Galaxy clusters grow primarily through the continuous accretion of group-scale haloes. Group galaxies experience preprocessing during their journey into clusters. A star-bursting compact group, the Blue Infalling Group (BIG), is plunging into the nea
rby cluster A1367. Previous optical observations reveal rich tidal features in the BIG members, and a long H$alpha$ trail behind. Here we report the discovery of a projected $sim 250$ kpc X-ray tail behind the BIG using Chandra and XMM-Newton observations. The total hot gas mass in the tail is $sim 7times 10^{10} {rm M}_odot$ with an X-ray bolometric luminosity of $sim 3.8times 10^{41}$ erg s$^{-1}$. The temperature along the tail is $sim 1$ keV, but the apparent metallicity is very low, an indication of the multi-$T$ nature of the gas. The X-ray and H$alpha$ surface brightnesses in the front part of the BIG tail follow the tight correlation established from a sample of stripped tails in nearby clusters, which suggests the multiphase gas originates from the mixing of the stripped interstellar medium (ISM) with the hot intracluster medium (ICM). Because thermal conduction and hydrodynamic instabilities are significantly suppressed, the stripped ISM can be long lived and produce ICM clumps. The BIG provides us a rare laboratory to study galaxy transformation and preprocessing.
We present the galaxy luminosity functions (LFs) of four Hickson Compact Groups using image data from the Subaru Hyper Suprime-Cam. A distinct dip appeared in the faint-ends of all the LFs at $M_gsim-12$. A similar dip was observed in the LFs of the
galaxy clusters Coma and Centaurus. However, LFs in the Virgo, Hydra, and the field had flatter slopes and no dips. As the relative velocities among galaxies are lower in compact groups than in clusters, the effect of galaxy-galaxy interactions would be more significant in compact groups. The $M_gsim-12$ dip of compact groups may imply that frequent galaxy-galaxy interactions would affect the evolution of galaxies, and the dip in LF could become a boundary between different galaxy populations.
Previous studies have revealed a population of galaxies in galaxy clusters with ram pressure stripped (RPS) tails of gas and embedded young stars. We observed 1.4 GHz continuum and HI emission with the Very Large Array in its B-configuration in two f
ields of the Coma cluster to study the radio properties of RPS galaxies. The best continuum sensitivities in the two fields are 6 and 8 $mu$Jy per 4 beam respectively, which are 4 and 3 times deeper than those previously published. Radio continuum tails are found in 10 (8 are new) out of 20 RPS galaxies, unambiguously revealing the presence of relativistic electrons and magnetic fields in the stripped tails. Our results also hint that the tail has a steeper spectrum than the galaxy. The 1.4 GHz continuum in the tails is enhanced relative to their H$alpha$ emission by a factor of $sim$7 compared to the main bodies of the RPS galaxies. The 1.4 GHz continuum of the RPS galaxies is also enhanced relative to their IR emission by a factor of $sim$2 compared to star-forming galaxies. The enhancement is likely related to ram pressure and turbulence in the tail. We furthermore present HI detections in three RPS galaxies and upper limits for the other RPS galaxies. The cold gas in D100s stripped tail is dominated by molecular gas, which is likely a consequence of the high ambient pressure. No evidence of radio emission associated with ultra-diffuse galaxies is found in our data.
