We present ALMA Band 9 observations of the C II158 μm emission for a sample of 10 main-sequence galaxies at redshift z ˜ 2, with typical stellar masses (log M⋆/M⊙ ˜ 10.0-10.9) and star formation ...rates (˜35-115 M⊙ yr-1). Given the strong and well-understood evolution of the interstellar medium from the present to z = 2, we investigate the behaviour of the C II emission and empirically identify its primary driver. We detect C II from six galaxies (four secure and two tentative) and estimate ensemble averages including non-detections. The C II-to-infrared luminosity ratio (C II/LIR) of our sample is similar to that of local main-sequence galaxies (˜2 × 10-3), and ˜10 times higher than that of starbursts. The C II emission has an average spatial extent of 4-7 kpc, consistent with the optical size. Complementing our sample with literature data, we find that the C II luminosity correlates with galaxies' molecular gas mass, with a mean absolute deviation of 0.2 dex and without evident systematics: the C II-to-H2 conversion factor (α _C II ˜ 30 M⊙/L⊙) is largely independent of galaxies' depletion time, metallicity, and redshift. C II seems therefore a convenient tracer to estimate galaxies' molecular gas content regardless of their starburst or main-sequence nature, and extending to metal-poor galaxies at low and high redshifts. The dearth of C II emission reported for z > 6-7 galaxies might suggest either a high star formation efficiency or a small fraction of ultraviolet light from star formation reprocessed by dust.
Context.
Molecular gas is a necessary fuel for star formation. The CO (1−0) transition is often used to deduce the total molecular hydrogen but is challenging to detect in low-metallicity galaxies in ...spite of the star formation taking place. In contrast, the C
II
λ
158
μ
m is relatively bright, highlighting a potentially important reservoir of H
2
that is not traced by CO (1−0) but is residing in the C
+
-emitting regions.
Aims.
Here we aim to explore a method to quantify the total H
2
mass (
M
H
2
) in galaxies and to decipher what parameters control the CO-dark reservoir.
Methods.
We present Cloudy grids of density, radiation field, and metallicity in terms of observed quantities, such as O
I
, C
I
, CO (1−0), C
II
,
L
TIR
, and the total
M
H
2
. We provide recipes based on these models to derive total
M
H
2
mass estimates from observations. We apply the models to the
Herschel
Dwarf Galaxy Survey, extracting the total
M
H
2
for each galaxy, and compare this to the H
2
determined from the observed CO (1−0) line. This allows us to quantify the reservoir of H
2
that is CO-dark and traced by the C
II
λ
158
μ
m.
Results.
We demonstrate that while the H
2
traced by CO (1−0) can be negligible, the C
II
λ
158
μ
m can trace the total H
2
. We find 70 to 100% of the total H
2
mass is not traced by CO (1−0) in the dwarf galaxies, but is well-traced by C
II
λ
158
μ
m. The CO-dark gas mass fraction correlates with the observed
L
C
II
/
L
CO(1−0)
ratio. A conversion factor for C
II
λ
158
μ
m to total H
2
and a new CO-to-total-
M
H
2
conversion factor as a function of metallicity are presented.
Conclusions.
While low-metallicity galaxies may have a feeble molecular reservoir as surmised from CO observations, the presence of an important reservoir of molecular gas that is not detected by CO can exist. We suggest a general recipe to quantify the total mass of H
2
in galaxies, taking into account the CO and C
II
observations. Accounting for this CO-dark H
2
gas, we find that the star-forming dwarf galaxies now fall on the Schmidt–Kennicutt relation. Their star-forming efficiency is rather normal because the reservoir from which they form stars is now more massive when introducing the C
II
measures of the total H
2
compared to the small amount of H
2
in the CO-emitting region.
