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Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Magnetic fields in halos of spiral galaxies
Observations of magnetic fields in halos of edge-on disk galaxies arediscussed in relation to the interstellar disk-halo interface in diskgalaxies. The distribution of extra-planar diffuse ionized gascorrelates on local and global scales with cosmic rays and magneticfields as inferred from observations of the non-thermal radio continuumradiation and its polarisation. From the polarisation a large-scale andwell-ordered magnetic field in these gaseous halos can be deduced. Forseveral objects a significant poloidal component of the halo field islikely. These observations indicate the presence of physical processeswhich generate and maintain magnetic fields on galactic scales. Theimportance of differential rotation of the gaseous halos for suchprocesses is briefly discussed.
| The structure of galactic disks. Studying late-type spiral galaxies using SDSS
Using imaging data from the SDSS survey, we present the g' and r' radialstellar light distribution of a complete sample of ~90 face-on tointermediate inclined, nearby, late-type (Sb-Sdm) spiral galaxies. Thesurface brightness profiles are reliable (1 σ uncertainty lessthan 0.2 mag) down to μ27 mag/''. Only ~10% of all galaxies havea normal/standard purely exponential disk down to our noise limit. Thesurface brightness distribution of the rest of the galaxies is betterdescribed as a broken exponential. About 60% of the galaxies have abreak in the exponential profile between 1.5-4.5 times thescalelength followed by a downbending, steeper outer region. Another~30% shows also a clear break between 4.0-6.0 times thescalelength but followed by an upbending, shallower outer region. A fewgalaxies have even a more complex surface brightness distribution. Theshape of the profiles correlates with Hubble type. Downbending breaksare more frequent in later Hubble types while the fraction of upbendingbreaks rises towards earlier types. No clear relation is found betweenthe environment, as characterised by the number of neighbours, and theshape of the profiles of the galaxies.
| ESO imaging survey: optical follow-up of 12 selected XMM-Newton fields
This paper presents the data recently released for the XMM-Newton/WFIsurvey carried out as part of the ESO Imaging Survey (EIS) project. Theaim of this survey is to provide optical imaging follow-up data in BVRIfor identification of serendipitously detected X-ray sources in selectedXMM-Newton fields. In this paper, fully calibrated individual andstacked images of 12 fields as well as science-grade catalogs for the 8fields located at high-galactic latitude are presented. These productswere created, calibrated and released using the infrastructure providedby the EIS Data Reduction system and its associated EIS/MVM imageprocessing engine, both of which are briefly described here. The datacovers an area of ~3 square degrees for each of the four passbands. Themedian seeing as measured in the final stacked images is 0.94 arcsec,ranging from 0.60 arcsec and 1.51 arcsec. The median limiting magnitudes(AB system, 2´´ aperture, 5σ detection limit) are25.20, 24.92, 24.66, and 24.39 mag for B-, V-, R-, and I-band,respectively. When only the 8 high-galactic latitude fields are includedthese become 25.33, 25.05, 25.36, and 24.58 mag, in good agreement withthe planned depth of the survey. Visual inspection of images andcatalogs, comparison of statistics derived from the present data withthose obtained by other authors and model predictions, as well as directcomparison of the results obtained from independent reductions of thesame data, demonstrate the science-grade quality of the automaticallyproduced final images and catalogs. These survey products, together withtheir logs, are available to the community for science exploitation inconjunction with their X-ray counterparts. Preliminary results from theX-ray/optical cross-correlation analysis show that about 61% of thedetected X-ray point sources in deep XMM-Newton exposures have at leastone optical counterpart within 2´´ radius down to R ≃25 mag, 50% of which are so faint as to require VLT observations therebymeeting one of the top requirements of the survey, namely to producelarge samples for spectroscopic follow-up with the VLT, whereas only 15%of the objects have counterparts down to the DSS limiting magnitude.
