Do LINER 2 galaxies harbour low-luminosity active galactic nuclei?

We use ROSAT HRI spatial data and ASCA spectral measurements for a sample of seven nearby, early-type spiral galaxies, to address the question of whether a low-luminosity active galactic nucleus (LLAGN) is present in galaxies that have a LINER 2 classification. The brightest discrete X-ray source in the ROSAT HRI observations is invariably found to be positionally coincident with the optical galactic nucleus, and in most cases its flux dominates the X-ray emission from the central region of the galaxy. All seven galaxies have X-ray spectra consistent with a two-component, soft thermal plus hard power-law, spectral form. If we exclude the two galaxies with relatively hard X-ray spectra, NGC 3628 and NGC 4594, for which there is supporting evidence for a LLAGN (or alternatively in the case of NGC 3628 a dominant ultraluminous X-ray binary), then the remaining galaxies show surprisingly similar X-ray spectral properties. Specifically the flux ratio F _X(0.5–1)/ F _X(2–5) , which measures the relative strengths of the thermal and non-thermal emission components, shows little scatter about a mean of 0.66, a value very similar to that measured in the classic starburst galaxy NGC 253. As there is no obvious reason why the luminosity of the hard power-law continuum emanating from a putative LLAGN should be very closely correlated with the thermal emission of the surrounding region, this suggests that that the broad-band (0.5–5 keV) X-ray emission from these LINER 2 galaxies may originate in a common set of processes probably associated with the starburst phenomenon. Conversely, it appears that in many LINER 2 galaxies and perhaps the majority, the nuclear X-ray luminosity does not derive directly from the presence of a LLAGN.     

Infrared–red cores in nearby elliptical galaxies

We present the Spitzer Space Telescope Infrared Array Camera observations for a sample of local elliptical galaxies to study later stages of active galactic nucleus (AGN) activity. A sample of 36 elliptical galaxies is selected from the Palomar spectroscopic survey. We detect nuclear non-stellar infrared emission in nine of them. There is unambiguous evidence of circumnuclear dust in these nine galaxies in their optical images. We also find a remarkable correlation between the infrared excess emission and the nuclear radio/X-ray emission, suggesting that infrared excess emission is tightly related to nuclear activity. The possible origin of infrared excess emission from hot dust heated by the central AGN is supported by the spectral indices of the infrared excess emission.     

The Origin of Infrared Emission from the Infrared Luminous Galaxy NGC 4418

We present a study of the origin of infrared (IR) emission in the optically normal, infrared luminous galaxy NGC 4418. By decomposing the stellar absorption features and continua in the range of 3600-8000 A from the Sloan Digital Sky Survey into a set of simple stellar populations, we derive the stellar properties for the nuclear region of NGC 4418. We compare the observed infrared luminosity with the one derived from the starburst model, and find that star-forming activity contributes only 7% to the total IR emission, that as the IR emission region is spatially very compact, the most possible source for the greater part of the IR emission is a deeply embedded AGN, though an AGN component is found to be unnecessary for fitting the optical spectrum.     

Photometric properties of the Arp 220 super star clusters

We investigate the photometric properties of six super stellar clusters (SSCs) seen in both the optical and near-infrared Hubble Space Telescope images of the local ultraluminous starburst galaxy Arp 220. Three of the SSCs are located in the central 0.5-kpc region. The remaining three are in the circumnuclear region between 0.5 and 2.5 kpc from the centre. Comparing the observed spectral energy distributions (SEDs) of the SSCs with the Starburst99 models of Leitherer et al., we confirm that all three nuclear SSCs are heavily obscured     Considering the results from this comparison in conjunction with measurements of the near-infrared CO absorption index and of millimetre CO linewidths and luminosities, we estimate the ages of the nuclear SSCs to be 10⁷–10⁸ yr. The bolometric luminosity of the three nuclear SSCs is at most one-fifth of the total bolometric luminosity of Arp 220. On the other hand, the circumnuclear SSCs have little internal extinction     These contribute negligibly to the total bolometric luminosity.     

