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Register a new userHigh-resolution CO and radio imaging of z~2 ULIRGs: extended CO structures, and implications for the universal star formation law
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M. S. Bothwell
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We present high spatial resolution (0.4, ~3.5 kpc) PdBI interferometric data on three ultra-luminous infrared galaxies (ULIRGs) at z~2: two submillimetre galaxies and one submillimetre faint star forming radio galaxy. The three galaxies have been ro- bustly detected in CO rotational transitions, either 12CO(J=4-3) or 12CO(J=3-2), allowing their sizes and gas masses to be accurately constrained. These are the highest spatial resolution observations observed to date (by a factor of ~2) for intermediate-excitation CO emission in z~2 ULIRGs. The galaxies appear extended over several resolution elements, having a mean radius of 3.7 kpc. High-resolution (0.3) combined MERLIN-VLA observations of their radio continua allow an analysis of the star formation behaviour of these galaxies, on comparable spatial scales to that of the CO observations. This matched beam approach sheds light on the spatial distribution of both molecular gas and star formation, and we can therefore calculate accurate star formation rates and gas surface densities: this allows us to place the three systems in the context of a Kennicutt-Schmidt (KS)-style star formation law. We find a difference in size between the CO and radio emission regions, and as such we suggest that using the spatial extent of the CO emission region to estimate the surface density of star formation may lead to error. This size difference also causes the star formation efficiencies within systems to vary by up to a factor of 5. We also find, with our new accurate sizes, that SMGs lie significantly above the KS relation, indicating that stars are formed more efficiently in these extreme systems than in other high-z star forming galaxies.
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We present a study of the formation of clustered, massive galaxies at large look-back times via spectroscopic imaging of CO in the unique GN20 proto-cluster at z = 4.05. Existing observations show that this is a dense concentration of gas-rich, very active star forming galaxies, including multiple bright submillimeter galaxies (SMGs). Using deep, high-resolution VLA CO(2-1) observations, we image the molecular gas with a resolution of ~1 kpc just 1.6 Gyr after the Big Bang. The SMGs GN20.2a and GN20.2b have deconvolved sizes of ~5 kpc X 3 kpc and ~8 kpc X 5 kpc (Gaussian FWHM) in CO(2-1), respectively, and we measure gas surface densities up to ~12,700/1,700X(sin i) (alpha_CO/0.8) M_sun/pc^2 for GN20.2a/GN20.2b in the highest-resolution maps. Dynamical mass estimates allow us to constrain the CO-to-H_2 conversion factor to alpha_CO = 1.7+/-0.8 M_sun (K km s^{-1} pc^2)^-1 for GN20.2a and alpha_CO = 1.1+/-^{1.5}_{1.1} M_sun (K km/s pc^2)^-1 for GN20.2b. We measure significant offsets (0.5-1) between the CO and optical emission, indicating either dust obscuration on scales of tens of kpc or that the emission originates from distinct galaxies. CO spectral line energy distributions imply physical conditions comparable to other SMGs and reveal further evidence that GN20.2a and GN20.2b are in different merging stages. We carry out a targeted search for CO emission from the 14 known B-band Lyman break galaxies (LBGs) in the field, tentatively detecting CO in a previously-undetected LBG and placing 3sigma upper limits on the CO luminosities of those that may lie within our bandpass. A blind search for emission line sources down to a 5sigma limiting CO luminosity of L_CO(2-1) = 8 X 10^9 K km/s pc^2 and covering Delta z = 0.0273 (~20 comoving Mpc) produces no other strong contenders associated with the proto-cluster.
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We have assembled a sample of 5 X-ray and submm-luminous z~2 QSOs which are therefore both growing their central black holes through accretion and forming stars copiously at a critical epoch. Hence, they are good laboratories to investigate the co-evolution of star formation and AGN. We have performed a preliminary analysis of the AGN and SF contributions to their UV-to-FIR SEDs, fitting them with simple direct (disk), reprocessed (torus) and star formation components. All three are required by the data and hence we confirm that these objects are undergoing strong star formation in their host galaxies at rates 500-2000 Msun/y. Estimates of their covering factors are between about 30 and 90%. In the future, we will assess the dependence of these results on the particular models used for the components and relate their observed properties to the intrinsice of the central engine and the SF material, as well as their relevance for AGN-galaxy coevolution.
We present the results of CO(J=3-2) on-the-fly mappings of two nearby non-barred spiral galaxies NGC 628 and NGC 7793 with the Atacama Submillimeter Telescope Experiment at an effective angular resolution of 25. We successfully obtained global distributions of CO(J=3-2) emission over the entire disks at a sub-kpc resolution for both galaxies. We examined the spatially-resolved (sub-kpc) relationship between CO(J=3-2) luminosities (LCO(3-2)) and infrared (IR) luminosities (LIR) for NGC 628, NGC 7793, and M 83, and compared it with global luminosities of JCMT Nearby Galaxy Legacy Survey sample. We found a striking linear LCO(3-2)-LIR correlation over the 4 orders of magnitude, and the correlation is consistent even with that for ultraluminous infrared galaxies and submillimeter selected galaxies. In addition, we examined the spatially-resolved relationship between CO(J=3-2) intensities (ICO(3-2)) and extinction-corrected star formation rates (SFRs) for NGC 628, NGC 7793, and M 83, and compared it with that for GMCs in M 33 and 14 nearby galaxy centers. We found a linear ICO(3-2)-SFR correlation with 1 dex scatter. We conclude that the CO(J=3-2) star formation law (i.e., linear LCO(3-2)-LIR and ICO(3-2)-SFR correlations) is universally applicable to various types and spatial scales of galaxies, from spatially-resolved nearby galaxy disks to distant IR-luminous galaxies, within 1 dex scatter.
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