Faculty Profiles - TOMITA Akihiko

Published Papers - TOMITA Akihiko

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Photometric properties of Kiso ultraviolet-excess galaxies in the Lynx-Ursa Major region

TT Takeuchi, A Tomita, K Nakanishi, TT Ishii, Iwata, I, M Saito

ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES ( UNIV CHICAGO PRESS ) 121 ( 2 ) 445 - 472 1999.04 [Refereed]

We have performed a systematic study of several regions in the sky where the number of galaxies exhibiting star formation (SF) activity is greater than average. We used Kiso ultraviolet-excess galaxies (KUGs) as our SE-enhanced sample. By statistically comparing the KUG and non-RUG distributions, we discovered four RUG-rich regions with sizes of similar to 10 degrees x 10 degrees. One of these regions corresponds spatially to a filament of length similar to 60 h(-1) Mpc in the Lynx-Ursa Major region (alpha similar to 9(h)-10(h), delta similar to 42 degrees-48 degrees). We call this "the Lynx-Ursa Major (LUM) filament." We obtained V(RI)(C) surface photometry of 11 of the KUGs in the LUM filament and used these to investigate the integrated colors, the distribution of SF regions, the morphologies, and the local environments. We found that these KUGs consist of distorted spiral galaxies and compact galaxies with blue colors. Their star formation occurs in the entire disk and is not confined to just the central regions. The colors of the SF regions imply that active star formation in the spiral galaxies occurred 10(7-8) yr ago, while that in the compact objects occurred 10(6-7) yr ago. Though the photometric characteristics of these KUGs are similar to those of-interacting galaxies or mergers, most of these KUGs do not show direct evidence of merger processes.

DOI

Vertical motion of molecular and ionized gas in nearby face-on galaxies

S Sakamoto, A Tomita

NEW PERSPECTIVES ON THE INTERSTELLAR MEDIUM ( ASTRONOMICAL SOC PACIFIC ) 168 283 - 286 1999

We have made high-resolution (CO)-C-12 J=1-0 images of NGC 628 (i = 6 degrees, D = 10 Mpc) to study the vertical motions of molecular gas, and high-dispersion scanning-slit spectroscopy of H alpha emission in NGC 628 and NGC 3938 (i = 10 degrees, D = 10 Mpc) to trace the vertical motions of ionieed gas. Supplemental images in narrow-band H alpha and J- and K' bands were also obtained.
The vertical velocity dispersion of molecular gas is 6.4 +/- 1.5 km s(-1) throughout the inner (0.5-6 kpc) disk without any systematic radial variation. The vertical velocity dispersion of molecular gas at 5 kpc from the center is smaller than that of atomic gas at the same distance by a factor of similar to 1.5 This difference naturally explains the fact that molecular disks in spiral galaxies are generally thinner than atomic gas disks by a factor of similar to 1.5 Although there is a significant point-to-point variation of the velocity dispersion, no simple correlations with CO brightness (gas surface density), K'-band brightness (stellar gravitational potential), or Her brightness (star-formation activity) are observed. There is no high-velocity (\Delta v(perpendicular to)\ > 30 km s(-1)) molecular gas detected with our 5 sigma sensitivity of similar to 5 X 10(5) M. per 680 pc beam. This suggests that disk-halo connection of ISM in the form of molecular gas is a rare phenomenon.
In contrast, high-dispersion H alpha spectroscopy of NGC 628 and NGC 3938 revealed that there are hints of peculiar vertical motion (\Delta v(perpendicular to)\ > 30 km s(-1)) of ionized gas probably related with disk-halo connection of the ISM. The regions with peculiar motions are associated with bright HII regions and may originate from expanding motions associated with massive star formation activity.

Star Formation Activity Depending on Size of Galaxy

Akihiko Tomita, Tsutomu T. Takeuchi, Hiroyuki Hirashita

Star Formation 1999, ed. Taishi Nakamoto (Nobeyama Radio Observatory) 50 - 51 1999

Photometric observations of star formation activity in early-type spiral galaxies

T Usui, M Saito, A Tomita

ASTRONOMICAL JOURNAL ( UNIV CHICAGO PRESS ) 116 ( 5 ) 2166 - 2176 1998.11 [Refereed]

We observationally study the current star formation activities of early-type spiral galaxies. We construct a complete sample of 15 early-type spiral galaxies having ratios of far-infrared (FIR) to optical B-band luminosity, log (L-FIR/L-B), larger than average for this type and present CCD imaging of the R and H alpha bands. The equivalent widths of H alpha emission increase with increasing L-FIR/L-B indicating that log (L-FIR/L-B) can be an indicator of star formation for such early-type spiral galaxies with star formation activities higher than average. For all of the observed early-type spiral galaxies, the extended H II regions exist at the central regions with some asymmetric features. H alpha emission is more concentrated to the galactic center than the R-band light, and the degree of the concentration increases with the star formation activity. We also analyze the relation between the star formation activities and the existence of companion galaxies in the sample galaxies and other bright early-type spiral galaxies. No correlation is found; this suggests that the interaction is not responsible for all of the star formation activities of early-type spiral galaxies.

