Contact.
The star formation rate (SFR) in high-redshift galaxies is expected due to competing physical processes. This stochastic variability might boost the luminosity of galaxies and might explain ...the over-abundance seen at
z
≳ 10 by the
James Webb
Space Telescope.
Aims.
We quantify the amplitude and timescales of this variability and identify the key physical processes.
Methods.
We selected 245
z
= 7.7 galaxies with stellar mass 5 × 10
6
≲
M
⋆
/
M
⊙
≲ 5 × 10
10
from
SERRA
, which is a suite of high-resolution radiation-hydrodynamic cosmological simulations. After fitting the average SFR trend, ⟨SFR⟩, we quantified the time-dependent variation,
δ
(
t
)≡logSFR/⟨SFR⟩, for each system and performed a periodogram analysis to search for periodicity modulations.
Results.
We find that
δ
(
t
) is distributed as a zero-mean Gaussian, with standard deviation
σ
δ
≃ 0.24 (corresponding to a UV magnitude s.d.
σ
UV
≃ 0.61) that is independent of
M
⋆
. However, the modulation timescale increases with stellar mass:
t
δ
∼ (9, 50, 100) Myr for
M
⋆
∼ (0.1, 1, 5)×10
9
M
⊙
, respectively. These timescales are imprinted on the SFR by different processes: (i) photoevaporation, (ii) supernova explosions, and (iii) cosmological accretion/merging dominating in low-, intermediate-, and high-mass systems, respectively.
Conclusions.
The predicted SFR variations cannot account for the required
z
≳ 10 UV luminosity function boost. Other processes, such as radiation-driven outflows clearing the dust, must then be invoked to explain the enhanced luminosity of super-early systems.
Abstract
The recent discovery of dusty galaxies well into the Epoch of Reionization (redshift z > 6) poses challenging questions about the properties of the interstellar medium in these pristine ...systems. By combining state-of-the-art hydrodynamic and dust radiative transfer simulations, we address these questions focusing on the recently discovered dusty galaxy A2744_YD4 (z = 8.38, Laporte et al.). We show that we can reproduce the observed spectral energy distribution (SED) only using different physical values with respect to the inferred ones by Laporte et al., i.e. a star formation rate of SFR = 78 $\mathrm{M}_{\odot } \rm yr^{-1}$, a factor ≈4 higher than deduced from simple SED fitting. In this case, we find: (i) dust attenuation (corresponding to τV = 1.4) is consistent with a Milky Way (MW) extinction curve; (ii) the dust-to-metal ratio is low, fd ∼ 0.08, implying that early dust formation is rather inefficient; (iii) the luminosity-weighted dust temperature is high, $T_{\rm d}=91\pm 23\, \rm K$, as a result of the intense (≈100 × MW) interstellar radiation field; and (iv) due to the high Td, the Atacama Large Millimeter/submillimeter Array Band 7 detection can be explained by a limited dust mass, Md = 1.6 × 106 M⊙. Finally, the high dust temperatures might solve the puzzling low infrared excess (IRX) recently deduced for high-z galaxies from the IRX–β relation.
We present zoom-in, adaptive mesh refinement, high-resolution (...30 pc) simulations of high-redshift (z ... 6) galaxies with the aim of characterizing their internal properties and interstellar ...medium. Among other features, we adopt a star formation model based on a physically sound molecular hydrogen prescription, and introduce a novel scheme for supernova feedback, stellar winds and dust-mediated radiation pressure. In the zoom-in simulation, the target halo hosts 'Dahlia', a galaxy with a stellar mass M... = 1.6 x 10 super( 10) M..., representative of a typical z ~ 6 Lyman-break galaxy. Dahlia has a total H2 mass of 108.5 M... that is mainly concentrated in a disc-like structure of effective radius ...0.6 kpc and scale height ...200 pc. Frequent mergers drive fresh gas towards the centre of the disc, sustaining a star formation rate per unit area of ...15 M...yr super( -1) kpc super( -2). The disc is composed of dense (n ... 25 cm super( -3)), metal-rich (Z ... 0.5 Z...) gas that is pressure supported by radiation. We compute the 158 ...m Cii emission arising from Dahlia, and find that ...95 per cent of the total Cii luminosity ($$L_...\rm C\,\small ...II......\simeq 10 greater than or equal to ..7.5...\,...\rm L..._...\odot ...$$) arises from the H2 disc. Although 30 per cent of the Cii mass is transported out of the disc by outflows, such gas negligibly contributes to Cii emission, due to its low density (n ... 10 cm super( -3)) and metallicity (Z ... 10 super( -1) Z...). Dahlia is underluminous with respect to the local Cii-SFR relation; however, its luminosity is consistent with upper limits derived for most z ~ 6 galaxies. (ProQuest: ... denotes formulae/symbols omitted.)
