2018 Fiscal Year Annual Research Report
Molecular gas in high redshift star-forming galaxies with ALMA
Publicly Offered Research
| Project Area | Why does the Universe accelerate? - Exhaustive study and challenge for the future - |
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| Project/Area Number |
18H04346
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| Research Institution | The University of Tokyo |
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Principal Investigator |
SILVERMAN John 東京大学, カブリ数物連携宇宙研究機構, 准教授 (90573030)
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| Project Period (FY) |
2018-04-01 – 2020-03-31
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| Keywords | starbursts / submillimeter |
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| Outline of Annual Research Achievements |
We have finished two key studies on the molecular gas properties of starburst galaxies at z ~ 1.5 with the Atacama Large Millimeter/submillimeter Array (ALMA). The first (Silverman et al. 2018, ApJ, 867, 92) is a full analysis of the total molecular gas content of twelve starbursts with CO 2-1 observations. The major result is that high-redshift starbursts have gas masses similar to more typical star forming galaxies thus their high star formation rates are driven by a more efficient conversion of gas to stars as opposed to a higher gas fraction. The second related work is a high-resolution study on 1 kpc scales of the gas as traced by CO 5-4 in a archetypal galaxy merger named PACS-787 (Silverman et al. 2018 ApJ, 868, 75). In this study, we show that starbursts in both galaxies, separated by 8 kiloparsec, in a merger can be efficiently triggered at early stages in the interaction. The gas shows disk-like rotation in both galaxies thus enabling us to measure the dynamical mass of the system and then estimate the amount of molecular gas, independent of the CO luminosity. This allows us to determine the conversion factor alpha_CO = gas mass / CO luminosity, a key ratio for deriving gas masses for CO detected samples in cases where dynamical masses are not available. In addition, we have begun the analysis of the CO 5-4 observations of two additional starbursts observed with ALMA at a resolution of 0.1" with a completion expected in FY2019.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
Overall, the project is going as planned. As described above, two major studies based on our ALMA observations of starbursts have been completed on schedule. The data analysis and publication of the two remaining starburst galaxies observed at 0.1" spatial resolution are now underway and close to completion. Regarding the ALPINE survey (PI: O. Le Fevre - LAM; co-PI J. Silverman), a large ALMA program to observe 122 galaxies in [CII] at very high redshifts 4 < z < 6 is well underway. All data has been prepared for science quality measurements. We are now determining sizes of the CII emitting regions, a key quantity used in the estimates of dynamical masses that are required to estimate gas mass. We have made progress on the ALMA archival study of FMOS galaxies serendipitously detected by ALMA, providing an upper limit or used for stacking the signal of a population of galaxies. The work on the individual detections is done. Given these accomplishments the current status of the project is set accordingly.
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| Strategy for Future Research Activity |
In early FY2019, we plan to complete the analysis on the molecular gas distribution in starbursts for two targets that represent late-stage mergers of galaxies close to coalescence at z~1.5. These observations nicely complement those of PACS-787 which is in an earlier stage of the merger. This work will measure the gas density in the central 1 kiloparsec region and the star formation thus indicating whether localized regions in starbursts follow the well established Schmidt-Kennicutt relation. With ALPINE, we plan to complete our measurements of the CII sizes. Seiji Fujimoto (ICRR) has been recruited to carry out this analysis. With the inclusion of the velocity widths of the CII lines, we will measure the dynamical masses and use the stellar masses from HSC to subtract off this component thus providing an estimate of the gas mass. With Seiji, we plan to published a paper in FY19 on the size - mass relation. We also plan to infer the ratio of dark matter to baryons based on the gas kinematics and dynamical mass estimates. This topic is in line with the aims of the Innovative aspect of the grant to provide further insight on the presence of dark matter in the early universe. We are also participating in joint studies with ALPINE collaborators to measure the gas fraction and star formation efficiencies of high-z galaxies in ALPINE. Finally, we will complete our project on the FIR continuum properties of star forming galaxies using the FMOS sample present in Kashino et al. 2019.
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