| The multi-phase gaseous halos of star forming late-type galaxies. I. XMM-Newton observations of the hot ionized medium
This study presents first results from an X-ray mini-survey carried outwith XMM-Newton to investigate the diffuse Hot Ionized Medium in thehalos of nine nearby star-forming edge-on spiral galaxies. Diffusegaseous X-ray halos are detected in eight of our targets, covering awide range of star formation rates from quiescent to starburst cases.For four edge-on spiral galaxies, namely NGC 3044, NGC 3221, NGC 4634,and NGC 5775, we present the first published high resolution/sensitivitydetections of extended soft X-ray halos. EPIC X-ray contour mapsoverlaid onto Hα imaging data reveals that in all cases thepresence of X-ray halos is correlated with extraplanar Diffuse IonizedGas. Moreover, these halos are also associated with non-thermal cosmicray halos, as evidenced by radio continuum observations. SupplementalUV-data obtained with the OM-telescope at 210 nm show Diffuse IonizedGas to be well associated with UV emission originating in the underlyingdisk. Beside NGC 891, NGC 4634 is the second non-starburst galaxy with adiffuse soft X-ray halo (|z|≤ 4 kpc). In case of NGC 3877, for whichwe also present the first high resolution X-ray imaging data, no haloemission is detectable. EPIC pn spectra (0.3-12 keV) of the diffuseX-ray emission are extracted at different offset positions from thedisk, giving evidence to a significant decrease of gas temperatures,electron densities, and gas masses with increasing distance to theplane. A comparison between dynamical and radiative cooling time scalesimplies that the outflow in all targets is likely to be sustained. Wefind very strong indications that spatially correlated multi-phasegaseous halos are created by star forming activity in the disk plane. Ina forthcoming paper, we will present multi-frequency luminosityrelations and evaluate key parameters which might trigger the formationof multi-phase galaxy halos.
| Astrophysical magnetic fields and nonlinear dynamo theory
Electronic Article Available from Elsevier Science.
| 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.
| X-ray bright optically inactive galaxies in XMM-Newton/Sloan Digital Sky Survey fields: more diluted than absorbed?
We explore the properties of X-ray bright optically inactive galaxies(XBONGs) detected in the 0.5-8 keV spectral band in 20 public XMM-Newtonfields overlapping with the Sloan Digital Sky Survey (SDSS). Weconstrain our sample to optically extended systems withlogfX/fopt > -2 that have spectroscopicidentifications available from the SDSS (r < 19.2 mag). The resultingsample contains 12 objects with LX(0.5-8 keV) = 5 ×1041-2 × 1044 erg s-1 in theredshift range 0.06 < z < 0.45. The X-ray emission in four casesis extended, suggesting the presence of hot gas associated with acluster or group of galaxies. The X-ray spectral fits show that twoadditional sources are best fit with a thermal component emission (kT~ 1keV). Three sources are most likely associated with active galacticnuclei (AGNs): their X-ray spectrum is described by a steep photon indexΓ~ 1.9 typical of unobscured AGNs, while they are very luminous inX-rays [LX(0.5-8 keV) ~ 1043-1044 ergs-1]. Finally, three more sources could be associated witheither normal galaxies or unobscured low-luminosity AGNs (LX< 1042 erg s-1). We find no evidence forsignificant X-ray absorbing columns in any of our XBONGs. The abovesuggest that XBONGs, selected in the total 0.5-8 keV band, comprise amixed bag of objects primarily including normal elliptical galaxies andtype 1 AGNs whose optical nuclear spectrum is probably diluted by thestrong stellar continuum. Nevertheless, as our sample is notstatistically complete we cannot exclude the possibility that a fractionof optically fainter XBONGs may be associated with heavily obscuredAGNs.
| 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.
| XMM-Newton Observations of Optically Selected Sloan Digital Sky Survey Clusters
We explore the X-ray properties of a subset of the optically selectedSloan Digital Sky Survey (SDSS) cluster sample of Goto et al. byanalyzing seven public XMM-Newton pointings, with exposure times rangingfrom ~4 to 46 ks. There are in total 17 SDSS clusters out of which onlyeight are detected at X-ray wavelengths withf0.5-2keV>~1.2×10-14 ergs cm-2s-1. For the remaining nine SDSS clusters, we estimate their3 σ luminosity upper limits (corresponding toLX<~5×1042 ergs s-1 in the0.5-2 keV band). This relatively low luminosity suggests that if theyare real structures, these galaxy aggregations correspond to poor groupsof galaxies. Using the SDSS photometric catalog, we also derive thecluster optical r-band luminosities. The resulting scaling relations(Lopt-LX, Lopt-TX) areconsistent with those of other recent studies.
