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HI content in galaxies in loose groups
Gas deficiency in cluster spirals is well known and ram-pressurestripping is considered the main gas removal mechanism. In some compactgroups too gas deficiency is reported. However, gas deficiency in loosegroups is not yet well established. Lower dispersion of the membervelocities and the lower density of the intragroup medium in small loosegroups favour tidal stripping as the main gas removal process in them.Recent releases of data from the HI Parkes All-Sky Survey (HIPASS) andcatalogues of nearby loose groups with associated diffuse X-ray emissionhave allowed us to test this notion. In this paper, we address thefollowing questions: (i) do galaxies in groups with diffuse X-rayemission statistically have lower gas content compared to the ones ingroups without diffuse X-ray emission? (ii) does HI deficiency vary withthe X-ray luminosity, LX, of the loose group in a systematicway? We find that (i) galaxies in groups with diffuse X-ray emission, onaverage, are HI deficient, and have lost more gas compared to those ingroups without X-ray emission; the latter are found not to havesignificant HI deficiency; (ii) no systematic dependence of the HIdeficiency with LX is found. Ram-pressure-assisted tidalstripping and evaporation by thermal conduction are the two possiblemechanisms to account for this excess gas loss.
| Extended Mid-Infrared Aromatic Feature Emission in M82
We present new images (ground-based optical and mid-infrared [MIR] fromthe Spitzer Space Telescope) and spectra (from Spitzer) of thearchetypal starburst galaxy M82. The Spitzer data show that the MIRemission extends at least 6 kpc along the minor axis of the galaxy. Weuse the optical and infrared data to demonstrate that the extendedemission is dominated by emission from dust. The colors of the MIRemission and the spectra indicate that there is a strong component ofaromatic feature emission (the MIR features commonly attributed topolycyclic aromatic hydrocarbons). The dust continuum and aromaticfeature emission are both strong in the well-known superwind region ofthis galaxy; clearly, the carrier of the aromatic features can survivein close proximity to the wind, far from the plane of the galaxy. Wealso see significant emission by dust well outside the superwind region,providing the clearest picture to date of the dust distribution in thehalo of this galaxy.
| The Intrinsic Shape of Spiral Galaxies in the 2MASS Large Galaxy Atlas
The apparent shapes of spiral galaxies in the Two Micron All Sky SurveyLarge Galaxy Atlas are used to constrain the intrinsic shapes of theirdisks. When the distribution of apparent axis ratios is estimated usinga nonparametric kernel method, the shape distribution is inconsistentwith axisymmetry at the 90% confidence level in the B band and at the99% confidence level in the Ks band. If spirals aresubdivided by Hubble type, the late-type spirals (Sc and later) areconsistent with axisymmetry, while the earlier spirals are stronglyinconsistent with axisymmetry. The distribution of disk ellipticity canbe fitted adequately with either a Gaussian or a lognormal distribution.The best fits for the late spirals imply a median ellipticity ofε~0.07 in the B band and ε~0.02 in the Ks band. Forthe earlier spirals, the best fits imply a median ellipticity ofε~0.18 in the B band and ε~0.30 in the Ks band. Theobserved scatter in the Tully-Fisher relation, for both late and earlyspirals, is consistent with the disk ellipticity measured in the B band.This indicates that excluding spirals of Hubble type earlier than Scwill minimize the intrinsic scatter in the Tully-Fisher relation used asa distance indicator.
| A new method to determine the thickness of non-edge-on disk galaxies
Aims.We present a new method to determine the thickness of non-edge-ondisk galaxies. This method allows us to investigate the mass-to-lightratio of the disk. Methods: .Our method is based on the comparisonof observations and theory of the distribution of the vertical velocitydispersion, which is obtained from the solution of three dimensionalPoisson equations and the galactic dynamical equation. Results:.As examples, the thickness and mass-to-light ratio of two diskgalaxies, NGC 1566 and NGC 5247, which have been extensively studied byspectroscopy, have been calculated. The calculated results areconsistent with observations and support the use of this method.However, due to the small sample size available, the results should beconfirmed on other samples of galaxies.
