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Register a new userVLA Mapping of the CO(1-0) Line in SMM J14011+0252
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Chelsea E. Sharon
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We present high-resolution CO(1-0) observations of the lensed submillimeter galaxy (SMG) SMM J14011+0252 at z=2.6. Comparison to the previously-detected CO(3-2) line gives an intensity ratio of r_3,1=0.97+/-0.16 in temperature units, larger than is typical for SMGs but within the range seen in the low-z ultraluminous infrared galaxy population. Combining our new data with previous mid-J CO observations, we perform a single-phase large velocity gradient (LVG) analysis to constrain the physical conditions of the molecular gas. Acceptable models have significant degeneracies between parameters, even when we rule out all models that produce optically thin emission, but we find that the bulk of the molecular gas has T_kin=20-60 K, n_{H_2}~10^4-10^5 cm^-3, and N_CO/Delta-v=10^{17.00+/-0.25} cm^-2 km^-1 s. For our best-fit models to self-consistently recover a typical CO-to-H_2 abundance and a plausible degree of virialization, the local velocity gradient in the molecular gas must be substantially larger than its galaxy-wide average. This conclusion is consistent with a scenario in which SMM J14011+0252 has a fairly face-on orientation and a molecular ISM composed of many unresolved clouds. Using previous H-alpha observations, we find that SMM J14011+0252 has a spatially resolved star formation rate vs. molecular gas surface density relation inconsistent with those of normal local star-forming galaxies, even if we adopt a local disk-like CO-to-H_2 conversion factor as motivated by our LVG analysis. This discrepancy supports the inference of a star formation relation for high-z starbursts distinct from the local relation that is not solely due to differing choices of gas mass conversion factor.
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We report the detection of CO(3-2) emission from the submillimeter-selected luminous galaxy SMM J14011+0252. The optical counterpart of the submillimeter source has been identified as a merger system with spectral characteristics consistent with a starburst at z=2.565. The CO emission confirms the optical identification of the submillimeter source and implies a molecular gas mass of 5x10^{10}h_{75}^{-2} M(sun), after correcting for a lensing amplification factor of 2.75. The large molecular gas mass and the radio emission are consistent with the starburst interpretation of the source. These results are similar to those found for SMM J02399-0136, which was the first submillimeter selected CO source found at high redshift. The CO detections of these two high-redshift submillimeter galaxies suggest the presence of massive reservoirs of molecular gas which is consistent with the inferred high rates of star-formation (1000 M(sun)/yr). These two systems appear to be associated with merger events which may evolve into present day luminous elliptical galaxies.
We used the IRAM Interferometer to detect CO(3-2), CO(7-6), and 1.3 mm dust continuum emission from the submillimeter galaxy SMM J14011+0252 at a redshift of 2.6. Contrary to a recent claim that the CO was extended over 6.6 arcsec (57 kpc), the new data yield a size of 2 x 0.5 arcsec for the CO and the dust. Although previous results placed the CO peak in a region with no visible counterpart, the new maps show the CO and dust are centered on the J1 complex seen on K-band and optical images. We suggest the CO is gravitationally lensed not only by the foreground cluster A1835, but also by an individual galaxy on the line of sight. Comparison of measured and intrinsic CO brightness temperatures indicates the CO size is magnified by a factor of 25 +/- 5. After correcting for lensing, we derive a true CO diameter of ~0.08 arcsec (700 pc), consistent with a compact circumnuclear disk of warm molecular gas similar to that in Arp 220. The high magnification means the true size, far-IR luminosity, star formation rate, CO luminosity, and molecular gas mass are all comparable with those in present-epoch ultraluminous IR galaxies, not with those of a huge, massive, early-universe galactic disk.
