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Bias-free Measurement of Giant Molecular Cloud Properties
We review methods for measuring the sizes, line widths, and luminositiesof giant molecular clouds (GMCs) in molecular-line data cubes with lowresolution and sensitivity. We find that moment methods are robust andsensitive, making full use of both position and intensity information,and we recommend a standard method to measure the position angle, majorand minor axis sizes, line width, and luminosity using moment methods.Without corrections for the effects of beam convolution and sensitivityto GMC properties, the resulting properties may be severely biased. Thisis particularly true for extragalactic observations, where resolutionand sensitivity effects often bias measured values by 40% or more. Wecorrect for finite spatial and spectral resolutions with a simpledeconvolution, and we correct for sensitivity biases by extrapolatingproperties of a GMC to those we would expect to measure with perfectsensitivity (i.e., the 0 K isosurface). The resulting method recoversthe properties of a GMC to within 10% over a large range of resolutionsand sensitivities, provided the clouds are marginally resolved with apeak signal-to-noise ratio greater than 10. We note that interferometerssystematically underestimate cloud properties, particularly the fluxfrom a cloud. The degree of bias depends on the sensitivity of theobservations and the (u,v) coverage of the observations. In an Appendixto the paper we present a conservative, new decomposition algorithm foridentifying GMCs in molecular-line observations. This algorithm treatsthe data in physical rather than observational units (i.e., parsecsrather than beams or arcseconds), does not produce spurious clouds inthe presence of noise, and is sensitive to a range of morphologies. As aresult, the output of this decomposition should be directly comparableamong disparate data sets.
| 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.
| Planetary nebulae as tracers of galaxy stellar populations
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
| Gas distribution, kinematics and star formation in faint dwarf galaxies
We compare the gas distribution, kinematics and the current starformation in a sample of 10 very faint (-13.37 < MB <-9.55) dwarf galaxies. For five of these galaxies we present fresh,high-sensitivity, Giant Metrewave Radio Telescope HI 21-cm observations.We find that the large-scale HI distribution in the galaxies istypically irregular and clumpy, with the peak gas density rarelyoccurring at the geometric centre. We also find that the velocity fieldsfor all the galaxies have an ordered component, although in general, thepatterns seen do not fit that expected from a rotating disc. For all ourgalaxies we construct maps of the HI column density at a constant linearresolution of ~300 pc; this forms an excellent data set to check for thepresence of a threshold column density for star formation. We find thatwhile current star formation (as traced by Hα emission) isconfined to regions with relatively large [NHI > (0.4-1.7)× 1021cm-2] HI column density, themorphology of the Hα emission is in general not correlated withthat of the high HI column density gas. Thus, while high column densitygas may be necessary for star formation, in this sample at least, it isnot sufficient to ensure that star formation does in fact occur. Weexamine the line profiles of the HI emission, but do not find a simplerelation between regions with complex line profiles and those withongoing star formation. Our sample includes examples of regions wherethere is ongoing star formation, but the profiles are well fitted by asingle Gaussian, as well as regions where there is no star formation butthe line profiles are complex. Finally, we examine the very fine scale(~20-100 pc) distribution of the HI gas, and find that at these scalesthe emission exhibits a variety of shell-like, clumpy and filamentaryfeatures. The Hα emission is sometimes associated withhigh-density HI clumps, sometimes the Hα emission lies inside ahigh-density shell, and sometimes there is no correspondence between theHα emission and the HI clumps. In summary, the interplay betweenstar formation and gas density in these galaxies does not seem to showthe simple large-scale patterns observed in brighter galaxies.
