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RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
| Hαkinematics of the SINGS nearby galaxies survey - I*
This is the first part of an Hαkinematics follow-up survey of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for28galaxies are presented. The observations were done on three differenttelescopes with Fabry-Perot of New Technology for the Observatoire dumont Megantic (FaNTOmM), an integral field photon-counting spectrometer,installed in the respective focal reducer of each telescope. The datareduction was done through a newly built pipeline with the aim ofproducing the most homogenous data set possible. Adaptive spatialbinning was applied to the data cubes in order to get a constantsignal-to-noise ratio across the field of view. Radial velocity andmonochromatic maps were generated using a new algorithm, and thekinematical parameters were derived using tilted-ring models.
| XMM-Newton observations of bright ROSAT selected active galactic nuclei with low intrinsic absorption
We present a sample of 21 ROSAT bright active galactic nuclei (AGNs),representing a range of spectral classes, and selected for follow-upsnapshot observations with XMM-Newton. The typical exposure was between5 and 10ks. The objects were primarily selected on the bases of X-raybrightness and not on hardness ratio; thus the sample cannot be strictlydefined as a `soft'sample. One of the main outcomes from the XMM-Newtonobservations was that all of the AGN, including 11 type 1.8-2 objects,required low levels of intrinsic absorption (NH<~1021cm-2). The low absorption in type 2 systems isa challenge to account for in the standard orientation-based unificationmodel, and we discuss possible physical and geometrical models whichcould elucidate the problem. Moreover, there does not appear to be anyrelation between the strength and shape of the soft excess, and thespectral classification of the AGN in this sample. We further identify anumber of AGN which deserve deeper observations or further analysis: forexample, the low-ionization nuclear emission regions (LINERs) NGC 5005and NGC 7331, where optically thin thermal and extended emission isdetected, and the narrow-line Seyfert 1 II Zw 177, which shows a broademission feature at ~ 5.8keV.
| 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.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| Warm Dust and Spatially Variable Polycyclic Aromatic Hydrocarbon Emission in the Dwarf Starburst Galaxy NGC 1705
We present Spitzer observations of the nearby dwarf starburst galaxy NGC1705 obtained as part of the Spitzer Infrared Nearby Galaxies Survey.The galaxy morphology is very different shortward and longward of ~5μm: optical and short-wavelength IRAC imaging shows an underlying redstellar population, with the central super star cluster (SSC) dominatingthe luminosity; longer wavelength IRAC and MIPS imaging reveals warmdust emission arising from two off-nuclear regions that are offset by~250 pc from the SSC and that dominate the far-IR flux of the system.These regions show little extinction at optical wavelengths. The galaxyhas a relatively low global dust mass (~2 × 105Msolar, implying a global dust-to-gas mass ratio ~2-4 timeslower than the Milky Way average, roughly consistent with themetallicity decrease). The off-nuclear dust emission appears to bepowered by photons from the same stellar population responsible for theexcitation of the observed Hα emission; these photons areunassociated with the SSC (although a contribution from embedded sourcesto the IR luminosity of the off-nuclear regions cannot be ruled out).Low-resolution IRS spectroscopy shows moderate-strength PAH emission inthe 11.3 μm band in the more luminous eastern peak; no PAH emissionis detected in the SSC or the western dust emission complex. There issignificant diffuse emission in the IRAC 8 μm band after starlighthas been removed by scaling shorter wavelength data; the fact that IRSspectroscopy shows spatially variable PAH emission strengths compared tothe local continuum within this diffuse gas suggests caution in theinterpretation of IRAC diffuse 8 μm emission as arising from PAHcarriers alone. The nebular metallicity of NGC 1705 falls at thetransition level of ~0.35 Zsolar found by Engelbracht andcollaborators, below which PAH emission is difficult to detect; the factthat a system at this metallicity shows spatially variable PAH emissiondemonstrates the complexity of interpreting diffuse 8 μm emission ingalaxies. NGC 1705 deviates significantly from the canonicalfar-infrared versus radio correlation, having significant far-infraredemission but no detected radio continuum.