| 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.
| A catalog of edge-on disk galaxies. From galaxies with a bulge to superthin galaxies
Spiral galaxies range from bulge-dominated early-type galaxies to latetypes with little or no bulge. Cosmological models do not predict theformation of disk-dominated, essentially bulgeless galaxies, yet theseobjects exist. A particularly striking and poorly understood example ofbulgeless galaxies are flat or superthin galaxies with large axisratios. We therefore embarked on a study aimed at a better understandingof these enigmatic objects, starting by compiling a statisticallymeaningful sample with well-defined properties. The disk axis ratios canbe most easily measured when galaxies are seen edge-on. We used datafrom the Sloan Digital Sky Survey (SDSS) in order to identify edge-ongalaxies with disks in a uniform, reproducible, automated fashion. Inthe five-color photometric database of the SDSS Data Release 1 (2099deg^2) we identified 3169 edge-on disk galaxies, which we subdividedinto disk galaxies with bulge, intermediate types, and simple diskgalaxies without any obvious bulge component. We subdivided these typesfurther into subclasses: Sa(f), Sb(f), Sc(f), Scd(f), Sd(f), Irr(f),where the (f) indicates that these galaxies are seen edge-on. Here wepresent our selection algorithm and the resulting catalogs of the 3169edge-on disk galaxies including the photometric, morphological, andstructural parameters of our targets. A number of incompleteness effectsaffect our catalog, but it contains almost a factor of four morebulgeless galaxies with prominent simple disks (flat galaxies) withinthe area covered here than previous optical catalogs, which were basedon the visual selection from photographic plates (cf. Karachentsev etal. 1999, Bull. Special Astrophys. Obs., 47, 5). We find thatapproximately 15% of the edge-on disk galaxies in our catalog are flatgalaxies, demonstrating that these galaxies are fairly common,especially among intermediate-mass star-forming galaxies. Bulgelessdisks account for roughly one third of our galaxies when also puffydisks and edge-on irregulars are included. Our catalog provides auniform database for a multitude of follow-up studies of bulgelessgalaxies in order to constrain their intrinsic and environmentalproperties and their evolutionary status.
| Discovery of PAHs in the halo of NGC 5907
We have used sensitive archival data from the Infrared Space Observatory(ISO) to make maps of the edge-on low SFR galaxy, NGC 5907, in 6different MIR bands: LW2, LW5, LW6, LW7, LW8, and LW10, covering thespectrum from 6.5 to 15.0 μm and including several narrow bands thatisolate the infrared aromatic spectral features commonly referred to asPAHs. Most of the MIR emission is dominated by PAHs and it is likelythat emission from VSGs contribute only negligibly except in the broadIRAS-equivalent band. The flux ratios are typical of galaxies with lowSFRs or quiesent regions within galaxies (e.g. M 83) and a very highPAH/continuum ratio is observed. The PAH emission follows the COdistribution and also shows some correlation within the disk with theλ850 μm distribution. However, the PAH emission also reacheslarger galactocentric radii than the CO and other correlations suggestthat the PAHs are also more widespread. A significant new discovery isthe presence of PAHs in the halo of the galaxy. In the narrow bands thatisolate single PAH features, the emission shows structure similar tohigh latitude features seen in other galaxies in other tracers. Thefeatures extend as far as 6.5 kpc from the plane but scale heights of3.5 kpc are more typical. The λ11.3/λ7.7 ratio alsoappears to increase with distance from the major axis. To our knowledge,this is the first time PAHs have been seen in the halo of an externalgalaxy. Just as significantly, they are seen in a low SFR galaxy,suggesting that strong SNe and winds are not necessary for these largemolecules to reach high latitudes.
| 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.
