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The dipole anisotropy of the 2 Micron All-Sky Redshift Survey
We estimate the acceleration on the Local Group (LG) from the 2 MicronAll-Sky Redshift Survey (2MRS). The sample used includes about 23200galaxies with extinction-corrected magnitudes brighter thanKs= 11.25 and it allows us to calculate the flux-weighteddipole. The near-infrared flux-weighted dipoles are very robust becausethey closely approximate a mass-weighted dipole, bypassing the effectsof redshift distortions and require no preferred reference frame. Thisis combined with the redshift information to determine the change indipole with distance. The misalignment angle between the LG and thecosmic microwave background (CMB) dipole drops to 12°+/- 7° ataround 50h-1Mpc, but then increases at larger distances,reaching 21°+/- 8° at around 130h-1Mpc. Exclusion ofthe galaxies Maffei 1, Maffei 2, Dwingeloo 1, IC342 and M87 brings theresultant flux dipole to 14°+/- 7° away from the CMB velocitydipole. In both cases, the dipole seemingly converges by60h-1Mpc. Assuming convergence, the comparison of the 2MRSflux dipole and the CMB dipole provides a value for the combination ofthe mass density and luminosity bias parametersΩ0.6m/bL= 0.40 +/- 0.09.
| The ultraluminous X-ray sources in the high-velocity system of NGC1275
We report the results of a study of X-ray point sources coincident withthe high-velocity system (HVS) projected in front of NGC1275. A verydeep X-ray image of the core of the Perseus cluster, made with theChandra X-ray Observatory, has been used. We find a population ofultraluminous X-ray sources [ULXs seven sources with LX(0.5 -7.0 keV) > 7 × 1039ergs-1]. As with theULX populations in the Antennae and Cartwheel galaxies, those in the HVSare associated with a region of very active star formation. Severalsources have possible optical counterparts found on the Hubble SpaceTelescope (HST) images, although the X-ray brightest one does not.Absorbed power-law models fit the X-ray spectra, with most having aphoton index between 2 and 3.
| Scale Heights of Non-Edge-on Spiral Galaxies
We present a method of calculating the scale height of non-edge-onspiral galaxies, together with a formula for errors. The method is basedon solving Poisson's equation for a logarithmic disturbance of matterdensity in spiral galaxies. We show that the spiral arms can not extendto inside the ``forbidden radius'' r0, due to the effect ofthe finite thickness of the disk. The method is tested by re-calculatingthe scale heights of 71 northern spiral galaxies previously calculatedby Ma, Peng & Gu. Our results differ from theirs by less than 9%. Wealso present the scale heights of a further 23 non-edge-on spiralgalaxies.
| A 2 Millimeter Spectral Line Survey of the Starburst Galaxy NGC 253
We present the first unbiased molecular line survey toward anextragalactic source, namely the nuclear region of the starburst galaxyNGC 253. The scan covers the frequency band from 129.1 to 175.2 GHz,i.e., most of the 2 mm atmospheric window. We identify 111 spectralfeatures as transitions from 25 different molecular species. Eight ofwhich (three tentatively) are detected for the first time in theextragalactic interstellar medium. Among these newly detected species,we detected the rare isotopomers 34SO andHC18O+. Tentative detections of two deuteratedspecies, DNC and N2D+, are reported for the firsttime from a target beyond the Magellanic Clouds. In addition, threehydrogen recombination lines are identified, while no organic moleculeslarger than methanol are detected. Column densities and rotationtemperatures are calculated for all the species, including an upperlimit to the ethanol abundance. A comparison of the chemical compositionof the nuclear environment of NGC 253 with those of selected nearbygalaxies demonstrates the chemical resemblance of IC 342 and NGC 4945 tothat of NGC 253. On the other hand, the chemistries characterizing NGC253 and M82 are clearly different. We also present a comparison of thechemical composition of NGC 253 with those observed in Galacticprototypical sources. The chemistry of NGC 253 shows a strikingsimilarity with the chemistry observed toward the Galactic centermolecular clouds, which are thought to be dominated by low-velocityshocks. This resemblance strongly suggests that the heating in thenuclear environment of NGC 253 is dominated by the same mechanism asthat in the central region of the Milky Way.
