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Register a new user[CII] 158 micron Emission from the Host Galaxies of Damped Lyman Alpha Systems
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Authors
Marcel Neeleman
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Gas surrounding high redshift galaxies has been studied through observations of absorption line systems toward background quasars for decades. However, it has proven difficult to identify and characterize the galaxies associated with these absorbers due to the intrinsic faintness of the galaxies compared to the quasars at optical wavelengths. Utilizing the Atacama Large Millimeter/Submillimeter Array, we report on detections of [CII] 158 micron line and dust continuum emission from two galaxies associated with two such absorbers at a redshift of z~4. Our results indicate that the hosts of these high-metallicity absorbers have physical properties similar to massive star-forming galaxies and are embedded in enriched neutral hydrogen gas reservoirs that extend well beyond the star-forming interstellar medium of these galaxies.
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We report on a search for the [CII] 158 micron emission line from galaxies associated with four high-metallicity damped Ly-alpha absorbers (DLAs) at z ~ 4 using the Atacama Large Millimeter/sub-millimeter Array (ALMA). We detect [CII] 158 micron emission from galaxies at the DLA redshift in three fields, with one field showing two [CII] emitters. Combined with previous results, we now have detected [CII] 158 micron emission from five of six galaxies associated with targeted high-metallicity DLAs at z ~ 4. The galaxies have relatively large impact parameters, ~16 - 45 kpc, [CII] 158 micron line luminosities of (0.36 - 30) x 10^8 Lsun, and rest-frame far-infrared properties similar to those of luminous Lyman-break galaxies, with star-formation rates of ~7 - 110 Msun yr-1. Comparing the absorption and emission line profiles yields a remarkable agreement between the line centroids, indicating that the DLA traces gas at velocities similar to that of the [CII] 158 micron emission. This disfavors a scenario where the DLA arises from gas in a companion galaxy. These observations highlight ALMAs unique ability to uncover a high redshift galaxy population that has largely eluded detection for decades.
The [CII] fine structure transition at 158 microns is the dominant cooling line of cool interstellar gas, and is the brightest of emission lines from star forming galaxies from FIR through meter wavelengths. With the advent of ALMA and NOEMA, capable of detecting [CII]-line emission in high-redshift galaxies, there has been a growing interest in using the [CII] line as a probe of the physical conditions of the gas in galaxies, and as a SFR indicator at z>4. In this paper, we use a semi-analytical model of galaxy evolution (G.A.S.) combined with the code CLOUDY to predict the [CII] luminosity of a large number of galaxies at 4< z<8. At such high redshift, the CMB represents a strong background and we discuss its effects on the luminosity of the [CII] line. We study the LCII-SFR and LCII-Zg relations and show that they do not strongly evolve with redshift from z=4 and to z=8. Galaxies with higher [CII] luminosities tend to have higher metallicities and higher star formation rates but the correlations are very broad, with a scatter of about 0.5 dex for LCII-SFR. Our model reproduces the LCII-SFR relations observed in high-redshift star-forming galaxies, with [CII] luminosities lower than expected from local LCII-SFR relations. Accordingly, the local observed LCII-SFR relation does not apply at high-z. Our model naturally produces the [CII] deficit, which appears to be strongly correlated with the intensity of the radiation field in our simulated galaxies. We then predict the [CII] luminosity function, and show that it has a power law form in the range of LCII probed by the model with a slope alpha=1. The slope is not evolving from z=4 to z=8 but the number density of [CII]-emitters decreases by a factor of 20x. We discuss our predictions in the context of current observational estimates on both the differential and cumulative luminosity functions.