Aims. The goal of this paper is to analyse the behaviour of the gas-to-dust mass ratio (G/D) of local Universe galaxies over a wide metallicity range. We especially focus on the low-metallicity part ...of the G/D vs metallicity relation and investigate several explanations for the observed relation and scatter. Methods. We assembled a total of 126 galaxies, covering a 2 dex metallicity range and with 30% of the sample with 12 + log(O/H)≤ 8.0. We homogeneously determined the dust masses with a semi-empirical dust model including submm constraints. The atomic and molecular gas masses have been compiled from the literature. We used two XCO scenarios to estimate the molecular gas mass: the Galactic conversion factor, XCO,MW, and a XCO that depends on the metallicity XCO,Z (∝Z-2). We modelled the observed trend of the G/D with metallicity using two simple power laws (slope of –1 and free) and a broken power law. Correlations with morphological type, stellar masses, star formation rates, and specific star formation rates are also discussed. We then compared the observed evolution of the G/D with predictions from several chemical evolution models and explored different physical explanations for the observed scatter in the G/D values. Results. We find that out of the five tested galactic parameters, metallicity is the main physical property of the galaxy driving the observed G/D. The G/D versus metallicity relation cannot be represented by a single power law with a slope of –1 over the whole metallicity range. The observed trend is steeper for metallicities lower than ~8.0. A large scatter is observed in the G/D values for a given metallicity: in metallicity bins of ~0.1 dex, the dispersion around the mean value is ~0.37 dex. On average, the broken power law reproduces the observed G/D best compared to the two power laws (slope of –1 or free) and provides estimates of the G/D that are accurate to a factor of 1.6. The good agreement of observed values of the G/D and its scatter with respect to metallicity with the predicted values of the three tested chemical evolution models allows us to infer that the scatter in the relation is intrinsic to galactic properties, reflecting the different star formation histories, dust destruction efficiencies, dust grain size distributions, and chemical compositions across the sample. Conclusions. Our results show that the chemical evolution of low-metallicity galaxies, traced by their G/D, strongly depends on their local internal conditions and individual histories. The large scatter in the observed G/D at a given metallicity reflects the impact of various processes occurring during the evolution of a galaxy. Despite the numerous degeneracies affecting them, disentangling these various processes is now the next step.
Aims. The purpose of this work is the characterization of the radial distribution of dust, stars, gas, and star-formation rate (SFR) in a sub-sample of 18 face-on spiral galaxies extracted from the ...DustPedia sample. Methods. This study is performed by exploiting the multi-wavelength DustPedia database, from ultraviolet (UV) to sub-millimeter bands, in addition to molecular (12CO) and atomic (Hi) gas maps and metallicity abundance information available in the literature. We fitted the surface-brightness profiles of the tracers of dust and stars, the mass surface-density profiles of dust, stars, molecular gas, and total gas, and the SFR surface-density profiles with an exponential curve and derived their scale-lengths. We also developed a method to solve for the CO-to-H2 conversion factor (αCO) per galaxy by using dust- and gas-mass profiles. Results. Although each galaxy has its own peculiar behavior, we identified a common trend of the exponential scale-lengths versus wavelength. On average, the scale-lengths normalized to the B-band 25 mag/arcsec2 radius decrease from UV to 70 μm, from 0.4 to 0.2, and then increase back up to ~0.3 at 500 microns. The main result is that, on average, the dust-mass surface-density scale-length is about 1.8 times the stellar one derived from IRAC data and the 3.6 μm surface brightness, and close to that in the UV. We found a mild dependence of the scale-lengths on the Hubble stage T: the scale-lengths of the Herschel bands and the 3.6 μm scale-length tend to increase from earlier to later types, the scale-length at 70 μm tends to be smaller than that at longer sub-mm wavelength with ratios between longer sub-mm wavelengths and 70 μm that decrease with increasing T. The scale-length ratio of SFR and stars shows a weak increasing trend towards later types. Our αCO determinations are in the range (0.3−9) M⊙ pc-2 (K km s-1)-1, almost invariant by using a fixed dust-to-gas ratio mass (DGR) or a DGR depending on metallicity gradient.
The rate at which interstellar gas is converted into stars, and its dependence on environment, is one of the pillars on which our understanding of the visible Universe is build. We present a ...comparison of the surface density of young stars (Σ⋆) and dust surface density (Σdust) across NGC 346 (N66) in 115 independent pixels of 6 × 6 pc2. We find a correlation between Σ⋆ and Σdust with a considerable scatter. A power-law fit to the data yields a steep relation with an exponent of 2.6 ± 0.2. We convert Σdust to gas surface density (Σgas) and Σ⋆ to star formation rate (SFR) surface densities (ΣSFR), using simple assumptions for the gas-to-dust mass ratio and the duration of star formation. The derived total SFR (4 ± 1×10−3 M⊙ yr−1) is consistent with SFR estimated from the Hα emission integrated over the Hα nebula. On small scales the ΣSFR derived using Hα systematically underestimates the count-based ΣSFR, by up to a factor of 10. This is due to ionizing photons escaping the area, where the stars are counted. We find that individual 36 pc2 pixels fall systematically above integrated disc galaxies in the Schmidt–Kennicutt diagram by on average a factor of ∼7. The NGC 346 average SFR over a larger area (90 pc radius) lies closer to the relation but remains high by a factor of ∼3. The fraction of the total mass (gas plus young stars) locked in young stars is systematically high (∼10 per cent) within the central 15 pc and systematically lower outside (2 per cent), which we interpret as variations in star formation efficiency. The inner 15 pc is dominated by young stars belonging to a centrally condensed cluster, while the outer parts are dominated by a dispersed population. Therefore, the observed trend could reflect a change of star formation efficiency between clustered and non-clustered star formation.
