arXiv subjects

M. Yagi

Publications and source records attributed to M. Yagi.

34 records · Page 2Linked to original sources

arXiv2003

The Halpha Luminosity Function and Star Formation Rate at z~0.24 Based on Subaru Deep Imaging Data

We have carried out a deep imaging survey for H$α$ emitting galaxies at $z\approx$0.24 using a narrowband filter tuned with the redshifted line. The total sky area covered is 706 arcmin$^2$ within a redshift range from 0.234 to 0.252 ($δz$=0.018). This corresponds to a volume of 3.9$\times10^3$ Mpc$^3$ when $Ω_{\rm matter}=0.3$, $Ω_Λ=0.7$, and $H_{\mathrm{0}}$=70 km s$^{-1}$ Mpc$^{-1}$ are adopted. We obtain a sample of 348 H$α$ emitting galaxies whose observed emission-line equivalent widths are greater than 12 Å. We find an extinction-corrected H$α$ luminosity density of $10^{39.65^{+0.08}_{-0.12}}$ ergs s$^{-1}$ Mpc$^{-3}$. Using the Kennicutt relation between the H$α$ luminosity and star formation rate, the star formation rate density in the covered volume is estimated as $0.036^{+0.006}_{-0.012}$ $M_{\sun}$ yr$^{-1}$ Mpc$^{-3}$. This value is higher by a factor of 3 than the local SFR density.

astro-ph↗

arXiv2003

Subaru Deep Survey. IV. Discovery of a Large-Scale Structure at Redshift \simeq 5

We report the discovery of a large-scale structure of Lyman alpha emitters (LAEs) at z=4.86 based on wide-field imaging with the prime-focus camera (Suprime-Cam) on the Subaru telescope. We observed a 25' x 45' area of the Subaru Deep Field in a narrow band (NB, lambdaC=7126 A and FWHM=73 A) together with R and i'. We isolate from these data 43 LAE candidates down to NB=25.5 mag using color criteria. Follow-up spectroscopy of five candidates suggests the contamination by low-z objects to be ~ 20%. We find that the LAE candidates are clustered in an elongated region on the sky of 20 Mpc in width and 50 Mpc in length at z=4.86, which is comparable in size to present-day large-scale structures (we adopt H0=70 km/s/Mpc, Omega0=0.3, lambda0=0.7). This elongated region includes a circular region of 12 Mpc radius of higher surface overdensity (delta=2), which may be the progenitor of a cluster of galaxies. Assuming this circular region to be a sphere with a spatial overdensity of 2, we compare our observation with predictions by Cold Dark Matter models. We find that an Omega0=0.3 flat model with sigma8=0.9 predicts the number of such spheres consistent with the observed number (one sphere in our survey volume) if the bias parameter of LAEs is b \simeq 6. This value suggests that the typical mass of dark haloes hosting LAEs at z \simeq 5 is of the order of 10^12 Msolar. Such a large mass poses an interesting question about the nature of LAEs.

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arXiv2003

A Photometric and Spectroscopic Study of Dwarf and Giant Galaxies in the Coma Cluster - IV. The Luminosity Function

A large spectroscopic survey is constructed of galaxies in the Coma cluster. The survey covers a wide area (1 deg$^2$) to deep magnitudes (R ~ 19.5), covering both the core (high density) and outskirts (intermediate to low density) of the cluster. The spectroscopic sample consists of a total of 1191 galaxies, of which, 760 galaxies are confirmed members of the Coma cluster. A statistical technique is developed to correct the spectroscopic sample for incompletness. The corrected sample is then used to construct R-band luminosity function (LF) spanning a range of 7 magnitudes (-23 < M_R < -16) both at the core and outskirts of the cluster. Dependence of the LF on local environment in Coma is explored. The LFs are found to be the same, within the errors, between the inner and outer regions and close to those from recent measurements for field galaxies. The total B-band LF for the Coma cluster, fitted to a Schecter form is also measured and shows a dip at M(B) = -18 mag., in agreement with previous studies. The implications of this feature are discussed. The LF is studied in B-R color intervals and shows a steep faint-end slope for red (B-R > 1.35) galaxies, both at the core and outskirts of the cluster. This population of low luminosity red galaxies has a higher surface density than the blue (B-R < 1.35) star-forming population, dominating the faint-end of the Coma cluster LF. It is found that relative number of high surface brightness galaxies is larger at the cluster core, implying destruction of low surface brightness galaxies in dense core environment.

