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($J$=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, $\Sigma$(FIR), is proportional linearly to the H$_2$ surface mass density, $\Sigma$(H$_2$), for the two samples; $\Sigma$(FIR) $\propto \Sigma$(H$_2$)$^{1.01\pm0.06}$ with a correlation coefficient of 0.96. It is often considered that $\Sigma$(FIR) provides a good measure of the star formation rate per unit area, $\Sigma$(SFR). It is also known that molecular gas is dominated in circumnuclear regions in the luminous starburst mergers; i.e., $\Sigma$(gas) $\simeq \Sigma$(H$_2$). Therefore, the above relationship suggests a star formation law; $\Sigma$(SFR) $\propto \Sigma$(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 $^{12}$CO to $^{13}$CO integrated intensity ratios at a transition $J$=1--0, $R = I(^{12}$CO)$/I(^{13}$CO) $\gtrsim 20$, in a sample of starburst merging galaxies such as Arp 220 are mainly attributed to the depression of $^{13}$CO emission with respect to $^{12}$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 $^{13}$CO luminosity. This implies that dust grains are also depressed in the high-$R$ starburst mergers, leading to the higher dust temperature ($T_{\rm d}$) in them because of the relative increase in the radiation density. Nevertheless, the average dust mass ($M_{\rm d}$) of the high-$R$ starburst mergers is higher significantly than that of non-high $R$ 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 $L$(FIR) $\propto M_{\rm d} T_{\rm d}^5$ given the dust emissivity law, $S_\nu \propto \lambda^{-1}$, the increases in both $M_{\rm d}$ and $T_{\rm d}$ 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

13CO(J=1−−0)^{13}CO(J = 1 -- 0) Depression in Luminous Starburst Mergers

It is known that the class of luminous starburst galaxies tends to have higher $R =^{12}CO(J=1--0)/^{13}CO(J=1--0)$ integrated line intensity ratios ($R>20$) than normal spiral galaxies ($R \sim 10$). Since most previous studies investigated only $R$, it remains uncertain whether the luminous starburst galaxies are overabundant in $^{12}$CO or underabundant in $^{13}$CO. Here we propose a new observational test to examine this problem. Our new test is to compare far-infrared luminosities [$L$(FIR)] with those of $^{12}$CO and $^{13}CO [L(^{12}CO)$ and $L(^{13}CO)$, respectively]. It is shown that there is a very tight correlation between $L(^{12}CO)$ and L(FIR), as found in many previous studies. However, we find that the $^{13}$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 $R$ values. Therefore, we conclude that the observed high $R$ values for the luminous starburst galaxies are attributed to their lower $^{13}$CO line intensities.Comment: 9 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for publication in Astrophysical Journal Letter

SDSSp J104433.04−-012502.2 at z=5.74z=5.74 is Gravitationally Magnified by an Intervening Galaxy

During the course of our optical deep survey program on L$\alpha$ emitters at $z \approx 5.7$ in the sky area surrounding the quasar SDSSp J104433.04$-$012502.2 at $z=5.74$, we found that a faint galaxy with $m_B$(AB) $\approx 25$ is located at \timeform{1".9} southwest of the quasar. Its broad-band color properties from $B$ to $z^\prime$ suggest that the galaxy is located at a redshift of $z \sim 1.5$ -- 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., $\sim$ 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