250 research outputs found
What Controls the Star Formation in Luminous Starburst Mergers ?
In order to understand what controls the star formation process in luminous
starburst mergers (e.g., NGC 6240, Arp 220, and so on), we investigate
observational properties of two samples of high-luminosity starburst galaxies
mapped in CO(=1--0) independently using both the Owens Valley Radio
Observatory (Scoville et al. 1991) and the IRAM interferometer (Downes &
Solomon 1998). We find that the surface density of far-infrared luminosity,
(FIR), is proportional linearly to the H surface mass density,
(H), for the two samples; (FIR) (H) with a correlation coefficient of 0.96. It is
often considered that (FIR) provides a good measure of the star
formation rate per unit area, (SFR). It is also known that molecular
gas is dominated in circumnuclear regions in the luminous starburst mergers;
i.e., (gas) (H). Therefore, the above relationship
suggests a star formation law; (SFR) (gas). We suggest
that this star formation law favors the gravitational instability scenario
rather than the cloud-cloud collision one.Comment: 14 pages, 2 figures. The Astrophysical Journal (Letters), in pres
An Origin of the Huge Far-Infrared Luminosity of Starburst Mergers
Recently Taniguchi and Ohyama found that the higher CO to CO
integrated intensity ratios at a transition =1--0, CO)CO) , in a sample of starburst merging
galaxies such as Arp 220 are mainly attributed to the depression of CO
emission with respect to CO. Investigating the same sample of galaxies
analyzed by Taniguchi & Ohyama, we find that there is a tight, almost linear
correlation between the dust mass and CO luminosity. This implies that
dust grains are also depressed in the high- starburst mergers, leading to
the higher dust temperature () in them because of the relative
increase in the radiation density. Nevertheless, the average dust mass () of the high- starburst mergers is higher significantly than that of
non-high galaxies. This is naturally understood because the galaxy mergers
could accumulate a lot of dust grains from their progenitor galaxies together
with supply of dust grains formed newly in the star forming regions. Since
(FIR) given the dust emissivity law, , the increases in both and
explain well why the starburst mergers are so bright in the FIR. We discuss
that the superwind activity plays an important role in destroying dust grains
as well as dense gas clouds in the central region of mergers.Comment: 10 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for
publication in Astrophysical Journal Letter
Detection of Polarized Broad Emission in the Seyfert 2 Galaxy Mrk 573
We report the discovery of the scattered emission from a hidden broad-line
region (BLR) in a Seyfert 2 galaxy, Mrk 573, based on our recent
spectropolarimetric observation performed at the Subaru Telescope. This object
has been regarded as a type 2 AGN without a hidden BLR by the previous
observations. However, our high quality spectrum of the polarized flux of Mrk
573 shows prominent broad (~3000 km/s) H_alpha emission, broad weak H_beta
emission, and subtle Fe II multiplet emission. Our new detection of these
indications for the presence of the hidden BLR in the nucleus of Mrk 573 is
thought to be owing to the high signal-to-noise ratio of our data, but the
possibility of a time variation of the scattered BLR emission is also
mentioned. Some diagnostic quantities such as the IRAS color, the radio power,
and the line ratio of the emission from the narrow-line region of Mrk 573 are
consistent with the distributions of such quantities of type 2 AGNs with a
hidden BLR. Mrk 573 is thought to be an object whose level of the AGN activity
is the weakest among the type 2 AGNs with a hidden BLR. In terms of the
systematic differences between the type 2 AGNs with and without a hidden BLR,
we briefly comment on an interesting Seyfert 2 galaxy, Mrk 266SW, which may
possess a hidden BLR but has been treated as a type 2 AGNs without a hidden
BLR.Comment: 9 pages including 6 figures, to appear in The Astronomical Journa
VV 655 and NGC 4418: Implications of an interaction for the evolution of a LIRG
VV 655, a dwarf irregular galaxy with HI tidal debris, is a companion to the
lenticular luminous infrared galaxy (LIRG) NGC 4418. NGC 4418 stands out among
nearby LIRGs due to its dense central concentration of molecular gas and the
dusty, bi-polar structures along its minor axis suggestive of a wind driven by
a central starburst and possible nuclear activity. We seek to understand the
consequences of the ongoing minor interaction between VV 655 and NGC 4418 for
the evolution of the LIRG, including the origin of the gas supply responsible
for its unusual nuclear properties. We investigate the structural, kinematic,
and chemical properties of VV 655 and NGC 4418 by analyzing archival imaging
data and optical spectroscopic observations from the SDSS-III and new spectra
from SALT-RSS. We characterize their gas-phase metal abundances and spatially
resolved, ionized gas kinematics, and demonstrate that the gas-phase
metallicity in NGC 4418 significantly exceeds that in VV 655. No kinematic
disturbances in the ionized gas are observed along the minor axis of NGC 4418,
but we see evidence for ionized gas outflows from VV 655 that may increase the
cross-section for gas stripping in grazing collisions. A faint, asymmetric
