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R. Morganti

Publications and source records attributed to R. Morganti.

At least 145 records · Page 8Linked to original sources
arXiv2010

The optical morphologies of the 2Jy sample of radio galaxies: evidence for galaxy interactions

We present deep GMOS-S/Gemini optical broad-band images for a complete sample of 46 southern 2Jy radio galaxies at intermediate redshifts (0.05<z<0.7). The high-quality observations show for the first time that the overall majority of PRGs at intermediate redshifts (78-85%) show peculiarities in their optical morphologies at relatively high levels of surface brightness ($μ$v=23.6 and Δ$μ$v~[21,26] mag arcsec-2) including tails, fans, bridges, shells, dust lanes, irregular features, amorphous haloes, and multiple nuclei. While the results for many of the galaxies are consistent with them being observed at, or after, the time of coalescence of the nuclei in a galaxy merger, we find that more than 1/3 of the sample are observed in a pre-coalescence phase of the merger, or following a close encounter between galaxies. By dividing the sample into Weak-Line Radio Galaxies (WLRGs; 11 objects) and Strong-Line Radio Galaxies (SLRGs; 35 objects) we find that only 27% of the former show clear evidence for interactions in their optical morphologies, in contrast to the SLRGs, of which at least 94% appear interacting. This is consistent with the idea that many WLRGs are fuelled/triggered by Bondi accretion of hot gas. Of the 28% of the sample that display evidence for significant starburst activity, we find that 92% present disturbed morphologies, following the same general trend as the total and SLRG samples. By comparing our PRGs with various samples of quiescent ellipticals from the literature, we conclude that the percentage of morphological disturbance that we find here exceeds that found for quiescent ellipticals when similar surface brightnesses are considered. Overall, our study indicates that galaxy interactions are likely to play a key role in the triggering of AGN/jet activity.

astro-ph.CO↗
arXiv2010

Formation of slowly rotating early-type galaxies via major mergers: a Resolution Study

We study resolution effects in numerical simulations of gas-rich and gas-poor major mergers, and show that the formation of slowly-rotating elliptical galaxies often requires a resolution that is beyond the present-day standards to be properly modelled. Our sample of equal-mass merger models encompasses various masses and spatial resolutions, ranging from about 200pc and 10^5 particles per component, typical of some recently published major merger simulations, to up to 32pc and 10^3 M_sun in simulations using 2.4 x 10^7 collisionless particles and 1.2 x 10^7 gas particles, among the highest resolutions reached so far for gas-rich major merger of massive disc galaxies. We find that the formation of fast-rotating early-type galaxies, that are flattened by a significant residual rotation, is overall correctly reproduced at all such resolutions. However, the formation of slow-rotating early-type galaxies, which have a low residual angular momentum and are supported mostly by anisotropic velocity dispersions, is strongly resolution-dependent. The evacuation of angular momentum from the main stellar body is largely missed at standard resolution, and systems that should be slow rotators are then found to be fast rotators. The effect is most important for gas-rich mergers, but is also witnessed in mergers with an absent or modest gas component. The effect is robust with respect to our initial conditions and interaction orbits, and originates in the physical treatment of the relaxation process during the coalescence of the galaxies. Our findings show that a high-enough resolution is required to accurately model the global properties of merger remnants and the evolution of their angular momentum. The role of gas-rich mergers of spiral galaxies in the formation of slow-rotating ellipticals may therefore have been underestimated.