| Dust in spiral galaxies: global properties
We present and analyse high-quality Submillimetre Common-User BolometerArray (SCUBA) 850- and 450-μm images of 14 local spiral galaxies,including the detection of dust well out into the extended disc in manycases. We use these data in conjunction with published far-infrared fluxdensities from IRAS and ISO, and millimetre-wave measurements fromground-based facilities to deduce the global properties of the dust inthese galaxies, in particular temperature and mass. We find that simpletwo-temperature greybody models of fixed dust emissivity index β= 2and with typical temperatures of 25 < Twarm < 40 K and10 < Tcold < 20 K provide good fits to the overallspectral energy distributions. The dust mass in the cold componentcorrelates with the mass in atomic hydrogen and the mass in the warmcomponent correlates with the mass in molecular hydrogen. These resultsthus fit the simple picture in which the cold dust is heatedpredominantly by the interstellar radiation field, while the hot dust isheated predominantly by OB stars in more active regions, although weargue that there is some mixing. The mean gas-to-dust mass ratio is 120+/- 60, very similar to that found within our own galaxy and roughly afactor of 10 lower than that derived from IRAS data alone. Thegas-to-dust mass ratios in the warm, molecular component are on averagehigher than those in the cold, atomic component. We compare ourmodelling results with similar results for more luminous spiral galaxiesselected at far-infrared wavelengths by the SCUBA Local Universe GalaxySurvey. We find that whilst the total dust mass distributions of the twosamples are indistinguishable, they have significantly different dusttemperature distributions in both the warm and cold components. Wesuggest that this difference might be related to the level of starformation activity in these systems, with the more active galaxieshaving more intense interstellar radiation fields and higher dusttemperatures.
| 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.
| High-Resolution Measurements of the Halos of Four Dark Matter-Dominated Galaxies: Deviations from a Universal Density Profile
We derive rotation curves for four nearby, low-mass spiral galaxies anduse them to constrain the shapes of their dark matter density profiles.This analysis is based on high-resolution two-dimensional Hαvelocity fields of NGC 4605, NGC 5949, NGC 5963, and NGC 6689 and COvelocity fields of NGC 4605 and NGC 5963. In combination with ourprevious study of NGC 2976, the full sample of five galaxies containsdensity profiles that span the range from αDM=0 toαDM=1.20, where αDM is the power-lawindex describing the central density profile. The scatter inαDM from galaxy to galaxy is 0.44, 3 times as large asin cold dark matter (CDM) simulations, and the mean density profileslope is αDM=0.73, shallower than that predicted by thesimulations. These results call into question the hypothesis that allgalaxies share a universal dark matter density profile. We show that oneof the galaxies in our sample, NGC 5963, has a cuspy density profilethat closely resembles those seen in CDM simulations, demonstrating thatwhile galaxies with the steep central density cusps predicted by CDM doexist, they are in the minority. In spite of these differences betweenobservations and simulations, the relatively cuspy density profiles wefind do not suggest that this problem represents a crisis for CDM.Improving the resolution of the simulations and incorporating additionalphysics may resolve the remaining discrepancies. We also find that fourof the galaxies contain detectable radial motions in the plane of thegalaxy. We investigate the hypothesis that these motions are caused by atriaxial dark matter halo and place lower limits on the ellipticity ofthe orbits in the plane of the disk of 0.043-0.175.Based on observations carried out at the WIYN Observatory. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory.
| Structure in the motions of the fastest halo stars
We analyzed the catalog published by Beers et al. (2000, ApJ, 119, 2866)of 2106 non-kinematically selected metal poor stars in the solarneighborhood, with the goal of quantifying the amount of substructure inthe motions of the fastest halo stars. We computed the two-pointvelocity correlation function for a subsample of halo stars within 1-2kpc of the Sun, and found statistical evidence of substructure with asimilar amplitude to that predicted by high resolution CDM simulations.The signal is due to a small kinematic group whose dynamical propertiesare compared to the stellar "stream" previously discovered by Helmi etal. (1999). If real, this high velocity moving group would providefurther support for the idea that substructures remain as fossils fromthe formation of the Galaxy as expected in the CDM scenario.
| Is the Galactic submillimeter dust emissivity underestimated?
