The sample of 566 molecular clouds identified in the CO(2–1) IRAM survey covering the disk of M 33 is explored in detail. The clouds were found using CPROPS and were subsequently catalogued in terms ...of their star-forming properties as non-star-forming (A), with embedded star formation (B), or with exposed star formation (C, e.g., presence of Hα emission). We find that the size-linewidth relation among the M 33 clouds is quite weak but, when comparing with clouds in other nearby galaxies, the linewidth scales with average metallicity. The linewidth and particularly the line brightness decrease with galactocentric distance. The large number of clouds makes it possible to calculate well-sampled cloud mass spectra and mass spectra of subsamples. As noted earlier, but considerably better defined here, the mass spectrum steepens (i.e., higher fraction of small clouds) with galactocentric distance. A new finding is that the mass spectrum of A clouds is much steeper than that of the star-forming clouds. Further dividing the sample, this difference is strong at both large and small galactocentric distances and the A vs. C difference is a stronger effect than the inner vs. outer disk difference in mass spectra. Velocity gradients are identified in the clouds using standard techniques. The gradients are weak and are dominated by prograde rotation; the effect is stronger for the high signal-to-noise clouds. A discussion of the uncertainties is presented. The angular momenta are low but compatible with at least some simulations. Finally, the cloud velocity gradients are compared with the gradient of disk rotation. The cloud and galactic gradients are similar; the cloud rotation periods are much longer than cloud lifetimes and comparable to the galactic rotation period. The rotational kinetic energy is 1–2% of the gravitational potential energy and the cloud edge velocity is well below the escape velocity, such that cloud-scale rotation probably has little influence on the evolution of molecular clouds.
We combine new sensitive, wide-field CO data from the HERACLES survey with ultraviolet and infrared data from GALEX and Spitzer to compare the surface densities of H2, Delta *SH2, and the recent star ...formation rate, Delta *SSFR, over many thousands of positions in 30 nearby disk galaxies. We more than quadruple the size of the galaxy sample compared to previous work and include targets with a wide range of galaxy properties. Even though the disk galaxies in this study span a wide range of properties, we find a strong, and approximately linear correlation between Delta *SSFR and Delta *SH2 at our common resolution of 1 kpc. This implies a roughly constant median H2 consumption time, Delta *tH2 Dep = Delta *SH2/ Delta *SSFR, of ~2.35 Gyr (including heavy elements) across our sample. At 1 kpc resolution, there is only a weak correlation between Delta *SH2 and Delta *tH2 Dep over the range Delta *SH2 5-100 M pc--2, which is probed by our data. We compile a broad set of literature measurements that have been obtained using a variety of star formation tracers, sampling schemes, and physical scales and show that overall, these data yield almost exactly the same results, although with more scatter. We interpret these results as strong, albeit indirect evidence that star formation proceeds in a uniform way in giant molecular clouds in the disks of spiral galaxies.
We present ~kiloparsec spatial resolution maps of the CO-to-H2 conversion factor (αCO) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for αCO and the ...DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H I column density to solve for both αCO and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, 12CO J = (2-1) maps from the IRAM 30 m large program HERACLES, and H I 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our αCO results on the more typically used 12CO J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for αCO and the DGR. On average, αCO = 3.1 M ⊙ pc-2 (K km s-1)-1 for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of αCO as a function of galactocentric radius. However, most galaxies exhibit a lower αCO value in the central kiloparsec—a factor of ~2 below the galaxy mean, on average. In some cases, the central αCO value can be factors of 5-10 below the standard Milky Way (MW) value of αCO, MW = 4.4 M ⊙ pc-2 (K km s-1)-1. While for αCO we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate αCO for studies of nearby galaxies.