astro-ph.CO↗
arXiv2010

Centaurus A: morphology and kinematics of the atomic hydrogen

We present new ATCA 21-cm line observations of the neutral hydrogen in the nearby radio galaxy Centaurus A. We image in detail (with a resolution down to 7", ~100pc) the distribution of HI along the dust lane. Our data have better velocity resolution and better sensitivity than previous observations. The HI extends for a total of ~15kpc. The data, combined with a titled-ring model of the disk, allow to conclude that the kinematics of the HI is that of a regularly rotating, highly warped structure down to the nuclear scale. The parameters (in particular the inclination) of our model are somewhat different from some of the previously proposed models but consistent with what was recently derived from stellar light in a central ring. The model nicely describes also the morphology of the dust lane as observed with Spitzer. There are no indications that large-scale anomalies in the kinematics exist that could be related to supplying material for the AGN. Large-scale radial motions do exist, but these are only present at larger radii r>6kpc). This unsettled gas is mainly part of a tail/arm like structure. The relatively regular kinematics of the gas in this structure suggests that it is in the process of settling down into the main disk. The presence of this structure further supports the merger/interaction origin of the HI in Cen A. From the structure and kinematics we estimate a timescale of 1.6-3.2*10^{8}yr since the merging event. No bar structure is needed to describe the kinematics of the HI. The comparison of the timescale derived from the large-scale HI structure and those of the radio structure together with the relative regularity of the HI down to the sub-kpc regions does not suggest a one-to-one correspondence between the merger and the phase of radio activity. Interestingly, the radial motions of the outer regions are such that the projected velocities are redshifted compared to the regular orbits. This means that the blueshifted absorption discovered earlier and discussed in our previous paper cannot be caused by out-moving gas at large radius projected onto the centre. Therefore, the interpretation of the blueshifted absorption, together with at least a fraction of the redshifted nuclear absorption, as evidence for a regular inner disk, still holds. Finally, we also report the discovery of two unresolved clouds detected at 5.2 and 11kpc away (in projection) from the HI disk. They are likely an other example of left-over of the merger that brought the HI gas.

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arXiv2010

Large-scale HI in nearby radio galaxies (II): the nature of classical low-power radio sources

An important aspect of solving the long-standing question as to what triggers various types of Active Galactic Nuclei involves a thorough understanding of the overall properties and formation history of their host galaxies. This is the second in a series of papers that systematically study the large-scale properties of cold neutral hydrogen (HI) gas in nearby radio galaxies. The main goal is to investigate the importance of gas-rich galaxy mergers and interactions among radio-loud AGN. In this paper we present results of a complete sample of classical low-power radio galaxies. We find that extended Fanaroff & Riley type-I radio sources are generally not associated with gas-rich galaxy mergers or ongoing violent interactions, but occur in early-type galaxies without large (> 10^8 M_sun) amounts of extended neutral hydrogen gas. In contrast, enormous discs/rings of HI gas (with sizes up to 190 kpc and masses up to 2 x 10^10 M_sun) are detected around the host galaxies of a significant fraction of the compact radio sources in our sample. This segregation in HI mass with radio source size likely indicates that these compact radio sources are either confined by large amounts of gas in the central region, or that their fuelling is inefficient and different from the fuelling process of classical FR-I radio sources. To first order, the overall HI properties of our complete sample (detection rate, mass and morphology) appear similar to those of radio-quiet early-type galaxies. If confirmed by better statistics, this would imply that low-power radio-AGN activity may be a short and recurrent phase that occurs at some point during the lifetime of many early-type galaxies.

astro-ph.CO↗
arXiv2010

Continuum surveys with LOFAR and synergy with future large surveys in the 1-2 GHz band

Radio astronomy is entering the era of large surveys. This paper describes the plans for wide surveys with the LOw Frequency ARray (LOFAR) and their synergy with large surveys at higher frequencies (in particular in the 1-2 GHz band) that will be possible using future facilities like Apertif or ASKAP. The LOFAR Survey Key Science Project aims at conducting large-sky surveys at 15, 30, 60, 120 and 200 MHz taking advantage of the wide instantaneous field of view and of the unprecedented sensitivity of this instrument. Four topics have been identified as drivers for these surveys covering the formation of massive galaxies, clusters and black holes using z>6 radio galaxies as probes, the study of the intercluster magnetic fields using diffuse radio emission and Faraday rotation measures in galaxy clusters as probes and the study of star formation processes in the early Universe using starburst galaxies as probes. The fourth topic is the exploration of new parameter space for serendipitous discovery taking advantage of the new observational spectral window open up by LOFAR. Here, we briefly discuss the requirements of the proposed surveys to address these (and many others!) topics as well as the synergy with other wide area surveys planned at higher frequencies (and in particular in the 1-2 GHz band) with new radio facilities like ASKAP and Apertif. The complementary information provided by these surveys will be crucial for detailed studies of the spectral shape of a variety of radio sources (down to sub-mJy sources) and for studies of the ISM (in particular HI and OH) in nearby galaxies.

astro-ph.CO↗
arXiv2009

An HI view of the on-going assembly of early-type galaxies: present and future observations

We present a preliminary analysis of the HI properties of early-type galaxies in the ATLAS3D sample. Using WSRT data for ~100 galaxies outside the Virgo cluster and data from the Alfalfa project for galaxies inside Virgo, we discuss the dependence of HI properties on environment. We detect HI in about half of the galaxies outside Virgo. For these systems, the HI morphology and kinematics change as a function of environment, going from regular, rotating systems around isolated galaxies to progressively more disturbed structures for galaxies with neighbours or in groups. In denser environment, inside Virgo, nearly none of the galaxies contains HI. We discuss future work in this field which will be enabled by next-generation, pre-SKA radio instruments. We present a simulated Apertif HI observation of an ATLAS3D early-type galaxy, showing how its appearance and detection level vary as a function of redshift.