We present detailed modeling of the spectral energy distribution (SED)of the spiral galaxies NGC 891, NGC 4013 and NGC 5907 in thefar-infrared (FIR) and sub-millimeter (submm) wavelengths. The modeltakes into account the emission of the diffuse dust component, which isheated by the UV and optical radiation field produced by the stars, aswell as the emission produced locally in star forming HII complexes.Radiative transfer simulations in the optical bands are used toconstrain the stellar and dust geometrical parameters and the dust mass.We find that the submm emission predicted by our model cannot accountfor the observed fluxes at these wavelengths. Two scenarios that couldaccount for the "missing" submm flux are examined. In the first scenariodust additional to that derived from the optical wavelengths is embeddedin the galaxy in the form of a thin disk. This additional dust disk,which is not detectable in the optical and which is associated with theyoung stellar population, gives rise to additional submm emission, andmakes the total flux match the observed values. The other scenarioexamines the possibility that the average emissivity at submmwavelengths of the dust grains found both in a diffuse component and indenser environments (e.g. molecular gas clouds) is higher than thevalues widely used in Galactic environments. This enhanced emissivityreproduces the observed FIR and submm fluxes with the dust mass equal tothat derived from the optical observations. In the second scenario, wetreat the submm emissivity as a free parameter and calculate its nominalvalue by fitting our model to the observed SED. We find a dustemissivity which is ~ 3 times the often-used values for our Galaxy. Bothscenarios can equally well reproduce the observed 850 μm surfacebrightness for all three galaxies. However, we argue that the scenarioof having more dust embedded in a second disk is not supported by thenear infrared observations. At 2.16 μm, the model images with asecond dust disk reveal a prominent dust lane which is not present inthe observations. Thus, the enhanced emissivity at submm wavelengths isa real possibility and the Galactic submillimeter dust emissivity may beunderestimated.
| Cosmic magnetic fields - as observed in the Universe, in galactic dynamos, and in the Milky Way
Cosmic magnetism has that exotic ``Je ne sais quoi''! Magnetism has beenobserved in various objects, located near the edge of the Universe andall the way down to the Milky Way's center. The observed magnetic fieldcan take the cell-type shape in randomly-oriented large blobs found inintracluster gas or outside of clusters of galaxies, the helix shape insynchrotron jets, the longitudinal shape in ram-pressured shocks inradio lobes near elliptical galaxies, the spiral shape of logarithmicarms in spiral galaxies, or the egg shape of an enlarged interstellarbubble. In strength, the magnetic field varies from 0.1 nG(cosmological), to 20 μG (galaxies, jets, superbubbles), and to 1 mGin the Milky Way filaments.Magnetism plays a small physical role in the formation of largestructures. It acts as a tracer of the dynamical histories ofcosmological and intracluster events, it guides the motion of theinterstellar ionised gas, and it aligns the charged dust particles.Batteries and dynamos are often employed in models to create and amplifyseed magnetic fields. Starting soon after the Big Bang (redshiftz>2000), this review covers the cosmological background surface(z~1100, distance ~4.3 Gpc), the epoch of first stars (z~20 distance~4.1 Gpc), the currently observable Universe (z~10, distance ~3.9 Gpc),superclusters of galaxies (size ~50 Mpc), intracluster gas (size ~10Mpc), galaxies (~30 kpc), spiral arms (~10 kpc), interstellarsuperbubbles (~100 pc), synchrotron filaments (~10 pc), and the MilkyWay's center.
| A faint red stellar halo around an edge-on disc galaxy in the Hubble Ultra Deep Field
We analyse the detailed structure of a highly-inclined (i>~ 80°)disc galaxy that lies within the Hubble Ultra Deep Field (UDF). Theunprecedented depth of the UDF data allow disc and extraplanar emissionto be traced reliably to surface brightness levels ofμV,i,z~ 29-30 mag arcsec-2 (corresponding torest-frame equivalents of μg,r,i~ 28-29 magarcsec-2) in this redshift z= 0.32 system. We detect excessemission above the disc, which is characterized by a moderatelyflattened (b/a~ 0.6) power law (I~R-2.6). The structure andcolour of this component are very similar to the stellar halo detectedin an SDSS stacking analysis of local disc galaxies and lend support tothe idea that we have detected a stellar halo in this distant system.Although the peculiar colours of the halo are difficult to understand interms of normal stellar populations, the consistency found between theUDF and SDSS analyses suggests that they cannot be easily discounted.
