The dust mass in z > 6 normal star-forming galaxies Mancini, Mattia; Schneider, Raffaella; Graziani, Luca ...
Monthly notices of the Royal Astronomical Society. Letters,
07/2015, Volume:
451, Issue:
1
Journal Article
Peer reviewed
Open access
We interpret recent ALMA observations of z > 6 normal star-forming galaxies by means of a seminumerical method, which couples the output of a cosmological hydrodynamical simulation with a chemical ...evolution model which accounts for the contribution to dust enrichment from supernovae, asymptotic giant branch stars and grain growth in the interstellar medium. We find that while stellar sources dominate the dust mass of small galaxies, the higher level of metal enrichment experienced by galaxies with M
star > 109 M⊙ allows efficient grain growth, which provides the dominant contribution to the dust mass. Even assuming maximally efficient supernova dust production, the observed dust mass of the z = 7.5 galaxy A1689-zD1 requires very efficient grain growth. This, in turn, implies that in this galaxy the average density of the cold and dense gas, where grain growth occurs, is comparable to that inferred from observations of quasar (QSO) host galaxies at similar redshifts. Although plausible, the upper limits on the dust continuum emission of galaxies at 6.5 < z < 7.5 show that these conditions must not apply to the bulk of the high-redshift galaxy population.
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.
Context.
Recently, there has been an increased interest in the study of the generation of low-energy cosmic rays (< 1 TeV) in shocks situated on the surface of a protostar or along protostellar jets. ...These locally accelerated cosmic rays offer an attractive explanation for the high levels of non-thermal emission and ionisation rates observed close to these sources.
Aims.
The high ionisation rate observed in some protostellar sources is generally attributed to shock-generated UV photons. The aim of this article is to show that when synchrotron emission and a high ionisation rate are measured in the same spatial region, a locally shock-accelerated cosmic-ray flux is sufficient to explain both phenomena.
Methods.
We assume that relativistic protons and electrons are accelerated according to the first-order Fermi acceleration mechanism, and we calculate their emerging fluxes at the shock surface. These fluxes are used to compute the ionisation rate and the non-thermal emission at centimetre wavelengths. We then apply our model to the star-forming region OMC-2 FIR 3/FIR 4. Using a Bayesian analysis, we constrain the parameters of the model and estimate the spectral indices of the non-thermal radio emission, the intensity of the magnetic field, and its degree of turbulence.
Results.
We demonstrate that the local cosmic-ray acceleration model makes it possible to simultaneously explain the synchrotron emission along the HOPS 370 jet within the FIR 3 region and the ionisation rate observed near the FIR 4 protocluster. In particular, our model constrains the magnetic field strength (∼250−450 μG), its turbulent component (∼20−40 μG), and the jet velocity in the shock reference frame for the three non-thermal sources of the HOPS 370 jet (between 350 km s
−1
and 1000 km s
−1
).
Conclusions.
Beyond the modelling of the OMC-2 FIR 3/FIR 4 system, we show how the combination of continuum observations at centimetre wavelengths and molecular transitions is a powerful new tool for the analysis of star-forming regions: These two types of observations can be simultaneously interpreted by invoking only the presence of locally accelerated cosmic rays, without having to resort to shock-generated UV photons.
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Abstract
With a metallicity of 12 + Log(O/H) ≈ 7.1–7.2, I Zw 18 is a canonical low-metallicity blue compact dwarf (BCD) galaxy. A growing number of BCDs, including I Zw 18, have been found to host ...strong, narrow-lined, nebular He
ii
(
λ
4686) emission with enhanced intensities compared to H
β
(e.g., He
ii
(
λ
4686)/H
β
> 1%). We present new observations of I Zw 18 using the Keck Cosmic Web Imager. These observations reveal two nebular He
ii
emission regions (or He
iii
regions) northwest and southeast of the He
iii
region in the galaxy’s main body investigated in previous studies. All regions exhibit He
ii
(
λ
4686)/H
β
greater than 2%. The two newly resolved He
iii
regions lie along an axis that intercepts the position of I Zw 18's ultraluminous X-ray (ULX) source. We explore whether the ULX could power the two He
iii
regions via shock activity and/or beamed X-ray emission. We find no evidence of shocks from the gas kinematics. If the ULX powers the two regions, the X-ray emission would need to be beamed. Another potential explanation is that a class of early-type nitrogen-rich Wolf–Rayet stars with low winds could power the two He
iii
regions, in which case the alignment with the ULX would be coincidental.
Abstract
We present resolved C
i
line intensities of 18 nearby galaxies observed with the SPIRE FTS spectrometer on the
Herschel Space Observatory
. We use these data along with resolved CO line ...intensities from
J
up
= 1 to 7 to interpret what phase of the interstellar medium the C
i
lines trace within typical local galaxies. A tight, linear relation is found between the intensities of the CO(4–3) and C
i
(2–1) lines; we hypothesize this is due to the similar upper level temperature of these two lines. We modeled the C
i
and CO line emission using large-velocity gradient models combined with an empirical template. According to this modeling, the C
i
(1–0) line is clearly dominated by the low-excitation component. We determine C
i
to molecular mass conversion factors for both the C
i
(1–0) and C
i
(2–1) lines, with mean values of
α
C
i
(1−0)
= 7.3
M
⊙
K
−1
km
−1
s pc
−2
and
α
C
i
(2−1)
= 34
M
⊙
K
−1
km
−1
s pc
−2
with logarithmic root-mean-square spreads of 0.20 and 0.32 dex, respectively. The similar spread of
to
(derived using the CO(2–1) line) suggests that C
i
(1–0) may be just as good a tracer of cold molecular gas as CO(2–1) in galaxies of this type. On the other hand, the wider spread of
α
C
i
(2−1)
and the tight relation found between C
i
(2–1) and CO(4–3) suggest that much of the C
i
(2–1) emission may originate in warmer molecular gas.
