Publication

1. -Please see the list here on INSPIRE.
   

2. -Or, alternatively, please see my CV (in PDF format; as of November 2015).
   

Research summary

I am working on particle astrophysics and cosmology. In particular, I am interested in dark matter problem in the Universe, and how to probe it using annihilation products such as energetic gamma rays and neutrinos. Although I am a theoretical astrophysicist, I always consider prospects of current or near future generation of experiments and telescopes.

I worked with many people of various backgrounds. My main collaborators are found in the following list (alphabetical).

1. •John Beacom (Ohio State U.)
   

2. •Aurélien Benoit-Lévy (UCL)
   

3. •Shunsaku Horiuchi (Virginia Tech)
   

4. •Koji Ishiwata (Kanazawa U.)
   

5. •Marc Kamionkowski (Johns Hopkins U.)
   

6. •Eiichiro Komatsu (Max-Planck Institute for Astrophysics)
   

7. •Alexander Kusenko (UCLA)
   

8. •Peter Meszaros (Penn State U.)
   

9. •Kohta Murase (Penn State U.)
   

10. •Daisuke Nagai (Yale U.)
    

11. •Ehud Nakar (Tel Aviv U.)
    

12. •Vasiliki Pavlidou (U. Crete)
    

13. •Stefano Profumo (UC Santa Cruz)
    

14. •Re’em Sari (Hebrew U.)
    

15. •Katsuhiko Sato (IPMU, U. Tokyo)
    

16. •Tomonori Totani (Kyoto U.)
    

17. •Christoph Weniger (U. Amsterdam)
    

Also, see here for my current and past students and postdocs and their publications.

Below, I summarize some of my research highlights.

Particle Dark Matter

Identity of dark matter in the Universe, although 80% of matter has to be hidden in dark, is completely unknown. There are, however, several candidates, most of them being some undiscovered particle. Weakly interacting massive particle (WIMP) such as neutralino predicted by supersymmetric theory is an attractive example. If that is the case, WIMP might annihilate with each other to yield annihilation signature such as GeV gamma rays potentially detectable with Fermi Gamma-Ray Space Telescope.

I have worked on several subjects on dark matter annihilation for Fermi. In particular, Eiichiro Komatsu and I proposed a new analysis technique to use anisotropy analysis of would-be-obtained all-sky gamma-ray map (Ando & Komatsu 2006). We worked out theoretical models of distributions of cosmological dark matter halos to predict the angular power spectrum and showed that the predicted anisotropy is large enough to be detected with Fermi if dark matter contribution to the gamma-ray background is no smaller than about 10% level (Ando et al. 2007).

I also analytically worked out the angular power spectrum from the Galactic substructure (Ando 2009). This results not only confirmed the previous studies by others but also enabled physical understanding of the possible signal in terms of substructure models. With Samuel Lee and Marc Kamionkowski, I also worked on one-point statistics of the gamma-ray background, i.e., probability distribution function of the gamma-ray flux, and pointed out that the signature from the Galactic substructure should be discriminated from that from the smooth dark-matter component (Lee et al. 2009). Michael Feyereisen, Lee, and I revisited this problem in the case of extragalactic dark matter halos (Feyereisen et al. 2015).

With Kamionkowski, Lee, and Savvas Koushiappas, I pointed out that the detectability of proper motions of small substructure through gamma rays is very small (Ando et al. 2008a). The discussion is based on the upper limit of the diffuse gamma-ray background measured with EGRET instrument on board the Compton Gamma-Ray Observatory.

I also pointed out that the signals from the Galactic center strongly constrain the dark-matter contribution to the gamma-ray background (Ando 2005).

The series of my papers on gamma-ray background motivated a dedicated data analysis by the Fermi-LAT collaboration in 2012, in which the angular power spectrum was detected for the first time and the level of anisotropy was found to be consistent with my prediction for astrophysical sources that I made in Ando et al. (2007). Komatsu and I interpreted this result to put the latest constraints on the dark matter annihilation cross section (Ando & Komatsu 2013).

