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Register a new userA search for CII-158$mu$m line emission in HCM,6A, a Lyman-$alpha$ emitter at $z=6.56$
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Nissim Kanekar
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We report a Plateau de Bure interferometer search for CII-158$mu$m emission from HCM6A, a lensed Lyman-$alpha$ emitter (LAE) at $z = 6.56$. Our non-detections of CII-158$mu$m line emission and 1.2mm radio continuum emission yield $3sigma$ limits of L$_{rm CII} < 6.4 times 10^7 times (Delta V/100 km s^{-1})^{1/2}$ L$_odot$ on the CII-158$mu$m line luminosity and S$_{rm 1.2mm} < 0.68$ mJy on the 1.2mm flux density. The local conversion factor between L$_{rm CII}$ and star formation rate (SFR) yields an SFR $< 4.7$ M$_odot$ yr$^{-1}$, $approx 2$ times lower than that inferred from the ultraviolet (UV) continuum, suggesting that the local factor may not be applicable in high-$z$ LAEs. The non-detection of 1.2mm continuum emission yields a total SFR $< 28$ M$_odot$/yr; any obscured star formation is thus within a factor of two of the visible star formation. Our best-fit model to the rest-frame UV/optical spectral energy distribution of HCM6A yields a stellar mass of $1.3 times 10^9$ M$_odot$ and an SFR of ~10 M$_odot$/yr, with negligible dust obscuration. We fortuitously detect CO J=3-2 emission from a $z=0.375$ galaxy in the foreground cluster Abell370, obtaining a CO line luminosity of L$^prime ({rm CO}) > (8.95 pm 0.79) times 10^8$ K km s$^{-1}$ pc$^2$, and a molecular gas mass of M$({rm H_2}) > (4.12 pm 0.36) times 10^9$ M$_odot$, for a CO-to-H$_2$ conversion factor of 4.6 M$_odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$.
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The [CII] 157.74 $mu$m transition is the dominant coolant of the neutral interstellar gas, and has great potential as a star formation rate (SFR) tracer. Using the Herschel KINGFISH sample of 46 nearby galaxies, we investigate the relation of [CII] surface brightness and luminosity with SFR. We conclude that [CII] can be used for measurements of SFR on both global and kiloparsec scales in normal star-forming galaxies in the absence of strong active galactic nuclei (AGN). The uncertainty of the $Sigma_{rm [CII]}-Sigma_{rm SFR}$ calibration is $pm$0.21 dex. The main source of scatter in the correlation is associated with regions that exhibit warm IR colors, and we provide an adjustment based on IR color that reduces the scatter. We show that the color-adjusted $Sigma_{rm[CII]}-Sigma_{rm SFR}$ correlation is valid over almost 5 orders of magnitude in $Sigma_{rm SFR}$, holding for both normal star-forming galaxies and non-AGN luminous infrared galaxies. Using [CII] luminosity instead of surface brightness to estimate SFR suffers from worse systematics, frequently underpredicting SFR in luminous infrared galaxies even after IR color adjustment (although this depends on the SFR measure employed). We suspect that surface brightness relations are better behaved than the luminosity relations because the former are more closely related to the local far-UV field strength, most likely the main parameter controlling the efficiency of the conversion of far-UV radiation into gas heating. A simple model based on Starburst99 population-synthesis code to connect SFR to [CII] finds that heating efficiencies are $1%-3%$ in normal galaxies.
The scatter in the relationship between the strength of [CII] 158$mu$m emission and the star formation rate at high-redshift has been the source of much recent interest. Although the relationship is well-established locally, several intensely star-forming galaxies have been found whose [CII] 158$mu$m emission is either weak, absent or spatially offset from the young stars. Here we present new ALMA data for the two most distant, gravitationally-lensed and spectroscopically-confirmed galaxies, A2744_YD4 at $z=$8.38 and MACS1149_JD1 at $z=$9.11, both of which reveal intense [OIII] 88$mu$m emission. In both cases we provide stringent upper limits on the presence of [CII] 158$mu$m with respect to [OIII] 88$mu$m. We review possible explanations for this apparent redshift-dependent [CII] deficit in the context of our recent hydrodynamical simulations. Our results highlight the importance of using several emission line diagnostics with ALMA to investigate the nature of the interstellar medium in early galaxies.
We study the effects of a metallicity variation on the thermal balance and [CII] fine-structure line strengths in interstellar photon dominated regions (PDRs). We find that a reduction in the dust-to-gas ratio and the abundance of heavy elements in the gas phase changes the heat balance of the gas in PDRs. The surface temperature of PDRs decreases as the metallicity decreases except for high density ($n>10^6$ cm$^{-3}$) clouds exposed to weak ($chi< 100$) FUV fields where vibrational H$_2$-deexcitation heating dominates over photoelectric heating of the gas. We incorporate the metallicity dependence in our KOSMA-$tau$ PDR model to study the metallicity dependence of [CII]/CO line ratios in low metallicity galaxies. We find that the main trend in the variation of the observed CII/CO ratio with metallicity is well reproduced by a single spherical clump, and does not necessarily require an ensemble of clumps as in the semi-analytical model presented by Bolatto et al. (1999).
We present new ALMA observations and physical properties of a Lyman Break Galaxy at z=7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, $M_{rm UV}approx-22.4$, and has been spectroscopically identified in Ly$alpha$ with a small rest-frame equivalent width of $approx4$ AA. Previous HST image has shown that the target is comprised of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [OIII] 88 $mu$m, [CII] 158 $mu$m, and their underlying dust continuum emission. In the whole system of B14-65666, the [OIII] and [CII] lines have consistent redshifts of $7.1520pm0.0003$, and the [OIII] luminosity, $(34.4pm4.1)times10^{8}L_{rm odot}$, is about three times higher than the [CII] luminosity, $(11.0pm1.4)times10^{8}L_{rm odot}$. With our two continuum flux densities, the dust temperature is constrained to be $T_{rm d}approx50-60$ K under the assumption of the dust emissivity index of $beta_{rm d}=2.0-1.5$, leading to a large total infrared luminosity of $L_{rm TIR}approx1times10^{12}L_{rm odot}$. Owing to our high spatial resolution data, we show that the [OIII] and [CII] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by $approx200$ km s$^{-1}$. We also find these two clumps have comparable UV, infrared, [OIII], and [CII] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major-merger. The merger interpretation is also supported by the large specific star-formation rate (defined as the star-formation rate per unit stellar mass), sSFR$=260^{+119}_{-57}$ Gyr$^{-1}$, inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [OIII]-to-[CII] luminosity ratio.
We consider the capabilities of ALMA and the ngVLA to detect and image the[CII] 158,$mu$m line from galaxies into the cosmic `dark ages ($z sim 10$ to 20). The [CII] line may prove to be a powerful tool in determining spectroscopic redshifts, and galaxy dynamics, for the first galaxies. In 40,hr, ALMA has the sensitivity to detect the integrated [CII] line emission from a moderate metallicity, active star-forming galaxy [$Z_A = 0.2,Z_{odot}$; star formation rate (SFR)= 5,$M_odot$,yr$^{-1}$], at $z = 10$ at a significance of 6$sigma$. The ngVLA will detect the integrated [CII] line emission from a Milky-Way like star formation rate galaxy ($Z_{A} = 0.2,Z_{odot}$, SFR = 1,$M_odot$,yr$^{-1}$), at $z = 15$ at a significance of 6$sigma$. Imaging simulations show that the ngVLA can determine rotation dynamics for active star-forming galaxies at $z sim 15$, if they exist. The [CII] detection rate in blind surveys will be slow (of order unity per 40,hr pointing.
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