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arXiv2002

Narrow band imaging in [OIII] and Halpha to search for ICPNe in the Virgo cluster

We have identified intracluster planetary nebulae in a Virgo cluster core field by imaging with the Subaru Suprime-Cam through two narrow band filters centered at the redshifted wavelengths of the [OIII] $λ=5007$ Åand the H$α$ $λ= 6563$ Ålines; broad-band images in V and R were acquired to check for emission in the adjacent continuum. Emission line objects in Virgo are then selected from the two-color diagram ([OIII] -- Halpha) vs. ([OIII] -- (V+R)), which is calibrated using PNe in M84 (Jacoby et al. 1990). Using both [OIII] and Halpha allows us to distinguish bona-fide planetary nebulae from high redshift emission-line galaxies at the bright end of the [OIII] luminosity function. Spectroscopic observations of a subsample of these objects were made at the TNG and at the VLT, in a region around M84 and in an intracluster field respectively. The observations confirm the efficiency of the combined [OIII]+H$α$ imaging to identify true PNe. We also obtained the first spectrum of an intracluster PN which shows the [OIII] doublet with S/N > 10 and its Halpha emission. From the results based on the spectroscopic follow-up, we derive a lower limit to the fraction of the Virgo cluster light contributed by the intracluster stars at the surveyed position in the cluster core: it amounts to 10%.

astro-ph↗

arXiv2002

Candidates for Intracluster Planetary Nebulae in the Virgo Cluster based on the Suprime-Cam Narrow-Band Imaging in O[III] and Halpha

We have identified 38 candidates of intracluster planetary nebulae (ICPNe) in a $34'\times27'$ field in the core of the Virgo cluster based on the Suprime-Cam imaging through two narrow-band filters centered at the redshifted wavelengths of the [OIII]$λ=5007Å$ and the Halpha λ=6563Ålines. Broad-band images in V and R bands are used to check for any emissions in the adjacent continuum. We describe the method briefly and present the list of intracluster planetary nebulae candidates, together with their finding charts. The ICPN candidates show a highly inhomogeneous distribution, which may suggest an association with the M86-M84 subcluster. Fraction of diffuse intracluster light with respect to total light in galaxies is estimated to be about 10%, leading to an estimate of about 20% for the baryon fraction. Spectroscopic follow up and a wider survey are critical to reveal the nature of intracluster stellar population.

astro-ph↗

arXiv2002

Evolution of Elliptical Galaxies at z >~ 1 Revealed from a Large, Multicolor Sample of Extremely Red Objects

We study the evolution of elliptical galaxies at z >~ 1 on the basis of a sample of 247 Extremely Red Objects (EROs) with R-Ks >= 3.35 (AB) and Ks <= 22.1 (AB) constructed from BVRi'z'JHKs multicolor data of a 114 arcmin2 area in the Subaru/XMM Deep Survey Field. By fitting template spectra of old ellipticals (OEs) and young, dusty starbursts (DSs) to the multicolor data, we classify EROs into these two classes and estimate their redshifts. We find that 58% of the EROs in our sample belong to the OE class and that 24% of the OEs are fit by a spectrum having a disk component with the B-band bulge-to-total luminosity ratio of <= 0.9. We compare the observed surface density of OEs with predictions from passive evolution models. We then derive rest-frame B-band luminosity functions (LFs) of OEs in our sample at z=1-1.5 and 1.5-2.5. We find that the LF at z=1-1.5 roughly agrees with the LF of local ellipticals if a dimming of 1.3 mag from z=1.25 to the present epoch is assumed. On the other hand, the amplitude of the LF at z=1.5-2.5 is found to be lower than that of the local LF by a factor of ~ 3 over the whole range of magnitude observed. Taking account of a strong decrease in the number density of morphologically classified early-type galaxies at z >~ 1.5 found by several authors, we conclude that the majority of ellipticals seen at present have not established either a red color or a smooth 1/4-law profile before z ~ 1.5$. (abridged)