| The Central Engines of 19 LINERs as Viewed by Chandra
Using archival Chandra observations of 19 LINERs, we explore the X-rayproperties of their inner kiloparsec to determine the origin of theirnuclear X-ray emission, to investigate the presence of an AGN, and toidentify the power source of the optical emission lines. The relativenumbers of LINER types in our sample are similar to those in opticalspectroscopic surveys. We find that diffuse, thermal emission is verycommon and is concentrated within the central few hundred parsecs. Theaverage spectra of the hot gas in spiral and elliptical galaxies arevery similar to those of normal galaxies. They can be fitted with athermal plasma (kT~0.5 keV) plus a power-law (photon index of 1.3-1.5)model. There are on average three detected point sources in their innerkiloparsec with 1037 ergss-1
| Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
| 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.
| [Si II], [Fe II], [C II], and H2 Emission from Massive Star-forming Regions
We calculate the [Si II] 34.8 μm, [Fe II] 26.0 μm, and [C II] 158μm infrared fine-structure emission that may arise from ionized gas(H II regions) and/or associated neutral gas (photodissociation regions[PDRs]) in massive star-forming regions. Assuming thermal pressurebalance between an H II region and a PDR, the relative amounts offine-structure line emission from the H II region and PDR depend on theelectron density and resulting thermal pressure in the H II region,gas-phase abundances of the emitting species, and the UV spectrum fromthe stellar population producing the H II region. For normalmetallicity, we find that [C II] emission is always dominated by PDRs,while [Si II] and [Fe II] are dominated by PDRs for H II regions withelectron density ne>~10 cm-3. We also calculatethe H2 0-0 S(0), 0-0 S(1), 0-0 S(2), and 0-0 S(3) purerotational line emission arising from the PDR at the atomic-to-molecularinterface. The overall intensity of the H2 line emissiondirectly traces warm molecular mass, while H2 line ratiosconstrain the PDR temperature, gas density, and far-ultraviolet fieldstrength. Models of the integrated emission of [Si II], [Fe II], [C II],and H2 from Galactic and extragalactic star-forming regionsare presented for use in interpreting observations with Spitzer, ISO,SOFIA, and the Herschel Space Observatory. We compare our results withobservations of the Galactic source NGC 2023 (an individual HII/photodissociation region in Orion), the inner regions of the MilkyWay, and the central regions of the nearby star-forming spiral galaxyNGC 7331. We also compare our results with recently published similarwork by Abel and coworkers.
| Spitzer Space Telescope IRAC and MIPS Observations of the Interacting Galaxies IC 2163 and NGC 2207: Clumpy Emission
IC 2163 and NGC 2207 are interacting galaxies that have been wellstudied at optical and radio wavelengths and simulated in numericalmodels to reproduce the observed kinematics and morphological features.Spitzer IRAC and MIPS observations reported here show over 200 brightclumps from young star complexes. The brightest IR clump is amorphologically peculiar region of star formation in the western arm ofNGC 2207. This clump, which dominates the Hα and radio continuumemission from both galaxies, accounts for ~12% of the total 24 μmflux. Nearly half of the clumps are regularly spaced along somefilamentary structure, whether in the starburst oval of IC 2163 or inthe thin spiral arms of NGC 2207. This regularity appears to influencethe clump luminosity function, making it peaked at a value nearly afactor of 10 above the completeness limit, particularly in the starburstoval. This is unlike the optical clusters inside the clumps, which havea luminosity function consistent with the usual power-law form. Thegiant IR clumps presumably formed by gravitational instabilities in thecompressed gas of the oval and the spiral arms, whereas the individualclusters formed by more chaotic processes, such as turbulencecompression, inside these larger scale structures.
| An Initial Look at the Far-Infrared-Radio Correlation within Nearby Star-forming Galaxies Using the Spitzer Space Telescope
We present an initial look at the far-infrared-radio correlation withinthe star-forming disks of four nearby, nearly face-on galaxies (NGC2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging,observed as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS),and Westerbork Synthesis Radio Telescope (WSRT) radio continuum data,taken for the WSRT SINGS radio continuum survey, we are able to probevariations in the logarithmic 24 μm/22 cm (q24) and 70μm/22 cm (q70) surface brightness ratios across each diskat subkiloparsec scales. We find general trends of decreasingq24 and q70 with declining surface brightness andwith increasing radius. The residual dispersion around the trend ofq24 and q70 versus surface brightness is smallerthan the residual dispersion around the trend of q24 andq70 versus radius, on average by ~0.1 dex, indicating thatthe distribution of star formation sites is more important indetermining the infrared/radio disk appearance than the exponentialprofiles of disks. We have also performed preliminary phenomenologicalmodeling of cosmic-ray electron (CR electron) diffusion using animage-smearing technique and find that smoothing the infrared mapsimproves their correlation with the radio maps. We find that exponentialsmoothing kernels work marginally better than Gaussian kernels,independent of projection for these nearly face-on galaxies. This resultsuggests that additional processes besides simple random walk diffusionin three dimensions must affect the evolution of CR electrons. Thebest-fit smoothing kernels for the two less active star-forming galaxies(NGC 2403 and NGC 3031) have much larger scale lengths than those of themore active star-forming galaxies (NGC 5194 and NGC 6946). Thisdifference may be due to the relative deficit of recent CR electroninjection into the interstellar medium for the galaxies that havelargely quiescent disks.
| Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
| Thermal and non-thermal components of the interstellar medium at sub-kiloparsec scales in galaxies
Aims. We present new radio continuum observations of ten BIMA SONGgalaxies, taken at 1.4 GHz with the Very Large Array. These observationsallow us to extend the study of the relationships between the radiocontinuum (RC) and CO emission to 22 CO luminous galaxies for whichsingle dish CO images have been added to interferometric data. NewSpitzer infrared (IR) images of six of these galaxies have beenreleased. The analysis of these high resolution images allowed us toprobe the RC-IR-CO correlations down to linear scales of a few hundredpc. Methods: .We compare the point-by-point RC, CO and mid-IRintensities across entire galaxy disks, producing radial profiles andspatially resolved images of the RC/CO and RC/mid-IR ratios.Results: .For the 22 galaxies analysed, the RC-CO correlation on scalesfrom ~10 kpc down to ~100 pc is nearly linear and has a scatter of afactor of two, i.e. comparable to that of the global correlations. Thereis no evidence for any severe degradation of the scatter below the kpcscale. This also applies to the six galaxies for which high-resolutionmid-IR data are available. In the case of NGC 5194,we find that the non-thermal radio spectral index is correlated with theRC/FIR ratio. Conclusions: .The scatter of the point-by-pointcorrelations does not increase significantly with spatial resolution. Wethus conclude that we have not yet probed the physical scales at whichthe correlations break down. However, we observe local deviations fromthe correlations in regions with a high star formation rate, such as thespiral arms, where we observe a flat radio spectrum and a low RC/FIRratio. In the intra-arm regions and in the peripheral regions of thedisk, the RC/FIR is generally higher and it is characterized by asteepening of the radio spectrum.
| The extragalactic Cepheid bias: a new test using the period-luminosity-color relation
We use the Period-Luminosity-Color relation (PLC) for Cepheids to testfor the existence of a bias in extragalactic distances derived from theclassical Period-Luminosity (PL) relation. We calculate the parametersof the PLC using several galaxies observed with the Hubble SpaceTelescope and show that this calculation must be conducted with a PLCwritten in a form where the parameters are independent. The coefficientsthus obtained are similar to those derived from theoretical models.Calibrating with a few unbiased galaxies, we apply this PLC to allgalaxies of the Hubble Space Telescope Key Program (HSTKP) and comparethe distance moduli with those published by the HSTKP team. The newdistance moduli are larger (more exactly, the larger the distance thelarger the difference), consistent with a bias. Further, the bias trendthat is observed is the same previously obtained from two independentmethods based either on the local Hubble law or on a theoretical modelof the bias. The results are quite stable but when we force the PLCrelation closer to the classical PL relation by using unrealisticparameters, the agreement with HSTKP distance moduli is retrieved. Thisalso suggests that the PL relation leads to biased distance moduli. Thenew distance moduli reduce the scatter in the calibration of theabsolute magnitude of supernovae SNIa at their maximum. This may alsosuggest that the relation between the amplitude at maximum and the decayof the light curve Δ m15 may not be as strong asbelieved.