Nuclear-to-disk rotation curves of galaxies in the H alpha and [N II] emission lines

Y Sofue, A Tomita, Y Tutui, M Honma, Y Takeda

PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN ( OXFORD UNIV PRESS ) 50 ( 5 ) 427 - 463 1998.10 [Refereed]

We have obtained optical CCD spectroscopy along the major axes of 22 nearby spiral galaxies of Sb and Sc types in order to analyze their rotation curves. By subtracting the stellar continuum emission, we have obtained position-velocity (PV) diagrams of the H alpha and [N II] lines. We point out that the H alpha line is often superposed by a broad stellar absorption feature (Balmer wing) in the nuclear regions, and, therefore, the [N II] line is a better tracer of kinematics in the central few hundred parsec regions. By applying the envelope-tracing technique to the H alpha and [N II] PV diagrams, we have derived nucleus-to-disk rotation curves of the observed galaxies. The rotation curves rise steeply within the central few hundred parsecs, indicating a rapidly rotating nuclear disk and mass concentration near the nucleus.

DOI

Hot velocity fields of H II regions in nearby dwarf irregular galaxies

A Tomita, K Ohta, K Nakanishi, TT Takeuchi, M Saito （Part： Lead author )

ASTRONOMICAL JOURNAL ( UNIV CHICAGO PRESS ) 116 ( 1 ) 131 - 145 1998.07 [Refereed]

We present H alpha velocity fields of 13 giant H II regions in four nearby dwarf irregular galaxies: NGC 2366, Holmberg II, IC 2574, and WLM (the Wolf-Lundmark-Melotte system). We classify the velocity features, as well as the morphologies, of the H II regions. The H alpha velocity features are divided into three categories; three H II regions show chaotic features with a typical scale of variation of a few hundred parsecs in size and a few times 10 km s(-1) in velocity, one region exhibits an expanding-bubble feature, and the remaining nine have calm velocity fields. There is a correlation between the H alpha velocity features and the morphology of the H II regions. We measured the bulk motion of the H II regions relative to the ambient H I velocity for the II Ir regions with calm velocity fields and found a typical velocity difference of about 5 km s(-1). We discuss a model for the origin of star-forming regions based on the presence of the velocity difference between H alpha and H I gas, as well as the H I characteristics.

Nuclear rise of rotation curves of galaxies

Y Sofue, Y Tutui, M Honma, A Tomita

CENTRAL REGIONS OF THE GALAXY AND GALAXIES ( SPRINGER ) 184 51 - 52 1998

Nuclear rotation curves of galaxies in the co-line emission

Y Sofue, Y Tutui, M Honma, A Tomita

ASTRONOMICAL JOURNAL ( AMER INST PHYSICS ) 114 ( 6 ) 2428 - 2436 1997.12 [Refereed]

We have obtained high-resolution position-velocity (PV) diagrams along the major axes of the central regions of nearby galaxies in the CO-line emission using the Nobeyama 45 m telescope and the Millimeter Array. Nuclear rotation curves for 14 galaxies have been derived based on the PV diagrams using the envelope-tracing method. The nuclear rotation curves for most of the galaxies show a steep rise within a few hundred pc, which indicates a high-density concentration of mass. (C) 1997 American Astronomical Society. [S0004-6256(97)02512-0].

