arXiv subjects

Raffaella Morganti

Publications and source records attributed to Raffaella Morganti.

At least 163 records · Page 9Linked to original sources

arXiv2009

Interaction and gas outflows in radio-loud AGN - Disruptive and constructive effects of radio jets

In the last years, Active Galactic Nuclei (AGN) have become more popular than ever. The possibility of using them to produce feedback effects that would help solving some of the still open questions connected to the hierarchical scenario of galaxy formation and evolution, has made them particularly popular among theorists. AGN-driven outflows can have different origin. Here, I will concentrate on the role that the radio-loud phase of nuclear activity (and the presence of radio plasma jets) can play in the evolution of a galaxy. I will focus on two aspects in which radio activity could be important. The first is exploring whether relativistic plasma jet associated with radio-loud galaxies could provide an effective way to produce gas outflows with characteristics that can be relevant in the evolution of the host galaxy. The second, is to investigate whether they can provide a mechanism for the triggering of star formation.

astro-ph.CO↗

arXiv2009

ESO 381-47, an early-type galaxy with extended HI and a star forming ring

ESO 381-47 is an early type galaxy with an extended HI disk. GALEX and very deep optical images reveal a distinct stellar ring far outside the optical body with a diameter of ~30 kpc, which has undergone recent star formation at 1.8 x 10^-4 Msun/yr/kpc^-2, consistent with other new results which detect low level star formation below the traditional Kennicutt relation in the outer parts of spiral galaxies. The morphology of this galaxy resembles the recently identified class of ultraviolet objects called extended ultraviolet disks, or XUV-disks. New HI observations of this galaxy taken at the ATCA and in the CnB array at the VLA show that the cold gas lies in an extended (diameter ~90 kpc) ring around the central S0 galaxy. The HI data cube can be well modeled by a warped ring. The faint ionized gas in the inner parts of the galaxy is kinematically decoupled from the stars and instead appears to exhibit velocities consistent with the rotation of the HI ring at larger radius. The peak of the stellar ring, as seen in the optical and UV, is slightly displaced to the inside relative to the peak of the HI ring. We discuss the manner in which this offset could be caused by the propagation of a radial density wave through an existing stellar disk, perhaps triggered by a galaxy collision at the center of the disk, or possibly due to a spiral density wave set up at early times in a disk too hot to form a stellar bar. Gas accretion and resonance effects due to a bar which has since dissolved are also considered to explain the presence of the star forming ring seen in the GALEX and deep optical data.

astro-ph.CO↗

arXiv2009

Broad HI absorption in the candidate binary black-hole 4C37.11 (B2 0402+379)

We report the discovery of extremely broad 21-cm HI absorption (FWZI ~1600 km/s) detected with the Westerbork Synthesis Radio Telescope in the radio source 4C37.11 (B2 0402+379). This object has been claimed to host a super-massive binary black hole (Rodriguez et al. 2006). The main features in the absorption profile are two components, separated by ~1100 km/s. The HI absorption in 4C37.11 is unusual because it is the first case where such broad absorption is found to be centred on the systemic velocity of the host galaxy and not asymmetric and blueshifted as is seen in all other galaxies with broad HI absorption. Given the large width of the absorption, we suggest that a possible explanation for the extreme properties of the HI absorption is that it is the kinematic signature of a binary black hole. If this interpretation is correct, the combined black hole mass derived from the absorption profile is consistent with that derived from the luminosity of the spheroid. If the broad absorption is indeed due to a binary black hole, this finding confirms the importance of the gaseous component in the merging process of supermassive black holes.

astro-ph.CO↗

arXiv2008

Circumnuclear HI disks in radio galaxies: The case of Cen A and B2 0258+35

New HI observations of the nearby radio-loud galaxies Centaurus A and B2 0258+35 show broad absorption (Delta_v=400km/s) against the unresolved nuclei. Both sources belong to the cases where blue- and redshifted absorption is observed at the same time. In previous Cen A observations only a relative narrow range of redshifted absorption was detected. We show that the data suggest in both cases the existence of a circumnuclear disk. For Cen A the nuclear absorption might be the atomic counterpart of the molecular circumnuclear disk that is seen in CO and H_2. Higher resolution observations are now needed to locate the absorption and to further investigate the structure and kinematics of the central region of the AGN and the way the AGN are fueled.

astro-ph↗

arXiv2008

Cold gas & mergers: fundamental difference in HI properties of different types of radio galaxies?