Recently, Aurélien Benoit-Lévy, Komatsu, and I proposed to take cross correlation between the all-sky gamma-ray map and nearby galaxy catalogs such as 2MASS in order to probe dark matter (Ando et al. 2014). We showed that taking cross correlations will be a very efficient method to reduce possible contributions from ordinary astrophysical sources, and to extract dark matter component. We predicted that the sensitivity due to future cross-correlation analyses will reach the canonical annihilation cross section for thermal freeze-out mechanism for dark matter with masses up to 1 TeV (Ando 2014).

The gamma-ray intensity from dark matter annihilation depends on the amount of substructure in dark matter halos. Richard Bartels and I revisited the modeling of these substructure boost factor, by taking tidal stripping properly into account (Bartels & Ando 2015). We found that the conventional estimate of the boost by using concentration-mass relation for field halos will end up underestimate of the gamma-ray flux by a factor of 2 to 5. With Roberta Diamanti and Maria Cabrera, I estimated the dependence of the subhalo boost on minimum mass of the smallest dark matter structure (Diamanti et al. 2015).

High-Energy Astrophysics

High-energy astrophysical sources such as active galactic nuclei (AGNs) and gamma-ray bursts (GRBs) are also fascinating sources by themselves. Their radiation mechanisms and particle acceleration mechanisms are not totally understood. I worked on several aspects on high-energy astrophysics, especially focusing on high-energy gamma rays and neutrinos.

With Beacom, I considered a model of GRB-like jets loaded with a lot more baryons. As the jets become optically thick, these sources are no longer observed as bright GRBs. They will, however, emit strong neutrino radiation in TeV energy regime, where large-volume neutrino telescopes such as IceCube will have good sensitivity. Our calculation shows with this model, IceCube expects about 30 neutrino events from such a source located at 10 Mpc (Ando & Beacom 2005).

With Ehud Nakar and Re’em Sari, I predicted number of expected detection of GRBs with Large Area Telescope on Fermi (Ando et al. 2008b). Our prediction was based on the fluence upper limits and detections due to EGRET, which was, albeit their importance, totally neglected in the previous studies. From this approach, we predicted the detection rate of about 10–15 per year, which turned out to be more or less the right number. During this study, we also pointed out that the effect of Compton scattering in the Klein-Nishina regime could significantly change spectra of the GRB prompt emission (Nakar et al. 2009).

Recent measurement of high-energy neutrinos with IceCube telescope at the South Pole in the TeV-PeV energy range indicated that they are of extragalactic origin. Irene Tamborra, Kohta Murase, and I studied the star-forming and starburst galaxies as potential sources of high-energy neutrinos (Tamborra et al. 2014). We in particular focused on synergy between gamma-ray and neutrino intensities, and found that most of the neutrinos can be explained with these sources, if cosmic rays can be efficiently accelerated and confined in the system. With Fabio Zandanel, Tamborra, Stefano Gabici, I studied clusters and groups of galaxies as a source of the IceCube neutrinos (Zandanel et al. 2015). Here we found that the radio number count of the clusters and groups provide very stringent constraints on potential neutrino emission from these sources, making them unlikely to be dominant in neutrinos. Furthermore, with Tamborra and Zandanel, I analyzed the recent cross-correlation measurement of the diffuse gamma-ray background with Fermi-LAT and several galaxy catalogs such as 2MASS, and applied the results on sources of high-energy neutrinos through hadronuclear interactions (Ando et al. 2015). We showed that such tomographic constraints are quite stringent, improving up to one order of magnitude compared with conventional spectral constraints. Tamborra and I also computed neutrino flux from GRBs (Tamborra & Ando 2015).