| A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion
We present results from a high-resolution X-ray imaging study of nearbyLINERs observed by ACIS on board Chandra. This study complements andextends previous X-ray studies of LINERs, focusing on the underexploredpopulation of nearby dust-enshrouded infrared-bright LINERs. The sampleconsists of 15 IR-bright LINERs (LFIR/LB>3),with distances that range from 11 to 26 Mpc. Combining our sample withprevious Chandra studies, we find that ~51% (28/55) of the LINERsdisplay compact hard X-ray cores. The nuclear 2-10 keV luminosities ofthe galaxies in this expanded sample range from ~2×1038to ~2×1044 ergs s-1. We find that the mostextreme IR-faint LINERs are exclusively active galactic nuclei (AGNs).The fraction of LINERs containing AGNs appears to decrease with IRbrightness and increase again at the highest values ofLFIR/LB. We find that of the 24 LINERs showingcompact nuclear hard X-ray cores in the expanded sample that wereobserved at Hα wavelengths, only eight actually show evidence of abroad line. Similarly, of the 14 LINERs showing compact nuclear hardX-ray cores with corresponding radio observations, only eight display acompact flat spectrum radio core. These findings emphasize the need forhigh-resolution X-ray imaging observations in the study of IR-brightLINERs. Finally, we find an intriguing trend in the Eddington ratioversus LFIR and LFIR/LB for theAGN-LINERs in the expanded sample that extends over 7 orders ofmagnitude in L/LEdd. This correlation may imply a linkbetween black hole growth, as measured by the Eddington ratio, and thestar formation rate, as measured by the far-IR luminosity andIR-brightness ratio. If the far-IR luminosity is an indicator of themolecular gas content in our sample of LINERs, our results may furtherindicate that the mass accretion rate scales with the host galaxy's fuelsupply. We discuss the potential implications of our results in theframework of black hole growth and AGN fueling in low-luminosity AGNs.
| X-ray emission from NGC 1808: more than a complex starburst
Earlier observations of NGC 1808 in various wavebands (X-ray, optical,near-infrared, radio) provided evidence for the existence of either astarburst or a Seyfert 2 nucleus. We here present the results ofmultiwavelength XMM-Newton and Chandra observations, which directlyprove the co-existence of thermal diffuse plasma and non-nuclearunresolved point-like sources associated with the starburst activity,along with a Low Luminosity Active Galactic Nucleus (LLAGN) or an UltraLuminous X-ray source (ULX). The broad bandwidth of XMM-Newton allows usto show that the unresolved nuclear source in NGC 1808 dominates thehard X-ray spectrum, while the emission in the soft regime, below 1 keV,is dominated by a thermal component associated to an extended starburst.Both EPIC and RGS data provide reliable detections of a number ofemission lines from heavy elements, with abundances ranging from roughly0.7 to 2.2 Z_ȯ for different elements. However, no 6.4 keV FeKα fluorescence line emission was detected. The analysis of thenuclear region of NGC 1808 allows us to detect and disentangle thecontribution of an unresolved nuclear X-ray source and the starburstregion, but the exact nature of the nucleus remains unknown. Theobserved luminosity of NGC 1808 is L2{-10keV}=(1.61±0.06)×1040 erg s-1. Acomparison of our OM 212 nm image with a CTIO 4-m telescope Hαframe shows a good general correspondence between the emission frommassive stars and warm ionized gas, with minor deviations near the endsof the bar in NGC 1808. An aditional, very soft thermal spectralcomponent with kT≃ 0.1 keV has been discovered in the XMM-Newtonspectral analysis, which most likely originates from the halo of NGC1808.
