Abstract
We study the photoevaporation of Jeans-unstable molecular clumps by isotropic FUV ($6\, {\rm eV} \lt {\rm h}\nu \lt 13.6\, {\rm eV}$) radiation, through 3D radiative transfer hydrodynamical ...simulations implementing a non-equilibrium chemical network that includes the formation and dissociation of H2. We run a set of simulations considering different clump masses ($M=10\!-\!200\, {\rm M}_{\odot }$) and impinging fluxes (G0 = 2 × 103 to 8 × 104 in Habing units). In the initial phase, the radiation sweeps the clump as an R-type dissociation front, reducing the H2 mass by a factor $40\!-\!90{{\ \rm per\ cent}}$. Then, a weak ($\mathcal {M}\simeq 2$) shock develops and travels towards the centre of the clump, which collapses while losing mass from its surface. All considered clumps remain gravitationally unstable even if radiation rips off most of the clump mass, showing that external FUV radiation is not able to stop clump collapse. However, the FUV intensity regulates the final H2 mass available for star formation: for example, for G0 < 104 more than 10 per cent of the initial clump mass survives. Finally, for massive clumps (${\gtrsim } 100\, {\rm M}_{\odot }$) the H2 mass increases by $25\!-\!50{{\ \rm per\ cent}}$ during the collapse, mostly because of the rapid density growth that implies a more efficient H2 self-shielding.
We present new ALMA observations of the O iii88 μm line and high angular resolution observations of the C ii158 μm line in a normal star forming galaxy at z = 7.1. Previous C ii observations of this ...galaxy had detected C ii emission consistent with the Lyα redshift but spatially slightly offset relative to the optical (UV-rest frame) emission. The new C ii observations reveal that the C ii emission is partly clumpy and partly diffuse on scales larger than about 1 kpc. O iii emission is also detected at high significance, offset relative to the optical counterpart in the same direction as the C ii clumps, but mostly not overlapping with the bulk of the C ii emission. The offset between different emission components (optical/UV and different far-IR tracers) is similar to that which is observed in much more powerful starbursts at high redshift. We show that the O iii emitting clump cannot be explained in terms of diffuse gas excited by the UV radiation emitted by the optical galaxy, but it requires excitation by in-situ (slightly dust obscured) star formation, at a rate of about 7 M⊙ yr-1. Within 20 kpc from the optical galaxy the ALMA data reveal two additional O iii emitting systems, which must be star forming companions. We discuss that the complex properties revealed by ALMA in the z ~ 7.1 galaxy are consistent with expectations by recent models and cosmological simulations, in which differential dust extinction, differential excitation and different metal enrichment levels, associated with different subsystems assembling a galaxy, are responsible for the various appearance of the system when observed with distinct tracers.
Abstract
To improve our understanding of high-z galaxies, we study the impact of H2 chemistry on their evolution, morphology and observed properties. We compare two zoom-in high-resolution (30 pc) ...simulations of prototypical M
⋆ ∼ 1010 M⊙ galaxies at z = 6. The first, ‘Dahlia’, adopts an equilibrium model for H2 formation, while the second, ‘Althæa’, features an improved non-equilibrium chemistry network. The star formation rate (SFR) of the two galaxies is similar (within 50 per cent), and increases with time reaching values close to 100 M⊙ yr−1 at z = 6. They both have SFR–stellar mass relation consistent with observations, and a specific SFR of ≃5 Gyr−1. The main differences arise in the gas properties. The non-equilibrium chemistry determines the H → H2 transition to occur at densities >300 cm−3, i.e. about 10 times larger than predicted by the equilibrium model used for Dahlia. As a result, Althæa features a more clumpy and fragmented morphology, in turn making SN feedback more effective. Also, because of the lower density and weaker feedback, Dahlia sits 3σ away from the Schmidt–Kennicutt relation; Althæa, instead nicely agrees with observations. The different gas properties result in widely different observables. Althæa outshines Dahlia by a factor of 7 (15) in C
$\scriptstyle \rm II$
157.74 μm (H217.03 μm) line emission. Yet, Althæa is underluminous with respect to the locally observed C
$\scriptstyle \rm II$
–SFR relation. Whether this relation does not apply at high-z or the line luminosity is reduced by cosmic microwave background and metallicity effects remain as an open question.
By combining high-resolution, radiative transfer cosmological simulations of z 6 galaxies with a subgrid multiphase model of their interstellar medium, we derive the expected intensity of several ...far-infrared emission lines (C ii 158, O i 63 and N ii 122 μm) for different values of the gas metallicity, Z. For Z = Z, the C ii spectrum is very complex due to the presence of several emitting clumps of individual sizes 3 kpc; the peak is displaced from the galaxy centre by 100 km s−1. While the O i spectrum is also similarly displaced, the N ii line comes predominantly from the central ionized regions of the galaxy. When integrated over ∼ 500 km s− 1, the C ii line flux is 185 mJy km s− 1; 95 per cent of such flux originates from the cold (T 250 K) H i phase and only 5 per cent from the warm (T 5000 K) neutral medium. The O i and N ii fluxes are ∼6 and ∼90 times lower than the C ii one, respectively. By comparing our results with observations of Himiko, the most extended and luminous Lyman alpha emitter at z = 6.6, we find that the gas metallicity in this source must be subsolar. We conclude that the C ii line from z 6 galaxies is detectable by the ALMA full array in 1.9 < t
ON < 7.7 h observing time, depending on Z.