We provide an overview of ancillary 24-, 70- and 160-μm data from the Multiband Imaging Photometer for Spitzer (MIPS) that are intended to complement the 70-500 μm Herschel Space Observatory ...photometry data for nearby galaxies obtained by the Herschel-SPIRE Local Galaxies Guaranteed Time Programs and the Herschel Virgo Cluster Survey. The MIPS data can be used to extend the photometry to wavebands that are not observed in these Herschel surveys and to check the photometry in cases where Herschel performs observations at the same wavelengths. Additionally, we measured globally integrated 24-160 μm flux densities for the galaxies in the sample that can be used for the construction of spectral energy distributions. Using MIPS photometry published by other references, we have confirmed that we are obtaining accurate photometry for these galaxies.
Aims.
The purpose of this work is the characterization of the main scaling relations between all of the interstellar medium (ISM) components, namely dust, atomic, molecular, and total gas, and ...gas-phase metallicity, as well as other galaxy properties, such as stellar mass (
M
star
) and galaxy morphology, for late-type galaxies in the Local Universe.
Methods.
This study was performed by extracting late-type galaxies from the entire DustPedia sample and by exploiting the large and homogeneous dataset available thanks to the DustPedia project. The sample consists of 436 galaxies with morphological stage spanning from
T
= 1−10,
M
star
from 6 × 10
7
to 3 × 10
11
M
⊙
, star formation rate from 6 × 10
−4
to 60
M
⊙
yr
−1
, and oxygen abundance from 12 + log(O/H) = 8−9.5. Molecular and atomic gas data were collected from the literature and properly homogenized. All the masses involved in our analysis refer to the values within the optical disks of galaxies. The scaling relations involving the molecular gas are studied by assuming both a constant and a metallicity-dependent CO-to-H
2
conversion factor (
X
CO
). The analysis was performed by means of the survival analysis technique, in order to properly take into account the presence of both detection and nondetection in the data.
Results.
We confirm that the dust mass correlates very well with the total gas mass, and find –for the first time– that the dust mass correlates better with the atomic gas mass than with the molecular one. We characterize important mass ratios such as the gas fraction, the molecular-to-atomic gas mass ratio, the dust-to-total gas mass ratio (DGR), and the dust-to-stellar mass ratio, and study how they relate to each other, to galaxy morphology, and to gas-phase metallicity. Only the assumption of a metallicity-dependent
X
CO
reproduces the expected decrease of the DGR with increasing morphological stage and decreasing gas-phase metallicity, with a slope of about 1. The DGR, the gas-phase metallicity, and the dust-to-stellar mass ratio are, for our galaxy sample, directly linked to galaxy morphology. The molecular-to-atomic gas mass ratio and the DGR show a positive correlation for low molecular gas fractions, but for galaxies rich in molecular gas this trend breaks down. To our knowledge, this trend has never been found before, and provides new constraints for theoretical models of galaxy evolution and a reference for high-redshift studies. We discuss several scenarios related to this finding.
Conclusions.
The DustPedia database of late-type galaxies is an extraordinary tool for the study of the ISM scaling relations, thanks to its homogeneous collection of data for the different ISM components. The database is made publicly available to the whole community.
We present new ISOCAM mid-infrared spectra of three starbursting nearby dwarf galaxies, NGC 1569, NGC 1140 and II Zw 40 and the 30 Dor region of the LMC and explore the properties of the ISM in ...low-metallicity environments, also using additional sources from the literature. We analyse the various components of the ISM probed by the mid-infrared observations and compare them with other Galactic and extragalactic objects. The MIR spectra of the low-metallicity starburst sources are dominated by the Ne IIIl15.56 km and S IVl10.51 km lines, as well as a steeply rising dust continuum. PAH bands are generaly faint, both locally and averaged over the full galaxy, in stark contrast to dustier starburst galaxies, where the PAH features are very prominant and even dominate on global scales. The hardness of the modeled interstellar radiation fields for the dwarf galaxies increases as the presence of PAH band emission becomes less pronounced. The Ne III/Ne II ratios averaged over the full galaxy are strikingly high, often > 10. Thus, the hard radiation fields are pronounced and pervasive. We find a prominent correlation between the PAHs/VSGs and the Ne III/Ne II ratios for a wide range of objects, including the low metallicity galaxies as well as Galactic H II regions and other metal-rich galaxies. This effect is consistent with the hardness of the interstellar radiation field playing a major role in the destruction of PAHs in the low metallicity ISM. We see a PAHs/VSGs and metallicity correlation, also found by Engelbracht et al. (2005, ApJ, 628, 29) for a larger survey. Combined effects of metallicity and radiation field seem to be playing important roles in the observed behavior of PAHs in the low metallicity systems.