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| Neutral hydrogen gas in 7 high-inclination spiral galaxies. I. The data
High-sensitivity interferometric H i line observations of a small sampleof seven galaxies with limiting column densities of a few times1019 cm-2 are presented. A tilted ring modelfitting routine was used to determine some global characteristics of theH i distribution and kinematics in the galaxy disks. 4 of the 7 galaxieshave low maximum rotation velocities of 125 km s-1,indicating that they are low-mass systems. Visual inspection shows thatat least one galaxy, NGC 4700, exhibits signs of extraplanar H iemission. An in-depth search for H i gas in the galaxy halos and thedetermination of halo gas properties, based on three-dimensionalmodeling, will follow in a separate publication. Companion galaxies weredetected in H i line emission near 3 of the 7 sample galaxies: NGC 1511,NGC 4565 and NGC 4700. One of these, NGC 1511, is found to be stronglyinteracting and is therefore not suitable for a study of the dependenceof its halo properties on the level of star formation activity in theunderlying disk. In the case of NGC 4700 the companion galaxy has novisible influence on its gas kinematics, while NGC 4565 might beaffected by its interaction with two small companions.Figures [see full text] and Appendix A are only available in electronicform at http://www.edpsciences.org
| Intergalactic neutral hydrogen gas in the Grus quartet of galaxies
Australia Telescope Compact Array multi-configuration mosaicing of theGrus quartet of galaxies reveals the presence of spectacular tidalstructures. 2.1×109 Mȯ of neutralatomic hydrogen (H I) gas, i.e. 11% of all H I in the group, are foundto be dragged from NGC 7582 into intergalactic space. About1.34×109 Mȯ of H I gas are contained ina tidal tail emanating from the north-western disk of NGC 7582, with aprojected length of about 85 kpc and width of up to 32 kpc and arelative velocity with respect to the centre of NGC 7582 of 130-140 kms-1. 7.7×108 Mȯ of H Ireside in an intergalactic H I cloud 48 kpc West of NGC 7582, whichmight originate from the disk of NGC 7582 as well and has no opticalcounterpart in a red Digital Sky Survey (DSS) image. These observationsprove that tidal stripping is occurring in the Grus quartet and thattidal features in compact groups can be potentially importantcontributors of metal-enriched matter to the intergalactic medium. Thetidal features around NGC 7582 cover an area of about 2000kpc2, almost doubling the group's cross-section forLyman-α absorption of light from background sources compared tothe optical extent of the member galaxies.
| On the alignment between binary spiral galaxies
We show some significance against the null hypothesis of randominteractions of binary spiral galaxies, and in favour of the alternativethat more interactions than expected occur for axes either nearlyparallel (spins being parallel or anti-parallel) or nearly orthogonal.We discuss this in the context of similar prior studies, using adifferent statistical focus in such a way that we are able toincorporate additional data.
| XMM-Newton Observations of Starburst Galaxies
We report on the results of XMM-Newton observations of nearbystarburst galaxies that form part of a multi-wavelength study of allphases of the extraplanar interstellar medium (ISM) in externalgalaxies. This study is conducted in order to assess the importance ofhalos as repositories of a metal-enriched medium and their significancein terms of galactic chemical evolution and possible metal enrichment ofthe intergalactic medium (IGM). Here we shortly summarize our findingsbased on XMM-Newton observations of NGC 1511 and NGC 1808 andpresent preliminary results for NGC 4666 and NGC 3628.
| XMM-Newton Observations of Nearby Edge-On Starburst Galaxies
We report on the results of XMM-Newton observations of nearby starburstgalaxies that form part of a multi-wavelength study of all phases ofextraplanar gas in external galaxies, which is conducted in order toassess the importance of halos as repositories of a metal-enrichedmedium and their significance in terms of galactic chemical evolutionand possible metal enrichment of the intergalactic medium (IGM).XMM-Newton observations of the starburst galaxy NGC 1511 revealed e.g.the presence of a previously unknown extended hot gaseous phase of itsinterstellar medium (ISM), which partly extends out of the disk plane.We also present preliminary results based on XMM-Newton observations ofNGC 1808, NGC 4666 and NGC 3628.
