Aims. We analyze the applicability of far-infrared fine-structure lines Cii 158 μm, Oi 63 μm, and Oiii 88 μm to reliably trace the star formation rate (SFR) in a sample of low-metallicity dwarf ...galaxies from the Herschel Dwarf Galaxy Survey and, furthermore, extend the analysis to a broad sample of galaxies of various types and metallicities in the literature. Methods. We study the trends and scatter in the relation between the SFR (as traced by GALEX FUV and MIPS 24 μm) and far-infrared line emission, on spatially resolved and global galaxy scales, in dwarf galaxies. We assemble far-infrared line measurements from the literature and infer whether the far-infrared lines can probe the SFR (as traced by the total infrared luminosity) in a variety of galaxy populations. Results. In metal-poor dwarfs, the Oi63 and Oiii88 lines show the strongest correlation with the SFR with an uncertainty on the SFR estimates better than a factor of 2, while the link between Cii emission and the SFR is more dispersed (uncertainty factor of 2.6). The increased scatter in the SFR–LCII relation toward low metal abundances, warm dust temperatures, and large filling factors of diffuse, highly ionized gas suggests that other cooling lines start to dominate depending on the density and ionization state of the gas. For the literature sample, we evaluate the correlations for a number of different galaxy populations. The Cii and Oi63 lines are considered to be reliable SFR tracers in starburst galaxies, recovering the star formation activity within an uncertainty of factor 2. For sources with composite and active galactic nucleus (AGN) classifications, all three FIR lines can recover the SFR with an uncertainty factor of 2.3. The SFR calibrations for ultra-luminous infrared galaxies (ULIRGs) are similar to starbursts/AGNs in terms of scatter but offset from the starburst/AGN SFR relations because of line deficits relative to their total infrared luminosity. While the number of detections of the FIR fine-structure lines is still very limited at high redshift for Oi63 and Oiii88, we provide an SFR calibration for Cii.
We have conducted ALMA CO isotopes and 1.3 mm continuum observations toward filamentary molecular clouds of the N159W-South region in the Large Magellanic Cloud with an angular resolution of ∼0 25 ...(∼0.07 pc). Although the previous lower-resolution (∼1″) ALMA observations revealed that there is a high-mass protostellar object at an intersection of two line-shaped filaments in 13CO with the length scale of ∼10 pc, the spatially resolved observations, in particular, toward the highest column density part traced by the 1.3 mm continuum emission, the N159W-South clump, show complicated hub-filamentary structures. We also discovered that there are multiple protostellar sources with bipolar outflows along the massive filament. The redshifted/blueshifted components of the 13CO emission around the massive filaments/protostars have complementary distributions, which is considered to be possible evidence for a cloud-cloud collision. We propose a new scenario in which the supersonically colliding gas flow triggers the formation of both the massive filament and protostars. This is a modification of the earlier scenario of cloud-cloud collision, by Fukui et al., that postulated the two filamentary clouds occur prior to the high-mass star formation. A recent theoretical study of the shock compression in colliding molecular flows by Inoue et al. demonstrates that the formation of filaments with hub structure is a usual outcome of the collision, lending support for the present scenario. The theory argues that the filaments are formed as dense parts in a shock compressed sheet-like layer, which resembles "an umbrella with pokes."
We present ALMA observations of CO isotopes and 1.3 mm continuum emission toward the N159E-Papillon Nebula in the Large Magellanic Cloud (LMC). The spatial resolution is 0 25-0 28 (0.06-0.07 pc), ...which is a factor of 3 higher than previous ALMA observations in this region. The high resolution allowed us to resolve highly filamentary CO distributions with typical widths of ∼0.1 pc (full width half maximum) and line masses of a few 100 M pc−1. The filaments (more than ten in number) show an outstanding hub-filament structure emanating from the nebular center toward the north. We identified for the first time two massive protostellar outflows of ∼104 yr dynamical age along one of the most massive filaments. The observations also revealed several pillar-like CO features around the Nebula. The H ii region and the pillars have a complementary spatial distribution and the column density of the pillars is an order of magnitude higher than that of the pillars in the Eagle nebula (M16) in the Galaxy, suggesting an early stage of pillar formation with an age younger than ∼105 yr. We suggest that a cloud-cloud collision triggered the formation of the filaments and protostar within the last ∼2 Myr. It is possible that the collision is more recent, as part of the kpc-scale H i flows come from the tidal interaction resulting from the close encounter between the LMC and SMC ∼200 Myr ago as suggested for R136 by Fukui et al.