| Hot Molecular Gas in the Nuclear Region of IC 342
We present the first interferometric detection of extragalacticNH3 (6,6) emission in the nearby galaxy IC 342 made using theVLA. The data have a resolution of 7.8"×5.0" and trace hot (T~412K) and dense (>104 cm-3) molecular gas. We havecovered a 170''×300'' area and detect twovery strong line emission peaks, likely associated with the twostrongest star formation regions of the central part of the galaxy. Wecompare these emission peaks to CO (1-0) and (2-1) emission data, whichare the most abundant CO transitions and trace spatially extendedemission. The NH3 (6,6) emission is also compared to emissiondata from three high-density, nitrogen-bearing tracers: HNC (1-0),HC3N (10-9), and N2H+ (1-0). Ourresults suggest that the molecular mass in the nuclear region of IC 342has at least two different components, a dense and cold component and aless dense and hotter component.
| Core-Collapse Very Massive Stars: Evolution, Explosion, and Nucleosynthesis of Population III 500-1000 Msolar Stars
We calculate evolution, collapse, explosion, and nucleosynthesis ofPopulation III very massive stars with 500 and 1000 Msolar.Presupernova evolution is calculated in spherical symmetry. Collapse andexplosion are calculated by a two-dimensional code, based on the bipolarjet models. We compare the results of nucleosynthesis with the abundancepatterns of intracluster matter, hot gases in M82, and extremelymetal-poor stars in the Galactic halo. It was found that both 500 and1000 Msolar models enter the region of pair instability butcontinue to undergo core collapse. In the presupernova stage,silicon-burning regions occupy a large fraction, more than 20% of thetotal mass. For moderately aspherical explosions, the patterns ofnucleosynthesis match the observational data of both the intraclustermedium and M82. Our results suggest that explosions of Population IIIcore-collapse very massive stars contribute significantly to thechemical evolution of gases in clusters of galaxies. For Galactic halostars our [O/Fe] ratios are smaller than the observational abundances.However, our proposed scenario is naturally consistent with thisoutcome. The final black hole masses are ~230 and ~500 Msolarfor the 500 and 1000 Msolar models, respectively. This resultmay support the view that Population III very massive stars areresponsible for the origin of intermediate-mass black holes, which wererecently reported to be discovered.
| Morphology of Spitzer 24 μm Detected Galaxies in the UDF: The Links between Star Formation and Galaxy Morphology
We have studied the morphologies of infrared-luminous galaxies at0.3<=z<1.4 in the HST Ultra Deep Field (UDF) by calculatingconcentration and asymmetry indices and comparing the results withsimilar calculations for: (1) galaxies at similar redshift that are lessinfrared-active, and (2) local luminous infrared galaxies [LIRGs;LIR(8-1000 μm)>1011 Lsolar]. Wefind that the high-redshift samples are dominated by galaxies withconcentrations similar to local late-type disk galaxies; however, theyare significantly more asymmetric than most local galaxies but aresimilar in both regards to local LIRGs. On average, the high-redshiftinfrared-active galaxies are slightly more asymmetric than the lessactive ones, although they do include a significantly higher portion ofhighly asymmetric (merging?) systems and a lower portion of moreconcentrated, symmetric ones. The morphological similarity ofinfrared-active and typical infrared-inactive galaxies at high redshiftsuggests that they may be from the same parent population, but are indifferent stages of an episodic star formation process. The similaritybetween high redshift and local LIRGs suggests that a certain level ofasymmetry is generally associated with LIRG-level activity.
| 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.
