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Register a new userINTERMEDIATE RESOLUTION SPECTROSCOPY OF THE RADIO GALAXY B2 0902+34 AT $Zapprox 3.4$
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We have carried out spectroscopic observations of the high redshift ($zapprox 3.4$) radio galaxy 0902+34 at intermediate resolution with the William Herschel Telescope. The dynamical spectral ranges covered are 4600-5480 AA and 5920-7680 AA with resolutions of 5.4 AA and 9.5 AA, respectively. We detect a continuum that is almost flat and resolve three emission lines: Ly$alpha$, C IV $lambda$1549 and He II $lambda$1640, the last one previously undetected. The line ratios are similar to the typical values found for narrow-line high redshift radio galaxies. Line ratios observed in different regions of the galaxy seem to indicate the presence of strong ionization and/or dust density gradients. We have not detected any Ly$alpha$ absorption at z=3.3968 (red wing of the Ly$alpha$ emission line) as might be expected from the absorption found at 21 cm by other authors using the VLA and Arecibo antennas. We discuss possible models for the H I absorbing cloud.
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Radio loud Active Galactic Nuclei are episodic in nature, cycling through periods of activity and quiescence. In this work we investigate the duty cycle of the radio galaxy B2~0258+35, which was previously suggested to be a restarted radio galaxy based on its morphology. The radio source consists of a pair of kpc-scale jets embedded in two large-scale lobes (~240 kpc) with relaxed shape and very low surface brightness, which resemble remnants of a past AGN activity. We have combined new LOFAR data at 145 MHz and new SRT data at 6600 MHz with available WSRT data at 1400 MHz to investigate the spectral properties of the outer lobes and derive their age. Interestingly, the spectrum of both the outer Northern and Southern lobes is not ultra-steep as expected for an old ageing plasma with spectral index values equal to $rm alpha_{1400}^{145}=0.48pm0.11$ and $rm alpha_{6600}^{1400}=0.69pm0.20$ in the outer Northern lobe, and $rm alpha_{1400}^{145}=0.73pm0.07$ in the outer Southern lobe. Moreover, despite the wide frequency coverage available for the outer Northern lobe (145-6600~MHz), we do not identify a significant spectral curvature (SPC$simeq$0.2$pm0.2$). While mechanisms such as in-situ particle reacceleration, mixing or compression can temporarily play a role in preventing the spectrum from steepening, in no case seem the outer lobes to be compatible with being very old remnants of past activity as previously suggested (with age $gtrsim$ 80 Myr). We conclude that either the large-scale lobes are still fuelled by the nuclear engine or the jets have switched off no more than a few tens of Myr ago. Our study shows the importance of combining morphological and spectral properties to reliably classify the evolutionary stage of low surface brightness, diffuse emission that low frequency observations are revealing around a growing number of radio sources.
The Southern Galactic Plane Survey (SGPS) is an HI and 1.4-GHz continuum survey of the 4th quadrant of the Galaxy at arcmin resolution. We present here results on linearly polarized continuum emission from an initial 28-square-degree Test Region for the SGPS, consisting of 190 mosaiced pointings of the Australia Telescope Compact Array, and covering the range 325.5 < l < 332.5, -0.5 < b < +3.5. Complicated extended structure is seen in linear polarization throughout the Test Region, almost all of which has no correlation with total intensity. We interpret the brightest regions of polarized emission as representing intrinsic structure in extended polarization, most likely originating in the Crux spiral arm at a distance of 3.5 kpc; fainter polarized structure is imposed by Faraday rotation in foreground material. Two large areas in the field are devoid of polarization. We argue that these voids are produced by foreground HII regions in which the magnetic field is disordered on scales of ~0.1-0.2 pc. We also identify a depolarized halo around the HII region RCW 94, which we suggest results from the interaction of the HII region with a surrounding molecular cloud.
Recently, Saxena et al. (2018) reported the discovery of a possible radio galaxy, J1530$+$1049 at a redshift of z=5.72. We observed the source with the European Very Long Baseline Interferometry Network at $1.7$ GHz. We detected two faint radio features with a separation of $sim 400$ mas. The radio power calculated from the VLA flux density by Saxena et al. (2018), and the projected source size derived from our EVN data place J1530$+$1049 among the medium-sized symmetric objects (MSOs) which are thought to be young counterparts of radio galaxies (An and Baan 2012). Thus, our finding is consistent with a radio galaxy in an early phase of its evolution as proposed by Saxena et al. (2018).
We report the first high spatial resolution submillimeter continuum observations of the Sagittarius B2 cloud complex using the Submillimeter Array (SMA). With the subarcsecond resolution provided by the SMA, the two massive star-forming clumps Sgr B2(N) and Sgr B2(M) are resolved into multiple compact sources. In total, twelve submillimeter cores are identified in the Sgr B2(M) region, while only two components are observed in the Sgr B2(N) clump. The gas mass and column density are estimated from the dust continuum emission. We find that most of the cores have gas masses in excess of 100 M$_{odot}$ and column densities above 10$^{25}$ cm$^{-2}$. The very fragmented appearance of Sgr B2(M), in contrast to the monolithic structure of Sgr B2 (N), suggests that the former is more evolved. The density profile of the Sgr B2(N)-SMA1 core is well fitted by a Plummer density distribution. This would lead one to believe that in the evolutionary sequence of the Sgr B2 cloud complex, a massive star forms first in an homogeneous core, and the rest of the cluster forms subsequently in the then fragmenting structure.
Star-forming galaxies (SFGs) with stellar masses below $10^{10},M_odot$ make up the bulk of the galaxy population at $z>2$. The properties of the cold gas in these galaxies can only be probed in very deep ALMA observations or by targeting strongly lensed galaxies. Here we report the results of a pilot survey using the Atacama Compact Array (ACA) of molecular gas in the most strongly magnified galaxies selected as giant arcs in optical data. The selection in rest-frame UV wavelengths ensures that sources are regular star forming galaxies, without a priori indications of intense dusty starburst activity. We conducted Band 4 and Band 7 observations to detect mid-$J$ CO, [C I] and thermal continuum as molecular gas tracers from four strongly lensed systems at $zapprox2-3$: our targets are SGAS J1226651.3+215220 (A and B), SGAS J003341.5+024217 and the Sunburst Arc. The measured molecular mass is then projected onto the source plane with detailed lens models developed from high resolution HST observations. Multiwavelength photometry is then used to obtain the intrinsic stellar mass and star formation rate via SED fitting. In only one of the sources are the three tracers robustly detected, while in the others they are either undetected or detected in continuum only. The implied molecular gass masses range from $4times 10^{9},M_odot$ in the detected source to an upper limit of $lesssim 10^9,M_odot$ in the most magnified source. The inferred gas fraction and gas depletion timescale are found to lie approximately 0.5 to 1.0 dex below the established scaling relations based on previous studies of unlensed massive galaxies. Our results indicate that the cold gas content of intermediate to low mass galaxies should not be extrapolated from the trends seen in more massive high-$z$ galaxies. (Abridged abstract)
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