2015 Fiscal Year Research-status Report
Systematic study of the stellar initial mass function and its variation
| Project/Area Number |
15K17600
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| Research Institution | The University of Tokyo |
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Principal Investigator |
MORE Surhud 東京大学, カブリ数物連携宇宙研究機構, 特任助教 (00646044)
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| Project Period (FY) |
2015-04-01 – 2017-03-31
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| Keywords | Weak lensing / Initial mass function / Assembly bias / Edges of galaxy clusters |
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| Outline of Annual Research Achievements |
The goal of the research proposal was to obtain evidence for variations in the stellar initial mass function using data from SDSS-III BOSS, MaNGA and the Hyper Suprime Cam (HSC) Survey. In FY2015, I have analyzed the stacked spectra of massive galaxies from the BOSS survey binned by velocity dispersion and stellar mass. This was the first observational step in order to address the stellar IMF of galaxies. As weak lensing catalogs from the HSC are in testing stage, I have measured the galaxy galaxy lensing signal from the public data in the Dark Energy Survey and the Red Sequence cluster survey to look at the total density profiles around cluster scale objects. I have also contributed to the measurements of the weak lensing signal from SDSS around red and blue galaxies, which will allow me to revisit the conflicting results found by Dutton et al. 2010 about the stellar IMF in early type galaxies using dynamical analysis. Finally, the most interesting results that were obtained in FY2015 was our detection of halo assembly bias, a long standing but hitherto unverified prediction of the standard model, and the edges of galaxy clusters. The last of these results have revealed discrepancies between the concordance structure formation model and observations -- either related to the systematics in the weak lensing measurements, cluster finding algorithms, or the nature of dark matter itself. The first one of which is the most relevant for this project as weak lensing systematics may result in an incorrect inference, while the last one is of great fundamental importance.
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| Current Status of Research Progress |
Current Status of Research Progress
3: Progress in research has been slightly delayed.
Reason
The progress in the research project is steady, given that the initial observational results are in hand. The interpretation of the stacked spectra of galaxies with respect to the initial mass function is however a little bit more involved. The main issue with the interpretation of stacked spectra is that indicators in the optical wavelengths in order to constrain the initial stellar mass function may not be optimal (based on some results by Dr. Song Huang). This would mean that the interpretation may turn out to be dependent on theoretical systematics rather than the observational uncertainties. Therefore the progress on the observational side could not be fully utilized. I have started focussing on the weak lensing aspects of this project. I am now involved with the weak lensing in many of the leading surveys as discussed above. The positive thing is that during the weak lensing exploration, I have stumbled upon the results of halo assembly bias and the splashback radius, both of which were totally unexpected research areas. Some of the discrepancies we have uncovered could be potentially significant in our understanding of the interpretation of the weak lensing signal or the nature of dark matter itself. In those respects, the research has progressed in areas which are broader than that in the original plan.
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| Strategy for Future Research Activity |
Given that the theoretical interpretation of the stacked spectra may be limited by theoretical systematics, I will be focussing on the weak lensing aspects of the project. As a contribution to the HSC weak lensing analysis on small scales relevant for the project, I have developed a new parameter called "deblendedness" which is now included in the HSC image reduction software. I will use it to quantify how the weak lensing measurements may be affected by nearby bright objects, crucial in order to use the weak lensing signal from very small scales. In addition, I will also investigate the impact of the new weak lensing measurements from the SDSS that I was involved in, on the stellar initial mass function (improving the results of Dutton et al. 2010). I have already developed the software to aid in this interpretation. As part of the halo assembly bias project, I have been able to secure two galaxy cluster samples which have the exact same halo mass but different halo assembly histories. The brightest central galaxies in these halos are expected to have different formation histories, and I will investigate if there are any signs of differences in the stellar populations of these galaxies. I have done preliminary analysis of the stacked spectra of these galaxies, which shows very little difference between the two, but this result needs vetting.
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| Causes of Carryover |
We could not secure the publication of the observational paper related to the IMF this year due to the aforementioned theoretical systematics, and the hardware was secured with a slightly smaller price tag.
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| Expenditure Plan for Carryover Budget |
The remaining amount would be used in the following year for the same purpose of inviting visitors who are involved with the theoretical interpretation of stacked spectra.
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[Journal Article] Strong bimodality in the host halo mass of central galaxies from galaxy-galaxy lensing2016
Author(s)
Mandelbaum, R. , Wang, W., Zu, Y., White, S., Henriques, B. and More, S.
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Journal Title
MNRAS
Volume: 457
Pages: 3200-3218
DOI
Peer Reviewed / Int'l Joint Research
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