| Imaging Molecular Gas in the Luminous Merger NGC 3256: Detection of High-Velocity Gas and Twin Gas Peaks in the Double Nucleus
Molecular gas in the merging starburst galaxy NGC 3256 has been imagedwith the Submillimeter Array at a resolution of1''×2'' (170×340 pc at 35 Mpc). Thisis the first interferometric imaging of molecular gas in the mostluminous galaxy within z=0.01. There is a large disk of molecular gas(r>3 kpc) in the center of the merger with a strong gas concentrationtoward the double nucleus. The gas disk having a mass of~3×109 Msolar in the central 3 kpc rotatesaround a point between the two nuclei that are 850 pc apart on the sky.The molecular gas is warm and turbulent and shows spatial variation ofthe intensity ratio between CO isotopomers. High-velocity molecular gasis discovered at the galactic center. Its velocity in our line of sightis up to 420 km s-1 offset from the systemic velocity of thegalaxy; the terminal velocity is twice as large as that due to therotation of the main gas disk. The high-velocity gas is most likely dueto a molecular outflow from the gas disk, entrained by thestarburst-driven superwind in the galaxy. The molecular outflow isestimated to have a rate of ~10 Msolar yr-1 and toplay a significant role in the dispersal or depletion of molecular gasfrom the galactic center. A compact gas concentration and steep velocitygradient are also found around each of the twin nuclei. They aresuggestive of a small gas disk rotating around each nucleus. If theseare indeed minidisks, their dynamical masses are ~109Msolar within a radius of 170 pc.
| The Polycyclic Aromatic Hydrocarbon Emission Deficit in Low-Metallicity Galaxies-A Spitzer View
Archival observations of 18 starburst galaxies that span a wide range inmetallicity reveal for the first time a correlation between the ratio ofemission-line fluxes of [Fe II] at 26 μm and [Ne II] at 12.8 μmand the 7.7 μm PAH strength, with the [Fe II]/[Ne II] flux ratiodecreasing with increasing PAH strength. We also find a strongcorrelation between the [Fe II]/[Ne II] flux ratio and the host galaxymetallicity, with the flux ratio decreasing with increasing metallicity.Since [Fe II] emission has been linked primarily to supernova shocks, weattribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies toenhanced supernova activity. We consider this to be a dominant mechanismfor PAH destruction, rather than grain destruction in photoionizedregions surrounding young massive stars. We also consider whether theextreme youth of the low-metallicity galaxies is responsible for thelack of PAH emission.
| An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources
We have studied the stellar and interstellar environments of twoluminous X-ray sources and five ultraluminous X-ray sources (ULXs) inorder to gain insight into their nature. Archival Hubble Space Telescopeimages were used to identify the optical counterparts of the ULXs Ho IXX-1 and NGC 1313 X-2, and to make photometric measurements of the localstellar populations of these and the luminous source IC 10 X-1. Weobtained high-dispersion spectroscopic observations of the nebulaearound these seven sources to search for He II λ4686 emission andto estimate the expansion velocities and kinetic energies of thesenebulae. Our observations did not detect nebular He II emission from anysource, with the exception of LMC X-1 this is either because we missedthe He III regions or because the nebulae are too diffuse to produce HeII surface brightnesses that lie within our detection limit. We comparethe observed ionization and kinematics of the supershells around theULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected fromthe underlying stellar population to assess whether additional energycontributions from the ULXs are needed. In both cases, we findinsufficient UV fluxes or mechanical energies from the stellarpopulation; thus these ULXs may be partially responsible for theionization and energetics of their supershells. All seven sources thatwe studied are in young stellar environments, and six of them haveoptical counterparts with masses >~7 Msolar thus, thesesources are most likely high-mass X-ray binaries.
