arXiv subjects

Tomotsugu Goto

Publications and source records attributed to Tomotsugu Goto.

At least 181 records · Page 10Linked to original sources

arXiv2008

Spatially-Resolved Medium Resolution Spectroscopy of an Interacting E+A (post-starburst) System with the Subaru telescope

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$δ$ equivalent width (EW) of the E+A galaxy is greater than 7Ågalaxy wide (8.5 kpc). The E+A galaxy have a weak [OIII] emission (EW$\sim$1Å) 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.

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arXiv2008

Timeline analysis and wavelet multiscale analysis of the AKARI All-Sky Survey at 90 micron

We present a careful analysis of the point source detection limit of the AKARI All-Sky Survey in the WIDE-S 90 $μ$m band near the North Ecliptic Pole (NEP). Timeline Analysis is used to detect IRAS sources and then a conversion factor is derived to transform the peak timeline signal to the interpolated 90 $μ$m flux of a source. Combined with a robust noise measurement, the point source flux detection limit at S/N $>5$ for a single detector row is $1.1\pm0.1$ Jy which corresponds to a point source detection limit of the survey of $\sim$0.4 Jy. Wavelet transform offers a multiscale representation of the Time Series Data (TSD). We calculate the continuous wavelet transform of the TSD and then search for significant wavelet coefficients considered as potential source detections. To discriminate real sources from spurious or moving objects, only sources with confirmation are selected. In our multiscale analysis, IRAS sources selected above $4σ$ can be identified as the only real sources at the Point Source Scales. We also investigate the correlation between the non-IRAS sources detected in Timeline Analysis and cirrus emission using wavelet transform and contour plots of wavelet power spectrum. It is shown that the non-IRAS sources are most likely to be caused by excessive noise over a large range of spatial scales rather than real extended structures such as cirrus clouds.

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arXiv2008

A Catalog of Local E+A(post-starburst) Galaxies selected from the Sloan Digital Sky Survey Data Release 5

E+A galaxies have been interpreted as post-starburst galaxies based on the presence of strong Balmer absorption lines combined with the absence of major emission lines ([OII] nor H$α$). As a population of galaxies in the midst of the transformation, E+A galaxies has been subject to an intense research activity. It has been, however, difficult to investigate E+A galaxies statistically since E+A galaxies are an extremely rare population of galaxies ($<$1% of all galaxies in the local Universe). Here, we present a large catalog of 564 E+A (post-starburst) galaxies carefully selected from half million spectra of the Sloan Digital Sky Survey Data Release 5. We define E+A galaxies as those with H$δ$ equivalent width $>$5Åand no detectable emission in [OII] $and$ H$α$. The catalog contains 564 E+A galaxies, and thus, is one of the largest of the kind to date. In addition, we have included the H$α$ line in the selection to remove dusty star-forming galaxies, which could have contaminated previous [OII]-based samples of E+A galaxies up to 52%. Thus, the catalog is one of the most homogeneous, let alone its size. The catalog presented here can be used for follow-up observations and statistical analyses of this rare population of galaxies.

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arXiv2008

Spatially Resolved Spectroscopy of Passive Spiral Galaxies

Passive spiral galaxies, despite their spiral morphological appearance, do not have any emission lines indicative of ongoing star formation in their optical spectra. Previous studies have suggested that passive spiral galaxies preferentially exist in infall regions of galaxy clusters, suggesting that the cluster environment is likely to be responsible for creating these galaxies. By carrying out spatially resolved long-slit spectroscopy on four nearby passive spiral galaxies with the Apache Point Observatory 3.5-m telescope, we investigated the stellar populations of passive spiral galaxies separately for their inner and outer regions. In the two unambiguously passive spiral galaxies among the four observed galaxies, H$δ$ absorption lines are more prominent in the outer regions of the galaxies, whereas the 4000-Å breaks (D$_{4000}$) are strongest in the inner regions of the galaxies. A comparison with a simple stellar population model for the two passive spiral galaxies indicates that the outer regions of the galaxies tend to harbour younger populations of stars. The strong H$δ$ absorption observed in the outer regions of the sample galaxies is consistent with that of galaxies whose star formation ceased a few Gyrs ago. Because of the large uncertainty in the absorption indices in our samples, further observations are needed in order to place constraints on the mechanisms that quench star formation in passive spiral galaxies.

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arXiv2008

Integrated field spectroscopy of E+A (post-starburst) galaxies with the Kyoto3DII

We have performed a two-dimensional spectroscopy of three nearby E+A (post-starburst) galaxies with the Kyoto3DII integral field spectrograph. In all the cases, Hdelta absorption is stronger at the centre of the galaxies, but significantly extended in a few kpc scale. For one galaxy (J1656), we found a close companion galaxy at the same redshift. The galaxy turned out to be a star-forming galaxy with a strong emission in Hgamma. For the other two galaxies, we have found that the central post-starburst regions possibly extend toward the direction of the tidal tails. Our results are consistent with the merger/interaction origin of E+A galaxies, where the infalling-gas possibly caused by a galaxy-galaxy merging creates a central-starburst, succeeded by a post-starburst (E+A) phase once the gas is depleted.

