Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

NGC 3998



Upload your image

DSS Images   Other Images

Related articles

The hot, warm and cold gas in Arp 227 - an evolving poor group
Arp 227 represents a prototypical example of an interacting mixed pairof galaxies located in a low-density environment. We investigate the gasproperties of the pair in the X-ray, Hα, HI and CO bands. We alsodetect two additional members of the group in HI which indicates thatthe pair constitutes the dominant members of a loose group.The HI distribution shows a tail of gas that connects the spiral member,NGC 470, to the lenticular, NGC 474, showing that the two main membersare currently undergoing interaction. The Hα emission reveals thepresence of secondary components at the centre of NGC 470, superposed onthe main component tracing the rotation of the galaxy. This latter mapsa nearly unperturbed velocity field. The dominant, nearly unperturbedtrend of the kinematics is confirmed by CO observations, althoughrestricted to the centre of the galaxy. The X-ray luminosity of NGC 470is comparable with that of a `normal' spiral galaxy. NGC 474 on theother hand is very gas-poor and has not been detected in Hα. ItsX-ray luminosity is consistent with the low end of the expected emissionfrom discrete sources.Arp 227 as a loose group shows several signatures of galaxy-galaxyinteraction. Our observations suggest the presence of signatures ofinteraction in the overall kinematics of the spiral companion. Theongoing interaction is clearly visible only in the outer HI halo of NGC470. While the large shell system of NGC 474 could be associated with anaccretion event, the secondary components in the Hα profile in thecentre of NGC 470 could be due to the interaction with the companion.The low X-ray luminosity of NGC 470 seems to be a characteristic ofdynamically young systems. All the above evidence suggest that Arp 227is an evolving group in the early phase of its evolution and that itsdrivers are the accretion of faint galaxies and the ongoing large-scaleinteraction between NGC 470 and 474.

On the Role of the Galactic Magnetic Halo in Ultra-High-Energy Cosmic-Ray Propagation
The study of the propagation of ultra-high-energy cosmic rays (UHECRs)is a key step to unveiling the secret of their origin. Up to now onlythe influence of the galactic and extragalactic magnetic fields wasconsidered. In this article we focus our analysis on the influence ofthe magnetic field of the galaxies standing between possible UHECRsources and us. Our main approach is to start from the well-known galaxydistribution up to 120 Mpc. We use the most complete galaxy catalog: theLEDA catalog. Inside a sphere of 120 Mpc, we extract 60,130 galaxieswith known positions. In our simulations we assign a halo dipolemagnetic field (HDMF) to each galaxy. The code developed is able toretro-propagate a charged particle from the arrival points of UHECR dataacross our galaxy sample. We present simulations in the case of theVirgo Cluster and show that there is a nonnegligible deviation in thecase of protons of 7×1019 eV, even if the B value isconservative. Then special attention is devoted to the AGASA triplet,where we find that NGC 3998 and NGC 3992 could be possible sourcecandidates.

Understanding the Nuclear Gas Dispersion in Early-Type Galaxies in the Context of Black Hole Demographics
The majority of nearby early-type galaxies contain detectable amounts ofemission-line gas at their centers. The nuclear gas kinematics form avaluable diagnostic of the central black hole (BH) mass. Here we analyzeand model Hubble Space Telescope STIS observations of a sample of 27galaxies; 16 Fanaroff-Riley Type I radio galaxies and 11 (more) normalearly-type galaxies. We focus here on what can be learned from thenuclear velocity dispersion (line width) of the gas as a complement tothe many studies dealing with gas rotation velocities. We find that thedispersion in a STIS aperture of ~0.1"-0.2" generally exceeds thelarge-scale stellar velocity dispersion of the galaxy. This isqualitatively consistent with the presence of central BHs but raises thequestions of whether the excess gas dispersion is of gravitational ornongravitational origin and whether the implied BH masses are consistentwith our current understanding of BH demography (as predicted by theM-σ relation between BH mass and stellar velocity dispersion). Toaddress this we construct purely gravitational axisymmetric dynamicalmodels for the gas, both thin-disk models and models with more generalaxis ratios and velocity anisotropies. For the normal galaxies thenuclear gas dispersions are adequately reproduced assuming disks aroundthe BHs with masses that follow the M-σ relation. In contrast, thegas dispersions observed for the radio galaxies generally exceed thosepredicted by any of the models. We attribute this to the presence ofnongravitational motions in the gas that are similar to or larger thanthe gravitational motions. The nongravitational motions are presumablydriven by the active galactic nucleus (AGN), but we do not find arelation between the radiative output of the AGN and thenongravitational dispersion. Given the uncertainties about the dynamicalstate of the gas, it is not possible to uniquely determine the BH massfor each galaxy from its nuclear gas dispersion. However, for the sampleas a whole the observed dispersions do not provide evidence forsignificant deviations from the M-σ relation.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.

On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.