| The Unusual Spectrum of the Ultraluminous X-Ray Source M82 X-1
The results of a spectral analysis, using XMM-Newton and Chandra data ofthe brightest ultraluminous X-ray source in the nearby galaxy M82, arepresented. The spectrum of M82 X-1 was found to be unusually hard(photon spectral index Γ~1) with a sharp cutoff at ~6 keV. Thedisk blackbody emission model requires a nonphysically high temperature.Instead, the spectrum is better described, with a lower reducedχ2, as emission due to the nearly saturatedComptonization of photons in an optically thick (τ~10-30, dependingon the geometry) plasma having a temperature kT~2 keV. This is incontrast to the high-energy spectra of other black hole systems, whichare relatively steeper (Γ>1.5) and hence are modeled as theunsaturated thermal and/or nonthermal Comptonization of soft photons, inan optically thin (τ~1) high-temperature plasma. An iron lineemission that is marginally resolved (σ~0.2 keV) is required tofit the data. We argue that the standard geometry for theX-ray-producing region, which consists of an optically thin inner diskor a uniform/patchy corona on top of a cold disk, is not applicable tothis source. Alternatively, the geometry of the X-ray-producing regioncould be a large sphere surrounding a cold accretion disk or anoptically thick inner disk region that cools by bremsstrahlungself-Comptonization. For the latter scenario, such an inner disk region,whose effective optical depth to absorption is less than unity, isexpected in the standard accretion disk theory for near-Eddingtonaccretion rates.
| Black Hole Mass of the Ultraluminous X-Ray Source M82 X-1
We report the first clear evidence for the simultaneous presence of alow-frequency break and a QPO in the fluctuation power spectrum of awell-known ultraluminous X-ray source (ULX) in M82 using long XMM-Newtonobservations. The break occurs at a frequency of34.2+6-3 mHz. The QPO has a centroid atνQPO=114.3+/-1.5 mHz, a coherenceQ≡νQPO/ΔνFWHM~=3.5, and anamplitude (rms) of 19% in the 2-10 keV band. The power spectrum isapproximately flat below the break frequency and then falls off abovethe break frequency as a power law with the QPO superposed. This form ofthe power spectrum is characteristic of the Galactic X-ray binaries(XRBs) in their high or intermediate states. M82 X-1 was likely in anintermediate state during the observation. The EPIC pn spectrum is welldescribed by a model comprising an absorbed power law (Γ~2) and aniron line at ~6.6 keV with a width σ~0.2 keV and an equivalentwidth of ~180 eV. Using the well-established correlations between thepower and energy spectral parameters for XRBs, we estimate a black holemass for M82 X-1 in the range of ~25-520 Msolar, includingsystematic errors that arise due to the uncertainty in the calibrationof the photon spectral index versus QPO frequency relation.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type
We study the nuclear star clusters (NCs) in spiral galaxies of variousHubble types using spectra obtained with the STIS on board the HubbleSpace Telescope (HST). We observed the nuclear clusters in 40 galaxies,selected from two previous HST WFPC2 imaging surveys. At a spatialresolution of ~0.2" the spectra provide a better separation of clusterlight from underlying galaxy light than is possible with ground-basedspectra. Approximately half of the spectra have a sufficiently highsignal-to-noise ratio for detailed stellar population analysis. For theother half we only measure the continuum slope, as quantified by the B-Vcolor. To infer the star formation history, metallicity, and dustextinction, we fit weighted superpositions of single-age stellarpopulation templates to the high signal-to-noise ratio spectra. We usethe results to determine the luminosity-weighted age, mass-to-lightratio, and masses of the clusters. Approximately half of the sampleclusters contain a population younger than 1 Gyr. Theluminosity-weighted ages range from 10 Myr to 10 Gyr. The stellarpopulations of NCs are generally best fit as a mixture of populations ofdifferent ages. This indicates that NCs did not form in a single event,but that instead they had additional star formation long after theoldest stars formed. On average, the sample clusters in late-typespirals have a younger luminosity-weighted mean age than those inearly-type spirals (L=8.37+/-0.25 vs.9.23+/-0.21). The average mass-weighted ages are older by ~0.7 dex,indicating that there often is an underlying older population that doesnot contribute much light but does contain most of the mass. The averagecluster masses are smaller in late-type spirals than in early-typespirals (logM=6.25+/-0.21 vs. 7.63+/-0.24) and exceed the masses typicalof globular clusters. The cluster mass correlates loosely with totalgalaxy luminosity. It correlates more strongly with both the Hubble typeof the host galaxy and the luminosity of its bulge. The lattercorrelation has the same slope as the well-known correlation betweensupermassive black hole mass and bulge luminosity. The properties ofboth nuclear clusters and black holes in the centers of spiral galaxiesare therefore intimately connected to the properties of the host galaxy,and in particular its bulge component. Plausible formation scenarioshave to account for this. We discuss various possible selection biasesin our results, but conclude that none of them can explain thedifferences seen between clusters in early- and late-type spirals. Theinability to infer spectroscopically the populations of faint clustersdoes introduce a bias toward younger ages, but not necessarily towardhigher masses.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555. These observations areassociated with proposals 9070 and 9783.
| Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field
We present Hubble Space Telescope Advanced Camera for Surveys images andcolor-magnitude diagrams for 25 nearby galaxies with radial velocitiesVLG<500 km s-1. Distances are determined basedon the luminosities of stars at the tip of the red giant branch thatrange from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, arefound to be the nearest known representatives of blue compact dwarfobjects. Using high-quality data on distances and radial velocities of110 nearby field galaxies, we derive their mean Hubble ratio to be 68 kms-1 Mpc-1 with a standard deviation of 15 kms-1 Mpc-1. Peculiar velocities of most of thegalaxies, Vpec=VLG-68D, follow a Gaussiandistribution with σv=63 km s-1 but with atail toward high negative values. Our data display the known correlationbetween peculiar velocity and galaxy elevation above the LocalSupercluster plane. The small observed fraction of galaxies with highpeculiar velocities, Vpec<-500 km s-1, may beunderstood as objects associated with nearby groups (Coma I, Eridanus)outside the local volume.
| Examining the Seyfert-Starburst Connection with Arcsecond-Resolution Radio Continuum Observations
We compare the arcsecond-scale circumnuclear radio continuum propertiesof five Seyfert and five starburst galaxies, concentrating on the searchfor any structures that could imply a spatial or causal connectionbetween the nuclear activity and a circumnuclear starburst ring. Noevidence is found in the radio emission for a link between thetriggering or feeding of nuclear activity and the properties ofcircumnuclear star formation. Conversely, there is no clear evidence ofnuclear outflows or jets triggering activity in the circumnuclear ringsof star formation. Interestingly, the difference in the angle betweenthe apparent orientation of the most elongated radio emission and theorientation of the major axis of the galaxy is on average larger inSeyfert galaxies than in starburst galaxies, and Seyfert galaxies appearto have a larger physical size scale of the circumnuclear radiocontinuum emission. The concentration, asymmetry, and clumpinessparameters of radio continuum emission in Seyfert galaxies andstarbursts are comparable, as are the radial profiles of radio continuumand near-infrared line emission. The circumnuclear star formation andsupernova rates do not depend on the level of nuclear activity. Theradio emission usually traces the near-infrared Brγ andH2 1-0 S(1) line emission on large spatial scales, butlocally their distributions are different, most likely because of theeffects of varying local magnetic fields and dust absorption andscattering.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
| Methanol detection in M 82
Context: .The nuclear starburst region in M 82 shows systematical lowabundances of some complex molecules when compared with other starburstgalaxies. This is likely related to a presumably photodissociationdominated environment. In particular, methanol is known to showrelatively low abundance because it is easily photodissociated.Aims: .We present a multilevel study of the emission of methanol,detected for the first time in this galaxy, and discuss the origin ofits emission. Methods: .Observations of three transitions of CH3OHtowards the center and two positions around the nucleus of M 82 arepresented. Two different components are found, one with high excitation(n(H_2) 10^6 cm-3, T_rot 20 K) and the other withlow excitation (n(H_2) 10^4 cm-3, T_rot 5 K).Results: .The high observed methanol abundance of a few 10-9can only be explained if injection of methanol from dust grains is takeninto account. While the overall [ CH3OH] /[ NH3] ratio is much largerthan observed towards other starbursts, the dense high excitationcomponent shows a similar value to that found in NGC 253 and Maffei 2. Conclusions: .Our observations suggest the molecular material inM 82 to be formed by dense warm cores, shielded from the UV radiationand similar to the molecular clouds in other starbursts, surrounded by aless dense photodissociated halo. The dense warm cores are likely thelocation of recent and future star formation within M 82.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| Large-scale molecular shocks in galaxies: the SiO interferometer map of IC 342