| The satellite distribution of M31
The spatial distribution of the Galactic satellite system plays animportant role in Galactic dynamics and cosmology, where its successfulreproduction is a key test of simulations of galaxy halo formation.Here, we examine its representative nature by conducting an analysis ofthe three-dimensional spatial distribution of the M31 subgroup ofgalaxies, the next closest system to our own. We begin by a discussionof distance estimates and incompleteness concerns, before revisiting thequestion of membership of the M31 subgroup. We constrain this byconsideration of the spatial and kinematic properties of the putativesatellites. Comparison of the distribution of M31 and Galacticsatellites relative to the galactic discs suggests that the Galacticsystem is probably modestly incomplete at low latitudes by ~=20 percent. We find that the radial distribution of satellites around M31 ismore extended than the Galactic subgroup; 50 per cent of the Galacticsatellites are found within ~100 kpc of the Galaxy, compared to ~200 kpcfor M31. We search for `ghostly streams' of satellites around M31, inthe same way others have done for the Galaxy, and find several,including some that contain many of the dwarf spheroidal satellites. Thelack of M31-centric kinematic data, however, means that we are unable toprobe whether these streams represent real physical associations.Finally, we find that the M31 satellites are asymmetrically distributedwith respect to our line of sight to this object, so that the majorityof its satellites are on its near side with respect to our line ofsight. We quantify this result in terms of the offset between M31 andthe centre of its satellite distribution, and find it to be significantat the ~ 3σ level. We discuss possible explanations for thisfinding, and suggest that many of the M31 satellites may have beenaccreted only relatively recently. Alternatively, this anisotropy may berelated to a similar result recently reported for the 2dFGRS, whichwould imply that the halo of M31 is not yet virialized. Until such timeas a satisfactory explanation for this finding is presented, however,our results warn against treating the M31 subgroup as complete, unbiasedand relaxed.
| Molecular Gas in the Low-Metallicity, Star-forming Dwarf IC 10
We present a complete survey of CO (1-->0) emission in the LocalGroup dwarf irregular IC 10. The survey, conducted with the BIMAinterferometer, covers the stellar disk and a large fraction of theextended H I envelope with the sensitivity and resolution necessary todetect individual giant molecular clouds (GMCs) at the distance of IC 10(950 kpc). We find 16 clouds with a total CO luminosity of1×106 K km s-1 pc2, equivalent to4×106 Msolar of molecular gas using theGalactic CO-to-H2 conversion factor. Observations with theARO 12 m find that BIMA may resolve out as much as 50% of the COemission, and we estimate the total CO luminosity as~2.2×106 K km s-1 pc2. We measurethe properties of 14 GMCs from high-resolution OVRO data. These cloudsare very similar to Galactic GMCs in their sizes, line widths,luminosities, and CO-to-H2 conversion factors, despite thelow metallicity of IC 10 (Z~1/5 Zsolar). Comparing the BIMAsurvey to the atomic gas and stellar content of IC 10, we find that mostof the CO emission is coincident with high surface density H I. IC 10displays a much higher star formation rate per unit molecular(H2) or total (H I+H2) gas than most galaxies.This could be a real difference or may be an evolutionary effect-thestar formation rate may have been higher in the recent past.
| 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 Cosmological Significance of High-Velocity Cloud Complex H
We have used new and archival infrared and radio observations to searchfor a dwarf galaxy associated with the high-velocity cloud (HVC) knownas `complex H.' Complex H is a large (Ω>~400 deg2)and probably nearby (d=27 kpc) HVC whose location in the Galactic planehas hampered previous investigations of its stellar content. The H Imass of the cloud is 2.0×107(d/27 kpc)2Msolar, making complex H one of the most massive HVCs if itsdistance is more than ~20 kpc. Virtually all similar H I clouds in othergalaxy groups are associated with low surface brightness dwarf galaxies.We selected mid-infrared sources observed by the MSX satellite in thedirection of complex H that appeared likely to be star-forming regionsand observed them at the wavelength of the CO J=1-->0 rotationaltransition in order to determine their velocities. Of the 60 observedsources, 59 show emission at Milky Way velocities, and we detected noemission at velocities consistent with that of complex H. We use theseobservations to set an upper limit on the ongoing star formation rate inthe HVC of <~5×10-4 Msolaryr-1. We also searched the 2MASS database for evidence of anydwarf-galaxy-like stellar population in the direction of the HVC andfound no trace of a distant red giant population, with an upper limit onthe stellar mass of ~106 Msolar. Given the lack ofevidence for either current star formation or an evolved population, weconclude that complex H cannot be a dwarf galaxy with properties similarto those of known dwarfs. Complex H is therefore one of the most massiveknown H I clouds that does not contain any stars. If complex H isself-gravitating, then this object is one of the few known dark galaxycandidates. These findings may offer observational support for the ideathat the cold dark matter substructure problem is related to thedifficulty of forming stars in low-mass dark matter halos;alternatively, complex H could be an example of a cold accretion flowonto the Milky Way.