| A radio census of nuclear activity in nearby galaxies
In order to determine the incidence of black hole accretion-drivennuclear activity in nearby galaxies, as manifested by their radioemission, we have carried out a high-resolution Multi-ElementRadio-Linked Interferometer Network (MERLIN) survey of LINERs andcomposite LINER/Hii galaxies from a complete magnitude-limited sample ofbright nearby galaxies (Palomar sample) with unknown arcsecond-scaleradio properties. There are fifteen radio detections, of which three arenew subarcsecond-scale radio core detections, all being candidate AGN.The detected galaxies supplement the already known low-luminosity AGN -low-luminosity Seyferts, LINERs and composite LINER/Hii galaxies - inthe Palomar sample. Combining all radio-detected Seyferts, LINERs andcomposite LINER/Hii galaxies (LTS sources), we obtain an overall radiodetection rate of 54% (22% of all bright nearby galaxies) and weestimate that at least ~50% (~20% of all bright nearby galaxies) aretrue AGN. The radio powers of the LTS galaxies allow the construction ofa local radio luminosity function. By comparing the luminosity functionwith those of selected moderate-redshift AGN, selected from the 2dF/NVSSsurvey, we find that LTS sources naturally extend the RLF of powerfulAGN down to powers of about 10 times that of Sgr A*.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| Radio polarization and sub-millimeter observations of the Sombrero galaxy (NGC 4594). Large-scale magnetic field configuration and dust emission
We observed the nearby early-type spiral galaxy NGC 4594 (M 104,Sombrero galaxy) with the Very Large Array at 4.86 GHz, with theEffelsberg 100-m telescope at 8.35 GHz as well as with the HeinrichHertz Telescope at 345 GHz in radio continuum. The 4.86 and 8.35 GHzdata contain polarization information and hence information about themagnetic fields: we detected a large-scale magnetic field which is toour knowledge the first detection of a large-scale magnetic field in anSa galaxy in the radio range. The magnetic field orientation in M 104 ispredominantly parallel to the disk but has also vertical components atlarger z-distances from the disk. This field configuration is typicalfor normal edge-on spiral galaxies. The 345 GHz data pertain to the colddust content of the galaxy. Despite the optical appearance of the objectwith the huge dust lane, its dust content is smaller than that of morelate-type spirals.
| 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.
| The evolution of actively star-forming galaxies in the mid-infrared
In this paper we analyze the evolution of actively star-forming galaxiesin the mid-infrared (MIR). This spectral region, characterized bycontinuum emission by hot dust and by the presence of strong emissionfeatures generally ascribed to polycyclic aromatic hydrocarbon (PAH)molecules, is the most strongly affected by the heating processesassociated with star formation and/or active galactic nuclei (AGNs).Following the detailed observational characterization of galaxies in theMIR by the Infrared Space Observatory (ISO), we have updated themodelling of this spectral region in our spectrophotometric modelGRASIL. In the diffuse component we have updated the treatment of PAHsaccording to the model by Li & Draine. As for the dense phase of theinterstellar medium associated with the star-forming regions, themolecular clouds, we strongly decrease the abundance of PAHs as comparedto that in the cirrus, based on the observational evidence of the lackor weakness of PAH bands close to the newly formed stars, possibly dueto the destruction of the molecules in strong ultraviolet fields. Therobustness of the model is checked by fitting near-infrared to radiobroad-band spectra and the corresponding detailed MIR spectra of a largesample of galaxies, at once. With this model, we have analyzed thelarger sample of actively star-forming galaxies by Dale et al. We showthat the observed trends of galaxies in the ISO-IRAS-radio colour-colourplots can be interpreted in terms of the different evolutionary phasesof star formation activity, and the consequent different dominance inthe spectral energy distribution of the diffuse or dense phase of theISM. We find that the observed colours indicate a surprising homogeneityof the starburst phenomenon, allowing only a limited variation of themost important physical parameters, such as the optical depth of themolecular clouds, the time-scale of the escape of young stars from theirfor mation sites, and the gas consumption time-scale. In this paper wedo not attempt to reproduce the far-infrared coolest region in thecolour-colour plots, as we concentrate on models meant to reproduceactive star-forming galaxies, but we discuss possible requirements of amore complex modelling for the coldest objects.