The host galaxy/AGN connection in nearby early-type galaxies. A new view of the origin of the radio-quiet/radio-loud dichotomy?
This is the third in a series of three papers exploring the connectionbetween the multiwavelength properties of AGN in nearby early-typegalaxies and the characteristics of their hosts. Starting from aninitial sample of 332 galaxies, we selected 116 AGN candidates requiringthe detection of a radio source with a flux limit of ~1 mJy, as measuredfrom 5 GHz VLA observations. In Paper I we classified the objects withavailable archival HST images into "core" and "power-law" galaxies,discriminating on the basis of the nuclear slope of their brightnessprofiles. We used HST and Chandra data to isolate the nuclear emissionof these galaxies in the optical and X-ray bands, thus enabling us (oncecombined with the radio data) to study the multiwavelength behaviour oftheir nuclei. The properties of the nuclei hosted by the 29 coregalaxies were presented in Paper II Core galaxies invariably host aradio-loud nucleus, with a median radio-loudness of Log R = 3.6 and anX-ray based radio-loudness parameter of Log RX = -1.3. Herewe discuss the properties of the nuclei of the 22 "power-law" galaxies.They show a substantial excess of optical and X-ray emission withrespect to core galaxies at the same level of radio luminosity.Conversely, their radio-loudness parameters, Log R ˜ 1.6 and LogRX ˜ -3.3, are similar to those measured in Seyfertgalaxies. Thus the radio-loudness of AGN hosted by early-type galaxiesappears to be univocally related to the host's brightness profile:radio-loud AGN are only hosted by core galaxies, while radio-quiet AGNare found only in power-law galaxies. The brightness profile isdetermined by the galaxy's evolution, through its merger history; ourresults suggest that the same process sets the AGN flavour. In thisscenario, the black holes hosted by the merging galaxies rapidly sinktoward the centre of the newly formed object, setting its nuclearconfiguration, described by e.g. the total mass, spin, mass ratio, orseparation of the SMBHs. These parameters are most likely at the originof the different levels of the AGN radio-loudness. This connection mightopen a new path toward understanding the origin of theradio-loud/radio-quiet AGN dichotomy and provide us with a further toolfor exploring the co-evolution of galaxies and supermassive black holes.

Radiative transfer modeling of three-dimensional clumpy AGN tori and its application to NGC 1068
Recent observations of NGC 1068 and other AGN support the idea of ageometrically and optically thick dust torus surrounding the centralsupermassive black hole and accretion disk of AGN. In type 2 AGN, thetorus is seen roughly edge-on, leading to obscuration of the centralradiation source and a silicate absorption feature near 10 {μ m}.While most of the current torus models distribute the dust smoothly,there is growing evidence that the dust must be arranged in clouds. Wedescribe a new method for modeling near- and mid-infrared emission of3-dimensional clumpy tori using Monte Carlo simulations. We calculatethe radiation fields of individual clouds at various distances from theAGN and distribute these clouds within the torus region. The propertiesof the individual clouds and their distribution within the torus aredetermined from a theoretical approach of self-gravitating clouds closeto the shear limit in a gravitational potential. We demonstrate thatclumpiness in AGN tori can overcome the problem of over-pronouncedsilicate features. Finally, we present model calculations for theprototypical Seyfert 2 galaxy NGC 1068 and compare them to recenthigh-resolution measurements. Our model is able to reproduce both theSED and the interferometric observations of NGC 1068 in the near- andmid-infrared.

Molecular hydrogen and [FeII] in active galactic nuclei - II. Results for Seyfert 2 galaxies
Near-infrared spectroscopy is used to study the kinematics andexcitation mechanisms of H2 and [FeII] lines in a sampledominated by Seyfert 2 galaxies. The spectra simultaneously cover theJHK bands, allowing us to compare line fluxes emitted in the interval0.8-2.4 μm and avoiding aperture and seeing effects. TheH2 lines are systematically narrower than the narrow-lineregion lines, suggesting that, very likely, the H2 does notoriginate from the same parcel of gas that forms the narrow-line region.Emission-line ratios between H2 lines favour thermalexcitation mechanisms for the molecular gas in active galactic nuclei.It was found that non-thermal excitation contributes, at most, 30 percent of the observed H2. Thermal excitation is also confirmedby the rather similar vibrational and rotational temperatures in theobjects (~2000 K). The mass of hot H2 ranges from102 to 103Msolar, with nearly half ofobjects showing values of <500 Msolar. It shows that thefraction of molecular mass present in the nuclear region and emitting inthe near-infrared is a very small fraction of the warm molecular masspresent in the centre. A diagnostic diagram composed of the line ratiosH2/Brγ and [FeII]/Paβ proves to be a useful toolin the near-infrared for separating emission-line objects by theirdegree of nuclear activity. We found that active galactic nuclei arecharacterized by H2 2.121 μm/Brγ and [FeII] 1.257μm/Paβ flux ratios between 0.6 and 2. Starburst/HII galaxiesdisplay line ratios <0.6 while low-ionization nuclear emission-lineregions are characterized by values larger than 2 in either ratio.