| The GEMS project: X-ray analysis and statistical properties of the group sample
The Group Evolution Multiwavelength Study (GEMS) involves amultiwavelength study of a sample of 60 galaxy groups, chosen to span awide range of group properties. Substantial ROSAT Position SensitiveProportional Counter (PSPC) observations, available for all of thesegroups, are used to characterize the state of the intergalactic mediumin each. We present the results of a uniform analysis of these ROSATdata and a statistical investigation of the relationship between X-rayand optical properties across the sample. Our analysis improves inseveral respects on previous work: (i) we distinguish between systems inwhich the hot gas is a group-scale medium and those in which it appearsto be just a hot halo associated with a central galaxy; (ii) weextrapolate X-ray luminosities to a fixed overdensity radius(r500) using fitted surface brightness models, in order toavoid biases arising from the fact that cooler systems are detectable tosmaller radii, and (iii) optical properties have been rederived in auniform manner from the NASA Extragalactic Database, rather than relyingon the data in the disparate collection of group catalogues from whichour systems are drawn.The steepening of the LX-TX relation in the groupregime reported previously is not seen in our sample, which fits well onto the cluster trend, albeit with large non-statistical scatter. Anumber of biases affect the fitting of regression lines under thesecircumstances, and until the impact of these has been thoroughlyinvestigated it seems best to regard the slope of the groupLX-TX relation as being poorly determined. Asignificant problem in comparing the properties of groups and clustersis the derivation of system radii, to allow different systems to becompared within regions having the same overdensity. We find evidencethat group velocity dispersion (σv) provides a veryunreliable measure of system mass (and hence radius), with a number ofgroups having remarkably low values of σv, given thatthey appear from their X-ray properties to be collapsed systems. Weconfirm that the surface brightness profiles of groups are significantlyflatter than those of clusters - the maximum value of theβfit parameter for our sample is 0.58, lower than thetypical value of 0.67 seen in clusters - however, we find no significanttendency within our sample for cooler groups to show flatter profiles.This result is inconsistent with simple universal pre-heating models.The morphology of the galaxies in the GEMS groups is correlated to theirX-ray properties in a number of ways: we confirm the very strongrelationship between X-ray emission and a dominant early-type centralgalaxy, which has been noted since the early X-ray studies of groups,and also find that spiral fraction is correlated with the temperature ofthe hot gas and hence the depth of the gravitational potential. A classof spiral-rich groups with little or no X-ray emission probablycorresponds to groups that have not yet fully collapsed.
| Haloes around edge-on disc galaxies in the Sloan Digital Sky Survey
We present a statistical analysis of halo emission for a sample of 1047edge-on disc galaxies imaged in five bands by the Sloan Digital SkySurvey (SDSS). Stacking the homogeneously rescaled images of thegalaxies, we can measure surface brightnesses as deep asμr~ 31 mag arcsec-2. The results stronglysupport the almost ubiquitous presence of stellar haloes around discgalaxies, whose spatial distribution is well described by a power lawρ~r-3, in a moderately flattened spheroid (c/a~ 0.6). Thecolour estimates in g-r and r-i, although uncertain, give a clearindication for extremely red stellar populations, hinting at old agesand/or non-negligible metal enrichment. These results support the ideaof haloes being assembled via early merging of satellite galaxies.