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arXiv2007

The Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 um, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcmin, and the pixel pitch is matches the diffraction limit of the telescope. Derived point spread functions (PSFs) from observations of asteroids are similar to the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and its performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

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arXiv2007

The Far-Infrared Properties of Spatially Resolved AKARI Observations

We present the spatially resolved observations of IRAS sources from the Japanese infrared astronomy satellite AKARI All-Sky Survey during the performance verification (PV) phase of the mission. We extracted reliable point sources matched with IRAS point source catalogue. By comparing IRAS and AKARI fluxes, we found that the flux measurements of some IRAS sources could have been over or underestimated and affected by the local background rather than the global background. We also found possible candidates for new AKARI sources and confirmed that AKARI observations resolved IRAS sources into multiple sources. All-Sky Survey observations are expected to verify the accuracies of IRAS flux measurements and to find new extragalactic point sources.

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arXiv2007

Abundance diagnosis of E+A (post-starburst) galaxies

E+A galaxies are characterized as a galaxy with strong Balmer absorption lines but without any [OII] or H$α$ emission lines. The existence of strong Balmer absorption lines means that E+A galaxies have experienced a starburst within the last $<$1-1.5 gigayear. However, the lack of [OII] or H$α$ emission lines indicates that E+A galaxies do not have any on-going star formation. Therefore, E+A galaxies are interpreted as a post-starburst galaxy. Morphologically, E+A galaxies appear as early-type galaxies, implying E+A galaxies may be one of the progenitors of present-day elliptical galaxies. However, there remained other possibilities such as the dusty starburst scenario, where E+A galaxies have on-going star formation, but optical emission lines are invisible due to the heavy obscuration by dust. Therefore, an additional evidence of the post-starburst phenomena has been eagerly awaited. Using one of the largest samples of 451 E+A galaxies carefully selected from the Sloan Digital Sky Survey Data Release 4, here we show the abundance diagnosis of E+A galaxies using Mg and Fe lines. Our findings are as follows :(i) E+A galaxies has enhanced $α$-element abundance ratio compared to the star-forming galaxies with similar Balmer absorption strength. Since the truncation of strong starburst is required to enhance the alpha element ratio, this is an additional evidence that E+A galaxies are in the post-starburst phase; (ii) the metallicity and $α$-element abundance of E+A galaxies are consistent with those of elliptical galaxies, suggesting that E+A galaxies could be one of the progenitors of present-day elliptical galaxies in terms of chemical abundances.

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arXiv2006

Deep J-band imaging of high redshift QSO candidates with the Himalayan Chandra Telescope

High redshift QSOs (redshift$>$5.7) are highly important objects. If such QSOs may be found, their spectra will reveal the onset of reionization of the intergalactic medium (Gunn-Peterson trough), and provide precious insights into the re-ionization epoch in the very early universe. Here we report our pilot attempt to follow-up high redshift QSOs with the Himalayan Chandra Telescope. Deep $J$-band imaging was performed on three high redshift QSO candidates color-selected from the SDSS, using the near-infrared imager. Although none of the targets turned out to be likely high redshift QSOs, careful data reduction shows that the data reach the required depth, proving that the Himalayan Chandra Telescope is a powerful tool to follow-up high redshift QSO candidates.

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arXiv2006

Discovery of a new high redshift QSO at z=5.96 with the Subaru telescope

We report a discovery of a new high redshift quasar at $z=5.96$, observed with the FOCAS long-slit spectrograph on board the Subaru telescope. The spectrum shows strong and broad Ly$α$+NV emission lines with a sharp discontinuity to the blue side. A Ly$β$+OVI emission line is also detected, providing a consistent redshift measurement with the Ly$α$+NV emission. The QSO has an absolute magnitude of $M_{AB,1450}=-26.9$ ($H_{0}=50$km s$^{-1}$ Mpc$^{-1}$, $q_0=0.5$). The spectrum shows significant flux in the region 8000-8300 $Å$ and thus does not show a complete Gunn-Peterson trough in the redshift range 5.58 to 5.82, along the line of sight to this $z=5.96$ QSO. Therefore the Universe was already highly ionized at $z=5.82$

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arXiv2006

Environmental Effects on Galaxy Evolution Based on the SDSS

We have constructed a large, uniform galaxy cluster catalog from the Sloan Digital Sky Survey data. By studying the morphology--cluster-centric-radius relation, we have found two characteristic environments where galaxy morphologies change dramatically, indicating there exist two different physical mechanisms responsible for the cluster galaxy evolution. We also found an unusual population of galaxies, which have spiral morphology but do not have any emission lines, indicating these spiral galaxies do not have any on-going star formation activity. More interestingly, these passive spiral galaxies preferentially exist in the cluster outskirts. Therefore, these passive spiral galaxies are likely to be a key galaxy population being transformed from blue, star-forming galaxies into red, early-type galaxies.