While molecular gas mass is usually derived from $^{12}$CO($J$=1-0) - the most fundamental line to explore molecular gas - it is often derived from $^{12}$CO($J$=2-1) assuming a constant $^{12}$CO($J$=2-1)/$^{12}$CO($J$=1-0) line ratio ($R_{2/1}$). We present variations of $R_{2/1}$ and effects of the assumption that $R_{2/1}$ is a constant in 24 nearby galaxies using $^{12}$CO data obtained with the Nobeyama 45-m radio telescope and IRAM 30-m telescope. The median of $R_{2/1}$ for all galaxies is 0.61, and the weighted mean of $R_{2/1}$ by $^{12}$CO($J$=1-0) integrated-intensity is 0.66 with a standard deviation of 0.19. The radial variation of $R_{2/1}$ shows that it is high (~0.8) in the inner ~1 kpc while its median in disks is nearly constant at 0.60 when all galaxies are compiled. In the case that the constant $R_{2/1}$ of 0.7 is adopted, we found that the total molecular gas mass derived from $^{12}$CO($J$=2-1) is underestimated/overestimated by ~20%, and at most by 35%. The scatter of a molecular gas surface density within each galaxy becomes larger by ~30%, and at most by 120%. Indices of the spatially resolved Kennicutt-Schmidt relation by $^{12}$CO($J$=2-1) are underestimated by 10-20%, at most 39% in 17 out of 24 galaxies. $R_{2/1}$ has good positive correlations with star-formation rate and infrared color, and a negative correlation with molecular gas depletion time. There is a clear tendency of increasing $R_{2/1}$ with increasing kinetic temperature ($T_{rm kin}$). Further, we found that not only $T_{rm kin}$ but also pressure of molecular gas is important to understand variations of $R_{2/1}$. Special considerations should be made when discussing molecular gas mass and molecular gas properties inferred from $^{12}$CO($J$=2-1) instead of $^{12}$CO($J$=1-0).
Both the CO(2-1) and CO(1-0) lines are used to trace the mass of molecular gas in galaxies. Translating the molecular gas mass estimates between studies using different lines requires a good understanding of the behaviour of the CO(2-1)-to-CO(1-0) ratio, $R_{21}$. We compare new, high quality CO(1-0) data from the IRAM 30-m EMPIRE survey to the latest available CO(2-1) maps from HERACLES, PHANGS-ALMA, and a new IRAM 30-m M51 Large Program. This allows us to measure $R_{21}$ across the full star-forming disc of nine nearby, massive, star-forming spiral galaxies at 27 (${sim} 1{-}2$ kpc) resolution. We find an average $R_{21} = 0.64pm0.09$ when we take the luminosity-weighted mean of all individual galaxies. This result is consistent with the mean ratio for disc galaxies that we derive from single-pointing measurements in the literature, $R_{rm 21, lit}~=~0.59^{+0.18}_{-0.09}$. The ratio shows weak radial variations compared to the point-to-point scatter in the data. In six out of nine targets the central enhancement in $R_{21}$ with respect to the galaxy-wide mean is of order $sim 10{-}20%$. We estimate an azimuthal scatter of $sim$20% in $R_{21}$ at fixed galactocentric radius but this measurement is limited by our comparatively coarse resolution of 1.5 kpc. We find mild correlations between $R_{21}$ and CO brightness temperature, IR intensity, 70-to-160$ mu$m ratio, and IR-to-CO ratio. All correlations indicate that $R_{21}$ increases with gas surface density, star formation rate surface density, and the interstellar radiation field.
We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations towards 27 low-redshift ($0.02< z<0.2$) star-forming galaxies taken from the Valparaiso ALMA/APEX Line Emission Survey (VALES). We perform stacking analyses of the $^{12}$CO($1-0$), $^{13}$CO($1-0$) and C$^{18}$O($1-0$) emission lines to explore the $L$ ($^{12}$CO($1-0$))/$L$($^{13}$CO($1-0$))) (hereafter $L$($^{12}$CO)/$L$($^{13}$CO)) and $L$($^{13}$CO($1-0$))/$L$(C$^{18}$O($1-0$)) (hereafter $L$($^{13}$CO)/$L$(C$^{18}$O) line luminosity ratio dependence as a function of different global galaxy parameters related to the star formation activity. The sample has far-IR luminosities $10^{10.1-11.9}$L$_{odot}$ and stellar masses of $10^{9.8-10.9}$M$_{odot}$ corresponding to typical star-forming and starburst galaxies at these redshifts. On average we find a $L$($^{12}$CO)/$L$($^{13}$CO) line luminosity ratio value of 16.1$pm$2.5. Galaxies with evidences of possible merging activity tend to show higher $L$($^{12}$CO)/$L$($^{13}$CO) ratios by a factor of two, while variations of this order are also found in galaxy samples with higher star formation rates or star formation efficiencies. We also find an average $L$($^{13}$CO)/$L$(C$^{18}$O) line luminosity ratio of 2.5$pm$0.6, which is in good agreement with those previously reported for starburst galaxies. We find that galaxy samples with high $L_{text{IR}}$, SFR and SFE show low $L$($^{13}$CO)/$L$(C$^{18}$O) line luminosity ratios with high $L$($^{12}$CO)/$L$($^{13}$CO) line luminosity ratios, suggesting that these trends are produced by selective enrichment of massive stars in young starbursts.
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