ABSTRACT
ALMA observations have revealed the presence of dust in galaxies in the Epoch of Reionization (EoR; redshift z > 6). However, the dust temperature, Td, remains unconstrained, and this ...introduces large uncertainties, particularly in the dust mass determinations. Using an analytical and physically motivated model, we show that dust in high-z, star-forming giant molecular clouds (GMCs), largely dominating the observed far-infrared luminosity, is warmer ($T_\mathrm{ d} \lower.5ex\hbox{$\,\, \buildrel\,\gt\, \over \sim \,\,$}60\ \mathrm{K}$) than locally. This is due to the more compact GMC structure induced by the higher gas pressure and turbulence characterizing early galaxies. The compactness also delays GMC dispersal by stellar feedback, thus $\sim 40$ per cent of the total UV radiation emitted by newly born stars remains obscured. A higher Td has additional implications: it (a) reduces the tension between local and high-z IRX–β relation, and (b) alleviates the problem of the uncomfortably large dust masses deduced from observations of some EoR galaxies.
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.
ABSTRACT
We study the impact of deviations from the Kennicutt–Schmidt relation (quantified by the ‘burstiness’ parameter κs), gas metallicity (Z), and density (n) on the observed O iii88 μm/C ii158 ...μm surface brightness ratios (ΣO iii/ΣC ii) in nine galaxies at z ≈ 6−9. We first discuss possible biases in the measured ΣO iii/ΣC ii ratios by comparing the data with zoom-in cosmological simulations and then use a Markov Chain Monte Carlo algorithm to derive the best-fitting values of (κs, Z, n). We find that (i) the strongest dependence of ΣO iii/ΣC ii is on κs; (ii) high ratios identify starburst galaxies with short gas depletion times ($t_{\rm dep}=6-49\, \rm Myr$); (iii) a secondary dependence on density is found, with ΣO iii/ΣC ii anticorrelating with n as a result of the lower O iii critical density; and (iv) the ratio weakly depends only on Z. The nine galaxies are significantly enriched (Z = 0.2−0.5 Z⊙) and dense n ≈ 101−3 cm−3. This lends further support to the starburst scenario in which a rapid enrichment of the interstellar medium is expected.
ABSTRACT
A tight relation between the C ii 158 $\mu$m line luminosity and star formation rate is measured in local galaxies. At high redshift (z > 5), though, a much larger scatter is observed, with ...a considerable (15–20 per cent) fraction of the outliers being C ii-deficient. Moreover, the C ii surface brightness ($\Sigma_{\rm C\, \small {II}}$) of these sources is systematically lower than expected from the local relation. To clarify the origin of such C ii-deficiency, we have developed an analytical model that fits local C ii data and has been validated against radiative transfer simulations performed with cloudy. The model predicts an overall increase of $\Sigma_{\rm C\, \small {II}}$ with ΣSFR. However, for ΣSFR ${\gtrsim} 1 \, \mathrm{M}_\odot \,{\rm yr}^{-1}\,{\rm kpc}^{-2}$, $\Sigma_{\rm C\, \small {II}}$ saturates. We conclude that underluminous C ii systems can result from a combination of three factors: (a) large upward deviations from the Kennicutt–Schmidt relation (κs ≫ 1), parametrized by the ‘burstiness’ parameter κs; (b) low metallicity; (c) low gas density, at least for the most extreme sources (e.g. CR7). Observations of C ii emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. O iii88 $\mu$m, C iii1909 Å, CO lines). Simple formulae are given to interpret available data for low- and high-z galaxies.