We present results of a study of large-scale neutral hydrogen (HI) gas in nearby radio galaxies. We find that the early-type host galaxies of different types of radio sources (compact, FR-I and FR-II) appear to contain fundamentally different large-scale HI properties: enormous regular rotating disks and rings are present around the host galaxies of a significant fraction of low power compact radio sources, while no large-scale HI is detected in low power, edge-darkened FR-I radio galaxies. Preliminary results of a study of nearby powerful, edge-brightened FR-II radio galaxies show that these systems generally contain significant amounts of large-scale HI, often distributed in tail- or bridge-like structures, indicative of a recent galaxy merger or collision. Our results suggest that different types of radio galaxies may have a different formation history, which could be related to a difference in the triggering mechanism of the radio source. If confirmed by larger studies with the next generation radio telescopes, this would be in agreement with previous optical studies that suggest that powerful FR-II radio sources are likely triggered by galaxy mergers and collisions, while the lower power FR-I sources are fed in other ways (e.g. through the accretion of hot IGM). The giant HI disks/rings associated with some compact sources could - at least in some cases - be the relics of much more advanced mergers.

astro-ph↗

arXiv2007

Stellar populations, neutral hydrogen and ionised gas in field early-type galaxies

We present a study of the stellar populations of a sample of 39 local, field early-type galaxies whose HI properties are known from interferometric data. Our aim is to understand whether stellar age and chemical composition depend on the HI content of galaxies. As a by-product of our analysis, we also study their ionised gas content and how it relates to the neutral hydrogen gas. Stellar populations and ionised gas are studied from optical long-slit spectra. We determine stellar age, metallicity and alpha-to-iron ratio by analysing a set of Lick/IDS line-strength indices measured from the spectra after modelling and subtracting the ionised-gas emission. We do not find any trend in the stellar populations parameters with M(HI). However, we do find that, at stellar velocity dispersion below 230 km/s, 2/3 of the galaxies with less than 100 million solar masses of HI are centrally rejuvenated, while none of the HI-richer systems are. Furthermore, none of the more massive (velocity-dispersion>230 km/s) objects are centrally rejuvenated independently on their HI mass. Concerning the ionised gas, we detect emission in 60% of the sample. This is generally extended and always carachterised by LINER-like emission-line ratios at any radius. We find that a large HI mass is necessary (but not sufficient) for a galaxy to host bright ionised-gas emission. A plausible interpretation of our results is that gas-rich mergers play a significant role in E/S0 formation, especially at lower mass. Within this picture, HI-poor, centrally-rejuvenated objects could form in mergers where gas angular-momentum removal (and therefore inflow) is efficient; HI-rich galaxies with no significant age gradients (but possibly uniformly young) could be formed in interactions characterised by high-angular momentum gas.

astro-ph↗

arXiv2007

Centaurus A: Morphology and kinematics of the HI disk

We present first results of new ATCA HI emission and absorption observations of Centaurus A. The large-scale disk is described via a tilted-ring model. A broader redshifted absorption than previously known as well as a blueshifted absorption component against the nucleus is found.

astro-ph↗

arXiv2007

The shape of the dark matter halo in the early-type galaxy NGC 2974

We present HI observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected HI disc in this galaxy (Kim et al. 1988) is in fact a ring. By studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape. We construct mass models of NGC 2974 by combining the HI rotation curve with the central kinematics of the ionised gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionised gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the HI gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and Modified Newtonian Dynamics (MOND), which fit the data marginally worse. The mass-to-light ratio M/L_I increases in NGC 2974 from 4.3 (M/L_I)sun at one effective radius to 8.5 (M/L_I)sun at 5 Re. This increase of M/L already suggests the presence of dark matter: we find that within 5 Re at least 55 per cent of the total mass is dark.

astro-ph↗

arXiv2007

The interplay between radio jets and ISM in sub-kpc radio sources

This paper reviews the evidence for interaction between radio jets and their environment in small and intermediate (sub-kpc) scale radio sources. Observations of gas (both neutral hydrogen and ionised gas) have shown the presence of fast (> 1000 km/s) outflows likely originating from this interaction. The characteristics of these AGN-driven outflows (e.g. mass outflow rate) indicate that they may have a relevant impact on the evolution of the host galaxy. We also report on the detection of large HI disks found around the host galaxies of nearby compact radio sources. Similar structures have so far not been detected around large radio sources. The presence of these structures in relation to the evolution of young compact radio sources is discussed.