Using the IRAM Plateau de Bure Interferometer, we report the detection of the 158 micron [CII] emission line and underlying dust continuum in the host galaxy of the quasar ULAS J112001.48+064124.3 (hereafter J1120+0641) at z=7.0842+/-0.0004. This is the highest redshift detection of the [CII] line to date, and allows us to put first constraints on the physical properties of the host galaxy. The [CII] line luminosity is (1.2+/-0.2)x10^9 Lsun, which is a factor ~4 lower than observed in a luminous quasar at z=6.42 (SDSS J1148+5251). The underlying far-infrared (FIR) continuum has a flux density of 0.61+/-0.16 mJy, similar to the average flux density of z~6 quasars that were not individually detected in the rest-frame FIR. Assuming the FIR luminosity of L_FIR = 5.8x10^11-1.8x10^12 Lsun is mainly powered by star-formation, we derive a star-formation rate in the range 160-440 Msun/yr and a total dust mass in the host galaxy of 6.7x10^7-5.7x10^8 Msun (both numbers have significant uncertainties given the unknown nature of dust at these redshifts). The [CII] line width of sigma_V=100+/-15 km/s is among the smallest observed when compared to the molecular line widths detected in z~6 quasars. Both the [CII] and dust continuum emission are spatially unresolved at the current angular resolution of 2.0x1.7 arcsec^2 (corresponding to 10x9 kpc^2 at the redshift of J1120+0641).
We study the average Ly$alpha$ emission associated with high-$z$ strong (log $N$(H I) $ge$ 21) damped Ly$alpha$ systems (DLAs). We report Ly$alpha$ luminosities ($L_{rm Lyalpha}$) for the full as well as various sub-samples based on $N$(H I), $z$, $(r-i)$ colours of QSOs and rest equivalent width of Si II$lambda$1526 line (i.e., $W_{1526}$). For the full sample, we find $L_{rm Lyalpha}$$< 10^{41} (3sigma) rm erg s^{-1}$ with a $2.8sigma$ level detection of Ly$alpha$ emission in the red part of the DLA trough. The $L_{rm Lyalpha}$ is found to be higher for systems with higher $W_{1526}$ with its peak, detected at $geq 3sigma$, redshifted by about 300-400 $rm km s^{-1}$ with respect to the systemic absorption redshift, as seen in Lyman Break Galaxies (LBGs) and Ly$alpha$ emitters. A clear signature of a double-hump Ly$alpha$ profile is seen when we consider $W_{1526} ge 0.4$ AA and $(r-i) < 0.05$. Based on the known correlation between metallicity and $W_{1526}$, we interpret our results in terms of star formation rate (SFR) being higher in high metallicity (mass) galaxies with high velocity fields that facilitates easy Ly$alpha$ escape. The measured Ly$alpha$ surface brightness requires local ionizing radiation that is 4 to 10 times stronger than the metagalactic UV background at these redshifts. The relationship between the SFR and surface mass density of atomic gas seen in DLAs is similar to that of local dwarf and metal poor galaxies. We show that the low luminosity galaxies will contribute appreciably to the stacked spectrum if the size-luminosity relation seen for H I at low-$z$ is also present at high-$z$. Alternatively, large Ly$alpha$ halos seen around LBGs could also explain our measurements.
The brightest observed emission line in many star-forming galaxies is the [CII] 158 micron line, making it detectable up to z~7. In order to better understand and quantify the [CII] emission as a tracer of star-formation, the theoretical ratio between the [NII] 205 micron emission and the [CII] 158 micron emission has been employed to empirically determine the fraction of [CII] emission that originates from the ionized and neutral phases of the ISM. Sub-kiloparsec measurements of the [CII] 158 micron and [NII] 205 micron line in nearby galaxies have recently become available as part of the Key Insights in Nearby Galaxies: a Far Infrared Survey with Herschel (KINGFISH) and Beyond the Peak (BtP) programs. With the information from these two far-infrared lines along with the multi-wavelength suite of KINGFISH data, a calibration of the [CII] emission line as a star formation rate indicator and a better understanding of the [CII] deficit are pursued. [CII] emission is also compared to PAH emission in these regions to compare photoelectric heating from PAH molecules to cooling by [CII] in the neutral and ionized phases of the ISM. We find that the [CII] emission originating in the neutral phase of the ISM does not exhibit a deficit with respect to the infrared luminosity and is therefore preferred over the [CII] emission originating in the ionized phase of the ISM as a star formation rate indicator for the normal star-forming galaxies included in this sample.
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