuclei to the ...outermost reaches of galaxy halos. They carry a substantial amount of material that would otherwise have been used to form new stars. These cool outflows may have a profound impact on the evolution of their host galaxies and environments. This article provides an overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe. The remaining outstanding issues that have not yet been resolved are summarized at the end of the review to inspire new research directions.
ABSTRACT We present a wide area ( 8 × 8 kpc), sensitive map of CO (2-1) emission around the nearby starburst galaxy M82. Molecular gas extends far beyond the stellar disk, including emission ...associated with the well-known outflow as far as 3 kpc from M82's midplane. Kinematic signatures of the outflow are visible in both the CO and H i emission: both tracers show a minor axis velocity gradient and together they show double peaked profiles, consistent with a hot outflow bounded by a cone made of a mix of atomic and molecular gas. Combining our CO and H i data with observations of the dust continuum, we study the changing properties of the cold outflow as it leaves the disk. While H2 dominates the ISM near the disk, the dominant phase of the cool medium changes as it leaves the galaxy and becomes mostly atomic after about a kpc. Several arguments suggest that regardless of phase, the mass in the cold outflow does not make it far from the disk; the mass flux through surfaces above the disk appears to decline with a projected scale length of 1-2 kpc. The cool material must also end up distributed over a much wider angle than the hot outflow based on the nearly circular isophotes of dust and CO at low intensity and the declining rotation velocities as a function of height from the plane. The minor axis of M82 appears so striking at many wavelengths because the interface between the hot wind cavity and the cool gas produces H , hot dust, polycyclic aromatic hydrocarbon emission, and scattered UV light. We also show the level at which a face-on version of M82 would be detectable as an outflow based on unresolved spectroscopy. Finally, we consider multiple constraints on the CO-to-H2 conversion factor, which must change across the galaxy but appears to be only a factor of 2 lower than the Galactic value in the outflow.
We measure the velocity dispersion, , and surface density, , of the molecular gas in nearby galaxies from CO spectral line cubes with spatial resolution 45-120 pc, matched to the size of individual ...giant molecular clouds. Combining 11 galaxies from the PHANGS-ALMA survey with four targets from the literature, we characterize ∼30,000 independent sightlines where CO is detected at good significance. and show a strong positive correlation, with the best-fit power-law slope close to the expected value for resolved, self-gravitating clouds. This indicates only a weak variation in the virial parameter vir ∝ 2/ , which is ∼1.5-3.0 for most galaxies. We do, however, observe enormous variation in the internal turbulent pressure Pturb ∝ 2, which spans ∼5 dex across our sample. We find , , and Pturb to be systematically larger in more massive galaxies. The same quantities appear enhanced in the central kiloparsec of strongly barred galaxies relative to their disks. Based on sensitive maps of M31 and M33, the slope of the - relation flattens at 10 M pc−2, leading to high for a given and high apparent vir. This echoes results found in the Milky Way and likely originates from a combination of lower beam-filling factors and a stronger influence of local environment on the dynamical state of molecular gas in the low-density regime.
ABSTRACT
We present the results of a blind search for intervening H i 21 cm absorption towards 260 radio sources in the redshift range 0 < z < 2.74 with the Green Bank Telescope. The survey has the ...sensitivity to detect sub-damped Ly α (DLA) systems for H i spin temperatures Ts/f = 100 K, and despite the successful re-detection of 10 known 21 cm absorbers in the sample, we detect no new absorption lines in the full survey. Sources detected in 21 cm absorption were also searched for hydroxyl (OH) 18 cm absorption and we re-detect 1667 MHz OH absorption towards PKS 1830-211. We searched for intervening H i 21 cm absorption along the line of sight in each source achieving a total redshift coverage of Δz = 88.64 (comoving absorption path of ΔX = 159.5) after removing regions affected by radio frequency interference. We compute a 95 per cent confidence upper limit on the column density frequency distribution f(NH i) and set a statistical constraint on the spin temperature Ts in the range 100–1000 K, consistent with prior redshifted optical DLA surveys and H i 21 cm emission observations at the same redshifts. We infer a value for the cosmological mass density of neutral gas, ΩH i. Through comparison with prior ΩH i measurements, we place a statistical constraint on the mean spin temperature of Ts/f = 175 K. Our derived ΩH i values support a relative mild evolution in ΩH i over the last 11 Gyr and are consistent with other methods that measure ΩH i.