Characteristics of Kiso Ultraviolet Excess Galaxies

A Tomita, TT Takeuchi, T Usui, M Saito （Part： Lead author )

ASTRONOMICAL JOURNAL ( AMER INST PHYSICS ) 114 ( 5 ) 1758 - 1770 1997.11 [Refereed]

We examined the general characteristics of the Kiso Ultraviolet Excess Galaxies (KUGs). We present for the first time the quantitative expressions for the criteria of the KUGs; the boundary color separating the KUGs from the non-KUGs is (B - V)(T) = 0.74 and the KUG degrees of UV strength are found to correlate with the mean (B - V)(T) colors. We investigate the nature of the KUGs, a sample of blue galaxy population, and show that (1) about a half of the KUGs are spiral galaxies with Sb to Scd, (2) the KUGs are biased to late-type galaxies and include early-type galaxies with young star populations, and (3) the KUGs are preferably found among less luminous galaxies with L-B<10(10)L.. The KUGs also contain the post-starburst galaxies, many of which are found among the blue galaxy population at intermediate redshifts. The analysis of the far-infrared data shows that a typical present-to-past star formation rate for a KUG is 0.4. (C) 1997 American Astronomical Society.

Catalog of Galaxies and Galaxy Candidates behind the Milky Way Vol.3 : Galactic Longitude 8 degree to 43 degree, Galactic Latitude -17 degree to 17 degree

Adel T. Roman, K. Nakanishi, A. Tomita, M. Saito

Contributions from the Department of Astronomy Kyoto University ( 570 ) 1997

A search for galaxies behind the Milky Way at Aquila and Sagittarius

AT Roman, K Nakanishi, A Tomita, M Saito

Publications of the Astronomical Society of Japan ( OXFORD UNIV PRESS ) 48 ( 5 ) 679 - 690 1996.10 [Refereed]

We searched for galaxies using Schmidt atlases in a Milky Way region of about 800 square degrees around l similar or equal to 8 degrees to 43 degrees and b similar or equal to -17 degrees to 17 degrees. The surveyed region is located between the northern Local void and the Ophiuchus void. We detected about 5300 galaxy candidates, which have galaxy-like images with sizes larger than 0'.1. Of these, only 30 have been given in galaxy catalogs and another 74 are associated with IRAS point sources containing 18 IRAS galaxies with known redshifts. We made a catalog of detected objects in which the position in (alpha, delta) and (l, b), size, and features of image and multiplicity are given. The sky distribution of detected objects shows the presence of two clusters of galaxies and a group of nearby galaxies located around l = 33 degrees, b = -15 degrees with cz = 1500 km s(-1).

DOI

Distribution of blue galaxies in a merging cluster of galaxies Abell 168

A Tomita, FE Nakamura, T Takata, K Nakanishi, M Takeuchi, K Ohta, T Yamada （Part： Lead author )

The Astronomical Journal ( IOP PUBLISHING LTD ) 111 ( 1 ) 42 - 52 1996.01 [Refereed]

The fraction of-star-forming galaxies in rich clusters of galaxies increases rapidly with the redshift. This is interpreted as the result of a rapid evolution of cluster galaxies, though its mechanism is not yet dear. One hypothesis is that if galaxies run into the dense intracluster medium (ICM) regions, starbursts can be induced due to the compression of molecular clouds in the galaxies by a raised ''external'' ICM pressure. In a merging cluster, there would be dense ICM regions and some galaxies may experience a rapid increase of external pressure. Thus, if the mechanism works, starbursts would occur in such galaxies. Such a situation is considered to be realized in Coma cluster, and galaxies showing a recent starburst have a characteristic spatial distribution; they populate an elongated region between two subclusters. We examined the above hypothesis by measuring the spatial distribution of blue galaxies, regarded as star-forming galaxies in Abell 168 to check whether a case Like Coma is realized in this recently merging cluster. However, we could not find an enhanced blue-galaxy distribution between two subclusters. We discuss the causes of our result, including the possibilities of surviving the hypothesis. (C) 1996 American Astronomical Society.

DOI

Post Starburst Nucleus of NGC 5195

Toru Yamada, Akihiko Tomita

Ground-Based Astronomy in Asia, 3rd East-Asian Meeting on Astronomy, ed. N. Kaifu (NAOJ, Tokyo) 268 - 271 1996

A variation in the present star formation activity of spiral galaxies

Akihiko Tomita, Yoshio Tomita, Mamoru Saito

Publications of the Astronomical Society of Japan ( Astronomical Society of Japan ) 48 ( 2 ) 285 - 303 1996