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arXiv2002

Searching for dark matter halos in the Suprime-Cam 2 sq deg field

We report the first result of weak gravitational lensing survey on a 2.1 sq deg Rc-band image taken with a wide field camera (Suprime-Cam) on the prime focus of 8.2 m Subaru Telescope. The weak lensing mass reconstruction is applied to the data to search for dark matter halos of cluster scale; M >= 10^14 solar mass. The reconstructed convergence field is divided by 1-sigma noise to obtain the signal-to-noise ratio map (S/N-map) of the detection. Local maxima and minima are searched on the S/N-map and the probability distribution function (PDF) of the peaks are created to compare with model predictions. We found excess over noise PDF created from the randomized realization on both positive and negative sides. Negative peaks imply the presence of voids in the dark matter distribution and this is the first report of the detection. Positive peaks, on the other hand, represent the dark matter halos and the number count of the halos on the 2.1 sq deg image is 4.9 +- 2.3 for S/N > 5 where the Gaussian smoothing radius of the convergence map is 1'. The result is consistent with the prediction assuming the Press-Schechter mass function and the NFW halo profile under the cluster normalized CDM cosmology. Although the present statistics is not enough due to the limited field of view, this work demonstrates that dark matter halo count via weak lensing is a promising way to test the paradigm of structure formation and cosmological model.

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arXiv2002

Subaru Deep Survey II. Luminosity Functions and Clustering Properties of Lya Emitters at z=4.86 in the Subaru Deep Field

We report on early results from deep and wide-field narrow-band imaging on a 543 arcmin^2 area of the Subaru Deep Field. We find 87 Lya emitters (LAEs) at z=4.86(+/-0.03) which are photometrically selected by a combination of two broad bands (R and i') and one narrow band (NB711; 7126A FWHM=73A). We derive the luminosity functions (LFs) of the LAEs at Lya luminosity and at UV-continuum (1700A) luminosity. The LFs show little evolution between z=3.4 and z=4.86 either in Lya or UV-continuum emission. The amplitude of the LAE LF tends to decline at the bright magnitudes more rapidly than that of the LBG LF at similar redshifts. We calculate the angular correlation function of our LAEs up to ~15 arcmin separations. The angular correlation w(theta) is found to increase with decreasing angular separations, showing a clear signal of clustering. It is also found that the distribution of LAEs shows a large density gradient with a scale of >~15 arcmin, which would indicate the existence of a large-scale structure of LAEs on >~20h^(-1) Mpc scales.We fit the observed correlation function by Aw theta^(-0.8) to find Aw=29arcsec^(0.8). The estimated correlation length is r0=3.5(+/-0.3)h^(-1)Mpc in comoving units (Omega_m=0.3 and Omega_Lambda=0.7), which is slightly larger than the value for z~4 LBGs with i'<26. We calculate the angular correlation function for two sub-samples of the 87 LAEs divided by Lya luminosity, UV-continuum luminosity, and Lya equivalent width (EW). The Lya bright sub-sample shows a larger correlation amplitude than the Lya faint sub-sample, while no significant difference is found for the sub-samples divided by UV-continuum luminosity or EW.

astro-ph↗

arXiv2001

Kinetic temperature gradient driven modes in inhomogeneous plasmas

New unstable temperature gradient driven modes in an inhomogeneous plasma are identified. These modes represent transient $ω\simeq k_{\Vert }v_{th}^{(e,i)}$ sound oscillations in magnetized plasma that are kinetically destabilized via Landau interactions. Electron and ion sound branches are unstable for large values of the Larmor radius parameter $% k_{\bot}ρ_{e,i}\gg 1,$ respectively. The instability occurs due to a specific plasma response that significantly deviates from Boltzmann distribution in the region $k_{\bot}ρ_{i,e}\gg 1$ . Pacs: 52.35 Kt, 52.35 Qz

physics.plasm-ph↗

arXiv2001

A photometric and spectroscopic study of dwarf and giant galaxies in the Coma cluster -- III. Spectral ages and metallicities