We present the first high-resolution (5.6 arcsec×5.1 arcsec )images of the emission of silicon monoxide (SiO) in the nucleus of thenearby spiral IC 342, obtained with the IRAM Plateaude Bure Interferometer (PdBI). Using a two-field mosaic, we havesimultaneously mapped the emission of the SiO(v=0, J=2-1) andH13CO+(J=1-0) lines in a region of 0.9 kpc× 1.3 kpc (RA × Dec) centered around the nucleus ofIC 342. The bulk of the emission in the two linescomes from a 290 pc spiral arm located to the North and a centralcomponent that forms the southern ridge of a {r80} pc nuclear ringthat was identified in other interferometer maps of the galaxy. Wedetect continuum emission at 86.8 GHz in a 80-180 pc centralsource. The continuum emission, dominated by thermal free-freebremsstrahlung, is mostly anticorrelated with the observed distributionof SiO clouds. The SiO-to-H13CO+ intensity ratiois seen to increase by an order of magnitude from the nuclear ring (0.3) to the spiral arm ( 3.3). Furthermore the gas kinematics showsignificant differences between SiO and H13CO+over the spiral arm, where the linewidths of SiO are a factor of 2larger than those of H13CO+. The average abundanceof SiO in the inner {r320} pc of IC 342 isX(SiO) ≳2×10-10. This shows that shock chemistryis at work in the inner molecular gas reservoir of IC342. To shed light on the nature of shocks in IC342, we have compared the emission of SiO with another tracerof molecular shocks: the emission of methanol (CH3OH). We find that thesignificant difference of the abundance of SiO measured between thespiral arm (X(SiO) a few 10-9) and the nuclear ring (X(SiO) 10-10) is not echoed by a comparable variation in theSiO-to-CH3OH intensity ratio. This implies that the typical shockvelocities should be similar in the two regions. In contrast, thefraction of shocked molecular gas should be 5-7 times larger in thespiral arm (up to 10% of the available molecular gas mass over the armregion) compared to the nuclear ring. In the light of these results, werevise the validity of the various scenarios that have been proposed toexplain the onset of shock chemistry in galaxies and study theirapplicability to the nucleus of IC 342. We concludethat the large-scale shocks revealed by the SiO map of IC342 are mostly unrelated to star formation and arise insteadin a pre-starburst phase. Shocks are driven by cloud-cloud collisionsalong the potential well of the IC 342 bar. Thegeneral implications for the current understanding of galaxy evolutionare discussed.
| CI and CO in the center of M 51
We present J=2{-}1, J=3{-}2, J=4{-}3 12CO maps as well asJ=2{-}1, J=3{-}2 13CO and 492 GHz [CI] measurements of thecentral region in M 51. The distribution of CO is strongly concentratedtowards the spiral arms. The center itself is poor in, though not devoidof, CO emission. The observed line intensities require modelling with amulti-component molecular gas. A dense component must be present(n(H2) ≈ 103) with kinetic temperature T_kin≈ 100 K, combined with either a less dense (≈ 102cm-3) component of the same temperature, or a more dense(n(H2) ≈ 3 × 103 cm-3) andmuch cooler (T_kin = 10-30 K) component. Atomic carbon amounts arebetween 5 and 10 times those of CO. Much of the molecular gas mass isassociated with the hot PDR phase. The center of M 51 has a face-on gasmass density of about 40±20 Mȯ pc-2,and a well-established CO-to-H{2} conversion ratio X four to five timeslower than the standard Galactic value.