The star formation rate in spiral galaxies is considered to be continuously decreasing with time on a time scale of 109 yr. The present star formation activity, on the other hand, occurs in molecular clouds with a time scale of 107 yr, and shows various degrees among galaxies. We have made a new data set of 1681 nearby spiral galaxies from available databases, and have studied the statistics of the present star formation activity. We analyzed the far-infrared and optical B-band surface brightnesses of the H II regions and the non-H II regions in M31, and have shown that the far-infrared-to-optical B-band flux ratio (fFIR/fB) is a useful indicator of the present star formation activity of spiral galaxies. For the sample galaxies, we made a distribution diagram of log(fFIR/fB) versus log LB for each morphological type. The distribution of fFIR/fB has a dispersion of one to two orders of magnitude, even within the same morphological type of galaxies, implying that the star formation activities of spiral galaxies change discontinuously within a short time scale. After analyzing the log(fFIR/fB) versus log LB correlation, we suggest that the most active star formation in galaxies does not continue longer than 108 yr. We also constructed a universal histogram of log(fFIR/fB) for each morphological type. The earlier-type spirals tend to show a larger variation in the present star formation activity. We discuss the correlation between the present star formation activity and the structures and environments of galaxies. We suggest that a short-term variation occurs primarily due to internal processes which may change with the morphological type of galaxies.

DOI

A variation in the present star formation activity of spiral galaxies

Akihiko Tomita, Yoshio Tomita, Mamoru Saito （Part： Lead author )

Publications of the Astronomical Society of Japan ( Astronomical Society of Japan ) 48 ( 2 ) 285 - 303 1996 [Refereed]

DOI

A Nuclear Ring in the Sa(r!) Galaxy NGC7742

K. Wakamatsu, M. Hamabe, M.T. Nishida, A. Tomita

The Astronomical Society of the Pacific Conference Series(Barred Galaxies, IAU Colloquium No.157) 91 83 - 85 1996

CO OBSERVATIONS OF THE HIGH-REDSHIFT RADIO GALAXY 53W002

T YAMADA, K OHTA, A TOMITA, T TAKATA

ASTRONOMICAL JOURNAL ( AMER INST PHYSICS ) 110 ( 4 ) 1564 - 1572 1995.10 [Refereed]

We observed a high redshift radio galaxy 53W002 at z=2.390 with the Nobeyama 45 m telescope aiming at a detection of a redshifted (CO)-C-12 (J=1-0) emission line. The galaxy was discovered in the Leiden Berkeley Deep Survey and is known to have a blue SED and is a candidate for a genuinely young galaxy. We detected a weak (similar to 5 mJy) emission-line feature at z=2.392; the feature was significant in our November 1993 observations and marginally confirmed in the follow-up observations held in December 1993 and February 1994. If the detected emission-line feature is really a redshifted CO emission line associated with 53W002, its luminosity is 1.2x10(11) h(-2) K km s(-1) pc(2) (q(0)=0.5), or, H(H-2)similar to 5 X 10(11) h(-2) H-circle dot, adopting the galactic CO-to-H-2 conversion factor. (C) 1995 American Astronomical Society.

DOI

NGC-7217 - A SPHEROID-DOMINATED, EARLY-TYPE RESONANCE RING SPIRAL GALAXY

R BUTA, W VANDRIEL, J BRAINE, F COMBES, K WAKAMATSU, Y SOFUE, A TOMITA

ASTROPHYSICAL JOURNAL ( UNIV CHICAGO PRESS ) 450 ( 2 ) 593 - 615 1995.09 [Refereed]

NGC 7217 is a well-known northern spiral galaxy which is characterized by flocculent spiral structure and a series of three optical ringlike zones: a nuclear ring 21 '' in diameter, a weak inner ring 63 '' in diameter, and a striking outer ring 2.'6 in diameter. The rings all have nearly the same shape and position angle in projection. The appearance of the galaxy suggests that it may be more axisymmetric than the typical spiral galaxy, since there is little evidence for the presence of a bar, oval, or stellar density wave. This makes the origin of the ring features uncertain. In an effort to understand this kind of ringed galaxy, which is by no means typical, we have obtained multicolor CCD BVRI images, accurate surface photometry, mappings of the CO and HI gas distributions, and rotational velocities from H alpha and HI spectral line data.
Our deep surface photometry has revealed an important feature of NGC 7217 that was missed in previous studies: The region occupied by the rings of the galaxy is surrounded-by an extensive, nearly circular luminous halo. This halo cannot be merely an extension of the disk component because it is much rounder than the inner regions. Instead, we believe the light represents either the outer regions of the bulge or a separate stellar halo component. We are able to successfully model the luminosity profile in terms of an r(1/4) ''spheroid'' and an exponential disk with a spheroid-to-total disk (including rings) luminosity ratio of 2.3-2.4. This makes NGC 7217 one of the most spheroid-dominated spirals known, and the finding has important implications for the recent discovery by Merrifield and Kuijken of a significant population of counter-rotating stars in the galaxy. Although the spiral structure of NGC 7217 is flocculent in blue light, there is a definite two-armed stellar spiral in the region of the outer ring. This ring includes about 4.4% of the total blue luminosity and is the locus of most of the recent star formation in the galaxy.
The ring is also where we find the HI gas to be concentrated. The galaxy is very gas poor (M(HI) = L(B)(0) = 0.024 M.IL.,(B)) for its morphological type. The HI rotational velocities agree well with published and our new H alpha-values.
Fourier analysis reveals a very weak possible oval distortion in the stellar mass distribution. Using the I-band light distribution to define the potential, we carried out simulations of gas streaming with no self-gravity. A model with a bulge-to-disk mass ratio of 2.4 reproduces the observed optical ring morphology very well. This suggests to us that in spite of the extreme weakness of the observed nonaxisymmetry of this galaxy, this nonaxisymmetry is still sufficient to torque the gas into the usual resonance rings identified in other, more obviously barred galaxies.
An additional noteworthy feature that we have identified in a B-I color index map is a symmetric, nuclear dust ring 17 '' in angular diameter. Other dust lanes are seen mainly on the near side of the galaxy.