astro-ph↗

arXiv2007

Dominant Nuclear Outflow Driving Mechanisms in Powerful Radio Galaxies

In order to identify the dominant nuclear outflow mechanisms in Active Galactic Nuclei, we have undertaken deep, high resolution observations of two compact radio sources (PKS 1549-79 and PKS 1345+12) with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope. Not only are these targets known to have powerful emission line outflows, but they also contain all the potential drivers for the outflows: relativistic jets, quasar nuclei and starbursts. ACS allows the compact nature (<0.15") of these radio sources to be optically resolved for the first time. Through comparison with existing radio maps we have seen consistency in the nuclear position angles of both the optical emission line and radio data. There is no evidence for bi-conical emission line features on the large-scale and there is a divergance in the relative position angles of the optical and radio structure. This enables us to exclude starburst driven outflows. However, we are unable to clearly distinguish between radiative AGN wind driven outflows and outflows powered by relativistic radio jets. The small scale bi-conical features, indicative of such mechanisms could be below the resolution limit of ACS, especially if aligned close to the line of sight. In addition, there may be offsets between the radio and optical nuclei induced by heavy dust obscuration, nebular continuum or scattered light from the AGN.

astro-ph↗

arXiv2007

Fast neutral outflows in nearby radio galaxies: a major source of feedback

Fast (~1000 km/s) outflows of neutral gas (from 21-cm HI absorption) are detected in strong radio sources. The outflows occur, at least in some cases, at distances from the radio core that range between few hundred parsecs and kpc. These HI outflows likely originate from the interaction between radio jets and the dense surrounding medium. The estimated mass outflow rates are comparable to those of moderate starburst-driven superwinds. The impact on the evolution of the host galaxies is discussed.

astro-ph↗

arXiv2006

The Westerbork SINGS Survey: I. Overview and Image Atlas

We have obtained moderately deep radio continuum imaging at 18 and 22cm with the Westerbork array of 34 nearby galaxies drawn from the Spitzer SINGS and Starburst samples to enable complimentary analysis. The sub-sample have an optical major axis diameter in excess of 5 arcmin and are North of Declination 12.5 deg. Sub-sample galaxies span a very wide range of morphological types and star formation rates. Resolved detection was possible for every galaxy. This constitutes a first time detection at GHz radio frequencies for about half of the sample. Analysis of both total intensity and polarization properties of the sample will be published in companion papers. Both the HI and OH main-lines of the target galaxies were within the observed band-pass, albeit with only coarse velocity resolution. Only two low mass elliptical galaxies were undetected in HI. Four of the sub-sample galaxies were detected in OH main-line absorption, including two new detections. The results are presented in the form of an image atlas for which a standard transfer function and image size are used throughout and whereby the radio continuum, DSS optical and integrated HI are displayed side-by-side. Continuum and HI line photometry are tabulated for all targets.

astro-ph↗

arXiv2006

Minkowski's Object: A Starburst Triggered by a Radio Jet, Revisited

We present neutral hydrogen, ultraviolet, optical and near-infrared imaging, and optical spectroscopy, of Minkowski's Object (MO), a star forming peculiar galaxy near NGC 541. The observations strengthen evidence that star formation in MO was triggered by the radio jet from NGC 541. Key new results are the discovery of a 4.9E8 solar mass double HI cloud straddling the radio jet downstream from MO, where the jet changes direction and decollimates; strong detections of MO, also showing double structure, in UV and H-alpha; and numerous HII regions and associated clusters in MO. In UV, MO resembles the radio-aligned, rest-frame UV morphologies in many high redshift radio galaxies (HzRGs), also thought to be caused by jet-induced star formation. MO's stellar population is dominated by a 7.5 Myr-old, 1.9E7 solar mass instantaneous burst, with current star formation rate 0.52 solar masses per year (concentrated upstream from where the HI column density is high). This is unlike the jet-induced star formation in Centaurus A, where the jet interacts with pre-existing cold gas; in MO the HI may have cooled out of a warmer, clumpy intergalactic or interstellar medium as a result of jet interaction, followed by collapse of the cooling clouds and subsequent star formation (consistent with numerical simulations). Since the radio source that triggered star formation in MO is much less luminous, and therefore more common, than powerful HzRGs, and because the environment around MO is not particularly special in terms of abundant dense, cold gas, jet-induced star formation in the early universe might be even more prevalent than previously thought.