| The visible matter -- dark matter coupling
In the inner parts of spiral galaxies, of high or low surfacebrightness, there is a close correlation between rotation curve shapeand light distribution. For any feature in the luminosity profile thereis a corresponding feature in the rotation curve and vice versa. Thisimplies that the gravitational potential is strongly correlated with thedistribution of luminosity: either the luminous mass dominates or thereis a close coupling between luminous and dark matter. In a similar way,the declining rotation curves observed in the outer parts of highluminosity systems are a clear signature of the stellar disk whicheither dominates or traces the distribution of mass.The notion that the baryons are dynamically important in the centres ofgalaxies, including LSBs, undermines the whole controversy over thecusps in CDM halos and the comparison with the observations. If thebaryons dominate in the central regions of all spirals, including LSBs,how can the CDM profiles be compared with the observations?Alternatively, if the baryons do not dominate but simply trace the DMdistribution, why, in systems of comparable luminosity, are some DMhalos cuspy (like the light) and others (also like the light) are not?
| Molecular Gas in the Edge-On Galaxy NGC 4013
Our OVRO observations at 300 pc resolution of the molecular gas disk inthe edge-on spiral galaxy NGC 4013 show no evidence for extraplanarmaterial at our sensitivity limit. The observed molecular gas kinematicsare in agreement with gas motion in a barred potential.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| The Glow of Primordial Remnants
We determine the expected surface brightness and photometric signatureof a white dwarf remnant population, evolved from primordial low-massstars formed at high redshifts, in present day galactic halos. Weexamine the radial dependence of such a contribution as well as itsredshift dependence. Such a halo diffuse radiation is below thedetection limit of present large-field ground-based surveys, but shouldbe observable with the Hubble Space Telescope (HST) and with the futureJames Webb Space Telescope project. Since the surface brightness doesnot depend on the distance, the integration of several galactic darkhalos along the line of sight will appreciably increase the chances ofdetection. Either the detection or the nondetection of such a remnantdiffuse radiation within relevant detection limits offers valuableinformation about the minimum mass for star formation in the earlyuniverse and the evolution of the stellar initial mass function.
| M31's Undisturbed Thin Disk of Globular Clusters
We show that there is a subsystem of the M31 globular clusters withthin-disk kinematics. These clusters span the entire metallicity rangeof the M31 globular cluster system, in contrast to the (thick) diskglobular clusters in the Milky Way, which are predominantly metal-rich.Disk globular clusters are found across the entire disk of M31 and form~40% of the clusters projected on its disk. The existence of such a disksystem suggests that there was a relatively large thin disk in placevery early in M31's history. Accurate measures of the ages of theseclusters will constrain the epoch of disk formation in M31. There iscurrently no strong evidence for differences in age between Milky Wayand M31 globular clusters. While age differences are subtle for oldpopulations, it is unlikely that disk clusters with [Fe/H] around -2.0were formed after significant star formation began in the galaxy, as theprotocluster gas would be enriched by supernova ejecta. Thus it islikely that M31 had a rather large disk in place at early epochs. Thevery existence of such a cold disk means that M31 has suffered nomergers with an object of 10% or more of the disk mass since theclusters were formed. This makes the suggestion of Brown et al. that M31could have suffered an equal-mass merger 6-8 Gyr ago less viable.
| Radio Continuum Observations of the Virgo Cluster Spiral NGC 4522: The Signature of Ram Pressure
Radio continuum observations at 20 and 6 cm of the highly inclined Virgospiral galaxy NGC 4522 are presented. Both 20 and 6 cm total emissiondistributions are asymmetric with an extended component to the west,where extraplanar atomic gas and Hα emission are found. The 6 cmpolarized emission is located at the eastern edge of the galactic disk.Its peak is located about 1 kpc to the east of the total emission peak.We argue that this phenomena is a characteristic feature for clustergalaxies that are experiencing significant pressure from theintracluster medium. Polarized radio continuum emission is thus apowerful tool to detect interactions of spiral galaxies with the clusterICM. The degree of polarization decreases from the east to the west. Theflattest spectral index between 20 and 6 cm coincides with the peak ofthe 6 cm polarized emission. These findings are consistent with apicture of a large-scale shock due to ram pressure located at the eastof the galaxy where cosmic rays are accelerated. We conclude that it islikely that the galaxy experiences active ram pressure.