In the framework of a systematic study with the ALMA interferometer of IR-selected main-sequence and starburst galaxies at
z
∼ 1 − 1.7 at typical ∼1″ resolution, we report on the effects of mid-IR- ...and X-ray-detected active galactic nuclei (AGN) on the reservoirs and excitation of molecular gas in a sample of 55 objects. We find widespread detectable nuclear activity in ∼30% of the sample. The presence of dusty tori influences the IR spectral energy distribution of galaxies, as highlighted by the strong correlation among the AGN contribution to the total IR luminosity budget (
f
AGN
=
L
IR, AGN
/
L
IR
), its hard X-ray emission, and the Rayleigh-Jeans to mid-IR (
S
1.2 mm
/
S
24 μm
) observed color, with evident consequences on the ensuing empirical star formation rate estimates. Nevertheless, we find only marginal effects of the presence and strength of AGN on the carbon monoxide CO (
J
= 2, 4, 5, 7) or neutral carbon (C I(
3
P
1
−
3
P
0
), C I(
3
P
2
−
3
P
1
)) line luminosities and on the derived molecular gas excitation as gauged by line ratios and the full spectral line energy distributions. The C I and CO emission up to
J
= 5, 7 thus primarily traces the properties of the host in typical IR luminous galaxies. However, our analysis highlights the existence of a large variety of line luminosities and ratios despite the homogeneous selection. In particular, we find a sparse group of AGN-dominated sources with the highest
L
IR, AGN
/
L
IR, SFR
ratios, ≳3, that are more luminous in CO (5−4) than what is predicted by the
L
′
CO(5-4)
−
L
IR, SFR
relation, which might be the result of the nuclear activity. For the general population, our findings translate into AGN having minimal effects on quantities such as gas and dust fractions and star formation efficiencies. If anything, we find hints of a marginal tendency of AGN hosts to be compact at far-IR wavelengths and to display 1.8 times larger dust optical depths. In general, this is consistent with a marginal impact of the nuclear activity on the gas reservoirs and star formation in average star-forming AGN hosts with
L
IR
> 5 × 10
11
L
⊙
, typically underrepresented in surveys of quasars and submillimeter galaxies.
Aims. Within the framework of the DustPedia project we investigate the properties of cosmic dust and its interaction with stellar radiation (originating from different stellar populations) for 814 ...galaxies in the nearby Universe, all observed by the Herschel Space Observatory. Methods. We take advantage of the widely used fitting code CIGALE, properly adapted to include the state-of-the-art dust model THEMIS. For comparison purposes, an estimation of the dust properties is provided by approximating the emission at far-infrared and sub-millimeter wavelengths with a modified blackbody. Using the DustPedia photometry we determine the physical properties of the galaxies, such as the dust and stellar mass, the star-formation rate, the bolometric luminosity, the unattenuated and the absorbed by dust stellar light, for both the old (> 200 Myr) and young (≤200 Myr) stellar populations. Results. We show how the mass of stars, dust, and atomic gas, as well as the star-formation rate and the dust temperature vary between galaxies of different morphologies and provide recipes to estimate these parameters given their Hubble stage (T). We find a mild correlation between the mass fraction of the small a-C(:H) grains with the specific star-formation rate. On average, young stars are very efficient in heating the dust, with absorption fractions reaching as high as ∼77% of the total unattenuated luminosity of this population. On the other hand, the maximum absorption fraction of old stars is ∼24%. Dust heating in early-type galaxies is mainly due to old stars, up to a level of ∼90%. Young stars progressively contribute more for “typical” spiral galaxies and they become the dominant source of dust heating for Sm-type and irregular galaxies, with ∼60% of their luminosity contributing to that purpose. Finally, we find a strong correlation of the dust heating fraction by young stars with morphology and the specific star-formation rate.
![The [C ii] emission as a mo...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0035-8711/small "The [C ii] emission as a molecular gas mass tracer in galaxies at low and high redshifts")
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