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arXiv2006

Post-starburst--AGN Connection: Spatially Resolved Spectroscopy of Hdelta-Strong AGNs

Ever since the co-existence of an active galactic nucleus (AGN) and a starburst was observationally discovered, there has been a significant controversy over whether there is a physical connection between starbursts and AGNs. If yes, it is a subject of interest to reveal which one triggers another. Here we bring a unique insight onto the subject by identifying 840 galaxies with both a post-starburst signature (strong Balmer absorption lines) and an AGN (based on the emission line ratio). These poststarburst-AGNs account for the 4.2% of all the galaxies in a volume-limited sample. The presence of a post-starburst phase with an active AGN itself is of importance, suggesting that AGNs may outlive starbursts in the starburst-AGN connection. In addition, we have performed spatially resolved spectroscopy of three of our poststarbusrst-AGN galaxies, obtaining some evidence that the post-starburst region is more extended, but sharply centred around the central AGN, confirming a spatial connection between the post-starburst and the AGN.

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arXiv2005

Morphological Classification of Galaxies using Photometric Parameters: the Concentration index versus the Coarseness parameter

We devise improved photometric parameters for the morphological classification of galaxies using a bright sample from the First Data Release of the Sloan Digital Sky Survey. In addition to using an elliptical aperture concentration index for classification, we introduce a new texture parameter, coarseness, which quantifies deviations from smooth galaxy isophotes. The elliptical aperture concentration index produces morphological classifications that are in appreciably better agreement with visual classifications than those based on circular apertures. With the addition of the coarseness parameter,the success rate of classifying galaxies into early and late types increases to \~88% with respect to the reference visual classification. A reasonably high success rate ~68% is also attained in classifying galaxies into three types, early-type galaxies (E+S0), early- (Sa+Sb) and late- (Sc+Sdm+Im) type spiral galaxies.

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arXiv2005

Are Passive Spiral Galaxies truly 'passive' and 'spiral'?: a Near-Infrared perspective

Passive spiral galaxies - unusual galaxies with spiral morphologies but without any sign of on-going star formation - have recently been shown to exist preferentially in cluster infalling regions. This discovery directly connects passive spiral galaxies to cluster galaxy evolution studies, such as the Butcher-Oemler effect or the morphology-density relation. Thus, detailed study of passive spiral galaxies could potentially yield new insight on the underlying physical mechanisms governing cluster galaxy evolution. However, in previous work, passive spiral galaxies were selected from low-resolution optical images with $\sim$1.5 arcsec seeing. Passive spirals could therefore be mis-identified as S0 galaxies, or as dusty-starburst galaxies which are not passive at all. To address this issue we performed deep, high-resolution, near-infrared imaging of 32 passive spiral galaxies with UKIRT.

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arXiv2005

Radial and 2D Colour Properties of E+A Galaxies

We investigated the radial colour gradient and two-dimensional (2D) colour properties of 22 E+A galaxies with 5.5 < Hdelta equivalent width(EW) < 8.5Åand 49 normal early-type galaxies as a control sample at a redshift of <0.2 in the Second Data Release of the Sloan Digital Sky Survey. We found that a substantial number of E+A galaxies exhibit positive slopes of radial colour gradient (bluer gradients toward the centre) which are seldom seen in normal early-type galaxies. We found irregular 'colour morphologies'-- asymmetrical and clumpy patterns -- at the centre of g-r and r-i 2D colour maps of E+A galaxies with positive slopes of colour gradient. Kolomogorov-Smirnov two-sample tests show that g-r and r-i colour gradient distributions of E+A galaxies differ from those of early-type galaxies with a more than 99.99 per cent significance level. We also found a tight correlation between radial colour gradients and colours, and between radial colour gradients and the 4000-Åbreak in the E+A sample; E+A galaxies which exhibit bluer colour or weaker D4000 tend to have positive slopes of radial colour gradient. We compared the GISSEL model and E+A observational quantities, Hdelta EW, D4000 and u-g colour, and found that almost all our E+A galaxies are situated along a single evolutionary track. Therefore, these results are interpreted as E+A galaxies evolving from Hdelta EW ~ 8 Åto Hdelta EW ~ 5 Å, with colour gradients changing from positive to negative, and with the irregular 2D colour map becoming smoother, during a time-scale of ~300 Myr. Our results favour the hypothesis that E+A galaxies are post-starburst galaxies caused by merger/interaction, having undergone a centralized violent starburst.