We present high-resolution large-scale observations of the molecular and atomic gas in the Local Group galaxy M 33. The observations were carried out using the HEterodyne Receiver Array (HERA) at the ...30 m IRAM telescope in the CO(2–1) line, achieving a resolution of 12″ × 2.6 km s-1, enabling individual giant molecular clouds (GMCs) to be resolved. The observed region is 650 square arcminutes mainly along the major axis and out to a radius of 8.5 kpc, and covers entirely the 2′ × 40′ radial strip observed with the HIFI and PACS Spectrometers as part of the HERM33ES Herschel key program. The achieved sensitivity in main-beam temperature is 20–50 mK at 2.6 km s-1 velocity resolution. The CO(2–1) luminosity of the observed region is 1.7 ± 0.1 × 107 K km s-1 pc2 and is estimated to be 2.8 ± 0.3 × 107 K km s-1 pc2 for the entire galaxy, corresponding to H2 masses of 1.9 × 108 M⊙ and 3.3 × 108 M⊙ respectively (including He), calculated with N(H2)/ICO(1 − 0) twice the Galactic value due to the half-solar metallicity of M 33. The H i 21 cm VLA archive observations were reduced, and the mosaic was imaged and cleaned using the multi-scale task in the CASA software package, yielding a series of datacubes with resolutions ranging from 5″ to 25″. The H i mass within a radius of 8.5 kpc is estimated to be 1.4 × 109 M⊙ . The azimuthally averaged CO surface brightness decreases exponentially with a scale length of 1.9 ± 0.1 kpc whereas theatomic gas surface density is constant at ΣHi = 6 ± 2 M⊙ pc-2 deprojected to face-on. For an N(H2)/ICO(1 − 0) conversion factor twice that of the Milky Way, the central kiloparsec H2 surface density is ΣH2 = 8.5 ± 0.2 M⊙ pc-2. The star formation rate per unit molecular gas (SF efficiency, the rate of transformation of molecular gas into stars), as traced by the ratio of CO to Hα and FIR brightness, is constant with radius. The SFE, with a N(H2)/ICO(1 − 0) factor twice galactic, appears 2–4 times greater than for large spiral galaxies. A morphological comparison of molecular and atomic gas with tracers of star formation is presented showing good agreement between these maps both in terms of peaks and holes. A few exceptions are noted. Several spectra, including those of a molecular cloud situated more than 8 kpc from the galaxy center, are presented.
The IRAM M 33 CO(2–1) survey Druard, C; Braine, J; Schuster, K F ...
Astronomy and astrophysics (Berlin),
07/2014, Letnik:
567
Journal Article
Recenzirano
Odprti dostop
To study the interstellar medium and the interplay between the atomic and molecular components in a low-metallicity environment, we present a complete high angular and spectral resolution map and ...position-position-velocity data cube of the sup 12 CO(J = 2-1) emission from the Local Group galaxy Messier 33. Its metallicity is roughly half-solar, such that we can compare its interstellar medium with that of the Milky Way with the main changes being the metallicity and the gas mass fraction. Using the CO(2-1) emission to trace the molecular gas, the probability distribution function of the Hsub 2 column density shows an excess at high column density above a log-normal distribution.
Pulsars in the Galactic centre promise to enable unparalleled tests of gravity theories and black hole physics and to serve as probes of the stellar formation history and evolution and the ...interstellar medium in the complex central region of the Milky Way. The community has surveyed the innermost region of the galaxy for decades without detecting a population of pulsars, which is puzzling. A strong scattering of the pulsed signals in this particular direction has been argued to be a potential reason for the non-detections. Scattering has a strong inverse dependence on observing frequency, therefore an effective way to alleviate its effect is to use higher frequencies in a survey for pulsars in the Galactic centre, in particular, close to the supermassive black hole Sagittarius A*. We present the first pulsar survey at short millimetre wavelengths, using several frequency bands between 84 and 156 GHz (λ = 3.5–1.92 mm), targeted to the Galactic centre. The observations were made with the Institut de Radioastronomie Millimétrique 30m Telescope in 28 epochs between 2016 December and 2018 May. This survey is the first that is essentially unaffected by scattering and therefore unbiased in population coverage, including fast-spinning pulsars that might be out of reach of lower-frequency Galactic centre surveys. We discovered no new pulsars and relate this result mainly to the decreased flux density of pulsars at high frequencies, combined with our current sensitivity. However, we demonstrate that surveys at these extremely high radio frequencies are capable of discovering new pulsars, analyse their sensitivity limits with respect to a simulated Galactic centre pulsar population, and discuss the main challenges and possible improvements for similar surveys in the future.