| Detection of Neutrinos from Supernovae in Nearby Galaxies
While existing detectors would see a burst of many neutrinos from aMilky Way supernova, the supernova rate is only a few per century. As analternative, we propose the detection of 1 neutrino per supernovafrom galaxies within 10 Mpc, in which there were at least 9core-collapse supernovae since 2002. With a future 1 Mton scaledetector, this could be a faster method for measuring the supernovaneutrino spectrum, which is essential for calibrating numerical modelsand predicting the redshifted diffuse spectrum from distant supernovae.It would also allow a ≳104 times more precise triggertime than optical data alone for high-energy neutrinos and gravitationalwaves.
| Astrophysical magnetic fields and nonlinear dynamo theory
Electronic Article Available from Elsevier Science.
| The nuclear molecular clouds of NGC4945
The 3-mm transitions of HCN, HCO+ and HNC in the densemolecular clouds near the nucleus of the southern starburst galaxyNGC4945 have been observed with three antennas of the AustraliaTelescope Compact Array (ATCA). Molecular-line emission was detectedwithin the velocity range (300-800kms-1) seen in single-dishspectra, although emission with extent greater than about 15arcsec wouldnot have been detected because of the lack of sufficiently small CompactArray (CA) baselines. Imaging the results at velocity intervals of22.5kms-1 and with a restoring beam of dimension 5.6 ×3.5arcsec2 yielded images of typical deconvolved dimensions 7× 3arcsec2 (130 × 60pc for an assumed galaxydistance of 4Mpc). The positions of the images vary systematically withvelocity, consistent with the result expected for a rotating molecularcloud ensemble viewed edge-on and inclined at a position angle (PA) of45°. Although the brightest images occurred at velocities of the twomajor features (~430 and ~710 km s-1) in Swedish-ESOSubmillimetre Telescope (SEST) spectra, the images at interveningvelocities were unexpectedly faint or even in absorption. The resultshave been interpreted in terms of an extended cloud component notdetected by the CA in conjunction with absorption against the nuclearcontinuum emission. Some of the HCO+ images are dominated byabsorption and the overall results do not truly represent the cloudstructure. The HNC results appear to contain little absorption, and theemission integrated over velocity yielded an elliptical image centred onan H2O `megamaser' located at the galaxy's nucleus. The HNCposition-velocity distribution along the major axis is consistent withan edge-on circumnuclear molecular ring with a rotational velocity of135kms-1. However, the ring radius of ~60kms-1 issignificantly less than previously derived from carbon monoxide (CO)observations. It can be reconciled with the CO results if the HNC ringtraces high-density gas concentrated around the inside of the molecularring, whereas the CO traces low-density gas that extends to higherradii. The HNC feature appears to be associated with features revealedin published infrared (IR) studies of the distribution of dust and starformation regions in the nuclear region of NGC4945. Limited results onthe variation of HNC/HCN abundance ratios within the nuclear molecularclouds support previous conclusions that the clouds are at an advancedevolutionary stage within a starburst period.
| BHαBAR: big Hα kinematical sample of barred spiral galaxies - I. Fabry-Perot observations of 21 galaxies
We present the Hα gas kinematics of 21 representative barredspiral galaxies belonging to the BHαBAR sample. The galaxies wereobserved with FaNTOmM, a Fabry-Perot integral-field spectrometer, onthree different telescopes. The three-dimensional data cubes wereprocessed through a robust pipeline with the aim of providing the mosthomogeneous and accurate data set possible useful for further analysis.The data cubes were spatially binned to a constant signal-to-noiseratio, typically around 7. Maps of the monochromatic Hα emissionline and of the velocity field were generated and the kinematicalparameters were derived for the whole sample using tilted-ring models.The photometrical and kinematical parameters (position angle of themajor axis, inclination, systemic velocity and kinematical centre) arein relative good agreement, except perhaps for the later-type spirals.
