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Register a new userAn empirical representation of a physical model for the ISM [CII], CO, and [CI] emission at redshift $1leq zleq9$
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Sub-millimeter emission lines produced by the interstellar medium (ISM) are strong tracers of star formation and are some of the main targets of line intensity mapping (LIM) surveys. In this work we present an empirical multi-line emission model that simultaneously covers the mean, scatter, and correlations of [CII], CO J=1-0 to J=5-4, and [CI] lines in redshift range $1leq zleq9$. We assume the galaxy ISM line emission luminosity versus halo mass relations can be described by double power laws with redshift-dependent log normal scatter. The model parameters are then derived by fitting to the state of the art semi-analytic simulation results that have successfully reproduced multiple sub-millimeter line observations at $0leq zlesssim6$. We cross check the line emission statistics predicted by the semi-analytic simulation and our empirical model, finding that at $zgeq1$ our model reproduces the simulated line intensities with fractional error less than about 10%. The fractional difference is less than 25% for the power spectra. Grounded on physically-motivated and self-consistent galaxy simulations, this computationally efficient model will be helpful in forecasting ISM emission line statistics for upcoming LIM surveys.
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We used the KOSMA 3m telescope to map the core 7x5 of the Galactic massive star forming region W3Main in the two fine structure lines of atomic carbon and four mid-J transitions of CO and 13CO. The maps are centered on the luminous infrared source IRS5 for which we obtained ISO/LWS data comprising four high-J CO transitions, CII, and OI at 63 and 145 micron. In combination with a KAO map of integrated line intensities of CII (Howe et al. 1991), this data set allows to study the physical structure of the molecular cloud interface regions where the occurence of carbon is believed to change from C+ to C0, and to CO. The molecular gas in W3Main is warmed by the far ultraviolet (FUV) field created by more than a dozen OB stars. Detailed modelling shows that most of the observed line intensity ratios and absolute intensities are consistent with a clumpy photon dominated region (PDR) of a few hundred unresolved clumps per 0.84pc beam, filling between 3 and 9% of the volume, with a typical clump radius of 0.025pc (2.2), and typical mass of 0.44Msun. The high-excitation lines of CO stem from a 100-200K layer, as also the CI lines. The bulk of the gas mass is however at lower temperatures.
A tight relation between the [CII]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%) fraction of the outliers being [CII]-deficient. Moreover, the [CII] surface brightness ($Sigma_{rm CII}$) of these sources is systematically lower than expected from the local relation. To clarify the origin of such [CII]-deficiency we have developed an analytical model that fits local [CII] data, and has been validated against radiative transfer simulations performed with CLOUDY. The model predicts an overall increase of $Sigma_{rm CII}$ with the surface star formation rate ($Sigma_*$). However, for $Sigma_* > 1 M_odot~{rm yr}^{-1}~{rm kpc}^{-2}$, $Sigma_{rm CII}$ saturates. We conclude that underluminous [CII] systems can result from a combination of three factors: (a) large upward deviations from the Kennicutt-Schmidt relation ($kappa_s gg 1$), parameterized by the burstiness parameter $kappa_s$; (b) low metallicity; (c) low gas density, at least for the most extreme sources (e.g. CR7). Observations of [CII] emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. [OIII]88$mu$m, CIII]1909A, CO lines). Simple formulae are given to interpret available data for low and high-$z$ galaxies.