The XMM-Newton Needles in the Haystack Survey: the local X-ray luminosity function of `normal' galaxies
In this paper we estimate the local (z < 0.22) X-ray luminosityfunction of `normal' galaxies derived from the XMM-Newton Needles in theHaystack Survey. This is an on-going project that aims to identifyX-ray-selected normal galaxies (i.e. non-AGN dominated) in the localUniverse. We are using a total of 70 XMM-Newton fields covering an areaof 11 deg2 which overlap with the Sloan Digital Sky SurveyData Release 2. Normal galaxies are selected on the basis of theirresolved optical light profile, their low X-ray-to-optical flux ratio[log(fx/fo) < - 2] and soft X-ray colours. Wefind a total of 28 candidate normal galaxies to the 0.5-8keV band fluxlimit of ~2 × 10-15ergcm-2s-1.Optical spectra are available for most sources in our sample (82 percent). These provide additional evidence that our sources are bona fidenormal galaxies with X-ray emission coming from diffuse hot gas emissionand/or X-ray binaries rather than a supermassive black hole. 16 of ourgalaxies have narrow emission lines or a late-type spectral energydistribution (SED) while the remaining 12 present only absorption linesor an early-type SED. Combining our XMM-Newton sample with 18 local (z< 0.22) galaxies from the Chandra Deep Field North and South surveys,we construct the local X-ray luminosity function of normal galaxies.This can be represented with a Schechter form with a break atL*~ 3+1.4-1.0×1041ergs-1 and a slope of α~ 1.78 +/- 0.12.Using this luminosity function and assuming pure luminosity evolution ofthe form ~(1 +z)3.3 we estimate a contribution to the X-raybackground from normal galaxies of ~10-20 per cent (0.5-8keV). Finally,we derive, for the first time, the luminosity functions for early- andlate-type systems separately.

Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.

The Relationship of Hard X-Ray and Optical Line Emission in Low-Redshift Active Galactic Nuclei
In this paper we assess the relationship of the population of activegalactic nuclei (AGNs) selected by hard X-rays to the traditionalpopulation of AGNs with strong optical emission lines. First, we studythe emission-line properties of a new hard-X-ray-selected sample of 47local AGNs (classified optically as Type 1 and 2 AGNs). We find that thehard X-ray (3-20 keV) and [O III] λ5007 optical emission-lineluminosities are well-correlated over a range of about 4 orders ofmagnitude in luminosity (mean luminosity ratio 2.15 dex with a standarddeviation of σ=0.51 dex). Second, we study the hard X-rayproperties of a sample of 55 local AGNs selected from the literature onthe basis of the flux in the [O III] line. The correlation between thehard X-ray (2-10 keV) and [O III] luminosity for the Type 1 AGNs isconsistent with what is seen in the hard-X-ray-selected sample. However,the Type 2 AGNs have a much larger range in the luminosity ratio, andmany are very weak in hard X-rays (as expected for heavily absorbedAGNs). We then compare the hard X-ray (3-20 keV) and [O III] luminosityfunctions of AGNs in the local universe. These have similar faint-endslopes, with a luminosity ratio of 1.60 dex (0.55 dex smaller than themean value for individual hard-X-ray-selected AGNs). We conclude that atlow redshift, selection by narrow optical emission lines will recovermost AGNs selected by hard X-rays (with the exception of BL Lacobjects). However, selection by hard X-rays misses a significantfraction of the local AGN population with strong emission lines.

The Swift/BAT High-Latitude Survey: First Results
We present preliminary results from the first 3 months of the SwiftBurst Alert Telescope (BAT) high Galactic latitude survey in the 14-195keV band. The survey reaches a flux of ~10-11 ergscm-2 s-1 and has ~2.7 arcmin (90% confidence)positional uncertainties for the faintest sources. This represents themost sensitive survey to date in this energy band. These data confirmthe conjectures that a high-energy-selected active galactic nucleus(AGN) sample would have very different properties from those selected inother bands and that it represents a ``true'' sample of the AGNpopulation. We have identified 86% of the 66 high-latitude sources.Twelve are Galactic-type sources, and 44 can be identified withpreviously known AGNs. All but five of the AGNs have archival X-rayspectra, enabling us to estimate the line-of-sight column densities andother spectral properties. Both of the z>0.11 objects are blazars.The median redshift of the others (excluding radio-loud objects) is0.012. We find that the column density distribution of these AGNs isbimodal, with 64% of the nonblazar sources having column densitiesNH>=1022 cm-2. None of the sourceswith logLX>43.5 (cgs units) show high column densities,and very few of the lower LX sources have low columndensities. Based on these data, we expect the final BAT catalog to have>200 AGNs and reach fluxes of less than ~10-11 ergscm-2 s-1 over the entire sky.