| Nuclear Stellar Populations in the Infrared Space Observatory Atlas of Bright Spiral Galaxies
To understand the nuclear stellar populations and star formationhistories of the nuclei of spiral galaxies, we have obtained K-bandnuclear spectra for 41 galaxies and H-band spectra for 20 galaxies inthe Infrared Space Observatory's Atlas of Bright Spiral Galaxies. In thevast majority of the subsample (80%), the near-infrared spectra suggestthat evolved red stars completely dominate the nuclear stellarpopulations and that hot young stars are virtually nonexistent. Thesignatures of recent star formation activity are only found in 20% ofthe subsample, even though older red stars still dominate the stellarpopulations in these galaxies. Given the dominance of evolved stars inmost galaxy nuclei and the nature of the emission lines in the galaxieswhere they were detected, we suggest that nuclear star formationproceeds in the form of instantaneous bursts. The stars produced bythese bursts comprise only ~2% of the total nuclear stellar mass inthese galaxies, but we demonstrate how the nuclear stellar populationsof normal spiral galaxies can be built up through a series of thesebursts. The bursts were detected only in Sbc galaxies and later, andboth bars and interactions appeared to be sufficient, but not necessary,triggers for the nuclear star formation activity. The vast majority ofgalaxies with nuclear star formation were classified as H II galaxies.With one exception, LINERs and transition objects were dominated byolder red stars, which suggested that star formation was not responsiblefor generating these galaxies' optical line emission.
| Mid-Infrared Observations of Extended Light in the Spiral Galaxy NGC 5907
We present deep Spitzer Space Telescope observations of the disk andsurrounding regions of the spiral galaxy NGC 5907. These images, whichwere taken in four bands from 3 to 9.5 microns by the Infrared ArrayCamera, are relatively insensitive to the dust extinction that prevailsin this late type edge-on galaxy. They therefore reveal much that is newabout the stellar content and morphology of NGC 5907. We findindications of extended emission that appears to have a scale heightintermediate between the thin disk and the long-sought dark halo.Implications of this finding are discussed.This work is based in part on observations made with the Spitzer SpaceTelescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology under NASA contract 1407. Support forthis work was provided by NASA through contract 1256790 issued byJPL/Caltech.
| The emissivity of dust grains in spiral galaxies
We use the radiation transfer simulation of Xilouris et al.(\cite{xilouris1999}) to constrain the quantity of dust in three nearbyspiral galaxies (NGC 4013, NGC 5907 and NGC 4565). The predicted visualoptical depth from the model is compared with the thermal continuumradiation detected from NGC 4013 and NGC 5907 at 850 μm and from NGC4565 at 1.2 mm. The former is based on SCUBA images of NGC 4013 and NGC5907, reduced and presented for the first time in this work. Thecomparison of visual optical depth and 850 μm (1.2 mm) emissionyields the emissivity of dust grains in the submillimeter (millimeter)waveband. We infer a value of 1.2 × 104 for theemissivity at 850 μm which is a factor 4 higher than the benchmark,semi-empirical model of Draine & Lee (\cite{draine1984}). At 1.2 mmour values are a factor 1.5 higher than this model. Our estimates aremore closely aligned with recent measurements carried out in thelaboratory on amorphous carbon and silicate particulates. A comparisonbetween the distribution of 850 μm (1.2 mm) surface brightness andthe intensity levels in the 12CO(1-0) and 21 cm linesunderlines the spatial association between dust detected in thesubmillimeter/millimeter waveband and molecular gas clouds. We suggestthat the relatively high emissivity values that we derive may beattributable to amorphous, fluffy grains situated in denser gasenvironments.