| Neon and Oxygen Abundances in M33
We present new spectroscopic observations of 13 H II regions in theLocal Group spiral galaxy M33. The regions observed range from 1 to 7kpc in distance from the nucleus. Of the 13 H II regions observed, the[O III] λ4363 line was detected in six regions. Electrontemperatures were thus able to be determined directly from the spectrausing the [O III] λλ4959, 5007/λ4363 line ratio.Based on these temperature measurements, oxygen and neon abundances andtheir radial gradients were calculated. For neon, a gradient of-0.016+/-0.017 dex kpc-1 was computed, which agrees with theNe/H gradient derived previously from ISO spectra. A gradient of-0.012+/-0.011 dex kpc-1 was computed for O/H, much shallowerthan was derived in previous studies. The newly calculated O/H and Ne/Hgradients are in much better agreement with each other, as expected frompredictions of stellar nucleosynthesis. We examine the correlationbetween the WC/WN ratio and metallicity, and find that the new M33abundances do not impact the observed correlation significantly. We alsoidentify two new He II-emitting H II regions in M33, the first to bediscovered in a spiral galaxy other than the Milky Way. In both casesthe nebular He II emission is not associated with Wolf-Rayet stars.Therefore, caution is warranted in interpreting the relationship betweennebular He II emission and Wolf-Rayet stars when both are observed inthe integrated spectrum of an H II region.
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| A Local Group Polar Ring Galaxy: NGC 6822
Star counts obtained from a 2° × 2° area centered on NGC6822 have revealed an optical image of this galaxy composed of twocomponents: in addition to the well-known H I disk with its youngstellar component, there is a spheroidal stellar structure as extensiveas its H I disk, but with its major axis at roughly right angles to it,that we traced to at least 36'. Radial velocities of over 100intermediate-age carbon stars found within this structure displaykinematics contrasting strongly with those of the H I disk. These Cstars belong to the spheroid. Although devoid of gas, the spheroidrotation is consistent with the I-band Tully-Fisher relation. Theorientation of the rotation axis that minimizes the stellar velocitydispersion coincides with the minor axis of the stellar populationellipsoid, lying very nearly in the plane of the H I disk. We concludethat the H I disk is a polar ring and that the spheroidal component isan erstwhile disk, a fossil remainder of a past close encounter episode.Based on observations obtained with MegaPrime/MegaCam, a joint projectof CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT),which is operated by the National Research Council (NRC) of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique (CNRS) of France, and the University of Hawaii.Based on observations acquired at the du Pont Telescope, from theObservatories of the Carnegie Institution of Washington.
| 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.
| Weak redshift discretisation in the Local Group of galaxies?