astro-ph.CO↗
arXiv2009

Emission line gas ionisation in young radio galaxies

This paper is the second in a series in which we present intermediate-resolution spectra for a complete sample of 14 compact radio sources, taken with the aim of investigating the impact of the nuclear activity on the cirumnuclear interstellar medium (ISM) in the early stages of radio source evolution. In the first paper we presented the kinematic results from the line modelling and reported fast outflows in the circumnuclear gas. Here, we use the line fluxes to investigate the physical conditions and dominant ionisation mechanisms of the emission line gas. We find evidence for large electron densities and high reddening in the nuclear regions, particularly in the broader, blueshifted components. These results are consistent with the idea that the young, recently triggered radio sources still reside in dense and dusty cocoons deposited by the recent activity triggering event. In addition, we find that the quiescent nuclear and extended narrow components are consistent with AGN photoionisation. For the nuclear broader and shifted components the results are less clear. Whilst there are suggestions that the broader components may be closer to shock plus precursor models on the diagnostic diagrams (with high electron temperatures and densities), we are unable to unambiguously distinguish the dominant ionisation mechanism using the optical emission line ratios. This is surprising given the strong evidence for jet-cloud interactions (broad emission lines, large outflow velocities and strong radio-optical alignments), which favours the idea that the warm gas has been accelerated in shocks driven by the radio lobes expanding through a dense cocoon of gas deposited during the triggering event.

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arXiv2009

The Hot and Cold Outflows of NGC 3079

Very deep neutral hydrogen (HI) observations of the edge-on spiral galaxy NGC 3079 with the Westerbork Synthesis Radio Telescope (WSRT) are presented. The galaxy has been studied extensively in different wavelengths and known for its several unique and complex features. However, the new data still revealed several new features and show that the galaxy is even more complicated and interesting than previously thought. In the new data a large stream of gas, encircling the entire galaxy, was discovered, while the data also show, for the first time, that not only hot gas is blown into space by the starburst/AGN, but also large amounts of cold gas, despite the high energies involved in the outflow.

astro-ph.CO↗
arXiv2009

Is cold gas fuelling the radio galaxy NGC 315?

We present WSRT, VLA and VLBI observations of the HI absorption in the radio galaxy NGC 315. The main result is that two HI absorbing systems are detected against the central region. In addition to the known highly redshifted, very narrow component, we detect relatively broad (FWZI 150 km/s) absorption. This broad component is redshifted by ~80 km/s compared to the systemic velocity, while the narrow absorption is redshifted ~ 490 km/s. Both HI absorption components are spatially resolved at the pc-scale of the VLBI observations. The broad component shows strong gradients in density (or excitation) and velocity along the jet. We conclude that this gas is physically close to the AGN, although the nature of the gas resulting in the broad absorption is not completely clear. The possibility that it is entrained by the radio jet appears unlikely. Gas located in a thick circum-nuclear toroidal structure cannot be completely ruled out although it appears difficult to reconcile with the observed morphology and kinematics of the HI. A perhaps more likely scenario is that the gas producing the broad absorption could be (directly or indirectly) connected with the fuelling of the AGN, i.e. gas that is falling into the nucleus. If this is the case, the accretion rate derived is similar to that found for other X-ray luminous elliptical galaxies, although lower than that derived from the radio core luminosity for NGC 315. The density distribution of the narrow component is, featureless. Moreover, we detect a small amount of HI in emission a few kpc SW of the AGN, coincident with faint optical absorption features and at velocities very similar to the narrow absorption. This suggests that the gas causing the narrow absorption is not close to the AGN and is more likely caused by clouds falling into NGC 315.

astro-ph.CO↗
arXiv2009

The disc-dominated host galaxy of FR-I radio source B2 0722+30

We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper we use HI observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the Active Galactic Nucleus and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [OIII] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely HI-rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio-AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy.

