Research Report

Hideki Tanimura

1 Revealing the internal structure in the cosmic web

The large scale structure in the Universe poses many challenges to our understanding. The standard cosmological model of structure formation predicts that the cosmic structure is organized in a web-like structure called Cosmic Web, which is demonstrated in cosmological simulations. The cosmological simulations also illustrates that the cosmic web is constituted by four types of substractures: nodes, filaments, sheets and voids. Nodes form at dense regions, which are interconnected by filaments and sheets, and most of regions are occupied by voids with extremely low-density. Nodes, where galaxy clusters are located, are prominent structures and easily identified, therefore they have been used to study the large-scale structure. However, it is predicted that the largest fraction of matter (around 50% in mass) resides in filamentary structures. Despite the dominant structure, the filamentary structure consists of a complex geometrical pattern and the study of the internal structure has been hindered by its faintness and structural uncertainty. In my PhD research, I searched for hot/warm gas in the filaments between ∼260,000 LRG pairs with the distance of 6 - 10 Mpc/h, given that numerical simulations indicate that ∼85% of cluster pairs with this range of distance is connected by straight filaments. This allowed to identify the locations of filaments through the positions of cluster pairs. However, the simulations also demonstrate that longer filaments are mostly curved and have more complicated shapes. Recently, the algorithms to identify the cosmic network with galaxy distribution have been developed, providing the locations of the filamentary structure. One project in the ByoPiC, which I belong to, is to apply one of the algorithms, called DisPerSE, to the SDSS galaxies, providing a catalogue of filaments with their positions and lengths. Using the catalogue of filaments, I have examined their internal structures such as density and temperature as follows.

1.1 Probing matter in filaments with CMB lensing One of the tracers to probe cosmic structures is gravitaional lensing. In particular, photons in the Cosmic Microwave Background (CMB) are deflected by the gravitational lensing effect due to massive cosmic structures as they travel through the Universe, called CMB lensing. It can be used to study the matter distribution in the Universe, however the deflection angle is subtle especially for low-dense regions. Therefore I have examined the cosmic filaments with the length of 30 - 100 Mpc by stacking method. The statistical analysis has confirmed the lensing signal at more than 7-sigma confidence level and has provided the average matter density in the filaments to be δ ∼ 5.7. There are some studies of filaments with the gravitational lensing effect on scales of ∼10 Mpc, however, this is the first estimate of the matter density in the cosmic-web filaments on such large scales.

1 1.2 Probing baryonic gas in filaments with the thermal Sunyaev Zel’dovich effect Hydrodynamical simulations predict that the majority of baryons exist in filaments in the form of hot plasma (gas), which is referred to as Warm Hot Intergalactic Medium (WHIM), though stars in galaxies only comprise less than 10% of baryons. One such probe to study the WHIM gas in filaments, is the thermal Sunyaev-Zel’dovich (tSZ) effect: inverse Compton scattering of CMB photons by free electrons along the line of sight. The tSZ effect provides a direct measure of the gas pressure in the cosmic structures. I have examined the same filaments, studied above with the CMB lensing, using the tSZ effect through stacking method and confirmed the tSZ signal at ∼ 3.7 sigma confidence level. This is the first detection of the gas in the cosmic-web filaments on such large scales. The tSZ measurement, combined with the measured matter densiy of δ ∼ 5.7 from the CMB lensing, provides the temperature of the gas, T ∼ 1.2 × 106 [K], under the assumption that the matter density is almost equivalent to the baryon density, which is supported by hydrodynamic simulations. Note that only tSZ data does not provide gas density due to the degeneracy between density and temperature in the tSZ (∝ gas pressure ∝ gas density × gas temperature).