(abr.) We present the analysis of the spectroscopic catalog of galaxies in the Coma cluster from Mobasher et al. This catalog comprises ~300 spectra of cluster members with magnitudes M_B=-20.5 to -14 in two areas of ~1 X 1.5 Mpc towards the center and the SW region of the cluster. In this paper the analysis is restricted to the 257 galaxies with no emission lines in their spectra. The strength of the age-sensitive indices is found to correlate with galaxy magnitude over the whole magnitude range. Similarly, the metallicity-sensitive indices anticorrelate with magnitude. By comparing the observed indices with model grids based on the Padova isochrones, we derive luminosity-weighted ages and metallicities. We present the distributions of ages and metallicities for galaxies in various magnitude bins. The mean metallicity decreases with galaxy magnitude and, at a given luminosity, appears to be generally lower for galaxies in the SW region of Coma as compared to the center of the cluster. A broad range of ages, from younger than 3 Gyr to older than 9 Gyr, is found in galaxies of any magnitude. However, systematic trends of age with luminosity are present among galaxies in the central field. In the central Mpc of Coma, a large fraction of galaxies at any luminosity (50-60% of the giants, >30 % of the dwarfs) show no evidence in their central regions of star formation occurred at redshift z<2, while the proportion of galaxies with significant star formation occurring at intermediate (0.35<z<2) and low (z<0.35) redshifts is found to depend on galaxy luminosity. Faint galaxies with young luminosity-weighted ages appear to have a bimodal metallicity distribution that, if confirmed, would point to a composite formation scenario. Finally, we present an interpretation of the index-magnitude relations observed.

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arXiv2001

Ages of S0 and elliptical galaxies in the Coma cluster

The ages of stellar populations in 52 elliptical and S0 galaxies in the Coma cluster are investigated using a new spectroscopic sample. More than 40% of the S0s are found to have experienced recent star formation in their central regions during the last ~5 Gyrs, while such activity is absent in the ellipticals. Galaxies in this sample have absolute magnitudes in the range -20.5 < M_B < -17.5, and the fraction of S0 galaxies with recent star formation is higher at fainter luminosities. The observed luminosity range of S0 galaxies with signs of recent star formation activity is consistent with them being the descendants of typical star-forming spirals at intermediate redshift whose star formation has been halted as a consequence of the dense environment.

astro-ph↗

arXiv2001

A photometric and spectroscopic study of dwarf and giant galaxies in the Coma cluster - II. Spectroscopic observations

This is the second paper in a series studying the photometric and spectroscopic properties of galaxies of different luminosities in the Coma cluster. The sample selection, spectroscopic observations and completeness functions are presented here. To study the spectral properties of galaxies as a function of their local environment, two fields were selected for spectroscopic observations to cover both the core (Coma1) and outskirts (ie. south-west of the core and centered on NGC4839)- (Coma3) of the cluster. Medium resolution spectroscopy (6-9 Å) was carried out for a total of 490 galaxies in both fields (302 in Coma1 and 188 in Coma3), using the WYFFOS multi-fiber spectrograph on the William Herschel Telescope. The galaxies cover a range of $12 < R < 20$, corresponding to -23 < M_R < -15 (H0=65 km/sec/Mpc). The redshifts are measured with an accuracy of 100 km/sec. The spectral line strengths and equivalent widths are also measured for the same galaxies and analysed in Poggianti et al (2001- paper III). A total of 189 (Coma1) and 90 (Coma3) galaxies are identified as members of the Coma cluster. An analysis of the colors show that only two members of the Coma cluster in our sample have B-R > 2. The completeness functions for the spectroscopic sample is presented.

astro-ph↗

arXiv2000

Deep-Imaging Observations of a Candidate of an Absorbed QSO at z=0.653, AX J131831+3341