Young Populations in the Nuclei of Barred Galaxies

We have conducted UBVRI and H_α CCD photometry of five barred galaxies (NGC 2523, NGC 2950, NGC 3412, NGC 3945 and NGC 5383),along with SPH simulations, in order to understand the origin of young stellar populations in the nuclei of barred galaxies. The H_α emission, which is thought to be emitted by young stellar populations, is either absent or strongly concentrated in the nuclei of early-type galaxies (NGC 2950, NGC 3412 and NGC 3945),while they are observed in the nuclei and circumnuclear regions of intermediate-type galaxies with strong bars (NGC 2523 and NGC 5383).SPH simulations of realistic mass models for these galaxies show that some disc material can be driven into the nuclear region by a strong bar potential. This implies that the young stellar populations in the circumnuclear regions of barred galaxies can be formed out of such gas. The existence of nuclear dust lanes is an indication of an ongoing gas inflow and extremely young stellar populations in these galaxies, because nuclear dust lanes such as those in NGC 5383 are not long-lasting features according to our simulations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nuclear Starburst Activity in the Seyfert 2 Galaxy NGC 2273

We present spectrophotometric results of the Seyfert 2 galaxy NGC 2273. The presence of high-order Balmer absorption lines (H8, H9, H10) and weak equivalent widths of Call K A3933, CN A4200, G-band A4300 and MgIb 5173 clearly indicate recent star-forming activity in the nuclear region. Using a simple stellar population synthesis model, we find that for the best fit, the contributions of a power-law featureless continuum, an intermediate-age (~ 108 yr) and an old (> 109yr) stellar population to the total light at the reference normalization wavelength are 10.0%, 33.4% and 56.6%, respectively. The existence of recent starburst activity is also consistent with its high far-infrared luminosity (log LFIR/L = 9.9), its infrared color indexes [a(25,60) = -1.81 and a(60,100) = -0.79, typical values for Seyfert galaxies with circumnuclear starburst], and its q-value (2.23, ratio of infrared to radio flux, very similar to that of normal spirals and starburst galaxies). Byrd et al. have suggested that NGC 2273 mig     

A new superwind galaxy: XMM–Newton observations of NGC 6810

We present the first imaging X-ray observation of the highly inclined  ( i = 78°)  Sab Seyfert 2 galaxy NGC 6810 using XMM–Newton , which reveals soft X-ray emission that extends out to a projected height of ∼7 kpc away from the plane of the galaxy. The soft X-ray emission beyond the optical disc of the galaxy is most plausibly extraplanar, although it could instead come from large galactic radius. This extended X-ray emission is spatially associated with diffuse Hα emission, in particular with a prominent 5-kpc-long Hα filament on the north-west of the disc. A fraction ≲35 per cent of the total soft X-ray emission of the galaxy arises from projected heights  | z | ≥ 2 kpc  . Within the optical disc of the galaxy the soft X-ray emission is associated with the star-forming regions visible in ground-based Hα and XMM–Newton optical monitor near-UV imaging. The temperature, supersolar α-element-to-iron abundance ratio, soft X-ray/Hα correlation, and X-ray to far-infrared (FIR) flux ratio of NGC 6810 are all consistent with local starbursts with winds, although the large base radius of the outflow would make NGC 6810 one of the few 'disc-wide' superwinds currently known. Hard X-ray emission from NGC 6810 is weak, and the total   E = 2–10 keV  luminosity and spectral shape are consistent with the expected level of X-ray binary emission from the old and young stellar populations. The X-ray observations provide no evidence of any active galactic nucleus activity. We find that the optical, IR and radio properties of NGC 6810 are all consistent with a starburst galaxy, and that the old classification of this galaxy as a Seyfert 2 galaxy is probably incorrect.     