The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.

Silicate Emissions in Active Galaxies: From LINERs to QSOs
We report the first detection of ~10 and ~18 μm silicate dustemissions in a low-luminosity active galactic nucleus (AGN), obtained inSpitzer IRS 7-37 μm spectroscopy of the type 1 LINER galaxy NGC 3998.Silicate emissions in AGNs have only recently been detected in severalquasars. Our detection counters suggestions that silicate emissions arepresent only in the most luminous AGNs. The silicate features may besignatures of a dusty ``obscuring torus'' viewed face-on as postulatedfor type 1 AGNs. However, the apparently cool (~200 K) dust isinconsistent with theoretical expectations of much hotter torus walls.Furthermore, not all type 1 objects are silicate emission sources.Alternatively, the silicate emission may originate in dust not directlyassociated with a torus. We find that the long-wavelength (>~20μm) tail of the emission in NGC 3998 is significantly weaker than inthe sample of bright QSOs recently presented by Hao et al. The 10 μmprofile in our NGC 3998 spectrum is inconsistent with ``standard''silicate ISM dust. This may indicate differences in the dustcomposition, grain size distribution, or degree of crystallization. Thedifferences between NGC 3998, QSOs, and Galactic templates suggest thatthere are significant environmental variations.

The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies
LINER nuclei, which are present in many nearby galactic bulges, may bethe manifestation of low-rate or low-radiative-efficiency accretion ontosupermassive central black holes. However, it has been unclear whetherthe compact UV nuclear sources present in many LINERs are clusters ofmassive stars, rather than being directly related to the accretionprocess. We have used the Hubble Space Telescope to monitor the UVvariability of a sample of 17 galaxies with LINER nuclei and compactnuclear UV sources. Fifteen of the 17 galaxies were observed more thanonce, with two to five epochs per galaxy, spanning up to a year. Wedetect significant variability in most of the sample, with peak-to-peakamplitudes from a few percent to 50%. In most cases, correlatedvariations are seen in two independent bands (F250W and F330W).Comparison to previous UV measurements indicates, for many objects,long-term variations by factors of a few over decade timescales.Variability is detected in LINERs with and without detected compactradio cores, in LINERs that have broad Hα wings detected in theiroptical spectra (``LINER 1s''), and in those that do not (``LINER 2s'').This variability demonstrates the existence of a nonstellar component inthe UV continuum of all types and sets a lower limit to the luminosityof this component. Interestingly, all the LINERs that have detectedradio cores have variable UV nuclei, as one would expect from bona fideactive galactic nuclei. We note a trend in the UV color (F250W/F330W)with spectral type-LINER 1s tend to be bluer than LINER 2s. This trendmay indicate a link between the shape of the nonstellar continuum andthe presence or the visibility of a broad-line region. In one target,the poststarburst galaxy NGC 4736, we detect variability in a previouslynoted UV source that is offset by 2.5" (~60 pc in projection) from thenucleus. This may be the nearest example of a binary active nucleus andof the process leading to black hole merging.Based on observations with the Hubble Space Telescope, which is operatedby AURA, Inc., under NASA contract NAS 5-26555.

A Simple Test for the Existence of Two Accretion Modes in Active Galactic Nuclei
By analogy to the different accretion states observed in black holeX-ray binaries (BHXBs), it appears plausible that accretion disks inactive galactic nuclei (AGNs) undergo a state transition between aradiatively efficient and inefficient accretion flow. If the radiativeefficiency changes at some critical accretion rate, there will be achange in the distribution of black hole masses and bolometricluminosities at the corresponding transition luminosity. To test thisprediction, I consider the joint distribution of AGN black hole massesand bolometric luminosities for a sample taken from the literature. Thesmall number of objects with low Eddington-scaled accretion ratesm˙<0.01 and black hole massesMBH<109Msolar constitutes tentativeevidence for the existence of such a transition in AGNs. Selectioneffects, in particular those associated with flux-limited samples,systematically exclude objects in particular regions of the(MBH,Lbol) plane. Therefore, they requireparticular attention in the analysis of distributions of black holemass, bolometric luminosity, and derived quantities such as theaccretion rate. I suggest further observational tests of the BHXB-AGNunification scheme that are based on the jet domination of the energyoutput of BHXBs in the hard state, and of the possible equivalence ofBHXB in the very high (or steep power-law) state showing ejections andefficiently accreting quasars and radio galaxies with powerful radiojets.