outer arm is detected in NGC 4418 of the type normally associated with
galaxy-galaxy interactions. The simplest model suggests that the minor
interaction between VV 655 and NGC 4418 produced the unusual nuclear properties
of the LIRG via tidal torquing of the interstellar medium of NGC 4418 rather
than through a significant gas transfer event. In addition to inducing a
central concentration of gas in NGC 4418, this interaction also produced an
enhanced star formation rate and an outer tidal arm in the LIRG. The VV 655-NGC
4418 system offers an example of the potential for minor collisions to alter
the evolutionary pathways of giant galaxies.Comment: 9 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Antiferromagnetic order in the FFLO state
We investigate the antiferromagnetic (AF) order in the d-wave superconducting
(SC) state at high magnetic fields. A two-dimensional model with on-site
repulsion U, inter-site attractive interaction V and antiferromagnetic exchange
interaction J is solved using the mean field theory. For finite values of U and
J, a first order transition occurs from the normal state to the FFLO state,
while the FFLO-BCS phase transition is second order, consistent with the
experimental results in CeCoIn_5. Although the BCS-FFLO transition is
continuous, the Ne'el temperature of AF order is discontinuous at the phase
boundary because the AF order in the FFLO state is induced by the Andreev bound
state localized in the zeros of FFLO order parameter, while the AF order hardly
occurs in the uniform BCS state. The spatial structure of the magnetic moment
is investigated for the commensurate AF state as well as for the incommensurate
AF state. The influence of the spin fluctuations is discussed for both states.
Since the fluctuations are enhanced in the normal state for incommensurate AF
order, this AF order can be confined in the FFLO state. The experimental
results in CeCoIn_5 are discussed.Comment: Proceedings of LT25 conference (Amsterdam, 2008
Depression in Luminous Starburst Mergers
It is known that the class of luminous starburst galaxies tends to have
higher integrated line intensity ratios
() than normal spiral galaxies (). Since most previous studies
investigated only , it remains uncertain whether the luminous starburst
galaxies are overabundant in CO or underabundant in CO. Here we
propose a new observational test to examine this problem. Our new test is to
compare far-infrared luminosities [(FIR)] with those of CO and
and , respectively]. It is shown that there
is a very tight correlation between and L(FIR), as found in many
previous studies. However, we find that the CO luminosities of the
high-R galaxies are lower by a factor of three on the average than those
expected from the correlation for the remaining galaxies with ordinary
values. Therefore, we conclude that the observed high values for the
luminous starburst galaxies are attributed to their lower CO line
intensities.Comment: 9 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for
publication in Astrophysical Journal Letter
SDSSp J104433.04012502.2 at is Gravitationally Magnified by an Intervening Galaxy
During the course of our optical deep survey program on L emitters at
in the sky area surrounding the quasar SDSSp
J104433.04012502.2 at , we found that a faint galaxy with (AB)
is located at \timeform{1".9} southwest of the quasar. Its
broad-band color properties from to suggest that the galaxy is
located at a redshift of -- 2.5. This is consistent with no strong
emission line in our optical spectroscopy. Since the counter image of the
quasar cannot be seen in our deep optical images, the magnification factor
seems not to be very high. Our modest estimate is that this quasar is
gravitationally magnified by a factor of 2.Comment: 11 pages, 5 figures, PASJ, in pres
Optical Versus Mid-Infrared Spectroscopic Classification of Ultraluminous Infrared Galaxies
The origin of huge infrared luminosities of ultraluminous infrared galaxies
(ULIGs) is still in question. Recently, Genzel et al. made mid-infrared (MIR)
spectroscopy of a large number of ULIGs and found that the major energy source
in them is massive stars formed in the recent starburst activity; i.e.,
70% -- 80% of the sample are predominantly powered by the starburst. However,
it is known that previous optical spectroscopic observations showed that the
majority of ULIGs are classified as Seyferts or LINERs (low-ionization nuclear
emission-line regions). In order to reconcile this difference, we compare types
of emission-line activity for a sample of ULIGs which have been observed in
both optical and MIR. We confirm the results of previous studies that the
majority of ULIGs classified as LINERs based on the optical emission-line
diagnostics turn to be starburst-dominated galaxies based on the MIR ones.
Since the MIR spectroscopy can probe more heavily-reddened, inner parts of the
ULIGs, it is quite unlikely that the inner parts are powered by the starburst
while the outer parts are powered by non-stellar ionization sources. The most
probable resolution of this dilemma is that the optical emission-line nebulae
with the LINER properties are powered predominantly by shock heating driven by
the superwind activity; i.e., a blast wave driven by a collective effect of a
large number of supernovae in the central region of galaxy mergers.Comment: 15 pages, 2 tables, and 3 eps figures. The Astrophysical Journal
(Part 1), in pres
- …