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Register a new userSubmillimeter Observations of Low Metallicity Galaxy NGC 4214
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Results of submillimeter (450 micron and 850 micron) observations of a nearby dwarf irregular galaxy NGC 4214 with SCUBA on JCMT are presented. We aimed at examining the far-infrared to submillimeter spectral energy distribution (SED) and properties of dust thermal emission in a low metallicity environment by choosing NGC 4214 of which gas metallicity (log O/H + 12) is 8.34. We found that the SED is quite similar to those of IRAS bright galaxies sample (IBGS) which are local bright star-forming galaxies with metallicity comparable to the solar abundance. A dust temperature and an emissivity index for NGC 4214 obtained by a fitting to the single temperature greybody model are T_d = 35 pm 0.8 K and beta = 1.4 pm 0.1, respectively, which are typical values for IBGS. Compiling the previous studies on similar nearby dwarf irregular galaxies, we found that NGC 1569 shows similar results to those of NGC 4214, while NGC 4449 and IC 10 SE show different SEDs and low emissivity indices. There seems to be a variety of SEDs among metal poor dwarf irregular galaxies. We examined dependence on intensity of interstellar radiation field as well as two-temperature model, but the origin of the difference is not clear. Some mechanism(s) other than metallicity and interstellar radiation field must be responsible for controlling dust emission property.
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We present LOFAR observations at 150 MHz of the borderline FRI/FRII giant radio galaxy NGC 6251. This paper presents the most sensitive and highest-resolution images of NGC 6251 at these frequencies to date, revealing for the first time a low-surface-brightness extension to the northern lobe, and a possible backflow associated with the southern lobe. The integrated spectra of components of NGC 6251 are consistent with previous measurements at higher frequencies, similar to results from other LOFAR studies of nearby radio galaxies. We find the outer structures of NGC 6251 to be either at equipartition or slightly electron dominated, similar to those of FRII sources rather than FRIs; but this conclusion remains tentative because of uncertainties associated with the geometry and the extrapolation of X-ray measurements to determine the external pressure distribution on the scale of the outer lobes. We place lower limits on the ages of the extension of the northern lobe and the backflow of the southern lobe of $t gtrsim 250$ Myr and $t gtrsim 210$ Myr respectively. We present the first detection of polarisation at 150 MHz in NGC 6251. Taking advantage of the high Faraday resolution of LOFAR, we place an upper limit on the magnetic field in the group of $B < 0.2 (Lambda_B / 10 {rm kpc})^{-0.5} mu$G for a coherence scale of $Lambda_B < 60 {rm kpc}$ and $B < 13 mu$G for $Lambda_B = 240$ kpc.
A giant star-forming region in a metal-poor dwarf galaxy has been observed in optical lines with the 10-m Gran Telescopio Canarias and in the emission line of CO(1-0) with the NOEMA mm-wave interferometer. The metallicity was determined to be 12+log(O/H)=7.83+-0.09, from which we estimate a conversion factor of alpha_CO~100 Msun/pc2/(K km/s) and a molecular cloud mass of ~2.9x10^7 Msun. This is an enormous concentration of molecular mass at one end of a small galaxy, suggesting a recent accretion. The molecular cloud properties seem normal: the surface density, 120 Msun/pc2, is comparable to that of a standard giant molecular cloud, the clouds virial ratio of ~1.8 is in the star-formation range, and the gas consumption time, 0.5 Gyr, at the present star formation rate is typical for molecular regions. The low metallicity implies that the cloud has an average visual extinction of only 0.8 mag, which is close to the threshold for molecule formation. With such an extinction threshold, molecular clouds in metal-poor regions should have high surface densities and high internal pressures. If high pressure is associated with the formation of massive clusters, then metal-poor galaxies such as dwarfs in the early universe could have been the hosts of metal-poor globular clusters.
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