ABSTRACT
Non-equilibrium chemistry is a key process in the study of the interstellar medium (ISM), in particular the formation of molecular clouds and thus stars. However, computationally, it is ...among the most difficult tasks to include in astrophysical simulations, because of the typically high (>40) number of reactions, the short evolutionary time-scales (about 104 times less than the ISM dynamical time), and the characteristic non-linearity and stiffness of the associated ordinary differential equations system (ODEs). In this proof of concept work, we show that Physics Informed Neural Networks (PINN) are a viable alternative to traditional ODE time integrators for stiff thermochemical systems, i.e. up to molecular hydrogen formation (9 species and 46 reactions). Testing different chemical networks in a wide range of densities (−2 < log n/cm−3 < 3) and temperatures (1 < log T/K < 5), we find that a basic architecture can give a comfortable convergence only for simplified chemical systems: to properly capture the sudden chemical and thermal variations, a Deep Galerkin Method is needed. Once trained (∼103 GPUhr), the PINN well reproduces the strong non-linear nature of the solutions (errors $\lesssim 10{{\ \rm per\ cent}}$) and can give speed-ups up to a factor of ∼200 with respect to traditional ODE solvers. Further, the latter have completion times that vary by about $\sim 30{{\ \rm per\ cent}}$ for different initial n and T, while the PINN method gives negligible variations. Both the speed-up and the potential improvement in load balancing imply that PINN-powered simulations are a very palatable way to solve complex chemical calculation in astrophysical and cosmological problems.
ABSTRACT After two Atacama Large Millimeter/submillimeter Array (ALMA) observing cycles, only a handful of C ii 158 m emission line searches in z > 6 galaxies have reported a positive detection, ...questioning the applicability of the local C ii-star formation rate (SFR) relation to high-z systems. To investigate this issue we use the Vallini et al. (V13) model,based on high-resolution, radiative transfer cosmological simulations to predict the C ii emission from the interstellar medium of a z 7 (halo mass Mh = 1.17 × 1011 M ) galaxy. We improve the V13 model by including (a) a physically motivated metallicity (Z) distribution of the gas, (b) the contribution of photodissociation regions (PDRs), and (c) the effects of cosmic microwave background (CMB) on the C ii line luminosity. We study the relative contribution of diffuse neutral gas to the total C ii emission (Fdiff/Ftot) for different SFR and Z values. We find that the C ii emission arises predominantly from PDRs: regardless of the galaxy properties, Fdiff/Ftot ≤ 10%, since at these early epochs the CMB temperature approaches the spin temperature of the C ii transition in the cold neutral medium (TCMB ∼ ∼ 20 K). Our model predicts a high-z C ii-SFR relation, consistent with observations of local dwarf galaxies (0.02 < Z/Z < 0.5). The C ii deficit suggested by actual data (LCii < 2.0 × 107 L in BDF3299 at z 7.1) if confirmed by deeper ALMA observations, can be ascribed to negative stellar feedback disrupting molecular clouds around star formation sites. The deviation from the local C ii-SFR would then imply a modified Kennicutt-Schmidt relation in z > 6 galaxies. Alternatively/in addition, the deficit might be explained by low gas metallicities (Z < 0.1 Z ).
Abstract
We study the CO line luminosity (LCO), the shape of the CO spectral line energy distribution (SLED), and the value of the CO-to-H2 conversion factor in galaxies in the Epoch of Reionization ...(EoR). For this aim, we construct a model that simultaneously takes into account the radiative transfer and the clumpy structure of giant molecular clouds (GMCs) where the CO lines are excited. We then use it to post-process state-of-the-art zoomed, high resolution (30 pc), cosmological simulation of a main-sequence (M* ≈ 1010 M⊙, SFR ≈ 100 M⊙ yr− 1) galaxy, ‘Althæa’, at z ≈ 6. We find that the CO emission traces the inner molecular disc (r ≈ 0.5 kpc) of Althæa with the peak of the CO surface brightness co-located with that of the C$\, \scriptstyle \rm II$ 158 μm emission. Its LCO(1-0) = 104.85 L⊙ is comparable to that observed in local galaxies with similar stellar mass. The high (Σgas ≈ 220 M⊙ pc− 2) gas surface density in Althæa, its large Mach number ($\mathcal {M}$ ≈ 30) and the warm kinetic temperature (Tk ≈ 45 K) of GMCs yield a CO SLED peaked at the CO(7–6) transition, i.e. at relatively high-J and a CO-to-H2 conversion factor $\alpha _{\rm CO}\approx 1.5 \, \rm M_{{\odot }} \rm (K\, km\, s^{-1}\, pc^2)^{-1}$ lower than that of the Milky Way. The Atacama Large Millimeter/submillimeter Array observing time required to detect (resolve) at 5σ the CO(7–6) line from galaxies similar to Althæa is ≈13 h (≈38 h).
PDF
![ON THE [C ii]-SFR RELATION ...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-637X_1538-4357/small "ON THE [C ii]-SFR RELATION IN HIGH REDSHIFT GALAXIES")