| The Superwind Galaxy NGC 4666: Gravitational Interactions and the Influence of the Resulting Starburst on the Interstellar Medium
We present high-resolution observations of the atomic and molecular gasphase of the ``superwind'' galaxy NGC 4666. Deep Very Large Array (VLA)H I observations of NGC 4666 and its surroundings reveal the presence ofprominent tidal arms, which provide clear evidence that the galaxy isinteracting with its neighbor NGC 4668 and a newly discovered dwarfcompanion. This interaction is also evident from a dynamical analysis,which shows that the diffuse H I envelope around NGC 4666 (radii >19kpc) is kinematically altered with regard to the central disk. Thisinteraction has likely caused the starburst activity in NGC 4666. A cutthrough the H I emission distribution perpendicular to the major axisprovides no evidence for the existence of an H I halo: this distributioncan best be described by an inner disk and two outer spiral arms. Ourhigh-resolution Owens Valley Radio Observatory (OVRO) CO mosaic showsthat the molecular gas is distributed relatively uniformly in thecentral ~7 kpc; the total molecular gas mass is~1.0×1010 Msolar. A kinematicallyinteresting CO feature that can be interpreted as an expanding molecularsupershell is detected near the footpoint of one of the Hαoutflows at the turnover of galactic rotation. About 1% of the totalenergy input of the starburst would be needed to create this feature. Astudy of the CO (2-->1) to CO (1-->0) line-transition ratio(obtained with the IRAM 30 m and Swedish-ESO Submillimeter Telescope[SEST] telescopes) shows that there is no significant variation of themolecular gas excitation over the central galaxy disk[S(2-->1)/S(1-->0)~=0.85] on scales of ~2.5 kpc. This can beexplained by the unusually uniform star formation rate across NGC 4666'sdisk, which sets this system apart from most other starburst systems.The total amount of molecular and atomic gas isMmol~=3.0×1010Msolar, whichimplies that NGC 4666 can sustain many similar starburst episodes(consuming ~108 Msolar each) in the future. Twonew dwarf companions in the NGC 4666 group are detected, with adetection limit of 107 Msolar, over an area of~1.6×104 kpc2. We speculate that the huge HI envelope around NGC 4666 may represent a low-redshift counterpart fordamped Lyα systems seen at higher redshifts.
| An Unbiased Census of Active Galactic Nuclei in the Two Micron All Sky Survey
We present an unbiased near-IR-selected AGN sample, covering 12.56deg2 down to Ks~15.5, selected from the Two MicronAll Sky Survey (2MASS). Our only selection effect is a moderate colorcut (J-Ks>1.2) designed to reduce contamination fromGalactic stars. We observed both pointlike and extended sources. Usingthe brute-force capabilities of the Two Degree Field multifiberspectrograph on the Anglo-Australian Telescope, we obtained spectra of65% of the target list: an unbiased subsample of 1526 sources. Eightypercent of the 2MASS sources in our fields are galaxies, with a medianredshift of 0.15. The remainder are K and M dwarf stars. We findtentative evidence that Seyfert 2 nuclei are more common in ourIR-selected survey than in blue-selected galaxy surveys. We estimatethat 5.1+/-0.7% of the galaxies have Seyfert 2 nuclei with Hαequivalent widths greater than 0.4 nm, measured over a spectroscopicaperture of radius ~2.5 kpc. Blue-selected galaxy samples only findSeyfert 2 nuclei meeting these criteria in ~1.5% of galaxies. We findthat 1.2+/-0.3% of our sources are broadline (type 1) AGNs, giving asurface density of 1.0+/-0.3 deg2, down toKs<15.0. This is the same surface density of type 1 AGNsas optical samples down to B<18.5. Our type 1 AGNs, however, mostlylie at low redshifts, and host galaxy light contamination would make~50% of them hard to find in optical QSO samples. We conclude that thetype 1 AGN population found in the near-IR is not dramatically differentfrom that found in optical samples. There is no evidence for a largepopulation of AGNs that could not be found at optical wavelengths,although we can only place very weak constraints on any population ofdusty high-redshift QSOs. In contrast, the incidence of type 2(narrow-line) AGNs in a near-IR-selected galaxy sample seems to behigher than in a blue-selected galaxy sample.
| The composite starburst/AGN nature of the superwind galaxy NGC 4666
We report the discovery of a Compton-thick AGN and of intensestar-formation activity in the nucleus and disk, respectively, of thenearly edge-on superwind galaxy NGC 4666. Spatially unresolved emissionis detected by BeppoSAX only at energies <10 keV, whereas spatiallyresolved emission from the whole disk is detected by XMM-Newton. Aprominent (EW 1-2 keV) emission line at 6.4 keV is detectedby both instruments. From the XMM-Newton data alone the line isspectrally localized at E ≃ 6.42 ± 0.03 keV, and seems tobe spatially concentrated in the nuclear region of NGC 4666. This,together with the presence of a flat (Γ 1.3) continuum inthe nuclear region, suggests the existence of a strongly absorbed (i.e.,Compton-thick) AGN, whose intrinsic 2-10 keV luminosity is estimated tobe L2-10 2 × 1041 erg s-1.At energies 1 keV the integrated (BeppoSAX) spectrum is dominatedby a 0.25 keV thermal gas component distributed throughout thedisk (resolved by XMM-Newton). At energies 2-10 keV, theintegrated spectrum is dominated by a steep (Γ 2) power-law(PL) component. The latter emission is likely due to unresolved sourceswith luminosity L 1038-1039 ergs-1 that are most likely accreting binaries (with BH masses≤8 Mȯ). Such binaries, which are known to dominatethe X-ray point-source luminosity in nearby star-forming galaxies, haveΓ 2 PL spectra in the relevant energy range. A Γ 1.8 PL contribution from Compton scattering of (the radio-emitting)relativistic electrons by the ambient FIR photons may add a trulydiffuse component to the 2-10 keV emission.