Deep optical imaging with both Hyper Suprime-Cam and Suprime-Cam on the 8.2 m Subaru Telescope reveals a number of outer faint structures around the archetypical Seyfert galaxy NGC 1068 (M 77). We find three ultra diffuse objects (UDOs) around NGC 10
68. Since these UDOs are located within the projected distance of 45 kpc from the center of NGC 1068, they appear to be associated with NGC 1068. Hereafter, we call them UDO-SW, UDO-NE, and UDO-SE where UDO = Ultra Diffuse Object, SW = south west, NE = north west, and SE = south east; note that UDO-SE was already found in the SDSS Stripe 82 data. Among them, both UDO-NE and UDO-SW appear to show a loop or stream structure around the main body of NGC 1068, providing evidence for the physical connection to NGC 1068. We consider that UDO-SE may be a tidal dwarf galaxy. We also find another UDO-like object that is 2 magnitudes fainter and smaller by a factor of 3 to 5 than those of the three UDOs. This object may belong to a class of low surface brightness galaxy. Since this object is located along the line connecting UDO-NE and UDO-SW, it is suggested that this object is related to the past interaction event that formed the loop by UDO-NE and UDO-SW, thus implying the physical connection to NGC 1068. Another newly-discovered feature is an asymmetric outer one-arm structure emanated from the western edge of the outermost disk of NGC 1068 together with a ripple-like structure at the opposite side. These structures are expected to arise in a late phase of a minor merger according to published numerical simulations of minor mergers. All these lines of evidence show that NGC 1068 experienced a minor merger several billions years ago. We then discuss the minor-merger driven triggering of nuclear activity in the case of NGC 1068.
We report the discovery of 854 ultra diffuse galaxies (UDGs) in the Coma cluster using deep R band images, with partial B, i, and Halpha band coverage, obtained with the Subaru telescope. Many of them (332) are Milky Way-sized with very large effecti
ve radii of r_e>1.5kpc. This study was motivated by the recent discovery of 47 UDGs by van-Dokkum et al. (2015); our discovery suggests >1,000 UDGs after accounting for the smaller Subaru field. The new UDGs show a distribution concentrated around the cluster center, strongly suggesting that the great majority are (likely longtime) cluster members. They are a passively evolving population, lying along the red sequence in the CM diagram with no Halpha signature. Star formation was, therefore, quenched in the past. They have exponential light profiles, effective radii re ~ 800 pc- 5 kpc, effective surface brightnesses mu_e(R)=25-28 mag arcsec-2, and stellar masses ~1x10^7 - 5x10^8Msun. There is also a population of nucleated UDGs. Some MW-sized UDGs appear closer to the cluster center than previously reported; their survival in the strong tidal field, despite their large sizes, possibly indicates a large dark matter fraction protecting the diffuse stellar component. The indicated baryon fraction ~<1% is less than the cosmic average, and thus the gas must have been removed from the possibly massive dark halo. The UDG population appears to be elevated in the Coma cluster compared to the field, indicating that the gas removal mechanism is related primarily to the cluster environment.
We report the discovery of a new dwarf galaxy (NGC6503-d1) during the Subaru extended ultraviolet (XUV) disk survey. It is a likely companion of the spiral galaxy NGC6503. The resolved images, in B, V, R, i, and Halpha, show an irregular appearance d
ue to bright stars with underlying, smooth and unresolved stellar emission. It is classified as the transition type (dIrr/dSph). Its structural properties are similar to those of the dwarfs in the Local Group, with a V absolute magnitude ~ -10.5, half-light radius ~400 pc, and central surface brightness ~25.2. Despite the low stellar surface brightness environment, one HII region was detected, though its Halpha luminosity is low, indicating an absence of any appreciable O-stars at the current epoch. The presence of multiple stellar populations is indicated by the color-magnitude diagram of ~300 bright resolved stars and the total colors of the dwarf, with the majority of its total stellar mass ~4x10^6 Msun in an old stellar population.
We report deep Subaru Halpha observations of the XUV disk of M83. These new observations enable the first complete census of very young stellar clusters over the entire XUV disk. Combining Subaru and GALEX data with a stellar population synthesis mod
el, we find that (1) the standard, but stochastically-sampled, initial mass function (IMF) is preferred over the truncated IMF, because there are low mass stellar clusters (10^{2-3}Msun) that host massive O-type stars; that (2) the standard Salpeter IMF and a simple aging effect explain the counts of FUV-bright and Halpha-bright clusters with masses >10^3Msun; and that (3) the Halpha to FUV flux ratio over the XUV disk supports the standard IMF. The Subaru Prime Focus Camera (Suprime-Cam) covers a large area even outside the XUV disk -- far beyond the detection limit of the HI gas. This enables us to statistically separate the stellar clusters in the disk from background contamination. The new data, model, and previous spectroscopic studies provide overall consistent results with respect to the internal dust extinction (Av~0.1 mag) and low metallicity (~0.2Zsun) using the dust extinction curve of SMC.
