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Register a new userGalactic interaction as the trigger for the young radio galaxy MRC B1221-423
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Mergers between a massive galaxy and a small gas-rich companion (minor mergers) have been proposed as a viable mechanism for triggering radio emission in an active galaxy. Until now the problem has been catching this sequence of events as they occur. With MRC B1221$-$423 we have an active radio galaxy that has only recently been triggered, and a companion galaxy that provides the smoking gun. Using spectroscopic data taken with the VIMOS Integral Field Unit detector on the European Southern Observatorys Very Large Telescope, we have examined the distribution, ionization state and kinematics of ionized gas in this interacting system. We have also modelled the stellar continuum with synthesised spectra of stellar populations of different ages. From our study of the ionized gas, we have derived preliminary models for the geometry of the interaction, analysed the kinematic behaviour of the ionized gas, and examined the ionization mechanisms at work throughout the system. Our modelling of the stellar continuum allowed us to identify and date distinct stellar populations within the galaxy pair. We find evidence of multiple episodes of widespread starburst activity, and by dating these populations, we provide tentative insight into the history of the interaction.
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We present BVRIK images and spectroscopic observations of the z=0.17 host galaxy of the compact steep-spectrum radio source MRC B1221-423. This is a young (~1e5 yr) radio source with double lobes lying well within the visible galaxy. The host galaxy is undergoing tidal interaction with a nearby companion, with shells, tidal tails, and knotty star-forming regions all visible. We analyse the images of the galaxy and its companion pixel-by-pixel, first using colour-magnitude diagrams, and then fitting stellar population models to the spectral energy distributions of each pixel. The pixels separate cleanly in colour-magnitude diagrams, with pixels of different colours occupying distinct regions of the host galaxy and its companion. We find three distinct groups of ages: an old population in the outskirts of the host galaxy; an intermediate-age population around the nucleus and tidal tail, and a young population in the nucleus and blue knots. The correlation of age with position suggests the two most recent episodes were triggered by tidal interactions with the companion galaxy. The evidence points to the AGN in the centre of B1221-423 having been caught in the act of ignition. However, none of the components we have identified is as young as the radio source, implying that the delay between the interaction and the triggering of the AGN is at least 300 My.
In the framework of hierarchical structure formation AGN feedback shapes the galaxy luminosity function. Low luminosity, galaxy-scale double radio sources are ideal targets to investigate the interplay between AGN feedback and star formation. We use VLA and BIMA observations to study the radio continuum emission of NGC 3801 between 1.4 and 112.4 GHz. We find a prominent spectral break at ~10 GHz, where the spectrum steepens as expected from cosmic-ray electron (CRe) ageing. Using the equipartition magnetic field and fitting JP models locally we create a spatially resolved map of the spectral age of the CRe population. The spectral age of tau_int = 2.0 +/- 0.2 Myr agrees within a factor of two with the dynamical age of the expanding X-ray emitting shells. The spectral age varies only little across the lobes, requiring an effective mixing process of the CRe such as a convective backflow of magnetized plasma. The jet termination points have a slightly younger CRe spectral age, hinting at in-situ CRe re-acceleration. Our findings support the scenario where the supersonically expanding radio lobes heat the ISM of NGC 3801 via shock waves, and, as their energy is comparable to the energy of the ISM, are clearly able to influence the galaxys further evolution.