Origin of Radio Emission from Nearby Low-Luminosity Active Galactic Nuclei
We use the observational data in radio, optical, and X-ray wave bandsfor a sample of active galactic nuclei (AGNs) with measured black holemasses to explore the origin of radio emission from nearbylow-luminosity active galactic nuclei (LLAGNs). The maximal luminosityof an advection-dominated accretion flow (ADAF) can be calculated for agiven black hole mass, as there is a critical accretion rate above whichthe ADAF is no longer present. We find that the radio luminosities arehigher than the maximal luminosities expected from the ADAF model formost sources in this sample. This implies that the radio emission ispredominantly from the jets in these sources. The radio emission from asmall fraction of the sources (15/60; referred to as radio-weak sources)in this sample can be explained by the ADAF model. However, comparingthe observed multiband emission data with the spectra calculated for theADAF or adiabatic inflow-outflow solution (ADIOS) cases, we find thatneither ADAF nor ADIOS models can reproduce the observed multibandemission simultaneously, with reasonable magnetic field strengths, forthese radio-weak sources. A variety of other possibilities arediscussed, and we suggest that the radio emission is probably dominatedby jet emission even in these radio-weak LLAGNs.

Probing the Dust Obscuration in Seyfert Galaxies using Infrared Spectroscopy. II. Implication for the Unification of Seyfert Galaxies
We report near-IR spectroscopic observations of 11 Seyfert galaxies (sixSeyfert 1s, one Seyfert 1.9, and four Seyfert 2s) and additionalgalaxies (four LINERs, two H II, and one type 2 transition) forcomparison, obtained using the Gemini twin-channel near-IR camera on theShane 3 m telescope at Lick Observatory. With the unique design of theGemini camera, full J and K spectra were taken simultaneously throughthe same slit. This produced accurate line ratios of hydrogenrecombination lines over a large wavelength baseline. For the Seyfert 1s(<=1.5), the line ratios of Paβ/Brγ are not onlycomparable in both broad- and narrow-line regions but also consistentwith case B recombination, indicating little or no reddening in bothnarrow- and broad-line regions. Seyfert 2 (>1.5) galaxies, however,show substantial reddening toward the narrow-line regions. We compareoptical reddening data from the literature and find significant supportfor the dichotomy between Seyfert 1s and Seyfert 2s, at least inlow-luminosity objects. Two different scenarios are explored to explainthe observed difference in reddening: a difference in reddening due toan extended dusty torus under active galactic nucleus unification, and adifference due to a different grain size distribution between the twoSeyfert types. We also discuss a similar potential difference found inthe strength of the 9.7 μm silicate line, along with a possiblecorrelation between the narrow-line reddening and the strength of thesilicate absorption line. We also analyzed CO band head absorptionfeatures longward of 2.3 μm to look for nonstellar contamination andevidence of recent star formation activity. The CO band head in Seyfert1s shows heavy contamination from nonstellar radiation, which iscorrelated with an H-K nuclear color excess. We confirm that the COspectroscopic indices in both Seyfert types do not show evidence ofrecent star formation. Taking the nonstellar contamination into account,there is little evidence from the CO index for a difference in starformation rates in the nuclei of Seyfert 1s and Seyfert 2s in ourlow-luminosity sample.

Silicate emission in active galaxies - From LINERs to QSOs.
Not Available

Extended very cold dust in the interacting HI ring galaxy pair NGC 2293 / 2292
The LGG 138 galaxy group members NGC 2292 and NGC 2293 were imaged withISOPHOT in the far-infrared (FIR) at {60 μ m}, {100 μ m}, and {200μ m}. While no FIR emission is seen at {60 μ m}, and only very lowlevel emission is present at {100 μ m}, compact FIR emission fromboth NGC 2292 and NGC 2293 galaxy centres and extended emission likelyassociated with tidally removed dust and the HI ring surrounding NGC2292 / 2293 is strongly detected at {200 μ m}. Additionally, acompact FIR source associated with the neighbouring galaxy NGC 2295 isstrongly detected at {200 μ m}. Remarkably, none of these threegalaxies have been detected individually in 21 cm HI emission. Thesteeply rising far-infrared spectral energy distribution of theapparently interacting NGC 2292 / 2293 pair towards longer wavelengthsindicates the thermal emission of very cold dust with a temperature of13 K, much lower than typical values of interacting systems or evenquiescent spiral galaxies. The FIR data of this galaxy group clearlyshows for the first time that there could be FIR dust emission notaccompanied by HI, that dust even in an interacting system can have avery low dust temperature, and furthermore that gravitationalinteraction can give rise to an extended diffuse dust distribution.