The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and life-cycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc ...resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and H alpha observations. We investigate the range of CO-to-Hsub 2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on XCO to be 6 x 10sup 20 cmsup -2 Ksup -1 kmsup -1 s in the LMC (Z = 0.5Z) at 15 pc resolution, and 4 x 1021 cmsup -2 Ksup -1 kmsup -1 s in the SMC (Z = 0.2Z) at 45 pc resolution. Our analysis demonstrates that obtaining robust ISM masses remains a non-trivial endeavor even in the local Universe using state-of-the-art maps of thermal dust emission.
The CO-dark molecular gas mass in 30 Doradus Chevance, Mélanie; Madden, Suzanne C; Fischer, Christian ...
Monthly notices of the Royal Astronomical Society,
06/2020, Volume:
494, Issue:
4
Journal Article
Peer reviewed
Open access
ABSTRACT
Determining the efficiency with which gas is converted into stars in galaxies requires an accurate determination of the total reservoir of molecular gas mass. However, despite being the most ...abundant molecule in the Universe, H2 is challenging to detect through direct observations and indirect methods have to be used to estimate the total molecular gas reservoir. These are often based on scaling relations from tracers such as CO or dust, and are generally calibrated in the Milky Way. Yet, evidence that these scaling relations are environmentally dependent is growing. In particular, the commonly used CO-to-H2 conversion factor (XCO) is expected to be higher in metal-poor and/or strongly UV-irradiated environments. We use new SOFIA/FIFI-LS observations of far-infrared fine-structure lines from the ionized and neutral gas and the Meudon photodissociation region model to constrain the physical properties and the structure of the gas in the massive star-forming region of 30 Doradus in the Large Magellanic Cloud, and determine the spatially resolved distribution of the total reservoir of molecular gas in the proximity of the young massive cluster R136. We compare this value with the molecular gas mass inferred from ground-based CO observations and dust-based estimates to quantify the impact of this extreme environment on commonly used tracers of the molecular gas. We find that the strong radiation field combined with the half-solar metallicity of the surrounding gas is responsible for a large reservoir of ‘CO-dark’ molecular gas, leaving a large fraction of the total H2 gas (≳75 per cent) undetected when adopting a standard XCO factor in this massive star-forming region.
High-resolution (<0.1 pc) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 30Dor-10 molecular cloud 15 pc north of R136 are presented. The 12CO 2-1 emission morphology contains ...clumps near the locations of known mid-IR massive protostars, as well as a series of parsec-long filaments oriented almost directly toward R136. There is elevated kinetic energy (line widths at a given size scale) in 30Dor-10 compared to other Large Magellanic Cloud and Galactic star formation regions, consistent with large-scale energy injection to the region. Analysis of the cloud substructures is performed by segmenting emission into disjoint approximately round "cores" using clumpfind, by considering the hierarchical structures defined by isointensity contours using dendrograms, and by segmenting into disjoint long thin "filaments" using Filfinder. Identified filaments have widths ∼0.1 pc. The inferred balance between gravity and kinematic motions depends on the segmentation method: entire objects identified with clumpfind are consistent with freefall collapse or virial equilibrium with moderate external pressure, whereas many dendrogram-identified parts of hierarchical structures have higher mass surface densities LTE than if gravitational and kinetic energies were in balance. Filaments have line masses that vary widely compared to the critical line mass calculated assuming thermal and nonthermal support. Velocity gradients in the region do not show any strong evidence for accretion of mass along filaments. The upper end of the "core" mass distribution is consistent with a power law with the same slope as the stellar initial mass function.
ABSTRACT We present the Herschel SPIRE Fourier Transform Spectroscopy (FTS) atlas for a complete flux-limited sample of local ultraluminous infrared galaxies (ULIRGs) as part of the HERschel Ultra ...Luminous InfraRed Galaxy Survey (HERUS). The data reduction is described in detail and was optimized for faint FTS sources ,with particular care being taken for the subtraction of the background, which dominates the continuum shape of the spectra. To improve the final spectra, special treatment in the data reduction has been given to any observation suffering from artifacts in the data caused by anomalous instrumental effects. Complete spectra are shown covering 200-671 m, with photometry in the SPIRE bands at 250, 350, and 500 m. The spectra include near complete CO ladders for over half of our sample, as well as fine structure lines from C i 370 m, C i 609 m, and N ii 205 m. We also detect H2O lines in several objects. We construct CO spectral line energy distributions (SLEDs) for the sample, and compare their slopes with the far-infrared (FIR) colors and luminosities. We show that the CO SLEDs of ULIRGs can be broadly grouped into three classes based on their excitation. We find that the mid-J (5 < J < 8) lines are better correlated with the total FIR luminosity, suggesting that the warm gas component is closely linked to recent star formation. The higher J transitions do not linearly correlate with the FIR luminosity, consistent with them originating in hotter, denser gas that is unconnected to the current star formation. We conclude that in most cases more than one temperature component is required to model the CO SLEDs.