Abstract
We analyze H
α
or CO rotation curves extending out to several galaxy effective radii for 100 massive, large, star-forming disk galaxies (SFGs) across the peak of cosmic galaxy star formation ...(
z
∼ 0.6–2.5), more than doubling the previous sample presented by Genzel et al. and Price et al. The observations were taken with SINFONI and KMOS integral-field spectrographs at the ESO-Very Large Telescope, LUCI-LBT, NOEMA-IRAM, and Atacama Large Millimeter/submillimeter Array. We fit the major-axis kinematics with beam-convolved, forward models of turbulent rotating disks with bulges embedded in dark matter (DM) halos, including the effects of pressure support. The fraction of dark to total matter within the disk effective radius (
R
e
∼ 5 kpc),
f
DM
(
R
e
) =
V
2
DM
(
R
e
)/
V
2
circ
(
R
e
) decreases with redshift: at
z
∼ 1 (
z
∼ 2) the median DM fraction is 0.38 ± 0.23 (0.27 ± 0.18), and a third (half) of all galaxies are
maximal
disks with
f
DM
(
R
e
) < 0.28. DM fractions correlate inversely with the baryonic surface density, and the low DM fractions can be explained with a flattened, or cored, inner DM density distribution. At
z
∼ 2, there is ≈40% less DM mass on average within
R
e
compared to expected values based on cosmological stellar-mass–halo-mass relations. The DM deficit is more evident at high star formation rate surface densities (≳2.5
M
⊙
yr
−1
kpc
2
) and galaxies with massive bulges (≥10
10
M
⊙
). A combination of stellar or active galactic nucleus feedback, and/or heating due to dynamical friction, may drive the DM from cuspy into cored mass distributions, pointing to an efficient buildup of massive bulges and central black holes at
z
∼ 2 SFGs.
The Neel IRAM KIDs Array (NIKA) is a fully integrated measurement system based on kinetic inductance detectors (KIDs) currently being developed for millimeter wave astronomy. The instrument includes ...dual-band optics allowing simultaneous imaging at 150 GHz and 220 GHz. The imaging sensors consist of two spatially separated arrays of KIDs. The first array, mounted on the 150 GHz branch, is composed of 144 lumped-element KIDs. The second array (220 GHz) consists of 256 antenna-coupled KIDs. Each of the arrays is sensitive to a single polarization; the band splitting is achieved by using a grid polarizer. The optics and sensors are mounted in a custom dilution cryostat, with an operating temperature of ~70 mK. Electronic readout is realized using frequency multiplexing and a transmission line geometry consisting of a coaxial cable connected in series with the sensor array and a low-noise 4 K amplifier. The dual-band NIKA was successfully tested in 2010 October at the Institute for Millimetric Radio Astronomy (IRAM) 30 m telescope at Pico Veleta, Spain, performing in-line with laboratory predictions. An optical NEP was then calculated to be around 2 X 10--16 W Hz--1/2 (at 1 Hz) while under a background loading of approximately 4 pW pixel--1. This improvement in comparison with a preliminary run (2009) verifies that NIKA is approaching the target sensitivity for photon-noise limited ground-based detectors. Taking advantage of the larger arrays and increased sensitivity, a number of scientifically relevant faint and extended objects were then imaged including the Galactic Center SgrB2 (FIR1), the radio galaxy Cygnus A, and the NGC1068 Seyfert galaxy. These targets were all observed simultaneously in the 150 GHz and 220 GHz atmospheric windows.
Context. We report on a successful, simultaneous observation and modeling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive (4 $\times$ ...106 $M_{\odot}$ ) black hole at the Galactic center. Aims. We study and model the physical processes giving rise to the variable emission of Sgr A*. Methods. Our non-relativistic modeling is based on simultaneous observations that have been carried out on 03 June, 2008. We used the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the LABOCA bolometer at the Atacama Pathfinder Experiment (APEX). We emphasize the importance of a multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare modeling. Results. The observations reveal strong flare activity in the 0.87 mm (345 GHz) sub-mm domain and in the 3.8 μ/2.2 μm NIR. Inspection and modeling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5 ± 0.5 h. We explain the flare emission delay by an adiabatic expansion of the source components. The derived physical quantities that describe the flare emission give a source component expansion speed of vexp ~ 0.005c, source sizes around one Schwarzschild radius with flux densities of a few Janskys, and spectral indices of α = 0.8 to 1.8, corresponding to particle spectral indices ~2.6 to 4.6. At the start of the flare the spectra of these components peak at frequencies of a few THz. Conclusions. These parameters suggest that the adiabatically expanding source components either have a bulk motion greater than vexp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*.
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