H i content and star formation in the interacting galaxy Arp86

We present the results of Giant Metrewave Radio Telescope (GMRT) observations of the interacting system Arp86 in both neutral atomic hydrogen, H  i , and in radio continuum at 240 606 and 1394 MHz. In addition to H  i emission from the two dominant galaxies, NGC 7752 and NGC 7753, these observations show a complex distribution of H  i tails and bridges due to tidal interactions. The regions of highest column density appear related to the recent sites of intense star formation. H  i column densities  ∼1–1.5 × 10²¹ cm⁻²  have been detected in the tidal bridge which is bright in Spitzer image as well. We also detect H  i emission from the galaxy 2MASX J23470758+2926531, which is shown to be a part of this system. We discuss the possibility that this could be a tidal dwarf galaxy. The radio continuum observations show evidence of a non-thermal bridge between NGC 7752 and NGC 7753, and a radio source in the nuclear region of NGC 7753 consistent with it having a low-ionization nuclear emission region nucleus.     

The active galaxy NGC 4151: Archetype or exception?

Summary. The Seyfert galaxy NGC 4151 harbors in its nucleus the most intensively studied AGN (Active Galactic Nucleus). Among the brightest AGN (in apparent luminosity) it is the most widely variable and the variations of its ultraviolet and X-ray spectrum have been studied on time scales ranging from hours to decades. These observations have formed the basis of methods and models which have been found to generally apply to broad emission line AGN: the rich and complex relation between the X-ray and UV variations, the comptonization model of the X-ray spectrum from medium X-ray to -rays, the reverberation mapping, the stratification in velocity and physical conditions of the gas in the broad line region, and a method to estimate the black hole mass from emission line variability. The large barred spiral which hosts this nucleus has been extensively studied especially in the central region. Inflow of gas along the and possibly also the orbits have been detected, but since the accretion disk is not in the galactic plane (as evidenced by the significant angle separating the radio axis and the rotation axis of the galaxy) the incoming gas seen on kpcs scale must, as it flows further inward, move out of the galactic plane, along trajectories which are entirely unknown. There is more to learn on NGC 4151. In fact, the best is yet to come. Three avenues of investigation appear particularly promising: 1) The variations in flux and spectral shape of the X-ray continuum and its relationship with the UV variations are the key to understanding the specifics of the Comptonization model. Progress on this point will come from repeated simultaneous observations of the UV spectrum and of the entire X-ray and -ray spectrum. This will also give insights on the structure of the disk in the last stable orbits, the formation and structure of the corona and in the end, the process of energy production. Exciting results on these topics are expected in the near future from Chandra-AXAF, XMM and INTEGRAL. The Chandra and XMM (which have short energy range) main contributions will, however, be line diagnostics and for Chandra, imaging of the soft diffuse emission. 2) The search for the gas inflow which merges into and/or forms the torus could finally be successful. Several powerful approaches are possible: observing molecular lines in emission with millimeter arrays of increasing baseline and collecting area; using the nuclear radio structure as background source to observe free-free and atomic or molecular lines in absorption. 3) The observations of NGC 4151 during a state of deep minimum will provide a unique oportunity to observe the X-ray spectrum of a Seyfert 1 nucleus at epochs of very low accretion rate, to identify the nature of the narrow variable lines, to determine the stellar population of a currently active nucleus, and measure the mass of the black hole from the stellar lines. NGC 4151 at minimum states should be a target of opportunity for all space missions. In addition, observations on time scales of 10 years or more, especially following a deep minimum, will allow one to map emitting regions of size up to 1pc, thereby overlapping with the linear scale directly mapped with large radio telescopes. Received 30 October 1999 / Published online: 24 March 2000     