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arXiv2005

Total Galaxy Magnitudes and Effective Radii from Petrosian Magnitudes and Radii

Petrosian magnitudes were designed to help with the difficult task of determining a galaxy's total light. Although these magnitudes (taken here as the flux within 2R_P, with the inverted Petrosian index 1/eta(R_P)=0.2) can represent most of an object's flux, they do of course miss the light outside of the Petrosian aperture (2R_P). The size of this flux deficit varies monotonically with the shape of a galaxy's light-profile, i.e., its concentration. In the case of a de Vaucouleurs R^{1/4} profile, the deficit is 0.20 mag; for an R^{1/8} profile this figure rises to 0.50 mag. Here we provide a simple method for recovering total (Sersic) magnitudes from Petrosian magnitudes using only the galaxy concentration (R_90/R_50 or R_80/R_20) within the Petrosian aperture. The corrections hold to the extent that Sersic's model provides a good description of a galaxy's luminosity profile. We show how the concentration can also be used to convert Petrosian radii into effective half-light radii, enabling a robust measure of the mean effective surface brightness. Our technique is applied to the SDSS DR2 Petrosian parameters, yielding good agreement with the total magnitudes, effective radii, and mean effective surface brightnesses obtained from the NYU--VAGC Sersic R^{1/n} fits by Blanton et al. (2005). Although the corrective procedure described here is specifically applicable to the SDSS DR2 and DR3, it is generally applicable to all imaging data where any Petrosian index and concentration can be constructed.

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arXiv2005

Optical Properties of 4248 IRAS Galaxies

We have investigated optical properties of 4248 infrared galaxies (IRGs) by positionally matching data from the IRAS and the SDSS DR3. Our sample includes 3 hyper luminous infrared galaxies (L >10^13Lsun) and 178 ultra luminous infrared galaxies (10^12Lsun < L < 10^13Lsun). Our findings are as follows: (i) more IR luminous galaxies tend to have smaller local galaxy density, being consistent with the picture where luminous IRGs are created by the merger-interaction of galaxies that happens more often in lower density regions; (ii) the fractions of AGNs increase as a function of Lir; (iii) there is a good correlation between Lir and L[OIII] for AGNs, suggesting both of the parameters can be a good estimator of the total power of AGNs; (iv) a good correlation is found between Lir and optically estimated star formation rate (SFR) for star-forming galaxies, suggesting Lir is a good indicator of galaxy SFR. However, caution is needed when SFR is estimated using Lir, i.e., high SFR galaxies selected by Lir is frequently merger/interaction, whereas high SFR galaxies selected by optical emission is often normal spiral or Magellanic-cloud like irregular galaxies. (v) more IR luminous galaxies have slightly larger Halpha/Hbeta ratio; (vi) more IR luminous galaxies have more centrally-concentrated morphology, being consistent with the morphological appearance of galaxy-galaxy merger remnants. Optical image of ultra/very luminous infrared galaxies also show frequent signs of merger/interaction. (vii) comparison with the SED synthesis models indicate that majority of luminous infrared galaxies (L > 10^11Lsun) may be in a post-starburst phase, sharing a similar (but not the same) merger/interaction origin with E+A galaxies. (abridged)

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arXiv2005

266 E+A Galaxies Selected from the Sloan Digital Sky Survey Data Release 2: The Origin of E+A Galaxies

E+A galaxies are characterized as a galaxy with strong Balmer absorption lines but without any [OII] nor H$α$ emission lines. The existence of strong Balmer absorption lines indicates that E+A galaxies have experienced starburst within recent one Gyr. However, the lack of [OII] and H$α$ emission lines indicates that E+A galaxies do not have any on-going star formation. Therefore, E+A galaxies are interpreted as a post-starburst galaxy. For many years, however, it has been a mystery why E+A galaxies started starburst and why they quenched the star formation abruptly. Using one of the largest samples of 266 E+A galaxies carefully selected from the Sloan Digital Sky Survey Data Release 2, we have investigated the environment of E+A galaxies from 50 kpc to 8 Mpc scale, i.e., from a typical distance to satellite galaxies to the scale of large scale structures. We found that E+A galaxies have excess of local galaxy density only at a scale of $<100$ kpc (with a two $σ$ significance), but not at the cluster scale ($\sim$1.5 Mpc) nor in the scale of large scale structure ($\sim$8 Mpc). These results indicate that E+A galaxies are not created by the physical mechanisms associated with galaxy clusters or the large scale structure, but are likely to be created by the dynamical interaction with closely accompanying galaxies at a $<$100 kpc scale. The claim is also supported by the morphology of E+A galaxies. We have found that almost all E+A galaxies have a bright compact core, and that $\sim$30% of E+A galaxies have dynamically disturbed signatures or the tidal tails, being quite suggestive of morphological appearance of merger/interaction remnants.

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