| Structure and kinematics of edge-on galaxy discs - V. The dynamics of stellar discs
In earlier papers in this series we determined the intrinsic stellardisc kinematics of 15 intermediate- to late-type edge-on spiral galaxiesusing a dynamical modelling technique. The sample covers a substantialrange in maximum rotation velocity and deprojected face-on surfacebrightness, and contains seven spirals with either a boxy orpeanut-shaped bulge. Here we discuss the structural, kinematical anddynamical properties. From the photometry we find that intrinsicallymore flattened discs tend to have a lower face-on central surfacebrightness and a larger dynamical mass-to-light ratio. This observationsuggests that, at a constant maximum rotational velocity, lower surfacebrightness discs have smaller vertical stellar velocity dispersions.Although the individual uncertainties are large, we find from thedynamical modelling that at least 12 discs are submaximal. The averagedisc contributes 53 +/- 4 per cent to the observed rotation at 2.2 discscalelengths (hR), with a 1σ scatter of 15 per cent.This percentage becomes somewhat lower when effects of finite discflattening and gravity by the dark halo and the gas are taken intoaccount. Since boxy and peanut-shaped bulges are probably associatedwith bars, the result suggests that at 2.2hR the submaximalnature of discs is independent of barredness. The possibility remainsthat very high surface brightness discs are maximal, as these discs areunderrepresented in our sample. We confirm that the radial stellar discvelocity dispersion is related to the galaxy maximum rotationalvelocity. The scatter in this σ versus vmax relationappears to correlate with the disc flattening, face-on central surfacebrightness and dynamical mass-to-light ratio. Low surface brightnessdiscs tend to be more flattened and have smaller stellar velocitydispersions. The findings are consistent with the observed correlationbetween disc flattening and dynamical mass-to-light ratio and cangenerally be reproduced by the simple collapse theory for disc galaxyformation. Finally, the disc mass Tully-Fisher relation is offset fromthe maximum-disc scaled stellar mass Tully-Fisher relation of the UrsaMajor cluster. This offset, -0.3 dex in mass, is naturally explained ifthe discs of the Ursa Major cluster spirals are submaximal.
| The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties
In a recently completed survey of the stellar population properties oflow-ionization nuclear emission-line regions (LINERs) and LINER/HIItransition objects (TOs), we have identified a numerous class ofgalactic nuclei which stand out because of their conspicuous108-9 yr populations, traced by high-order Balmer absorptionlines and other stellar indices. These objects are called `young-TOs',because they all have TO-like emission-line ratios. In this paper weextend this previous work, which concentrated on the nuclear properties,by investigating the radial variations of spectral properties inlow-luminosity active galactic nuclei (LLAGNs). Our analysis is based onhigh signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distancesof typically up to 850 pc from the nucleus with a resolution of ~100 pc(~1 arcsec) and S/N ~ 30. Stellar population gradients are mapped by theradial profiles of absorption-line equivalent widths and continuumcolours along the slit. These variations are further analysed by meansof a decomposition of each spectrum in terms of template galaxiesrepresentative of very young (<=107 yr), intermediate age(108-9 yr) and old (1010 yr) stellar populations.This study reveals that young-TOs also differ from old-TOs andold-LINERs in terms of the spatial distributions of their stellarpopulations and dust. Specifically, our main findings are as follows.(i) Significant stellar population gradients are found almostexclusively in young-TOs. (ii) The intermediate age population ofyoung-TOs, although heavily concentrated in the nucleus, reachesdistances of up to a few hundred pc from the nucleus. Nevertheless, thehalf width at half-maximum of its brightness profile is more typically100 pc or less. (iii) Objects with predominantly old stellar populationspresent spatially homogeneous spectra, be they LINERs or TOs. (iv)Young-TOs have much more dust in their central regions than otherLLAGNs. (v) The B-band luminosities of the central <~1 Gyr populationin young-TOs are within an order of magnitude of MB=-15,implying masses of the order of ~107-108Msolar. This population was 10-100 times more luminous in itsformation epoch, at which time young massive stars would have completelyoutshone any active nucleus, unless the AGN too was brighter in thepast.
| First Results from THINGS: The HI Nearby Galaxy Survey
We describe The HI Nearby Galaxy Survey (THINGS), the largestprogramever undertaken at the Very Large Array to perform 21-cm HIobservations of thehighest quality ( 7'', ≤ 5 km s^{-1}resolution) ofnearby galaxies. The goal of THINGS is to investigatekeycharacteristics related to galaxy morphology, star formation andmassdistribution across the Hubble sequence. A sample of 34 objectswithdistances between 3 and 10 Mpc will be observed, covering a widerangeof evolutionary stages and properties. Data from THINGSwillcomplement SINGS, the Spitzer Infrared Nearby Galaxy Survey. Forthe THINGS sample, high-quality observations at comparable resolutionwillthus be available from the X-ray regime through to the radio partofthe spectrum. THINGS data can be used to investigate issues such asthesmall-scale structure of the ISM, its three-dimensional structure,the(dark) matter distribution and processes leading to starformation. Todemonstrate the quality of the THINGS data products, wepresent someprelimary HI maps here of four galaxies from the THINGSsample.