Nuclear star formation in the hotspot galaxy NGC 2903

We present high-resolution near-infrared imaging obtained using adaptive optics and HST /NICMOS, and ground-based spectroscopy of the hotspot galaxy NGC 2903. Our near-infrared resolution imaging enables us to resolve the infrared hotspots into individual young stellar clusters or groups of these. The spatial distribution of the stellar clusters is not coincident with that of the bright H  ii regions, as revealed by the HST /NICMOS Pa α image. Overall, the circumnuclear star formation in NGC 2903 shows a ring-like morphology with an approximate diameter of 625 pc.
The star formation properties of the stellar clusters and H  ii regions have been studied using the photometric and spectroscopic information in conjunction with evolutionary synthesis models. The population of bright stellar clusters shows a very narrow range of ages, 4–7×10⁶ yr after the peak of star formation, or absolute ages 6.5–9.5×10⁶ yr (for the assumed short-duration Gaussian bursts), and luminosities similar to the clusters found in the Antennae interacting galaxy. This population of young stellar clusters accounts for some 7–12 per cent of the total stellar mass in the central 625 pc of NGC 2903. The H  ii regions in the ring of star formation have luminosities close to that of the supergiant H  ii region 30 Doradus, they are younger than the stellar clusters, and they will probably evolve into bright infrared stellar clusters similar to those observed today. We find that the star formation efficiency in the central regions of NGC 2903 is higher than in normal galaxies, approaching the lower end of infrared luminous galaxies.     

Star formation in southern Seyfert galaxies

We have produced radio maps, using the Australia Telescope Compact Array, of the central regions of six southern type 2 Seyfert galaxies (NGC 1365, 4945, 6221, 6810, 7582 and Circinus) with circumnuclear star formation, to estimate the relative contribution of star formation activity compared to activity from the active galactic nucleus (AGN). The radio morphologies range from extended diffuse structures to compact nuclear emission, with no evidence, even in the relatively compact sources, for synchrotron self-absorption. In each case the radio to far-infrared (FIR) ratio has a value consistent with star formation, and in all but one case the radio to [Fe  II ] ratio is also consistent with star formation. We derive supernova rates and conclude that, despite the presence of a Seyfert nucleus in these galaxies, the radio, FIR and [Fe  II ] line emissions are dominated by processes associated with the circumnuclear star formation (i.e. supernova remnants and H  II regions) rather than with the AGN.     

Chandra HRC and HST observations of NGC 6240: resolving the active galactic nucleus and starburst

We present high spatial resolution X-ray Chandra HRC and HST WFPC2 H α observations of the prototypical infrared-luminous galaxy NGC 6240. The central region of this system shows a remarkably complex morphology, with filaments and loops observed in the optical and X-rays. The total X-ray luminosity is dominated by the extended emission. Both nuclei are clearly detected in the HRC image and both appear to be extended. The energetics of the nuclei imply that the southern nucleus is the more plausible counterpart to the obscured active galactic nucleus. The overall spectral energy distribution of the galaxy is in good agreement with a blend of starburst and AGN components that have similar bolometric luminosities,   L _bol∼5×10⁴⁵ erg s^-1  , with the starburst dominating the observed continuum in the near-infrared ( K band), optical and soft X-ray bands.     

The X-ray spectra of Compton-thick Seyfert 2 galaxies as seen by BeppoSAX

Results from BeppoSAX observations of Compton-thick Seyfert 2 galaxies are summarized and reviewed, and their general properties derived and discussed. In five out of the seven observed sources, the nucleus is directly visible at high X-ray energies, where the photons penetrate absorbers with column densities in the range 1.1–4.3×10²⁴ cm⁻² (in the other two sources, NGC 1068 and NGC 7674, the nucleus is instead totally obscured at all energies, implying even larger column densities). In most sources there is unambiguous evidence of a reflection component from optically thick, cold matter, while in two (or maybe four) cases there is also evidence of reflection from ionized matter. For the sources with a measured X-ray luminosity, a comparison with the infrared luminosity is made; while in two cases (the Circinus galaxy and NGC 4945) the IR emission appears to be dominated by starburst activity, in the other three sources (NGC 6240, Mrk 3 and TOL 0109-383) it is likely to be dominated by reprocessing of the UV and X-ray photons emitted by an active galactic nucleus.     