2 Missing baryon problem

Recent CMB observations indicate that baryons comprise 4.6% of the total energy density of the Universe and it is in agreement with the detected baryons at early times (z > 2), for example, through the measurements of quasar absorption lines. However, at late times (z < 2), ∼30 % of baryons has not been confirmed (”Missing Baryon Problem”) due to their complicated thermal states, caused by the cosmic evolution and interactions. According to hydrodynamical simulations, most of the missing baryons are expected to be in the form of WHIM with the temperature of 105-107 K and located in filaments between galaxy clusters.

2.1 WHIM in filaments Hydrodynamic simulations also indicate that the WHIM gas is elusive to detect due to its low density, which I have derived to be δ ∼ 5.7 in Section 1, by assuming that the matter density is equivalent to the baryonic gas density. The total amount of the detected WHIM gas provides the baryon fraction of ∼ 7% in the filaments with the distance of 30 - 100 Mpc, based on the geometry given by the filament catalogue. In addition, in my PhD research, I detected the WHIM gas in the filaments between 260,000 SDSS LRG pairs with the separation distance of 6 - 10 Mpc/h using the tSZ effect, and derived the gas density along with the analysis of the BAHAMAS cosmological hydrodynamic simulations. The result provides the measured baryon fraction of ∼ 2.6%. Adding these measured bayrons in the large-scale and small-scale filaments, the total baryon fraction amounts to ∼10%, which is consistent with the other independent tSZ measurements using almost one million galaxy pairs at higher redshift.

2.2 WHIM in superclusters I also have searched for the WHIM gas in superclusters. Superclusters have complex inner structures with multiple galaxy clusters and there are studies, showing that they are connected by

2 chains of filaments each other. Therefore I examined the WHIM gas in superclusters, but outside galaxy clusters by the tSZ effect. I have detected the residual tSZ signal in ∼700 superclusters at 2.5-sigma confidence level by stacking method and it provides the baryon fraction of ∼ 7 - 21 % with large uncertainties. These uncertainties are caused by the unresolved distribution of gas in such crowded environments and the unclear gas temperature in the environments with galaxy clusters and filaments entangled. Despite these difficulties, this result provides insights for future observations to search for the WHIM gas in superclusters.

3 Gas distribution in galaxy groups

Under the assumption that the galaxy-formation process is dominated by gravity, electron (gas) pressure and mass in halos should follow a simple power-law relation, called self-similar scaling relation. Any deviation from this relation points to the presence of more complex processes over gravity such as baryonic effects. For example, the AGN feedback is expected to have a wide range of impacts on the evolution of galaxies and the formation of large-scale structure. X-ray and SZ observations demonstrate that massive halos almost follow the self-similar scaling relation. On the other hand, numerous X-ray observations of galaxy groups find a deficit of baryons inside low- mass halos compared to the cosmic average baryon fraction, implying that the deviation from the self-simular scaling relation. The gas distribution in and around low-mass halos provides more deeper understandings of the internal structure in filaments. The detail study using numerical simulations demonstrate that 13.6−13.7 low-mass halos with M < 10 M are mostly located in filaments. Therefore, if the gas in the low-mass halos is confined inside, the internal structure in filaments is organized by chunks of low-mass halos, such as seen by optical observations of galaxies along filaments. On the other hand, if not, the gas can be distributed more widely in filaments. It can not be distinguished with the tSZ signal I have detected for the filaments, due to the limited angular resolution of the tSZ map produced by the Planck team. In my PhD research, I examined the average gas (pressure) distribution around ∼ 66,000 SDSS 13.5 LRGs, most of which belong to low-mass halos with the average mass of M ∼ 10 M , using the tSZ effect. The result demonstrated that the gas distribution of low-mass halos can be modeled by the gas distribution of massive halos, scaled by almost the self-similar relation. It can be explained by the Cosmo-OWLS cosmological hydrodynamic simulations, showing that the deviation 13.5 becomes significant below M ∼ 10 M , which almost corresponds to the average mass of my samples. These studies indicate that the importance of future observations for lower-mass halos 13.5 below M ∼ 10 M to probe the baryonic effects in galaxy clusters.

3