astro-ph.GA↗
arXiv2009

Gas and stars in compact (young) radio sources

Gas can be used to trace the formation and evolution of galaxies as well as the impact that the nuclear activity has on the surrounding medium. For nearby compact radio sources, we have used observations of neutral hydrogen - that we detected in emission distributed over very large scales - combined with the study of the stellar population and deep optical images to investigate the history of the formation of their host galaxy and the triggering of the activity. For more distant and more powerful compact radio sources, we have used optical spectra and HI - in absorption - to investigate the presence of fast outflows that support the idea that compact radio sources are young radio loud AGN observed during the early stages of their evolution and currently shredding their natal cocoons through extreme circumnuclear outflows. We will review the most recent results obtained from these projects.

astro-ph.GA↗
arXiv2008

The origin of the infrared emission in radio galaxies II: analysis of mid- to far-infrared Spitzer observations of the 2Jy sample

We present an analysis of deep mid- to far-infrared (MFIR) Spitzer photometric observations of the southern 2Jy sample of powerful radio sources (0.05 < z < 0.7), conducting a statistical investigation of the links between radio jet, AGN, starburst activity and MFIR properties. This is part of an ongoing extensive study of powerful radio galaxies that benefits from both complete optical emission line information and a uniquely high detection rate in the far-infrared (far-IR). We find tight correlations between the MFIR and [OIII] emission luminosities, which are significantly better than those between MFIR and extended radio luminosities, or between radio and [OIII] luminosities. Since [OIII] is a known indicator of intrinsic AGN power, these correlations confirm AGN illumination of the circum-nuclear dust as the primary heating mechanism for the dust producing thermal MFIR emission at both 24 and 70 microns. We demonstrate that AGN heating is energetically feasible, and identify the narrow line region clouds as the most likely location of the cool, far-IR emitting dust. Starbursts make a major contribution to the heating of the cool dust in only 15-28% of our targets. We also investigate the orientation dependence of the continuum properties, finding that the broad- and narrow-line objects in our sample with strong emission lines have similar distributions of MFIR luminosities and colours. Therefore our results are entirely consistent with the orientation-based unified schemes for powerful radio galaxies. However, the weak line radio galaxies (WLRG) form a separate class of objects with intrinsically low luminosity AGN in which both the optical emission lines and the MFIR continuum are weak.

astro-ph↗
arXiv2008

Kinematic modeling of disk galaxies III. The warped "Spindle" NGC 2685

This is the third of a series of papers in which the structure and kinematics of disk galaxies is studied. Employing direct tilted-ring fits to the data cube as introduced in Paper I, we perform an analysis of the "Spindle" galaxy NGC 2685, previously regarded as two-ringed polar ring galaxy. Deep HI and optical (i'-band) observations are presented. The HI observations strongly suggest that the gaseous structure of NGC 2685 does not consist of two separate mutually inclined regions, but forms a coherent, extremely warped disk, the appearance of two rings being due to projection effects. By comparing the HI total-intensity maps with the optical image we demonstrate that at large radii a faint stellar disk is well aligned with the outer HI disk. The shape of the dust-lanes obscuring the NE part of the inner stellar body indicates that also at smaller radii NGC 2685 possesses a disk containing gas, dust, and stars in which the various constituents are aligned. At smaller radii, this disk is kinematically decoupled from the central stellar body. Hence, in the region of the bright, central stellar body, NGC 2685 appears to consist of two disks that share a common centre, but have different orientation: a bright stellar lenticular body apparently devoid of dust and gas, and a heavily warped low-surface brightness disk containing stars, gas and dust. The low-surface-brightness disk changes its orientation gradually and at large radii assumes the orientation of the central stellar S0 disk. Since, according to our analysis, the intrinsic orientation of the low-surface-brightness disk changes through 70 degrees, the gaseous disk is coherent, and is at no radius orientated perpendicularly with respect to the central stellar body, NGC 2685 is likely not a classical polar-ring galaxy.

astro-ph↗
arXiv2008

From major merger to radio galaxy: low surface-brightness stellar counterpart to the giant HI ring around B2 0648+27

