Enabling the next generation of cm-wavelength studies of high-redshift molecular gas with the SKA

The Square Kilometre Array will be a revolutionary instrument for the study of gas in the distant Universe. SKA1 will have sufficient sensitivity to detect and image atomic 21 cm HI in individual galaxies at significant cosmological distances, complementing ongoing ALMA imaging of redshifted high-J CO line emission and far-infrared interstellar medium lines such as [CII] 157.7 um. At frequencies below ~50 GHz, observations of redshifted emission from low-J transitions of CO, HCN, HCO+, HNC, H2O and CS provide insight into the kinematics and mass budget of the cold, dense star-forming gas in galaxies. In advance of ALMA band 1 deployment (35 to 52 GHz), the most sensitive facility for high-redshift studies of molecular gas operating below 50~GHz is the Karl G. Jansky Very Large Array (VLA). Here, we present an overview of the role that the SKA could play in molecular emission line studies during SKA1 and SKA2, with an emphasis on studies of the dense gas tracers directly probing regions of active star-formation.

Karl G. Jansky Very Large Array observations of cold dust and molecular gas in starbursting quasar host galaxies at z~4.5

We present Karl G. Jansky Very Large Array (VLA) observations of 44 GHz continuum and CO J=2-1 line emission in BR1202-0725 at z=4.7 (a starburst galaxy and quasar pair) and BRI1335-0417 at z=4.4 (also hosting a quasar). With the full 8 GHz bandwidth capabilities of the upgraded VLA, we study the (rest-frame) 250 GHz thermal dust continuum emission for the first time along with the cold molecular gas traced by the Low-J CO line emission. The measured CO J=2-1 line luminosities of BR1202-0725 are L(CO) = (8.7+/-0.8)x10^10 K km/s pc^2 and L(CO) = (6.0+/-0.5)x10^10 K km/s pc^2 for the submm galaxy (SMG) and quasar, which are equal to previous measurements of the CO J=5-4 line luminosities implying thermalized line emission and we estimate a combined cold molecular gas mass of ~9x10^10 Msun. In BRI1335-0417 we measure L(CO) = (7.3+/-0.6)x10^10 K km/s pc^2. We detect continuum emission in the SMG BR1202-0725 North (S(44GHz) = 51+/-6 microJy), while the quasar is detected with S(44GHz) = 24+/-6 microJy and in BRI1335-0417 we measure S(44GHz) = 40+/-7 microJy. Combining our continuum observations with previous data at (rest-frame) far-infrared and cm-wavelengths, we fit three component models in order to estimate the star-formation rates. This spectral energy distribution fitting suggests that the dominant contribution to the observed 44~GHz continuum is thermal dust emission, while either thermal free-free or synchrotron emission contributes less than 30%.

[CII] line emission in massive star-forming galaxies at z=4.7

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [CII] 157.7micron fine structure line and thermal dust continuum emission from a pair of gas-rich galaxies at z=4.7, BR1202-0725. This system consists of a luminous quasar host galaxy and a bright submm galaxy (SMG), while a fainter star-forming galaxy is also spatially coincident within a 4 (25 kpc) region. All three galaxies are detected in the submm continuum, indicating FIR luminosities in excess of 10^13 Lsun for the two most luminous objects. The SMG and the quasar host galaxy are both detected in [CII] line emission with luminosities, L([CII]) = (10.0 +/- 1.5)x10^9 Lsun and L([CII]) = (6.5+/-1.0)x10^9 Lsun, respectively. We estimate a luminosity ratio, L([CII])/L(FIR) = (8.3+/-1.2)x10^-4 for the starburst SMG to the North, and L([CII])/L(FIR) = (2.5+/-0.4)x10^-4 for the quasar host galaxy, in agreement with previous high-redshift studies that suggest lower [CII]-to-FIR luminosity ratios in quasars than in starburst galaxies. The third fainter object with a flux density, S(340GHz) = 1.9+/-0.3 mJy, is coincident with a Ly-Alpha emitter and is detected in HST ACS F775W and F814W images but has no clear counterpart in the H-band. Even if this third companion does not lie at a similar redshift to BR1202-0725, the quasar and the SMG represent an overdensity of massive, infrared luminous star-forming galaxies within 1.3 Gyr of the Big Bang.

Radio and X-ray properties of submillimeter galaxies in the A2125 field

We present the radio and X-ray properties of 1.2 mm MAMBO source candidates in a 1600 sq. arcmin field centered on the Abell 2125 galaxy cluster at z=0.247. The brightest, non-synchrotron mm source candidate in the field has a photometric redshift, z = 3.93^+1.11_-0.80, and is not detected in a 31 ks Chandra X-ray exposure. These findings are consistent with this object being an extremely dusty and luminous starburst galaxy at high-redshift, possibly the most luminous yet identified in any blank-field mm survey. The deep 1.4 GHz VLA imaging identifies counterparts for 83% of the 29 mm source candidates identified at >=4-sigma S(1.2mm) = 2.7 - 52.1 mJy, implying that the majority of these objects are likely to lie at z <~ 3.5. The median mm-to-radio wavelength photometric redshift of this radio-detected sample is z~2.2 (first and third quartiles of 1.7 and 3.0), consistent with the median redshift derived from optical spectroscopic surveys of the radio-detected subsample of bright submm galaxies (S(850um) > 5 mJy). Three mm-selected quasars are confirmed to be X-ray luminous in the high resolution Chandra imaging, while another mm source candidate with potential multiple radio counterparts is also detected in the X-ray regime. Both of these radio counterparts are positionally consistent with the mm source candidate. One counterpart is associated with an elliptical galaxy at z = 0.2425, but we believe that a second counterpart associated with a fainter optical source likely gives rise to the mm emission at z~1.

An EVLA search for water megamaser emission in the submm galaxy SMM J16359+6612 at z=2.5

Using the Expanded Very Large Array, we have conducted a search for 22.2 GHz H2O megamaser emission in the strongly lensed submm galaxy, SMM J16359+6612 at z=2.517. This object is lensed into three components, and after a correction for magnification is applied to its submm-wavelength flux density, it is typical of the bulk of the high-redshift, submm galaxy population responsible for the 850 um extragalactic background (S(850um)~1 mJy). We do not detect any H2O megamaser emission, but the lensing allows us to place an interesting constraint on the luminosity of any megamasers present, L(H2O) < 5305 solar luminosities for an assumed linewidth of 80 km/s. Because the far-infrared luminosity in submm galaxies is mainly powered by star formation, and very luminous H2O megamasers are more commonly associated with quasar activity, it could be that blind searches for H2O megamasers will not be an effective means of determining redshifts for less luminous members of the submm galaxy population.