We present the results of deep imaging and spectroscopic observations of very extended ionized gas (EIG) around four member galaxies of the Coma cluster of galaxies: RB199, IC4040, GMP2923 and GMP3071. The EIGs were serendipitously found in an H-alph
a narrow band imaging survey of the central region of the Coma cluster. The relative radial velocities of the EIGs with respect to the systemic velocities of the parent galaxies from which they emanate increase almost monotonically with the distance from the nucleus of the respective galaxies, reaching -400 - -800 km/s at around 40 - 80 kpc from the galaxies. The one-sided morphologies and the velocity fields of the EIGs are consistent with the predictions of numerical simulations of ram pressure stripping. We found a very low-velocity filament (v_rel = -1300 km/s) at the southeastern edge of the disk of IC4040. Some bright compact knots in the EIGs of RB199 and IC4040 exhibit blue continuum and strong H-alpha emission. The equivalent widths of the H-alpha emission exceed 200 A, and are greater than 1000 A for some knots. The emission line intensity ratios of the knots are basically consistent with those of sub-solar abundance HII regions. These facts indicate that intensive star formation occurs in the knots. Some filaments, including the low velocity filament of the IC4040 EIG, exhibit shock-like emission line spectra, suggesting that shock heating plays an important role in ionization and excitation of the EIGs.
In order to investigate the evolution of E+A galaxies, we observed a galaxy SDSS J160241.00+521426.9, a possible E+A progenitor which shows both emission and strong Balmer absorptions, and its neighbor galaxy. We used the integral field spectroscopic
mode of the Kyoto Tridimensional Spectrograph (Kyoto3DII), mounted on the University of Hawaii 88-inch telescope located on Mauna Kea, and the slit-spectroscopic mode of the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope. We found a strong Balmer absorption region in the center of the galaxy and an emission-line region located 2 kpc from the center, in the direction of its neighbor galaxy. The recession velocities of the galaxy and its neighbor galaxy differ only by 100 km s^-1, which suggests that they are a physical pair and would have been interacting. Comparing observed Lick indices of Balmer lines and color indices with those predicted from stellar population synthesis models, we find that a suddenly quenched star-formation scenario is plausible for the star-formation history of the central region. We consider that star formation started in the galaxy due to galaxy interactions and was quenched in the central region, whereas star formation in a region offset from the center still continues or has begun recently. This work is the first study of a possible E+A progenitor using spatially resolved spectroscopy.
We have performed a spatially-resolved medium resolution long-slit spectroscopy of a nearby E+A (post-starburst) galaxy system, SDSSJ161330.18+510335.5. This E+A galaxy has an obvious companion galaxy 14kpc in front with the velocity difference of 61
.8 km/s. Both galaxies have obviously disturbed morphology We have found that H$delta$ equivalent width (EW) of the E+A galaxy is greater than 7AA galaxy wide (8.5 kpc). The E+A galaxy have a weak [OIII] emission (EW$sim$1AA) by $sim$2.6 kpc offset from the peak of the Balmer absorption lines. We detected a rotational velocity in the companion galaxy of $>$175km/s. The progenitor of the companion may have been a rotationally-supported, but yet passive S0 galaxy. The age of the E+A galaxy after quenching the star formation is estimated to be 100-500 Myr, with its centre having slightly younger stellar population. These findings are inconsistent with a simple picture where the dynamical interaction creates infall of the gas reservoir that causes the central starburst/post-starburst. Instead, our results present an important example where the galaxy-galaxy interaction can trigger a galaxy-wide post-starburst phenomena.
