Constraint on intergalactic dust from thermal history of intergalactic medium

This Letter investigates the amount of dust in the intergalactic medium (IGM). The dust photoelectric heating can be the most efficient heating mechanism in the IGM where the density is very small and there are a lot of hard ultraviolet photons. Comparing the observational thermal history of IGM with a theoretical one taking into account the dust photoelectric heating, we can put an upper limit on the dust-to-gas ratio, ${\cal D}$, in the IGM. Since the rate of the dust photoelectric heating depends on the size of dust, we find the following results: If the grain size is \ga 100 \AA, ${\cal D}$ at $z \sim 3$ is \la 1/100 Galactic value corresponding to \Omega_{\rm dust}^{\rm IGM}\la 10^{-5}. On the other hand, if the grain size is as small as $\sim 10$ \AA, ${\cal D}$ is \la 1/1000 Galactic value corresponding to \Omega_{\rm dust}^{\rm IGM}\la 10^{-6}.Comment: 5 pages, 2 figures; accepted for publication in MNRAS pink page

Anomalous Star-Formation Activity of Less-Luminous Galaxies in Cluster Environment

We discuss a correlation between star formation activity (SFA) and luminosity of star-forming galaxies at intermediate redshifts of $0.2\le z\le 0.6$ in both cluster and field environments. Equivalent width (EW) of [O{\sc ii}] is used for measurement of the SFA, and $R$-band absolute magnitude, $M_R$, for the luminosity. In less-luminous (M_R \gsim -20.7) galaxies, we find : (1) the mean EW([O{\sc ii}]) of cluster galaxies is smaller than that of field galaxies; but (2) some cluster galaxies have as large EW([O{\sc ii}]) as that of actively star-forming field galaxies. Based on both our results, we discuss a new possible mechanism for the Butcher-Oemler (BO) effect, assuming that the luminosity of a galaxy is proportional to its dynamical mass. Our proposal is that BO galaxies are less-massive cluster galaxies with smaller peculiar velocities. They are then stable against Kelvin-Helmholtz instability (KHI), and are not affected by tidal interaction between clusters and themselves. Their interstellar medium (ISM) would be hardly stripped, and their SFA would be little suppressed. Hence, as long as such galaxies keep up their SFA, the fraction of blue galaxies in a cluster does not decrease. As a cluster becomes virialized, however, such galaxies become more accelerated, the ISM available for SFA is stripped by KHI, and their color evolves redward, which produces the BO effect.Comment: accepted for publication in ApJ

Dynamical condition of neutral hydrogen envelopes of dwarf galaxies and their possible morphological evolution

We investigate the star-formation history of gas-rich dwarf galaxies, taking account of the dynamical evolution of their neutral hydrogen (H{\sc i}) envelope. Gas-rich dwarfs are classified into blue compact dwarfs (BCDs) and dwarf irregulars (dIrrs). In this paper, their H{\sc i} envelope is clearly shown not to be blown away by their stellar feedback. This is concluded since the observed star-formation rate (SFR) of gas-rich dwarfs is generally smaller than a critical SFR, $\psi_{\rm crit}$, at which stellar feedback accelerates the H{\sc i} envelope to the escape velocity. From this standpoint and the chemical property of sample BCDs, we suggest two possibilities; (1) The H{\sc i} gas in the envelope of BCDs is consumed to fuel their star-formation; and (2) BCDs have a similar star-formation history. We also discuss morphological evolution among dwarf galaxies. As long as gas-rich dwarfs are isolated, it is difficult for them to evolve into dwarf ellipticals (dEs). When the H{\sc i} envelope in gas-rich dwarfs is consumed in subsequent star-formation, a morphological exchange between BCDs and dIrrs is still expected, consistent with previous studies. If the SFR of gas-rich dwarfs was much higher than $\psi_{\rm crit}$ in the past, interestingly, an evolutionary scenario from dEs to gas-rich dwarfs is possible.Comment: 15 pages, 2 figures, accepted by A&A

Evolution of Hydromagnetic Disturbances in Low Ionized Cosmic Plasmas

We consider the propagation of hydromagnetic waves generated by a compact turbulent source in low ionized plasmas, applying the Lighthill theory. We assume the plasma to be isothermal, and adopt a uniform, stationary medium thread by ordered magnetic fields as an initial condition. Then, the distinct properties of the hydromagnetic waves originating from a source oscillating with a fixed frequency are studied in the linear regime. As is well known, in low ionized plasmas, the generated waves dissipate due to ion-neutral damping. In this paper, the dependence of the dissipation rate on the frequency of the oscillating source is investigated. The larger the frequency becomes, the more substantial is the wave dissipation. Implications of our results on the energy source in molecular clouds are also discussed. Interestingly, since the outflow lobes associated with young stellar objects act as compact turbulent sources, hydromagnetic waves are generated by them. From our order-estimations, about 70% of the energy of the outflow itself propagates as waves or turbulences, while the remaining 30% dissipates and heats the neutrals via ion-neutral damping. Then, we confirm that the outflows are significant energy sources in molecular clouds in the context of the Lighthill theory.Comment: 17 pages LaTeX, 3 PostScript figures, accepted, PASJ (Vol. 51, No. 3, pp. 337 - 344, 1999