We present results from a narrow-band survey of the field around the high redshift radio galaxy MRC 2104-242. We have selected Halpha emitters in a 7sq.arcmin field and compared the measured number density with that of a field sample at similar redshift. We find that MRC 2104-242 lies in an overdensity of galaxies that is 8.0 +/- 0.8 times the average density of a blank field, suggesting it resides in a large-scale structure that may eventually collapse to form a massive cluster. We find that there is more dust obscured star formation in the protocluster galaxies than in similarly selected control field galaxies and there is tentative evidence of a higher fraction of starbursting galaxies in the denser environment. However, on average we do not find a difference between the star formation rate (SFR)-mass relations of the protocluster and field galaxies and so conclude that the SFR of these galaxies at z~2.5 is governed predominantly by galaxy mass and not the host environment. We also find that the stellar mass distribution of the protocluster galaxies is skewed towards higher masses and there is a significant lack of galaxies at M < 10^10Msun within our small field of view. Based on the level of overdensity we expect to find ~22 star forming galaxies below 10^10Msun in the protocluster and do not detect any. This lack of low mass galaxies affects the level of overdensity which we detect. If we only consider high mass (M > 10^10.5Msun) galaxies, the density of the protocluster field increases to ~55 times the control field density.
We report the discovery of shocked molecular and ionized gas resulting from jet-driven feedback in the compact radio galaxy 4C 31.04 using near-IR imaging spectroscopy. 4C 31.04 is a $sim 100$ pc double-lobed Compact Steep Spectrum source believed to be a very young AGN. It is hosted by a giant elliptical with a $sim 10^{9}~rm M_odot$ multi-phase gaseous circumnuclear disc. We used high spatial resolution, adaptive optics-assisted $H$- and $K$-band integral field Gemini/NIFS observations to probe (1) the warm ($sim 10^3~rm K$) molecular gas phase, traced by ro-vibrational transitions of $rm H_2$, and (2), the warm ionized medium, traced by the [Fe II]$_{1.644~rm mu m}$ line. The [Fe II] emission traces shocked gas ejected from the disc plane by a jet-blown bubble $300-400~rm pc$ in diameter, whilst the $rm H_2$ emission traces shock-excited molecular gas in the interior $sim 1~rm kpc$ of the circumnuclear disc. Hydrodynamical modelling shows that the apparent discrepancy between the extent of the shocked gas and the radio emission can occur when the brightest regions of the synchrotron-emitting plasma are temporarily halted by dense clumps, whilst less bright plasma can percolate through the porous ISM and form an energy-driven bubble that expands freely out of the disc plane. This bubble is filled with low surface-brightness plasma not visible in existing VLBI observations of 4C 31.04 due to insufficient sensitivity. Additional radial flows of jet plasma may percolate to $sim rm kpc$ radii in the circumnuclear disc, driving shocks and accelerating clouds of gas, giving rise to the $rm H_2$ emission.
We present results from the Ponos simulation suite on the early evolution of a massive, $M_{rm vir}(z=0)=1.2times 10^{13}$ M$_{odot}$ galaxy. At $zgtrsim6$, before feedback from a central supermassive black hole becomes dominant, the main galaxy has a stellar mass $sim 2times 10^{9}$ M$_{odot}$ and a star formation rate $sim 20$ M$_{odot}$ yr$^{-1}$. The galaxy sits near the expected main sequence of star-forming galaxies at those redshifts, and resembles moderately star-forming systems observed at $z>5$. The high specific star formation rate results in vigorous heating and stirring of the gas by supernovae feedback, and the galaxy develops a thick and turbulent disc, with gas velocity dispersion $sim 40$ km s$^{-1}$, rotation to dispersion ratio $sim 2$, and with a significant amount of gas at $sim 10^5$ K. The Toomre parameter always exceeds the critical value for gravito-turbulence, $Qsim 1.5-2$, mainly due to the contribution of warm/hot gas inside the disc. Without feedback, a nearly gravito-turbulent regime establishes with similar gas velocity dispersion and lower $Q$. We propose that the hot and turbulent disc regime seen in our simulations, unlike the cold and turbulent gravito-turbulent regime of massive clumpy disc galaxies at $zsim 1-2$, is a fundamental characterisation of main sequence galaxies at $zgtrsim 6$, as they can sustain star formation rates comparable to those of low-mass starbursts at $z=0$. This results in no sustained coherent gas inflows through the disc, and in fluctuating and anisotropic mass transport, possibly postponing the assembly of the bulge and causing the initial feeding of the central black hole to be highly intermittent.
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