| The star-forming environment of an ultraluminous X-ray source in NGC4559: an optical study
We have studied the candidate optical counterparts and the stellarpopulation in the star-forming complex around the bright ultraluminousX-ray source (ULX) in the western part of the spiral galaxy NGC4559,using the HST Wide Field Planetary Camera 2 (WFPC2), XMM-Newton/OpticalMonitor and ground-based data. We find that the ULX is located near asmall group of OB stars, but is not associated with any massive youngclusters nor with any extraordinary massive stars. The brightest pointsource in the Chandra error circle is consistent with a single bluesupergiant (BSG) of mass ~20Msolar and age ~10 Myr. A fewother stars are resolved inside the error circle: mostly BSGs and redsupergiants (RSGs) with inferred masses ~10-15Msolar and ages~20 Myr. This is consistent with the interpretation of this ULX as ablack hole (BH) accreting from a high-mass donor star in its supergiantphase, with mass transfer occurring via Roche-lobe overflow. Theobserved optical colours and the blue-to-red supergiant ratio suggest alow metal abundance for the stellar population: 0.2<~Z/Zsolar<~ 0.4 (using the Padua tracks), or 0.05<~Z/Zsolar<~ 0.2 (using the Geneva tracks). The age ofthe star-forming complex is <~30 Myr. Hα images show that thisstar-forming region has a ring-like appearance. We propose that it is anexpanding wave of star formation, triggered by an initial densityperturbation, in a region where the gas was only marginally stable togravitational collapse. We also suggest that the most likely trigger wasa collision with a satellite dwarf galaxy going through the gas-richouter disc of NGC4559 less than 30 Myr ago. The culprit could be thedwarf galaxy visible a few arcsec north-west of the complex. If this isthe case, this system is a scaled-down version of the Cartwheel galaxy.The X-ray data favour a BH more massive (M > 50Msolar)than typical Milky Way BH candidates. The optical data favour a young BHoriginating in the recent episode of massive star formation; however,they also rule out an association with young massive star clusters (noneare present in the X7 field). We speculate that other mechanisms maylead to the formation of relatively massive BHs (perhaps M~50-100Msolar) from stellar evolution processes inlow-metallicity environments, or when star formation is triggered bygalactic collisions.
| Infrared Luminosities of Local-Volume Galaxies
Based on data from the Two-Micrometer All-Sky Survey (2MASS), weanalyzed the infrared properties of 451 Local-Volume galaxies atdistances D ≤ 10 Mpc. We determined the K-band luminosity functionof the galaxies in the range of absolute magnitudes from -25m to -11m.The local luminosity density within 8 Mpc is 6.8 × 108 L ȯMpc-3, a factor of 1.5 ± 0.1 higher than the global mean K-bandluminosity density. We determined the ratios of the virial mass to theK-band luminosity for nearby groups and clusters of galaxies. In theluminosity range from 5 × 1010 to 2 × 1013 L ȯ, thedependence log(M/L K) ∝ (0.27 ± 0.03) log L K with adispersion of 0.1 comparable to the measurement errors of themasses and luminosities of the systems of galaxies holds for the groupsand clusters of galaxies. The ensemble-averaged ratio, ≃ (20 25) M ȯ/L ȯ, was found to be much smaller than theexpected global ratio, (80 90)M ȯ/L ȯ, in the standard modelwith Ωm = 0.27. This discrepancy can be eliminated if the bulk ofthe dark matter in the Universe is not associated with galaxies andtheir systems.
| Supermassive Black Holes: Relation to Dark Halos
Estimates of the masses of supermassive black holes (M bh ) in thenuclei of disk galaxies with known rotation curves are compared withestimates of the rotational velocities V m and the“indicative” masses of the galaxies M i . Although there isa correlation between M bh and V m or M i , it is appreciably weakerthan the correlation with the central velocity dispersion. The values ofM bh for early-type galaxies (S0-Sab), which have more massive bulges,are, on average, higher than the values for late-type galaxies with thesame rotational velocities. We conclude that the black-hole masses aredetermined primarily by the properties of the bulge and not therotational velocity or the mass of the galaxy.
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