Interpretation of the expansion law of planetary nebulae

We reproduce the expansion velocity--radius ($V_{\rm{exp}}$--$R_{\rm{n}}$) relation in planetary nebulae by considering a simple dynamical model, in order to investigate the dynamical evolution and formation of planetary nebulae. In our model, the planetary nebula is formed and evolving by interaction of a fast wind from the central star with a slow wind from its progenitor, the AGB star. In particular, taking account of the mass loss history of the AGB star makes us succeed in the reproduction of the observed $V_{\rm{exp}}$-$R_{\rm{n}}$ sequence. As a result, examining the ensemble of the observational and theoretical evolution models of PNe, we find that if the AGB star pulsates and its mass loss rate changes with time (from $\sim 10^{-6.4}M_{\odot}$ yr$^{-1}$ to $\sim 10^{-5}M_{\odot}$ yr$^{-1}$), the model agrees with the observations. In terms of observation, we suggest that there are few planetary nebulae with larger expansion velocity and smaller radius because the evolutionary time-scale of such nebulae is so short and the size of nebulae is so compact that it is difficult for us to observe them.Comment: 16 pages, 18 figure1, PASJ in pres

On the Possibility of Observing H2 Emission from Primordial Molecular Cloud Kernels

We study the prospects for observing H$_2$ emission during the assembly of primordial molecular cloud kernels. The primordial molecular cloud cores, which resemble those at the present epoch, can emerge around $1+z=20$ according to recent numerical simulations. The kernels form inside the cores, and the first stars will appear inside the kernels. A kernel typically contracts to form one of the first generation stars with an accretion rate that is as large as $\sim 0.01 M_\odot$ year$^{-1}$. This occurs due to the primordial abundances that result in a kernel temperature of order 1000K, and the collapsing kernel emits H$_2$ line radiation at a rate $\sim 10^{35}$ erg sec$^{-1}$. Principally $J=5-3$ (v=0) rotational emission of H$_2$ is expected. At redshift $1+z=20$, the expected flux is $\sim 0.01~\mu$Jy for a single kernel. While an individual object is not observable by any facilities available in the near future, the expected assembly of primordial star clusters on sub-galactic scales can result in fluxes at the sub-mJy level. This is marginally observable with ASTRO-F. We also examine the rotational $J=2-0$ (v=0) and vibrational $\delta v = 1$ emission lines. The former may possibly be detectable with ALMA.Comment: 15 pages, 1 figure. MNRAS (Submitted

Spectra from Forming Region of the First Galaxies : The Effect of Aspherical Deceleration

Ly$\alpha$ line emission from the Loeb-Rybicki (LR) halo, which is the expanding HI IGM (intergalactic medium) around the first star clusters and the ionized interstellar medium, is investigated by solving a radiative transfer problem. While the initial scattering optical depth is $\sim 10^5$ for the Ly$\alpha$ photons, most of the Ly$\alpha$ photons can escape when the cumulative frequency-shift due to the expansion of the HI IGM becomes significantly large. The current paper improves upon previous treatments of the scattering processes and the opacity for the Ly$\alpha$ transfer. Confirming the previous results of the LR halo, we investigate the effect of the aspherical expansion of the IGM. The asphericity is hypothesized to follow the initial stage of the gravitational deceleration to form the large scale filamentary structure of the Universe. According to our results, the effect of the asphericity lets the peak wavelength of the line profile shift to longer wavelengths and the FWHM of the profile become wider than those of the spherically expanding model. To detect these features is meaningful if we are interested in the initial evolution of the large scale structure, since they reflect the dynamical properties of the IGM at that time. Furthermore, given the recent discovery of the high redshift cosmological reionization, we briefly comment on the effects of the redshift and the cosmological parameters on the line profile.Comment: 18 pages, 8 figures, accepted for publication in the Astrophysical Journa

Star Formation Efficiency in the Central 1 kpc Region of Early-Type Spiral Galaxies

It has been reported recently that there are some early-type spiral (Sa--Sab) galaxies having evident star-forming regions which concentrate in their own central 1-kpc. In such central region, is the mechanism of the star formation distinct from that in disks of spiral galaxies? To reveal this, we estimate the star formation efficiency (SFE) in this central 1-kpc star-forming region of some early-type spiral galaxies, taking account of the condition for this 1-kpc region to be self-gravitating. Using two indicators of present star formation rate (H$\alpha$ and infrared luminosity), we estimate the SFE to be a few percents. This is equivalent to the observational SFE in the disks of late-type spiral (Sb--) galaxies. This coincidence may support the universality of the mean SFE of spiral galaxies reported in the recent studies. That is, we find no evidence of distinct mechanism of the star formation in the central 1-kpc region of early-type galaxies. Also, we examine the structure of the central star-forming region, and discuss a method for estimating the mass of star-forming regions.Comment: accepted by A