We present spectroscopic follow-up observations of CR7 with ALMA, targeted at constraining the infrared (IR) continuum and C ii line-emission at high spatial resolution matched to the HST/WFC3 ...imaging. CR7 is a luminous Ly emitting galaxy at z = 6.6 that consists of three separated UV-continuum components. Our observations reveal several well-separated components of C ii emission. The two most luminous components in C ii coincide with the brightest UV components (A and B), blueshifted by km s−1 with respect to the peak of Ly emission. Other C ii components are observed close to UV clumps B and C and are blueshifted by and 80 km s−1 with respect to the systemic redshift. We do not detect FIR continuum emission due to dust with a 3 limiting luminosity . This allows us to mitigate uncertainties in the dust-corrected SFR and derive SFRs for the three UV clumps A, B, and C of 28, 5, and 7 yr−1. All clumps have C ii luminosities consistent within the scatter observed in the local relation between SFR and , implying that strong Ly emission does not necessarily anti-correlate with C ii luminosity. Combining our measurements with the literature, we show that galaxies with blue UV slopes have weaker C ii emission at fixed SFR, potentially due to their lower metallicities and/or higher photoionization. Comparison with hydrodynamical simulations suggests that CR7's clumps have metallicities of . The observed ISM structure of CR7 indicates that we are likely witnessing the build up of a central galaxy in the early universe through complex accretion of satellites.
ALMA suggests outflows in z ~ 5.5 galaxies Gallerani, S.; Pallottini, A.; Feruglio, C. ...
Monthly notices of the Royal Astronomical Society,
2018, Letnik:
473, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present the first attempt to detect outflows from galaxies approaching the Epoch of Reionization (EoR) using a sample of nine star-forming (SFR = 31 ± 20Mo˙ yr-1) z ~ 5.5 galaxies for which the C ...II158 μm line has been previously obtained with Atacama Large Millimeter Array (ALMA). We first fit each line with a Gaussian function and compute the residuals by subtracting the best-fitting model from the data. We combine the residuals of all sample galaxies and find that the total signal is characterized by a flux excess of ~0.5mJy extended over ~1000 km s-1. Although we cannot exclude that part of this signal is due to emission from faint satellite galaxies, we show that the most probable explanation for the detected flux excess is the presence of broad wings in the CII lines, signatures of starburst-driven outflows. We infer an average outflow rate of M˙ = 54 ± 23Mo˙ yr-1, providing a loading factor η = M˙/SFR = 1.7 ± 1.3 in agreement with observed local starbursts. Our interpretation is consistent with outcomes from zoomed hydrosimulations of Dahlia, a z ~ 6 galaxy (SFR ~ 100Mo˙ yr-1), whose feedback-regulated star formation results into an outflow rate M ~ 30Mo˙ yr-1. The quality of the ALMA data is not sufficient for a detailed analysis of the C II line profile in individual galaxies. Nevertheless, our results suggest that starburst-driven outflows are in place in the EoR and provide useful indications for future ALMA campaigns. Deeper observations of the CII line in this sample are required to better characterize feedback at high-z and to understand the role of outflows in shaping early galaxy formation.
The intrinsic strength of the Ly alpha line in young, star-forming systems makes it a special tool for studying high-redshift galaxies. However, interpreting observations remains challenging due to ...the complex radiative transfer involved. Here, we combine state-of-the-art hydrodynamical simulations of 'Althaea', a prototypical Lyman Break Galaxy (LBG; stellar mass M-star similar or equal to 10(10) M-circle dot) at z = 7.2, with detailed radiative transfer computations of dust/continuum, C II 158 mu m, and Ly alpha to clarify the relation between the galaxy properties and its Ly alpha emission. Althaea exhibits low (f(alpha) < 1 per cent) Ly alpha escape fractions and equivalent widths, EW less than or similar to 6 angstrom for the simulated lines of sight, with a large scatter. The correlation between escape fraction and inclination is weak, as a result of the rather chaotic structure of high-redshift galaxies. Low f(alpha) values persist even if we artificially remove neutral gas around star-forming regions to mimic the presence of H II regions. The high attenuation is primarily caused by dust clumps co-located with young stellar clusters. We can turn Althaea into a Lyman Alpha Emitter (LAE) only if we artificially remove dust from the clumps, yielding EWs up to 22 angstrom. Our study suggests that the LBG-LAE duty-cycle required by recent clustering measurements poses the challenging problem of a dynamically changing dust attenuation. Finally, we find an anticorrelation between the magnitude of Ly alpha-C II line velocity shift and Ly alpha luminosity.