ASCA spectroscopy of the luminous infrared galaxy NGC 6240: X-ray emission from a starburst and a buried active nucleus

We present an X-ray spectroscopic study of the prototype far-infrared galaxy NGC 6240 from ASCA . The soft X-ray spectrum (below 2 keV) shows clear signatures of thermal emission well described by a multitemperature optically thin plasma, which probably originates in a powerful starburst. Strong hard X-ray emission is also detected with ASCA and its spectrum above 3 keV is extremely flat with a prominent iron K line complex, very similar to that seen in the Seyfert 2 galaxy NGC 1068 but about an order of magnitude more luminous ( L _3−10keV ≈ 1.4 × 10⁴² erg s⁻¹). The hard X-ray spectrum indicates that only reflected X-rays of an active galactic nucleus (AGN) buried in a heavy obscuration ( N _H > 2 × 10²⁴ cm⁻²) are visible. This is evidence for an AGN in NGC 6240, emitting possibly at a quasar luminosity (∼ 10⁴⁵ erg s⁻¹), and suggests its significant contribution to the far-infrared luminosity.     

A ROSAT survey of Wolf–Rayet galaxies — II. The extended sample

We present results from an ongoing X-ray survey of Wolf–Rayet (WR) galaxies, a class of objects believed to be very young starbursts. This paper extends the first X-ray survey of WR galaxies by Stevens &38; Strickland by studying WR galaxies identified subsequent to the original WR galaxy catalogue of Conti.   Out of a sample of 40 new WR galaxies a total of 10 have been observed with the ROSAT PSPC, and of these seven have been detected (NGC 1365, NGC 1569, I Zw 18, NGC 3353, NGC 4449, NGC 5408 and a marginal detection of NGC 2366). Of these, all are dwarf starbursts except for NGC 1365, which is a barred spiral galaxy possibly with an active nucleus. We also report on observations of the related emission-line galaxy IRAS 0833+6517.   The X-ray properties of these galaxies are broadly in line with those found for the original sample; they are X-ray overluminous compared with their blue luminosity and have thermal spectra with typically kT  ∼ 0.4 − 1.0 keV. There are some oddities: NGC 5408 is very overluminous in X-rays, even compared with other WR galaxies; I Zw 18 has a harder X-ray spectrum; NGC 1365, although thought to contain an active nucleus, has X-ray properties that are broadly similar to other WR galaxies, and we suggest that the X-ray emission from NGC 1365 is due to starburst activity.   A good correlation between X-ray and blue luminosity is found for the WR galaxy sample as a whole. However, when just dwarf galaxies are considered there is little evidence of correlation. We discuss the implications of these results on our understanding of the X-ray emission from WR galaxies and suggest that the best explanation for the X-ray activity is starburst activity from a young starburst region.     

The Seyfert-Liner Galaxy NGC 7213: An XMM-Newton Observation

We examine the XMM X-ray spectrum of the low-ionisation nuclear emission-line region (LINER)-AGN NGC 7213, which is best fit with a power law, Kα emission lines from Fe i, Fe xxv and Fe xxvi and a soft X-ray collisionally ionised thermal plasma with kT = 0.18^+0.03_−0.01 keV. We find a luminosity of 7× 10⁻⁴ L_Edd, and a lack of soft X-ray excess emission, suggesting a truncated accretion disc. NGC 7213 has intermediate X-ray spectral properties, between those of the weak AGN found in the LINER M 81 and higher luminosity Seyfert galaxies. This supports the notion of a continuous sequence of X-ray properties from the Galactic Centre through LINER galaxies to Seyferts, likely determined by the amount of material available for accretion in the central regions. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).     