The under-abundance of very massive galaxies in the Universe is frequently attributed to the effect of galactic winds. Although ionized galactic winds are readily observable, most of the expelled ...mass (that is, the total mass flowing out from the nuclear region) is likely to be in atomic and molecular phases that are cooler than the ionized phases. Expanding molecular shells observed in starburst systems such as NGC 253 (ref. 12) and M 82 (refs 13, 14) may facilitate the entrainment of molecular gas in the wind. Although shell properties are well constrained, determining the amount of outflowing gas emerging from such shells and the connection between this gas and the ionized wind requires spatial resolution better than 100 parsecs coupled with sensitivity to a wide range of spatial scales, a combination hitherto not available. Here we report observations of NGC 253, a nearby starburst galaxy (distance ∼ 3.4 megaparsecs) known to possess a wind, that trace the cool molecular wind at 50-parsec resolution. At this resolution, the extraplanar molecular gas closely tracks the Hα filaments, and it appears to be connected to expanding molecular shells located in the starburst region. These observations allow us to determine that the molecular outflow rate is greater than 3 solar masses per year and probably about 9 solar masses per year. This implies a ratio of mass-outflow rate to star-formation rate of at least 1, and probably ∼3, indicating that the starburst-driven wind limits the star-formation activity and the final stellar content.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
NGC 253 hosts the nearest nuclear starburst. Previous observations show a region rich in molecular gas, with dense clouds associated with recent star formation. We used the Atacama Large ...Submillimeter/Millimeter Array (ALMA) to image the 350 GHz dust continuum and molecular line emission from this region at 2 pc resolution. Our observations reveal ∼14 bright, compact (∼2-3 pc FWHM) knots of dust emission. Most of these sources are likely to be forming super star clusters (SSCs) based on their inferred dynamical and gas masses, association with 36 GHz radio continuum emission, and coincidence with line emission tracing dense, excited gas. One source coincides with a known SSC, but the rest remain invisible in Hubble near-infrared (IR) imaging. Our observations imply that gas still constitutes a large fraction of the overall mass in these sources. Their high brightness temperature at 350 GHz also implies a large optical depth near the peak of the IR spectral energy distribution. As a result, these sources may have large IR photospheres, and the IR radiation force likely exceeds L/c. Still, their moderate observed velocity dispersions suggest that feedback from radiation, winds, and supernovae are not yet disrupting most sources. This mode of star formation appears to produce a large fraction of stars in the burst. We argue for a scenario in which this phase lasts ∼1 Myr, after which the clusters shed their natal cocoons but continue to produce ionizing photons. The strong feedback that drives the observed cold gas and X-ray outflows likely occurs after the clusters emerge from this early phase.
Observations show that star formation is an inefficient and slow process. This result can be attributed to the injection of energy and momentum by stars that prevents free-fall collapse of molecular ...clouds. The mechanism of this stellar feedback is debated theoretically; possible sources of pressure include the classical warm H II gas, the hot gas generated by shock heating from stellar winds and supernovae, direct radiation of stars, and the dust-processed radiation field trapped inside the H II shell. In this paper, we measure observationally the pressures associated with each component listed above across the giant H II region 30 Doradus in the Large Magellanic Cloud. We exploit high-resolution, multi-wavelength images (radio, infrared, optical, ultraviolet, and X-ray) to map these pressures as a function of position. We find that radiation pressure dominates within 75 pc of the central star cluster, R136, while the H II gas pressure dominates at larger radii. By contrast, the dust-processed radiation pressure and hot gas pressure are generally weak and not dynamically important, although the hot gas pressure may have played a more significant role at early times. Based on the low X-ray gas pressures, we demonstrate that the hot gas is only partially confined and must be leaking out the H II shell. Additionally, we consider the implications of a dominant radiation pressure on the early dynamics of 30 Doradus.
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 report on the results of a search for serendipitous sources in CO emission in 110 cubes targeting CO(2 − 1), CO(3 − 2), and CO(6 − 5) at
z
∼ 1–2 from the second Plateau de Bure High-
z
...Blue Sequence Survey (PHIBSS2). The PHIBSS2 observations were part of a 4 yr legacy program at the IRAM Plateau de Bure Interferometer aimed at studying early galaxy evolution from the perspective of molecular gas reservoirs. We present a catalog of 67 candidate secondary sources from this search, with 45 of the 110 data cubes showing sources in addition to the primary target that appear to be field detections, unrelated to the central sources. This catalog includes redshifts, line widths, and fluxes, as well as an estimation of their reliability based on their false-positive probability. We perform a search in the 3D
Hubble Space Telescope
/CANDELS catalogs for the secondary CO detections and tentatively find that ∼64% of these have optical counterparts, which we use to constrain their redshifts. Finally, we use our catalog of candidate CO detections to derive the CO(2 − 1), CO(3 − 2), CO(4 − 3), CO(5 − 4), and CO(6 − 5) luminosity functions over a range of redshifts, as well as the molecular gas mass density evolution. Despite the different methodology, these results are in very good agreement with previous observational constraints derived from blind searches in deep fields. They provide an example of the type of “deep-field” science that can be carried out with targeted observations.
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