POSSIBLE DETECTION OF A CO EMISSION-LINE FROM A QUASAR AT Z=2.88

T YAMADA, K AOKI, A TOMITA, T TAKATA, K OHTA, M YOSHIDA

ASTROPHYSICAL JOURNAL ( UNIV CHICAGO PRESS ) 438 ( 1 ) L5 - L7 1995.01 [Refereed]

We observed a high-redshift luminous radio-quiet quasar Q1235+0857 at z = 2.880 with the Nobeyama 45 m radio telescope aiming at detection of a redshifted (CO)-C-12 (J = 3-2) emission line. We have marginally (3-5 sigma significance) detected an emission-line-like feature at similar to 89.8 GHz (z = 2.883) with similar to 150 km s(-1) width (FWHM). Adopting the standard CO to H-2 conversion factor obtained for galactic giant molecular clouds, the CO luminosity can be roughly converted to H-2 gas mass of similar to 10(12) M., which is much larger than the molecular gas content of any galaxies at preent epoch and nearly equal to the total stellar mass of a giant elliptical galaxy. Although we should note that a confirmation of the detection by an independent facility is absolutely needed, the results discussed in this Letter would support the idea that luminous quasars at high redshift are associated with the massive galaxies in their forming phase.

DOI

RADIAL-VELOCITY DISTRIBUTION OF THE GALAXIES IN THE PUPPIS HIDDEN CONCENTRATION BEHIND THE MILKY-WAY

T YAMADA, A TOMITA, M SAITO, P CHAMARAUX, KAZES, I

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY ( WILEY-BLACKWELL ) 270 ( 1 ) 93 - 105 1994.09 [Refereed]

We investigate the radial velocity distribution of the galaxies in the Puppis region behind the Milky Way around (l, b) approximately (245-degrees, 0-degrees), where a concentration of galaxies was recently recognized through the systematic search for galaxies behind the zone of avoidance. Using the lower limit sample of the 60-mum flux-limited sample of IRAS galaxies brighter than f60 = 0.6 Jy, we find a large nearby clustering of galaxies at about 20 h-1 Mpc, whose peak spatial density at 7.5 h-1 Mpc scale is at least twice the whole-sky average. This Puppis concentration is probably associated with the S1 supercluster at (l, b) = (220-degrees, -15-degrees) detected in the QDOT survey, and this association is likely to be comparable to other nearby superclusters such as the Virgo, the Hydra, the Centaurus and the Fornax-Eridanus superclusters. Consequently, the effect of the Puppis concentration on the peculiar motions of the Local Group and other nearby galaxies should be considerable. There is no prominent individual cluster in the Puppis region, however, although some galaxies are concentrated into the regions around (l, b) = (245-degrees, -7-degrees) and (237-degrees, -15-degrees); the richness of these individual clusters in IRAS galaxies is as large as that of the Fornax cluster, and perhaps half or more of that of the Virgo cluster. We also study the radial velocity distribution of the galaxies selected by an optical limiting diameter, although uncertainty in the selection is large because of galactic extinction. The distribution of these diameter-selected galaxies shows good agreement with that of the IRAS-selected ones.

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Published Papers - TOMITA Akihiko