Abstract
Through adaptive optics (AO) imaging with the SOUL+LUCI instrument at the Large Binocular Telescope we were able to resolve, for the first time, individual stars in the gas-rich galaxy DDO68 ...C. This system was already suggested to be interacting with the extremely metal-poor dwarf DDO68, but its nature has remained elusive so far because of the presence of a bright foreground star close to its line of sight that hampers a detailed study of its stellar population and distance. In our study, we turned this interloper star into an opportunity to have a deeper insight on DDO68 C, using it as a guide star for the AO correction. Although the new data do not allow for a direct distance measurement through the red giant branch tip method, the combined analysis of the resolved-star color–magnitude diagram, of archival GALEX far-UV and near-UV photometry, and of H
α
data provides a self-consistent picture in which DDO68 C is at the same ∼13 Mpc distance as its candidate companion DDO68. These results indicate that DDO68 is a unique case of a low-mass dwarf, less massive than the Magellanic Clouds, interacting with three satellites (DDO68 C and two previously confirmed accreting systems), providing useful constraints on cosmological models and a potential explanation for its anomalous extremely low metallicity.
The
Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS)
, a proposed Astrophysics MIDEX-class mission concept, has an innovative 14-meter diameter inflatable primary mirror that ...will provide the sensitivity to study far-infrared continuum and line emission from galaxies at all redshifts with high spectral resolution heterodyne receivers.
OASIS
will have the sensitivity to follow the water trail from galaxies to the comets that create oceans. It will bring an understanding of the role of water in galaxy evolution and its part of the oxygen budget, by measuring water emission from local to intermediate redshift galaxies, observations that have not been possible from the ground. Observation of the ground-state HD line will accurately measure gas mass in a wide variety of astrophysical objects. Thanks to its exquisite spatial resolution and sensitivity,
OASIS
will, during its one-year baseline mission, detect water in galaxies with unprecedented statistical significance. This paper reviews the extragalactic science achievable and planned with
OASIS
.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Context. As a result of their relation to massive stars, long-duration gamma-ray bursts (GRBs) allow the pinpointing of star formation in galaxies independent of redshift, dust obscuration, or galaxy ...mass/size, thus providing a unique tool to investigate star formation history over cosmic time. Aims. About half of the optical afterglows of long-duration GRBs are missed owing to dust extinction and are primarily located in the most massive GRB hosts. It is important to investigate the amount of obscured star formation in these GRB host galaxies to understand this bias. Methods. Radio emission of galaxies correlates with star formation, but does not suffer extinction as do the optical star formation estimators. We selected 11 GRB host galaxies with either large stellar mass or large UV-based and optical-based star formation rates (SFRs) and obtained radio observations of these with the Australia Telescope Compact Array and the Karl Jansky Very Large Array. Results. Despite intentionally selecting GRB hosts with expected high SFRs, we do not find any radio emission related to star formation in any of our targets. Our upper limit for GRB 100621A implies that the earlier reported radio detection was due to afterglow emission. We detect radio emission from the position of GRB 020819B, but argue that it is in large part, if not completely, due to afterglow contamination. Conclusions. Half of our sample has radio-derived SFR limits, which are only a factor 2–3 above the optically measured SFRs. This supports other recent studies that the majority of star formation in GRB hosts is not obscured by dust.
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We present low- and high-resolution Spitzer/IRS spectra, supplemented by Infrared Array Camera and Multiband Imaging Photometer for Spitzer measurements, of 22 blue compact dwarf (BCD) galaxies. The ...BCD sample spans a wide range in oxygen abundance (12+log(O/H) between 7.4 and 8.3), and hardness of the interstellar radiation field (ISRF). The Infrared Spectrograph (IRS) spectra provide us with a rich set of diagnostics to probe the physics of star and dust formation in very low metallicity environments. We find that metal-poor BCDs have harder ionizing radiation than metal-rich galaxies: O IV emission is {approx}>4 times as common as Fe II emission. They also have a more intense ISRF, as indicated by the 71 to 160 {mu}m luminosity ratio. Two-thirds of the sample (15 BCDs) show polycyclic aromatic hydrocarbon (PAH) features, although the fraction of PAH emission normalized to the total infrared (IR) luminosity is considerably smaller in metal-poor BCDs ({approx}0.5%) than in metal-rich star-forming galaxies ({approx}10%). We find several lines of evidence for a deficit of small PAH carriers at low metallicity, and attribute this to destruction by a hard, intense ISRF, only indirectly linked to metal abundance. Our IRS spectra reveal a variety of H{sub 2} rotational lines, and more than a third of the objects in our sample (eight BCDs) have {approx}>3sigma detections in one or more of the four lowest-order transitions. The warm gas masses in the BCDs range from 10{sup 3} M{sub sun} to 10{sup 8} M{sub sun}, and can be comparable to the neutral hydrogen gas mass; relative to their total IR luminosities, some BCDs contain more H{sub 2} than Spitzer Nearby Galaxy Survey galaxies.
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