The results of deep-imaging observations of a candidate of an absorbed QSO at z=0.653, AX J131831+3341, are presented. AX J131831+3341 was found during the course of optical follow-up observations of the ASCA Large Sky Survey, and has an X-ray luminosity of 10^{45} erg s^{-1} (2-10keV), which corresponds to those of QSOs. Its optical spectrum shows no significant broad H_beta emission line, suggesting that the object is an absorbed QSO. Deep R and V band images reveal the presence of a point-like nucleus and an asymmetric extended component. The nuclear component has a blue color, and the optical magnitude is much fainter than that expected from the observed X-ray flux for typical type-1 AGNs. These photometric properties and the presence of broad MgII 2800A emission can be explained simultaneously if the observed nuclear light is dominated by scattered nuclear light, though there is a possibility that the nuclear component is a slightly absorbed nucleus if its intrinsic X-ray to optical flux ratio is the largest among X-ray selected AGNs. The size of the extended component, which is thought to be the host galaxy of the QSO, is larger than those of normal disk galaxies at z=0-0.75, and the absolute magnitude is similar to those of the brightest host galaxies of QSOs at redshifts smaller than 0.5. The V-R and R-I colors of the component are consistent with a 1 Gyr-old stellar population model without absorption.

astro-ph↗

arXiv2000

Optical Identification of the ASCA Large Sky Survey

We present results of optical identification of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources which were detected with the SIS in the 2-7 keV band above 3.5 sigma. The sources are identified with 30 AGNs, 2 clusters of galaxies, and 1 galactic star. Only 1 source is still unidentified. The flux limit of the sample corresponds to 1 x 10^{-13} erg s^{-1} cm^{-2} in the 2-10 keV band. Based on the sample, the paper discusses optical and X-ray spectral properties of the AGNs, contribution of the sources to the Cosmic X-ray Background, and redshift and luminosity distributions of the AGNs. An interesting result is that the redshift distribution of the AGNs suggests a deficiency of high-redshift (0.5 < z < 2) and luminous (Lx(2-10 keV) > 10^{44} erg s^{-1}) absorbed narrow-line AGNs (so called type 2 QSOs).

astro-ph↗

arXiv1998

The Morphology Dependence of Luminosity Segregation in the Coma Cluster

We carry out CCD photometry of galaxies in the 5.25 square region centered on Coma cluster down to $M_R=-16.0$, beyond the limit of conventional morphological classification. We use the angular two-point correlation function as well as radial profiles in order to characterize the luminosity segregation. We find strong luminosity segregation for our total sample over the magnitude range of $-20 \leq M_R \leq -16$, which is not entirely accounted for in terms of the morphology-density relation that is known to exist only for bright galaxies. We use a single consistent parameter, the degree of luminosity concentration, to parameterize the morphology of galaxies over the wide magnitude range, where both giant and dwarf galaxies are included. Galaxies with high central concentration (HCC) show strong luminosity segregation, i.e. their clustering strength depends strongly on luminosity while those with low central concentration (LCC) show almost no luminosity segregation. Radial density profile shows that brighter HCC-type galaxies tend to more strongly concentrate near the cluster center while LCC-type galaxies do not show such a dependence on luminosity. We show that these results are tenable against the contamination by field galaxies and uncertainties in our method of classification.

astro-ph↗

arXiv1998

Optical Identification of the Hardest X-ray Source in the ASCA Large Sky Survey

We report the optical identification of the hardest X-ray source (AX J131501+3141) detected in an unbiased wide-area survey in the 0.5--10 keV band, the ASCA Large Sky Survey. The X-ray spectrum of the source is very hard and is well reproduced by a power-law component (Gamma = 1.5^+0.7_-0.6) with N_H = 6^+4_-2 *10^22 cm^-2 (Sakano et al. 1998). We have found a galaxy with R=15.62 mag near the center of the error circle for the X-ray source. The optical spectrum of the galaxy shows only narrow emission lines whose ratios correspond to those of a type 2 Seyfert galaxy at z = 0.072, implying an absorption-corrected X-ray luminosity of 2*10^43 erg sec^-1 (2--10 keV) and M_B = -20.93 mag. A radio point source is also associated with the center of the galaxy. We thus identify the X-ray source with this galaxy as an obscured AGN. The hidden nature of the nucleus of the galaxy in the optical band is consistent with the X-ray spectrum. These results support the idea that the obscured AGNs/QSOs contribute significantly to the cosmic X-ray background in the hard band at the faint flux level.

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