We present the detection of a low surface-brightness stellar counterpart to an enormous (190 kpc) ring of neutral hydrogen (HI) gas that surrounds the nearby radio galaxy B2 0648+27. This system is currently in an evolutionary stage between major merger and (radio-loud) early-type galaxy. In a previous paper we investigated in detail the timescales between merger, starburst and AGN activity in B2 0648+27, based on its unusual multi-wavelength properties (large-scale HI ring, dominating post-starburst stellar population and infra-red luminosity). In this Research Note we present deep optical B- and V-band imaging that provides further evidence for the merger origin of B2 0648+27. The host galaxy shows a distorted optical morphology and a broad tidal arm is clearly present. A low surface-brightness stellar tail or partial ring curls around more than half the host galaxy at a distance of up to 55 kpc from the centre of the galaxy, following the large-scale, ring-like HI structure that we detected previously around this system. The gas and stars in this ring are most likely tidally expelled material that slowly fell back onto the host galaxy after the merger event. There also appear to be sites of star formation in the HI ring that may have formed within the gaseous tidal debris after the merger. We argue that the observed properties of the gas and stars in B2 0648+27, as well as the apparent time-delay between the merger and the starburst event, may be the logical result of a merger between two gas-rich disk galaxies with a prominent bulge, or of a merger between an elliptical and a gas-rich spiral galaxy. There also appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity.

astro-ph↗
arXiv2008

Enormous disc of cool gas surrounding the nearby powerful radio galaxy NGC 612 (PKS 0131-36)

We present the detection of an enormous disc of cool neutral hydrogen (HI) gas surrounding the S0 galaxy NGC 612, which hosts one of the nearest powerful radio sources (PKS 0131-36). Using the Australia Telescope Compact Array, we detect M_HI = 1.8 x 10^9 M_sun of HI emission-line gas that is distributed in a 140 kpc wide disc-like structure along the optical disc and dust-lane of NGC 612. The bulk of the gas in the disc appears to be settled in regular rotation with a total velocity range of 850 km/s, although asymmetries in this disc indicate that perturbations are being exerted on part of the gas, possibly by a number of nearby companions. The HI disc in NGC 612 suggests that the total mass enclosed by the system is M_enc ~ 2.9 x 10^12 sin^-2(i) M_sun, implying that this early-type galaxy contains a massive dark matter halo. We also discuss an earlier study by Holt et al. that revealed the presence of a prominent young stellar population at various locations throughout the disc of NGC 612, indicating that this is a rare example of an extended radio source that is hosted by a galaxy with a large-scale star-forming disc. In addition, we map a faint HI bridge along a distance of 400 kpc in between NGC 612 and the gas-rich (M_HI = 8.9 x 10^9 M_sun) barred galaxy NGC 619, indicating that likely an interaction between both systems occurred. From the unusual amounts of HI gas and young stars in this early-type galaxy, in combination with the detection of a faint optical shell and the system's high infra-red luminosity, we argue that either ongoing or past galaxy interactions or a major merger event are a likely mechanism for the triggering of the radio source in NGC 612. This paper is part of an ongoing study to map the large-scale neutral hydrogen properties of nearby radio galaxies. --abridged--

astro-ph↗
arXiv2008

A circumnuclear disk of atomic hydrogen in Centaurus A

We present new observations, performed with the Australia Telescope Compact Array, of the HI absorption in the central regions of Centaurus A. For the first time, absorption is detected against the radio core at velocities blueshifted with respect to the systemic velocity. Moreover, the data show that the nuclear redshifted absorption component is broader than reported before. With these new results, the kinematics of the HI in the inner regions of Cen A appears very similar to that observed in emission for the molecular circumnuclear disk. This suggests that the central HI absorption is not, as was previously claimed, evidence of gas infall into the AGN, but instead is due to a cold, circumnuclear disk.

astro-ph↗
arXiv2008

The origin of the infrared emission in radio galaxies I: new mid- to far-infrared and radio observations of the 2Jy sample

As part of a large study to investigate the nature of the longer wavelength continuum emission of radio-loud AGN, we present new mid to far-infrared (MFIR) and high frequency radio observations for a complete sample of 2Jy powerful, southern radio galaxies at intermediate redshifts (0.05<z<0.7). Utilizing the sensitivity of the Spitzer Space Telescope, we have made deep MIPS observations at the wavelengths of 24, 70 and 160 microns, detecting 100% of our sample at 24 microns, and 90% at 70 microns. This high detection rate at MFIR wavelengths is unparallelled in samples of intermediate redshift radio galaxies. Complementing these results, we also present new high frequency observations (15 to 24Ghz) from the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA), which are used to investigate the potential contamination of the MFIR continuum by non-thermal synchrotron emission. With the latter observations we detect the compact cores in 59% of our complete sample, and deduce that non-thermal contamination of the MFIR continuum is significant in a maximum of 30% of our total sample. MFIR fluxes, radio fluxes and spectral energy distributions for the complete sample are presented here, while in a second paper we will analyse these data and discuss the implications for our understanding of the heating mechanism for the warm/cool dust, star formation in the host galaxies, and the unified schemes for powerful radio sources.

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