| Deprojecting spiral galaxies using Fourier analysis. Application to the Ohio sample
We use two new methods developed recently (Barberàet al.\cite{bar03}, A&A, 415, 849), as well as information obtained fromthe literature, to calculate the orientation parameters of the spiralgalaxies in the Ohio State University Bright Galaxy Survey. We comparethe results of these methods with data from the literature, and find ingeneral good agreement. We provide a homogeneous set of mean orientationparameters which can be used to approximately deproject the disks of thegalaxies and facilitate a number of statistical studies of galaxyproperties.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/421/595
| 2-10 keV luminosity of high-mass binaries as a gauge of ongoing star-formation rate
Based on recent work on spectral decomposition of the emission ofstar-forming galaxies, we assess whether the integrated 2-10 keVemission from high-mass X-ray binaries (HMXBs),L2-10HMXB, can be used as a reliable estimator ofongoing star formation rate (SFR). Using a sample of 46 local (z 0.1) star-forming galaxies, and spectral modeling of ASCA, BeppoSAX, andXMM-Newton data, we demonstrate the existence of a linear SFR -L2-10^ HMXB relation which holds over 5 decades in X-rayluminosity and SFR. The total 2-10 keV luminosity is not a precise SFRindicator because at low SFR (i.e., in normal andmoderately-starbursting galaxies) it is substantially affected by theemission of low-mass X-ray binaries, which do not trace the current SFRdue to their long evolution lifetimes, while at very high SFR (i.e., forvery luminous FIR-selected galaxies) it is frequently affected by thepresence of strongly obscured AGNs. The availability of purelySB-powered galaxies - whose 2-10 keV emission is mainly due to HMXBs -allows us to properly calibrate the SFR -L2-10HMXB relation. The SFR -L2-10HMXB relation holds also for distant (z 1) galaxies in the Hubble Deep Field North sample, for which we lackspectral information, but whose SFR can be estimated from deep radiodata. If confirmed by more detailed observations, it may be possible touse the deduced relation to identify distant galaxies that are X-rayoverluminous for their (independently estimated) SFR, and are thereforelikely to hide strongly absorbed AGNs.Appendix A is only available in electronic form athttp://www.edpsciences.org
| An XMM-Newton hard X-ray survey of ultraluminous infrared galaxies
XMM-Newton observations of 10 ultraluminous infrared galaxies (ULIRGs)from a 200-ks mini-survey programme are reported. The aim is toinvestigate in hard X-rays a complete ULIRG sample selected from thebright IRAS 60-μm catalogue. All sources are detected in X-rays, fiveof which for the first time. These observations confirm that ULIRGs areintrinsically faint X-ray sources, their observed X-ray luminositiesbeing typically L2-10keV<= 1042-1043erg s-1, whereas their bolometric (mostly infrared)luminosities are Lbol > 1045 ergs-1. In all sources we find evidence for thermal emissionfrom hot plasma with a rather constant temperature kT~= 0.7 keV,dominating the X-ray spectra below 1 keV, and probably associated with anuclear or circumnuclear starburst. This thermal emission appearsuncorrelated with the far-infrared luminosity, suggesting that, inaddition to the ongoing rate of star formation, other parameters mayalso affect it. The soft X-ray emission appears to be extended on ascale of ~30 kpc for Mrk 231 and IRAS 19254-7245, possible evidence ofgalactic superwinds. In these two sources, IRAS 20551-4250 and23128-5919, we find evidence for the presence of hidden active galacticnuclei (AGNs), while a minor AGN contribution may be suspected also inIRAS 20100-4156. In particular, we have detected a strong (EW ~ 2 keV)Fe K line at 6.4 keV in the spectrum of IRAS 19254-7245 and a weaker onein Mrk 231, suggestive of deeply buried AGNs. For the other sources, theX-ray luminosities and spectral shapes are consistent with hot thermalplasma and X-ray binary emissions of mainly starburst origin. We findthat the 2-10 keV luminosities in these sources, most probably due tohigh-mass X-ray binaries, are correlated with LFIR: bothluminosities are good indicators of the current global star formationrate in the Galaxy. The composite nature of ULIRGs is then confirmed,with hints for a predominance of the starburst over the AGN phenomenonin these objects even when observed in hard X-rays.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| An Hα survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