A chemically decoupled nucleus and inner polar ring of the SBb galaxy NGC 4548

Our inzvestigation of the central region in NGC 4548, a bright Sb galaxy with a large-scale bar, using the Multipupil Field Spectrograph of the 6-m telescope revealed a chemically decoupled compact stellar nucleus with [Fe/H]=+0.6 and [Mg/Fe]=+0.1...+0.2 and with a mean stellar-population age of 5 Gyr. This nucleus, a probable circumnuclear disk coplanar with the global galactic disk, is embedded in the bulge whose stars are generally also young, T≈4 Gyr, although they are a factor of 2.5 more metal-poor. The bulge of NGC 4548 is triaxial and has a de Vaucouleurs surface-brightness profile; the unusual characteristics of its stellar population suggest the bulge formation or completion in the course of secular evolution in the triaxial potential of the global bar. The ionized gas within 3″ of the NGC 4548 nucleus rotates in a plane inclined to the principal symmetry plane of the galaxy, possibly, even in its polar plane, which may also result from the action of the large-scale bar.     

Circumnuclear kinematics in NGC 5248: the origin of nuclear spiral arms

We present for the first time a two-dimensional velocity field of the central region of the grand-design spiral galaxy NGC 5248, at 0.9-arcsec spatial resolution. The H α velocity field is dominated by circular rotation. While no systematic streaming motions are seen in the area of the nuclear grand-design spiral or the circumnuclear ring, the amplitude of residual velocities, after subtracting a model circular velocity field, reaches 20 km s⁻¹ in projection. The rotation curve levels out at around 140 km s⁻¹, after a well-resolved and rather shallow rise. We have generated an analytical model for the nuclear spiral and fitted it to our observations to obtain estimates of the pattern speed of the spiral and the speed of sound in the central region of NGC 5248. Our results are consistent with a low pattern speed, suggesting that the nuclear spiral rotates with the same rate as the main spiral structure in NGC 5248, and thus that the spiral structure is coupled from scales of a few hundred parsecs to several kiloparsecs. We have also compared the observed structure and kinematics between the nuclear regions of NGC 5248 and M100. Several similarities and differences are discussed, including the location of the peak emission regions on major and minor axes, and the spiral arm streaming motions. We find no kinematic evidence for the presence of a nuclear bar in NGC 5248.     

The host galaxies of nearby Seyfert 1 nuclei

We use optical and infrared imaging data of a complete hard X-ray selected sample of AGN to investigate the host galaxies of nearby Seyfert 1 nuclei. We disentangle the emission due to the unresolved point source and the resolved host galaxy by means of luminosity profile fitting. The stellar colours in the nucleus are redder inB — V andV — K than in normal spiral bulges or current stellar evolutionary models, probably due to dust obscuration.J — H colour is quite normal butH — K usually very red, probably indicating a large contribution from reradiating hot dust to the IR colours. No significant difference in IR colours oi the stellar component is found with respect to a comparison sample of Liners and starbursts, except that the reradiating dust in Seyferts tends to be hotter and lie closer to the nucleus.B — V colour gets slightly bluer towards the nucleus, whereasV — K andJ — K have red gradients.B — V andJ — K gradients are not correlated and are likely to be caused by different mechanisms. There is a clear correlation between the AGN and host luminosity. Finally, we present optical colour ratio maps of two Seyfert nuclei, NGC 3227 and NGC 7469. The blue, ringlike or elongated structure in the maps and its correspondence with molecular CO geometry is interpreted as evidence for circumnuclear star formation in these AGN.     

Near-Infrared Spectral Mapping of NGC 1614

We present the spatial distribution of the bright near-infrared emission lines, Br, H₂, He I, [Fe II], and the CO band longwards of 2.3 m, for the luminous infrared galaxy NGC 1614. The morphology of the ionised gas is different from that of the stellar light, and possibly forms a circumnuclear ring. Our data imply that the stellar population is older and the extinction is lower in the nucleus relative to the surrounding circumnuclear ring. We suggest that NGC 1614 is a galaxy whose recent interaction triggered massive star formation in the nucleus, which in turn caused a radially outward progression of star formation thereby producing the circumnuclear ring we observe today. There is no evidence for a buried AGN, and it is difficult to reconcile our data with the simple evolutionary model of ultraluminous galaxies proposed by Sanders et al (1988).