The host galaxy/AGN connection in nearby early-type galaxies. Sample selection and hosts brightness profiles
This is the first of a series of three papers exploring the connectionbetween the multiwavelength properties of AGNs in nearby early-typegalaxies and the characteristics of their hosts. We selected twosamples, both with high resolution 5 GHz VLA observations available andproviding measurements down to 1 mJy level, reaching radio-luminositiesas low as 1019 W Hz-1. We focus on the 116radio-detected galaxies as to boost the fraction of AGN with respect toa purely optically selected sample. Here we present the analysis of theoptical brightness profiles based on archival HST images, available for65 objects. We separate early-type galaxies on the basis of the slope oftheir nuclear brightness profiles, into core and power-law galaxiesfollowing the Nuker's scheme, rather than on the traditionalmorphological classification (i.e. into E and S0 galaxies). Our sampleof AGN candidates is indistinguishable, when their brightness profilesare concerned, from galaxies of similar optical luminosity but hostingweaker (or no) radio-sources. We confirm previous findings thatrelatively bright radio-sources (Lr > 1021.5 WHz-1) are uniquely associated to core galaxies. However,below this threshold in radio-luminosity core and power-law galaxiescoexist and they do not show any apparent difference in theirradio-properties. Not surprisingly, since our sample is deliberatelybiased to favour the inclusion of active galaxies, we found a higherfraction of optically nucleated galaxies. Addressing the multiwavelengthproperties of these nuclei will be the aim of the two forthcomingpapers.

Radio sources in low-luminosity active galactic nuclei. IV. Radio luminosity function, importance of jet power, and radio properties of the complete Palomar sample
We present the completed results of a high resolution radio imagingsurvey of all ( 200) low-luminosity active galactic nuclei (LLAGNs) andAGNs in the Palomar Spectroscopic Sample of all ( 488) bright northerngalaxies. The high incidences of pc-scale radio nuclei, with impliedbrightness temperatures ≳107 K, and sub-parsec jetsargue for accreting black holes in ≳50% of all LINERs andlow-luminosity Seyferts; there is no evidence against all LLAGNs beingmini-AGNs. The detected parsec-scale radio nuclei are preferentiallyfound in massive ellipticals and in type 1 nuclei (i.e. nuclei withbroad Hα emission). The radio luminosity function (RLF) of PalomarSample LLAGNs and AGNs extends three orders of magnitude below, and iscontinuous with, that of “classical” AGNs. We find marginalevidence for a low-luminosity turnover in the RLF; nevertheless LLAGNsare responsible for a significant fraction of present day massaccretion. Adopting a model of a relativistic jet from Falcke &Biermann, we show that the accretion power output in LLAGNs is dominatedby the kinetic power in the observed jets rather than the radiatedbolometric luminosity. The Palomar LLAGNs and AGNs follow the samescaling between jet kinetic power and narrow line region (NLR)luminosity as the parsec to kilo-parsec jets in powerful radio galaxies.Eddington ratios {l_Edd} (=L_Emitted/L_Eddington) of≤10-1{-}10-5 are implied in jet models of theradio emission. We find evidence that, in analogy to Galactic black holecandidates, LINERs are in a “low/hard” state (gas poornuclei, low Eddington ratio, ability to launch collimated jets) whilelow-luminosity Seyferts are in a “high” state (gas richnuclei, higher Eddington ratio, less likely to launch collimated jets).In addition to dominating the radiated bolometric luminosity of thenucleus, the radio jets are energetically more significant thansupernovae in the host galaxies, and are potentially able to depositsufficient energy into the innermost parsecs to significantly slow thegas supply to the accretion disk.

A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125

Close stars and accretion in low-luminosity active galactic nuclei
Quasar accretion discs are believed to form stars by self-gravity.Low-luminosity active galactic nuclei (LLAGNs) are much dimmer galacticcentres, and are often believed to be quasars that ran out of gaseousfuel. LLAGN accretion discs should thus coexist with thousands tomillions of stars or protostars left from the previous strongeraccretion activity. In principle, these stars may produce severalimportant effects: (i) contribute to the optical/ultraviolet spectra ofsome LLAGNs; (ii) the dusty discs could reprocess stellar radiation inthe infrared frequencies and then dominate the LLAGN spectra in thatregion; (iii) deplete the (accretion) gas disc much faster than it canaccrete on to the supermassive black hole; (iv) stars, individually orin groups, may slow down and modulate the accretion flow significantlydue to their inertia. In this way they may produce the LLAGN cut-offdiscs; (v) alternatively, frequent enough stellar collisions andresulting stellar disruptions could keep the inner disc empty. Here weexplore these ideas. We find that, despite `low' luminosities of LLAGNs,unrealistically high stellar densities are required to make a sizableradiative contribution to the Hubble Space Telescope optical/ultravioletspectra of these galactic nuclei. Stellar contribution to the infraredspectrum is more likely. Further, if LLAGNs are in a quasi-steady statefor as long as 107 yr or more, too high stellar densitieswould again be required to significantly affect the dynamics ofaccretion flow. However, if LLAGNs are `short-lived' phenomena, e.g.t<~ 105 yr, the low-activity states of quiescence-outburstcycles, then embedded stars may be much more important through the masseffects (iii)-(v). With observations of LLAGNs becoming progressivelybetter, it will be more and more difficult to neglect the presence ofclose stars in and around nuclear accretion discs.