In a series of two papers we present results of a new Hα imagingsurvey, aiming at the detection of extraplanar diffuse ionized gas inhalos of late-type spiral galaxies. We have investigated a sample of 74nearby edge-on spirals, covering the northern and southern hemisphere.In 30 galaxies we detected extraplanar diffuse emission at meandistances of |z| ~ 1-2 kpc. Individual filaments can be traced out to|z|<=6 kpc in a few cases. We find a good correlation between the FIRflux ratio (S60/S100) and the SFR per unit area(LFIR/D225), based on thedetections/non-detections. This is actually valid for starburst, normaland for quiescent galaxies. A minimal SFR per unit area for the lowestS60/S100 values, at which extended emission hasbeen detected, was derived, which amounts to dotEA25thres = (3.2+/-0.5)*E40ergs-1 kpc-2. There are galaxies where extraplanaremission was detected at smaller values ofLFIR/D225, however, only in combinationwith a significantly enhanced dust temperature. The results corroboratethe general view that the gaseous halos are a direct consequence of SFactivity in the underlying galactic disk.Based on observations collected at the European Southern Observatory,Chile (ESO No. 63.N-0070, ESO No. 64.N-0034, ESO No. 65.N.-0002).
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| Milestones in the Observations of Cosmic Magnetic Fields
Magnetic fields are observed everywhere in the universe. In this review,we concentrate on the observational aspects of the magnetic fields ofGalactic and extragalactic objects. Readers can follow the milestones inthe observations of cosmic magnetic fields obtained from the mostimportant tracers of magnetic fields, namely, the star-lightpolarization, the Zeeman effect, the rotation measures (RMs,hereafter)of extragalactic radio sources, the pulsar RMs, radio polarizationobservations, as well as the newly implemented sub-mm and mmpolarization capabilities. The magnetic field of the Galaxy was firstdiscovered in 1949 by optical polarization observations. The localmagnetic fields within one or two kpc have been well delineated bystarlight polarization data. The polarization observations of diffuseGalactic radio background emission in 1962 confirmed unequivocally theexistence of a Galactic magnetic field.The bulk of the presentinformation about the magnetic fields in the Galaxy comes from analysisof rotation measures of extragalactic radio sources and pulsars, whichcan be used to construct the 3-D magnetic field structure in theGalactic halo and Galactic disk. Radio synchrotron spurs in the Galacticcenter show a poloidal field, and the polarization mapping of dustemission and Zeeman observation in the central molecular zone reveal atoroidal magnetic field parallel to the Galactic plane. For nearbygalaxies, both optical polarization and multifrequency radiopolarization data clearly show the large-scale magnetic field followingthe spiral arms or dust lanes. For more distant objects, radiopolarization is the only approach available to show the magnetic fieldsin the jets or lobes of radio galaxies or quasars. Clusters of galaxiesalso contain widely distributed magnetic fields, which are reflected byradio halos or the RM distribution of background objects. Theintergalactic space could have been magnetized by outflows or galacticsuperwinds even in the early universe. The Zeeman effect andpolarization of sub-mm and mm emission can be used for the study ofmagnetic fields in some Galactic molecular clouds but it is observedonly at high intensity. Both approaches together can clearly show therole that magnetic fields play in star formation and cloud structure,which in principle would be analogous to galaxy formation fromprotogalactic clouds. The origin of the cosmic magnetic fields is anactive field of research. A primordial magnetic field has not been asyet directly detected, but its existence must be considered to give theseed field necessary for many amplification processes that have beendeveloped. Possibly, the magnetic fields were generated in protogalacticplasma clouds by the dynamo process, and maintained again by the dynamoafter galaxies were formed.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
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