ABSTRACT
We present a detailed analysis of the X-ray, infrared, and carbon monoxide (CO) emission for a sample of 35 local (z ≤ 0.15), active (LX ≥ 1042 erg s−1) galaxies. Our goal is to infer the ...contribution of far-ultraviolet (FUV) radiation from star formation (SF), and X-ray radiation from the active galactic nuclei (AGNs), respectively, producing photodissociation regions (PDRs) and X-ray-dominated regions (XDRs), to the molecular gas heating. To this aim, we exploit the CO spectral line energy distribution (CO SLED) as traced by Herschel, complemented with data from single-dish telescopes for the low-J lines, and high-resolution ALMA images of the mid-J CO emitting region. By comparing our results to the Schmidt–Kennicutt relation, we find no evidence for AGN influence on the cold and low-density gas on kpc-scales. On nuclear (r = 250 pc) scales, we find weak correlations between the CO line ratios and either the FUV or X-ray fluxes: this may indicate that neither SF nor AGN radiation dominates the gas excitation, at least at r = 250 pc. From a comparison of the CO line ratios with PDR and XDR models, we find that PDRs can reproduce observations only in presence of extremely high gas densities (n > 105 cm−3). In the XDR case, instead, the models suggest moderate densities (n ≈ 102−4 cm−3). We conclude that a mix of the two mechanisms (PDR for the mid-J, XDR, or possibly shocks for the high-J) is necessary to explain the observed CO excitation in active galaxies.
We present the first attempt to detect outflows from galaxies approaching the Epoch of Reionization (EoR) using a sample of nine star-forming (SFR = 31 +/- 20M(circle dot) yr(-1)) z similar to 5.5 ...galaxies for which the C II 158 mu m line has been previously obtained with Atacama Large Millimeter Array (ALMA). We first fit each line with a Gaussian function and compute the residuals by subtracting the best-fitting model from the data. We combine the residuals of all sample galaxies and find that the total signal is characterized by a flux excess of similar to 0.5 mJy extended over similar to 1000 km s(-1). Although we cannot exclude that part of this signal is due to emission from faint satellite galaxies, we show that the most probable explanation for the detected flux excess is the presence of broad wings in the C II lines, signatures of starburst-driven outflows. We infer an average outflow rate of. M(over dot) = 54 +/- 23M(circle dot) yr(-1), providing a loading factor eta = M(over dot)/SFR = 1.7 +/- 1.3 in agreement with observed local starbursts. Our interpretation is consistent with outcomes from zoomed hydrosimulations of Dahlia, a z similar to 6 galaxy (SFR similar to 100M(circle dot) yr(-1)), whose feedback-regulated star formation results into an outflow rate. M(over dot) similar to 30M(circle dot) yr(-1). The quality of the ALMA data is not sufficient for a detailed analysis of the C II line profile in individual galaxies. Nevertheless, our results suggest that starburst-driven outflows are in place in the EoR and provide useful indications for future ALMA campaigns. Deeper observations of the C II line in this sample are required to better characterize feedback at high-z and to understand the role of outflows in shaping early galaxy formation.
Abstract
We present our new Atacama Large Millimeter/Submillimeter Array (ALMA) observations targeting O
iii
88
μ
m, C
ii
158
μ
m, N
ii
122
μ
m, and dust-continuum emission for three Lyman break ...galaxies at
z
= 6.0293–6.2037, identified in the Subaru/Hyper Suprime-Cam survey. We clearly detect O
iii
and C
ii
lines from all of the galaxies at 4.3–11.8
σ
levels, and identify multi-band dust-continuum emission in two of the three galaxies, allowing us to estimate infrared luminosities and dust temperatures simultaneously. In conjunction with previous ALMA observations for six galaxies at
z
> 6, we confirm that all the nine
z
= 6–9 galaxies have high O
iii
/C
ii
ratios of
, ∼10 times higher than
z
∼ 0 galaxies. We also find a positive correlation between the O
iii
/C
ii
ratio and the Ly
α
equivalent width (EW) at the ∼90% significance level. We carefully investigate physical origins of the high O
iii
/C
ii
ratios at
z
= 6–9 using Cloudy, and find that high density of the interstellar medium, low C/O abundance ratio, and the cosmic microwave background attenuation are responsible to only a part of the
z
= 6–9 galaxies. Instead, the observed high O
iii
/C
ii
ratios are explained by 10–100 times higher ionization parameters or low photodissociation region (PDR) covering fractions of 0%–10%, both of which are consistent with our N
ii
observations. The latter scenario can be reproduced with a density-bounded nebula with PDR deficit, which would enhance the Ly
α
, Lyman continuum, and
ionizing photons escape from galaxies, consistent with the O
iii
/C
ii
-Ly
α
EW correlation we find.
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