Structural parameters of nearby emission-line galaxies
We present the results of an investigation on the main structuralproperties derived from VRI and Hα surface photometry of galaxieshosting nuclear emission-line regions [including Seyfert 1, Seyfert 2,low-ionization nuclear emission region (LINER) and starburst galaxies]as compared with normal galaxies. Our original sample comprises 22active galaxies, four starbursts and one normal galaxy and has beenextended with several samples obtained from the literature. Bulge anddisc parameters, along with the bulge-to-disc luminosity ratio, havebeen derived applying an iterative procedure. The resulting parametershave been combined with additional data in order to reach astatistically significant sample. We find some differences in the bulgedistribution across the different nuclear types that could implyfamilies of bulges with different physical properties. Bulge and disccharacteristic colours have been defined and derived for our sample andcompared with a control sample of early-type objects. The resultssuggest that bulge and disc stellar populations are comparable in normaland active galaxies.

Cold Dust in Early-Type Galaxies. I. Observations
We describe far-infrared observations of early-type galaxies selectedfrom the Infrared Space Observatory (ISO) archive. This ratherinhomogeneous sample includes 39 giant elliptical galaxies and 14 S0 (orlater) galaxies. These galaxies were observed with the array photometerPHOT on-board the ISO satellite using a variety of different observingmodes-sparse maps, mini-maps, oversampled maps, and singlepointings-each of which requires different and often rather elaboratephotometric reduction procedures. The ISO background data agree wellwith the COBE-DIRBE results to which we have renormalized ourcalibrations. As a further check, the ISO fluxes from galaxies at 60 and100 μm agree very well with those previously observed with IRAS atthese wavelengths. The spatial resolution of ISO is several timesgreater than that of IRAS, and the ISO observations extend out to 200μm, which views a significantly greater mass of colder dust notassessable to IRAS. Most of the galaxies are essentially point sourcesat ISO resolution, but a few are clearly extended at FIR wavelengthswith image sizes that increase with FIR wavelength. The integratedfar-infrared luminosities do not correlate with optical luminosities,suggesting that the dust may have an external, merger-related origin. Ingeneral, the far-infrared spectral energy distributions can be modeledwith dust at two temperatures, ~43 and ~20 K, which probably representlimits of a continuous range of temperatures. The colder dust componentdominates the total mass of dust, 106-107Msolar, which is typically more than 10 times larger than thedust masses previously estimated for the same galaxies using IRASobservations. For S0 galaxies we find that the optically normalizedfar-infrared luminosity LFIR/LB correlatesstrongly with the mid-infrared luminosityL15μm/LB, but that correlation is weaker forelliptical galaxies.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and United Kingdom) and with the participation of ISAS andNASA.

A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.

Testing Radiatively Inefficient Accretion Flow Theory: An XMM-Newton Observation of NGC 3998
We present the results of a 10 ks XMM-Newton observation of NGC 3998, a``type I'' LINER galaxy (i.e., with significant broad Hαemission). Our goal is to test the extent to which radiativelyinefficient accretion flow (RIAF) models and/or scaled-down activegalactic nuclei (AGNs) models are consistent with the observedproperties of NGC 3998. A power-law fit to the XMM-Newton spectraresults in a power-law slope of Γ=1.9 and 2-10 keV flux of1.1×10-11 ergs cm-2 s-1, inexcellent agreement with previous hard X-ray observations. The OM UVflux at 2120 Å appears to be marginally resolved, with ~50% of theflux extended beyond 2". The nuclear component of the 2120 Å fluxis consistent with an extrapolation of the X-ray power law, although~50% of the flux may be absorbed. The OM U flux lies significantly abovethe X-ray power-law extrapolation and contains a significantcontribution from extragalactic emission. The upper limit for narrow FeK emission derived from the XMM-Newton spectra is 33 eV (forΔχ2=2.7). The upper limit for narrow Fe K emissionderived from a combined fit of the XMM-Newton and BeppoSAX spectra is 25eV, which is one of the strictest limits to date for any AGN. Thissignificantly rules out Fe K emission, which is expected to be observedin typical Seyfert 1 galaxies. The X-ray flux of NGC 3998 has not beenobserved to vary significantly (at >30% level) within the X-rayobservations, and only between observations at a level of ~50%, which isalso in contrast to typical Seyfert 1 galaxies. The lack of anyreflection features suggests that any optically thick, geometricallythin accretion disk must be truncated, probably at a radius of order100-300 (in Schwarzschild units). RIAF models fit the UV to X-rayspectral energy distribution of NGC 3998 reasonably well. In thesemodels the mid-IR flux also constrains the emission from any outer thindisk component that might be present. The UV to X-ray spectral energydistribution (SED) is also consistent with a Comptonized thin disk witha very low accretion rate (M<10-5MEdd), inwhich case the lack of Fe K emission may be due to an ionized accretiondisk. Accretion models in general do not account for the observed radioflux of NGC 3998, and the radio flux may be due to a jet. Recent jetmodels may also be consistent with the nuclear fluxes of NGC 3998 ingeneral, including the X-ray, optical/UV, and mid-IR bands. The(ground-based) near-IR to optical photometric data for the nuclearregion of NGC 3998 contain large contributions from extranuclearemission. We also derive nuclear fluxes using archival Hubble SpaceTelescope WFPC2 data, resulting in meaningful constraints to the nuclearSED of NGC 3998 in the optical band.