| The H II Region/PDR Connection: Self-consistent Calculations of Physical Conditions in Star-forming Regions
We have performed a series of calculations designed to reproduceinfrared diagnostics used to determine physical conditions instar-forming regions. We self-consistently calculate the thermal andchemical structure of an H II region and photodissociation region (PDR)that are in pressure equilibrium. This differs from previous work, whichused separate calculations for each gas phase. Our calculations span awide range of stellar temperatures, gas densities, and ionizationparameters. We describe improvements made to the spectral synthesis codeCloudy that made these calculations possible. These include the additionof a molecular network with ~1000 reactions involving 68 molecularspecies and improved treatment of the grain physics. Data from theSpitzer First Look Survey, along with other archives, are used to deriveimportant physical characteristics of the H II region and PDR. Theseinclude stellar temperatures, electron densities, ionization parameters,UV radiation flux (G0), and PDR density. Finally, wecalculate the contribution of the H II region to PDR emission linediagnostics, which allows for a more accurate determination of physicalconditions in the PDR.
| Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities.
| Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope
Full low-resolution (65
| The Baryonic Tully-Fisher Relation of Galaxies with Extended Rotation Curves and the Stellar Mass of Rotating Galaxies
I investigate the baryonic Tully-Fisher relation for a sample ofgalaxies with extended 21 cm rotation curves spanning the range 20 kms-1<~Vf<=300 km s-1. A variety ofscalings of the stellar mass-to-light ratio Υ* areconsidered. For each prescription for Υ*, I give fitsof the form Md=AVxf.Presumably, the prescription that comes closest to the correct valuewill minimize the scatter in the relation. The fit with minimum scatterhas A=50 Msolar km-4 s4 andx=4. This relation holds over five decades in mass. Galaxy color,stellar fraction, and Υ* are correlated with eachother and with Md, in the sense that more massivegalaxies tend to be more evolved. There is a systematic dependence ofthe degree of maximality of disks on surface brightness. High surfacebrightness galaxies typically have Υ*~3/4 of themaximum disk value, while low surface brightness galaxies typicallyattain ~1/4 of this amount.
| Secular Evolution via Bar-driven Gas Inflow: Results from BIMA SONG
We present an analysis of the molecular gas distributions in the 29barred and 15 unbarred spirals in the BIMA CO (J=1-0) Survey of NearbyGalaxies (SONG). For galaxies that are bright in CO, we confirm theconclusion by Sakamoto et al. that barred spirals have higher moleculargas concentrations in the central kiloparsec. The SONG sample alsoincludes 27 galaxies below the CO brightness limit used by Sakamoto etal. Even in these less CO-bright galaxies we show that high central gasconcentrations are more common in barred galaxies, consistent withradial inflow driven by the bar. However, there is a significantpopulation of early-type (Sa-Sbc) barred spirals (6 of 19) that have nomolecular gas detected in the nuclear region and have very little out tothe bar corotation radius. This suggests that in barred galaxies withgas-deficient nuclear regions, the bar has already driven most of thegas within the bar corotation radius to the nuclear region, where it hasbeen consumed by star formation. The median mass of nuclear moleculargas is over 4 times higher in early-type bars than in late-type (Sc-Sdm)bars. Since previous work has shown that the gas consumption rate is anorder of magnitude higher in early-type bars, this implies that theearly types have significantly higher bar-driven inflows. The loweraccretion rates in late-type bars can probably be attributed to theknown differences in bar structure between early and late types. Despitethe evidence for bar-driven inflows in both early and late Hubble-typespirals, the data indicate that it is highly unlikely for a late-typegalaxy to evolve into an early type via bar-induced gas inflow.Nonetheless, secular evolutionary processes are undoubtedly present, andpseudobulges are inevitable; evidence for pseudobulges is likely to beclearest in early-type galaxies because of their high gas inflow ratesand higher star formation activity.
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