Supernova Neutrinos

Neutrino is the only particle that shows features beyond the standard model such as nonzero masses and mixings. I started my research career by studying supernova neutrinos from various aspects. For example, with Katsuhiko Sato, I pointed out that the neutrinos detected with Super-Kamiokande would tell us the direction of supernova with the precision of 5–10 degrees (Ando & Sato 2001), and also theoretically calculated the flavor conversion involving nonzero magnetic moment showing that the feature is very characteristic in both energy and time information (Ando & Sato 2003a, b, c).

The diffuse neutrino background emitted from past supernovae is an fascinating target to work on, since the experimental sensitivity is coming closer to the theoretical prediction. With Sato and Tomonori Totani, I predicted the flux and event rate of the diffuse supernova neutrino background taking into account the effect of neutrino oscillation (Ando et al. 2003).

This model was analyzed in light of the Super-K data to set an encouraging upper bound. Sato and I further worked on this subject motivated by such an encouraging upper limit. We performed more detailed analysis with various neutrino oscillation models (Ando & Sato 2003d), and I pointed out that the very stringent limit on neutrino lifetime on nonradiative decays could be obtained from the Super-K result (Ando 2003). Sato and I wrote a review article on this subject, too (Ando & Sato 2004).

With John Beacom and Hasan Yuksel, I considered detection prospects of supernova neutrinos from nearby galaxies (Ando et al. 2005). Using the updated galaxy catalog and supernova catalog, we showed that the rate of detection with future Mton-scale detectors is more than 1 per year, and can be significantly improved with a 5-Mton detector (Kistler et al. 2008).

Cosmology

Many of my papers are related to some cosmological problems in the Universe. For example, the dark matter mentioned above is of course one such example. There are also number of cosmological issues including mystery of dark energy, how to probe primordial power from microwave background or large-scale structure etc.

If dark energy is made of some classical scalar field such as “quintessence” and interacts with ordinary particle, then we might see some feature of dark-energy interaction in the visible sector. Neutrino might couple to dark energy, and if so, the neutrino oscillation phenomenology will change. With Kamionkowski and Irina Mocioiu, I worked out this assuming some certain type of dark-energy–neutrino interaction (Ando et al. 2009). We found that such kind of interactions generally modifies neutrino oscillation probability during propagation in a way similar to the matter effect. Furthermore, we pointed out that the future measurement of cosmogenic neutrinos will improve the current limit on this kind of interactions by several orders of magnitude. And we also pointed out that if the anomaly seen in the measurement were indeed coming from secret dark-energy interaction, there would be particular type of directional dependence of the oscillation probability, and we worked out to derive concrete formula of this dependence.

The most popular way of trying to understand properties of dark energy is to obtain its equation of state w, by measuring the cosmic expansion precisely. There are several approaches to achieve this, but using clusters of galaxies is one such example. However, in order to do so, we have to measure their masses precisely, as the evolution of mass distribution function is sensitive to the expansion history. If one tries to estimate the mass using hydrostatic equilibrium between pressure support and gravitational attraction, any nonthermal pressure components will give systematic uncertainty. With Daisuke Nagai, I pointed out that if cosmic rays in clusters give significant contribution to the pressure so that they give systematic uncertainty to mass measurements, then these clusters should be bright in GeV gamma rays and be observable by Fermi (Ando & Nagai 2008). We estimated the Fermi sensitivity to the cosmic-ray pressure, and showed that it can constrain the cosmic-ray pressure at a few percent level, which would be extremely useful for cluster mass measurement. Zandanel and I analyzed the Fermi data for the Coma cluster and put stringent constraints on its cosmic ray content (Zandanel & Ando 2014).

With Kamionkowski, I also studied primordial (statistical) anisotropy of the density fluctuations plausibly induced by some inflation mechanisms (Ando & Kamionkowski 2008). We worked out the nonlinear evolution of such density fluctuations, and showed that the nonlinear effect does suppress the primordial anisotropy, but by no larger than several percent. Thus the detection would likely imply existence of primordial anisotropic power.