| Thick disks of lenticular galaxies. 3D-photometric thin/thick disk decomposition of eight edge-on s0 galaxies
Thick disks are faint and extended stellar components found aroundseveral disk galaxies including our Milky Way. The Milky Way thick disk,the only one studied in detail, contains mostly old disk stars (≈10Gyr), so that thick disks are likely to trace the early stages of diskevolution. Previous detections of thick disk stellar light in externalgalaxies have been originally made for early-type, edge-on galaxies butdetailed 2D thick/thin disk decompositions have been reported for only ascant handful of mostly late-type disk galaxies. We present in thispaper for the first time explicit 3D thick/thin disk decompositionscharacterising the presence and properties (e.g. scalelength andscaleheight) for a sample of eight lenticular galaxies by fitting 3Ddisk models to the data. For six out of the eight galaxies we were ableto derive a consistent thin/thick disk model. The mean scaleheight ofthe thick disk is 3.6 times larger than that of the thin disk. Thescalelength of the thick disk is about twice, and its central luminositydensity between 3-10% of, the thin disk value. Both thin and thick diskare truncated at similar radii. This implies that thick disks extendover fewer scalelengths than thin disks, and turning a thin disk into athick one requires therefore vertical but little radial heating. Allthese structural parameters are similar to thick disk parameters forlater Hubble-type galaxies previously studied. We discuss our data inrespect to present models for the origin of thick disks, either as pre-or post-thin-disk structures, providing new observational constraints.Based on observations obtained at the European Southern Observatory,Chile.Full appendices are only available in electronic form athttp://www.edpsciences.org
| The ISOPHOT 170 μm Serendipity Survey II. The catalog of optically identified galaxies%
The ISOPHOT Serendipity Sky Survey strip-scanning measurements covering≈15% of the far-infrared (FIR) sky at 170 μm were searched forcompact sources associated with optically identified galaxies. CompactSerendipity Survey sources with a high signal-to-noise ratio in at leasttwo ISOPHOT C200 detector pixels were selected that have a positionalassociation with a galaxy identification in the NED and/or Simbaddatabases and a galaxy counterpart visible on the Digitized Sky Surveyplates. A catalog with 170 μm fluxes for more than 1900 galaxies hasbeen established, 200 of which were measured several times. The faintest170 μm fluxes reach values just below 0.5 Jy, while the brightest,already somewhat extended galaxies have fluxes up to ≈600 Jy. For thevast majority of listed galaxies, the 170 μm fluxes were measured forthe first time. While most of the galaxies are spirals, about 70 of thesources are classified as ellipticals or lenticulars. This is the onlycurrently available large-scale galaxy catalog containing a sufficientnumber of sources with 170 μm fluxes to allow further statisticalstudies of various FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) areMPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena,Imperial College London.Full Table 4 and Table 6 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/39
| Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies
We present new calculations of the attenuation of stellar light fromspiral galaxies using geometries for stars and dust which can reproducethe entire spectral energy distribution from the ultraviolet (UV) to theFar-infrared (FIR)/submillimeter (submm) and can also account for thesurface brightness distribution in both the optical/Near-infrared (NIR)and FIR/submm. The calculations are based on the model of Popescu et al.(\cite{Popescu2000}), which incorporates a dustless stellar bulge, adisk of old stars with associated diffuse dust, a thin disk of youngstars with associated diffuse dust, and a clumpy dust componentassociated with star-forming regions in the thin disk. The attenuations,which incorporate the effects of multiple anisotropic scattering, arederived separately for each stellar component, and presented in the formof easily accessible polynomial fits as a function of inclination, for agrid in optical depth and wavelength. The wavelength range considered isbetween 912 {Å} and 2.2 μm, sampled such that attenuation canbe conveniently calculated both for the standard optical bands and forthe bands covered by GALEX. The attenuation characteristics of theindividual stellar components show marked differences between eachother. A general formula is given for the calculation of compositeattenuation, valid for any combination of the bulge-to-disk ratio andamount of clumpiness. As an example, we show how the optical depthderived from the variation of attenuation with inclination depends onthe bulge-to-disk ratio. Finally, a recipe is given for aself-consistent determination of the optical depth from theHα/Hβ line ratio.Tables \ref{tab4}-\ref{tab6} and Appendix A are only available inelectronic form at http://www.edpsciences.org }
| Cold Dust in Galaxies: Sub-mm Observations with the Heinrich-Hertz-Telescope
We have studied the cold dust component of a sample of nearby galaxiesvia its thermal continuum emission at 870 mu m wavelength.
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