The Stellar Populations of Low-Luminosity Active Galactic Nuclei. II. Space Telescope Imaging Spectrograph Observations
We present a study of the stellar populations of low-luminosity activegalactic nuclei (LLAGNs). Our goal is to search for spectroscopicsignatures of young and intermediate-age stars and to investigate theirrelationship with the ionization mechanism in LLAGNs. The method used isbased on the stellar population synthesis of the optical continuum ofthe innermost (20-100 pc) regions in these galaxies. For this purpose,we have collected high spatial resolution optical (2900-5700 Å)STIS spectra of 28 nearby LLAGNs that are available in the Hubble SpaceTelescope archive. The analysis of these data is compared with a similaranalysis also presented here for 51 ground-based spectra of LLAGNs. Ourmain findings are as follows: (1) No features due to Wolf-Rayet starswere convincingly detected in the STIS spectra. (2) Young starscontribute very little to the optical continuum in the ground-basedaperture. However, the fraction of light provided by these stars ishigher than 10% in most of the weak-[O I] ([OI]/Hα<=0.25) LLAGNSTIS spectra. (3) Intermediate-age stars contribute significantly to theoptical continuum of these nuclei. This population is more frequent inobjects with weak than with strong [O I]. Weak-[O I] LLAGNs that haveyoung stars stand out for their intermediate-age population. (4) Most ofthe strong-[O I] LLAGNs have predominantly old stellar population. A fewof these objects also show a featureless continuum that contributessignificantly to the optical continuum. These results suggest that youngand intermediate-age stars do not play a significant role in theionization of LLAGNs with strong [O I]. However, the ionization inweak-[O I] LLAGNs with young and/or intermediate-age populations couldbe due to stellar processes. A comparison of the properties of theseobjects with Seyfert 2 galaxies that harbor a nuclear starburst suggeststhat weak-[O I] LLAGNs are the lower luminosity counterparts of theSeyfert 2 composite nuclei.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. Based on observations made with the Nordic OpticalTelescope (NOT), operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrofísica deCanarias.

Inner-truncated Disks in Galaxies
We present an analysis of the disk brightness profiles of 218 spiral andlenticular galaxies. At least 28% of disk galaxies exhibit innertruncations in these profiles. There are no significant trends oftruncation incidence with Hubble type, but the incidence among barredsystems is 49%, more than 4 times that for nonbarred galaxies. However,not all barred systems have inner truncations, and not allinner-truncated systems are currently barred. Truncations represent areal dearth of disk stars in the inner regions and are not an artifactof our selection or fitting procedures nor the result of obscuration bydust. Disk surface brightness profiles in the outer regions are wellrepresented by simple exponentials for both truncated and nontruncateddisks. However, truncated and nontruncated systems have systematicallydifferent slopes and central surface brightness parameters for theirdisk brightness distributions. Truncation radii do not appear tocorrelate well with the sizes or brightnesses of the bulges. Thissuggests that the low angular momentum material apparently missing fromthe inner disk was not simply consumed in forming the bulge population.Disk parameters and the statistics of bar orientations in our sampleindicate that the missing stars of the inner disk have not simply beenredistributed azimuthally into bar structures. The sharpness of thebrightness truncations and their locations with respect to othergalactic structures suggest that resonances associated with diskkinematics, or tidal interactions with the mass of bulge stars, might beresponsible for this phenomenon.

Statistical properties of local active galactic nuclei inferred from the RXTE 3-20 keV all-sky survey
We compiled a sample of 95 AGNs serendipitously detected in the 3-20 keVband at Galactic latitude |b|>10o during the RXTE slewsurvey (XSS, Revnivtsev et al. 2004), and utilize it to study thestatistical properties of the local population of AGNs, including theX-ray luminosity function and absorption distribution. We find thatamong low X-ray luminosity (L3-20< 1043.5 ergs-1) AGNs, the ratio of absorbed (characterized by intrinsicabsorption in the range 1022cm-21041 erg s-1 estimated here issmaller than the earlier estimated total X-ray volume emissivity in thelocal Universe, suggesting that a comparable X-ray flux may be producedtogether by lower luminosity AGNs, non-active galaxies and clusters ofgalaxies. Finally, we present a sample of 35 AGN candidates, composed ofunidentified XSS sources.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/469

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Constellation:Ursa Major
Right ascension:11h57m56.30s
Aparent dimensions:2.818′ × 2.291′

Catalogs and designations:
Proper Names   (Edit)
NGC 2000.0NGC 3998

→ Request more catalogs and designations from VizieR