The aim of our study is to investigate the physical properties of the star-forming interstellar medium (ISM) in the Large Magellanic Cloud (LMC) by separating the origin of the emission lines spatially and spectrally. Following Okada et al. (2015, Paper I), we investigate different phases of the ISM traced by carbon-bearing species in four star-forming regions in the LMC, and model the physical properties using the KOSMA-tau PDR model. We mapped 3--13 arcmin$^2$ areas in 30 Dor, N158, N160 and N159 along the molecular ridge of the LMC in [CII]158um with GREAT on board SOFIA, and in CO(2-1) to (6-5), $^{13}$CO(2-1) and (3-2), [CI]3P1-3P0 and 3P2-3P1 with APEX. In all four star-forming regions, the line profiles of CO, $^{13}$CO, and [CI] emission are similar, whereas [CII] typically shows wider line profiles or an additional velocity component. For selected positions in N159 and 30 Dor, we observed the velocity-resolved [OI] 145um and 63um lines for the first time with upGREAT. At some positions, the [OI] line profiles match those of CO, at other positions they are more similar to the [CII] profiles. We interpret the different line profiles of CO, [CII] and [OI] as contributions from spatially separated clouds and/or clouds in different physical phases, which give different line ratios depending on their physical properties. We model the emission from the CO, [CI], [CII], and [OI] lines and the far-infrared continuum emission using the latest KOSMA-tau PDR model, which treats the dust-related physics consistently and computes the dust continuum SED together with the line emission of the chemical species. We find that the line and continuum emissions are not well-reproduced by a single clump ensemble. Toward the CO peak at N159~W, we propose a scenario that the CO, [CII], and [OI] 63um emission are weaker than expected because of mutual shielding among clumps. (abridged for arXiv)
We present results on the properties of neon emission in $zsim2$ star-forming galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Doubly-ionized neon ([NeIII]3869) is detected at $geq3sigma$ in 61 galaxies, representing $sim$25% of the MOSDEF sample with H$alpha$, H$beta$, and [OIII]$5007$ detections at similar redshifts. We consider the neon emission-line properties of both individual galaxies with [NeIII]3869 detections and composite $zsim2$ spectra binned by stellar mass. With no requirement of [NeIII]3869 detection, the latter provide a more representative picture of neon emission-line properties in the MOSDEF sample. The [NeIII]3869/[OII]3727 ratio (Ne3O2) is anti-correlated with stellar mass in $zsim2$ galaxies, as expected based on the mass-metallicity relation. It is also positively correlated with the [OIII]$5007$/[OII]$3727$ ratio (O32), but $zsim2$ line ratios are offset towards higher Ne3O2 at fixed O32, compared with both local star-forming galaxies and individual H~II regions. Despite the offset towards higher Ne3O2 at fixed O32 at $zsim2$, biases in inferred Ne3O2-based metallicity are small. Accordingly, Ne3O2 may serve as an important metallicity indicator deep into the reionization epoch. Analyzing additional rest-optical line ratios including [NeIII]$3869$/[OIII]$5007$ (Ne3O3) and [OIII]$5007$/H$beta$ (O3H$beta$), we conclude that the nebular emission-line ratios of $zsim2$ star-forming galaxies suggest a harder ionizing spectrum (lower stellar metallicity, i.e., Fe/H) at fixed gas-phase oxygen abundance, compared to systems at $zsim0$. These new results based on neon lend support to the physical picture painted by oxygen, nitrogen, hydrogen, and sulfur emission, of an ionized ISM in high-redshift star-forming galaxies irradiated by chemically young, $alpha$-enhanced massive stars.
The sensitive infrared telescopes, Spitzer and Herschel, have been used to target low-metallicity star-forming galaxies, allowing us to investigate the properties of their interstellar medium (ISM) in unprecedented detail. Interpretation of the observations in physical terms relies on careful modeling of those properties. We have employed a multiphase approach to model the ISM phases (HII region and photodissociation region) with the spectral synthesis code Cloudy. Our goal is to characterize the physical conditions (gas densities, radiation fields, etc.) in the ISM of the galaxies from the Herschel Dwarf Galaxy Survey. We are particularly interested in correlations between those physical conditions and metallicity or star-formation rate. Other key issues we have addressed are the contribution of different ISM phases to the total line emission, especially of the [CII]157um line, and the characterization of the porosity of the ISM. We find that the lower-metallicity galaxies of our sample tend to have higher ionization parameters and galaxies with higher specific star-formation rates have higher gas densities. The [CII] emission arises mainly from PDRs and the contribution from the ionized gas phases is small, typically less than 30% of the observed emission. We also find correlation - though with scatter - between metallicity and both the PDR covering factor and the fraction of [CII] from the ionized gas. Overall, the low metal abundances appear to be driving most of the changes in the ISM structure and conditions of these galaxies, and not the high specific star-formation rates. These results demonstrate in a quantitative way the increase of ISM porosity at low metallicity. Such porosity may be typical of galaxies in the young Universe.
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