astro-ph↗

arXiv2005

Neutral hydrogen in radio galaxies: results from nearby, importance for far away

The study of neutral hydrogen emission and absorption in radio galaxies is giving new and important insights on a variety of phenomena observed in these objects. Such observations are helping to understand the origin of the host galaxy, the effects of the interaction between the radio jet and the ISM, the presence of fast gaseous outflows as well as jet-induced star formation. Recent results obtained on these phenomena are summarized in this review. Although the HI observations concentrate on nearby radio galaxies, the results also have relevance for the high-z objects as all these phenomena are important, and likely even more common, in high-redshift radio sources.

astro-ph↗

arXiv2005

Probing the nature of the ISM in Active Galactic Nuclei through HI absorption

The physical and kinematical conditions of the gas surrounding an active galactic nucleus (AGN) offer key diagnostics for understanding the processes occurring in the inner few kpc around the nucleus. Neutral hydrogen can give important insights on these regions. Apart from probing the presence of gas in relatively settled conditions (i.e. circumnuclear disks/tori) it can also trace the presence of extreme outflows. Some examples of these phenomena are briefly presented. For the study of the neutral hydrogen around AGN the high resolution offered by the VLBI is crucial in order to locate the regions where the absorption occurs and to study in detail the kinematics of the gas. Recent VLBI results are discussed here.

astro-ph↗

arXiv2005

The giant star forming halo associated with the radio galaxy PKS1932-46

We report the discovery of a giant (~160 kpc) knotty extended emission line nebula associated with the radio galaxy PKS1932-46 at z=0.23. The 2-d long slit spectra, obtained with VLT-FORS2 at a large angle (~63 degr) to the radio source axis, shows that the nebula extends well beyond the radio structure and the ionization cones of the active nucleus. This is one of the largest ionized nebulae yet detected around a radio galaxy at any redshift. The analysis of the ionization, morphological and kinematic properties of the knots suggests that these are star forming objects, probably compact HII galaxies. We propose that the giant structure is a star forming halo associated with the debris of the merger that triggered the activity. This study reinforces the view that radio galaxies are activated by major mergers which also trigger substantial star formation. The star formation activity can extend on the scale of a galaxy group, beyond the old stellar halo of the host galaxy.

astro-ph↗

arXiv2004

The interplay between radio-activity and the ISM in radio galaxies

Radio-loud AGNs can inhabit regions with a very rich ISM. The presence of this rich medium is likely related to the origin and evolution of the host galaxy and of the active nucleus. Recent observations show that a large fraction of radio galaxies contains a significant young stellar population. This supports the idea that mergers are responsible for both the starburst phase and the triggering of the nuclear activity. The gas that reaches the central regions can have quite disturbed kinematics, likely due to the effects of the AGN activity and in particular of the powerful radio jets. The recent detection of fast nuclear gas outflows, observed both in ionised and neutral gas, is giving new and important insights into the physical conditions of the gaseous medium around the nucleus and the interaction between the AGN and this medium. Finally, as another example of the interplay between the radio activity and the ISM, the possibility of star formation induced by the passage of the radio jet will be discussed.

astro-ph↗

arXiv2003

HI on large and small scales in starburst radio galaxies

The study of the optical continuum of radio galaxies shows that about 30% have a young stellar population component. Among them are the most far-IR bright radio galaxies. A further indication of the relatively gas rich environment of these galaxies (possibly related to the recent merger from which they originate) is the high fraction being detected in HI. Here we present recent results obtained from the study of neutral hydrogen (detected either in emission or absorption) in a group of starburst radio galaxies. In some objects, large-scale (tens of kpc) structures involving HI masses exceeding 10^9 M_sun are observed. In these cases, the HI can be used to study the origin and evolution of these systems and the timescales involved. In this respect, the parameters obtained from the study of the stellar populations and from the HI can be complementary. In other objects, very broad (> 1000 km/s), mostly blueshifted HI is detected in absorption. This result shows that, despite the extremely energetic phenomena occurring near an AGN - including the powerful radio jet - some of the outflowing gas remains, or becomes again, neutral. This can give new and important insights in the physical conditions of the gaseous medium around an AGN. The possible origin of the extreme kinematics is discussed.

astro-ph↗