Abstract
We present a first look at the MRS observations of the nucleus of the nearby galaxy M83, taken with MIRI on board JWST. The observations show a rich set of emission features from the ionized ...gas, warm molecular gas, and dust. To begin dissecting the complex processes in this part of the galaxy, we divide the observations into four different regions. We find that the strength of the emission features varies strongly from region to region, with the southeast region displaying the weakest features tracing the dust continuum and interstellar medium (ISM) properties. Comparison between the cold molecular gas traced by the
12
CO (1–0) transition with the Atacama Large Millimeter/submillimeter Array and the H
2
S(1) transition shows a similar spatial distribution. This is in contrast to the distribution of the much warmer H
2
emission from the S(7) transition found to be concentrated around the optical nucleus. We use the rotational emission lines and model the H
2
excitation to estimate a total molecular gas mass accounting for the warm H
2
component of
M
(>50 K)
H
2
= 67.90 (±5.43) × 10
6
M
⊙
. We compare this value to the total gas mass inferred by probing the cold H
2
gas through the
12
CO (1–0) emission,
M
(CO)
H
2
= 17.15 × 10
6
M
⊙
. We estimate that ∼75% of the total molecular gas mass is contained in the warm H
2
component. We also identify O
iv
25.89
μ
m and Fe
ii
25.99
μ
m emission. We propose that the diffuse Fe
ii
25.99
μ
m emission might be tracing shocks created during the interactions between the hot wind produced by the starburst and the much cooler ISM above the galactic plane. More detailed studies are needed to confirm such a scenario.
We present the results of near-infrared-to-mid-infrared (NIR-to-MIR) imaging and NIR spectroscopic observations of two galaxy mergers, NGC 2782 (Arp 215) and NGC 7727 (Arp 222), with the Infrared ...Camera on board AKARI. NGC 2782 shows extended MIR emission in the eastern side of the galaxy, which corresponds to the eastern tidal tail seen in the H i 21 cm map, while NGC 7727 shows extended MIR emission in the north of the galaxy, which is similar to the plumes seen in the residual image at the K-band after subtracting a galaxy model. Both extended structures are thought to have formed in association with their merger events. They show excess emission at 7-15 m, which can be attributed to emission from polycyclic aromatic hydrocarbons (PAHs), while the observed spectral energy distributions (SEDs) decline longward of 24 m, suggesting that very small grains (VSGs) are deficient. These characteristics of the observed MIR SED may be explained if PAHs are formed by fragmentation of VSGs during merger events. The star formation rate is estimated from the MIR PAH emission in the eastern tail region of NGC 2782 and it is in fair agreement with those estimated from H and C ii 158 m. MIR observations are efficient for the study of dust processing and structures formed during merger events.
Abstract
In this work, we present 299 candidate young stellar objects (YSOs) in 30 Doradus discovered using Spitzer and Herschel point-source catalogs, 276 of which are new. We study the parental ...giant molecular clouds in which these YSO candidates form using recently published Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 7 observations of
12
CO and
13
CO. The threshold for star formation in 30 Doradus inferred by the LTE-based mass surface density is 178
M
⊙
pc
−2
, 40% higher than the threshold for star formation in the Milky Way. This increase in star formation threshold in comparison to the Milky Way and increase in line width seen in clumps 11 pc away in comparison to clumps 45 pc away from the R136 super star cluster could be due to injected turbulent energy, increase in interstellar medium pressure, and/or local magnetic field strength. Of the 299 YSO candidates in this work, 62% are not associated with
12
CO molecular gas. This large fraction can be explained by the fact that 75%–97% of the H
2
gas is not traced by CO. We fit a Kroupa initial mass function to the YSO candidates and find that the total integrated stellar mass is 18,000
M
⊙
and that the region has a star formation rate (SFR) of 0.18
M
⊙
yr
−1
. The initial mass function determined here applies to the four 150″ × 150″ (37.5 pc × 37.5 pc) subfields and one 150″ × 75″ (37.5 pc × 18.8 pc) subfield observed with ALMA. The SFR in 30 Doradus has increased in the past few million years.
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