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
| Massive Star Cluster Populations in Irregular Galaxies as Probable Younger Counterparts of Old Metal-rich Globular Cluster Populations in Spheroids
Peak metallicities of metal-rich populations of globular clusters(MRGCs) belonging to early-type galaxies and spheroidal subsystems ofspiral galaxies (spheroids) of different mass fall within the somewhatconservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible ageeffects are taken into account, this metallicity range might becomesmaller. Irregular galaxies such as the Large Magellanic Cloud (LMC),with longer timescales of formation and lower star formation (SF)efficiency, do not contain old MRGCs with [Fe/H]>-1.0, but they areobserved to form populations of young/intermediate-age massive starclusters (MSCs) with masses exceeding 104 Msolar.Their formation is widely believed to be an accidental process fullydependent on external factors. From the analysis of available data onthe populations and their hosts, including intermediate-age populousstar clusters in the LMC, we find that their most probable meanmetallicities fall within -0.7<=[Fe/H]<=-0.3, as the peakmetallicities of MRGCs do, irrespective of signs of interaction.Moreover, both the disk giant metallicity distribution function (MDF) inthe LMC and the MDFs for old giants in the halos of massive spheroidsexhibit a significant increase toward [Fe/H]~-0.5. That is in agreementwith a correlation found between SF activity in galaxies and theirmetallicity. The formation of both the old MRGCs in spheroids and MSCpopulations in irregular galaxies probably occurs at approximately thesame stage of the host galaxies' chemical evolution and is related tothe essentially increased SF activity in the hosts around the samemetallicity that is achieved very early in massive spheroids, later inlower mass spheroids, and much later in irregular galaxies. Changes inthe interstellar dust, particularly in elemental abundances in dustgrains and in the mass distribution function of the grains, may be amongthe factors regulating star and MSC formation activity in galaxies.Strong interactions and mergers affecting the MSC formation presumablyplay an additional role, although they can substantially intensify theinternally regulated MSC formation process. Several implications of oursuggestions are briefly discussed.
| Mid-Infrared Images of Stars and Dust in Irregular Galaxies
We present mid-IR to optical properties of 22 representative irregulargalaxies: 18 irregular (Im) galaxies, 3 blue compact dwarfs, and 1Magellanic-type spiral galaxy. The mid-IR is based on images from theSpitzer Space Telescope archives. The 3.6 and 4.5 μm bands and theUBVJHK images are used to examine disk morphology and the integrated andazimuthally averaged magnitudes and colors of stars. The nonstellarcontribution to the 4.5 μm images is used to trace hot dust. The 5.8and 8.0 μm images reveal emission from hot dust and polycyclicaromatic hydrocarbons (PAHs), and both may contribute to thesepassbands, although we refer to the nonstellar emission as PAH emission.We compare the 8.0 μm images to Hα. Im galaxies have no hiddenbars, and those with double-exponential optical light profiles have thesame at mid-IR. Most galaxies have similar optical and mid-IR scalelengths. Four galaxies have super star clusters that are not visible atoptical bands. Galaxies with higher area-normalized star formation rateshave more dust and PAH emission relative to starlight. Hot dust and PAHemission is found mostly in high surface brightness H II regions,implying that massive stars are the primary source of heating. Galaxieswith intense, widespread star formation have more extended PAH emission.The ratio of PAH to Hα emission is not constant on small scales.PAHs are associated with shells and giant filaments, so they are notdestroyed during shell formation.This work is based in part on archival data obtained with the SpitzerSpace Telescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under a contract with NASA.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33
We present UBVRI photometry obtained from Mosaic images of M31 and M33using the Kitt Peak National Observatory 4 m telescope. We describe ourdata reduction and automated photometry techniques in some detail, as wewill shortly perform a similar analysis of other Local Group galaxies.The present study covered 2.2 deg2 along the major axis ofM31 and 0.8 deg2 on M33, chosen so as to include all of theregions currently active in forming massive stars. We calibrated ourdata using photometry from the Lowell 1.1 m telescope, and this externalmethod resulted in millimagnitude differences in the photometry ofoverlapping fields, providing some assurance that our photometry isreliable. The final catalog contains 371,781 and 146,622 stars in M31and M33, respectively, where every star has a counterpart in (at least)the B, V, and R passbands. Our survey goes deep enough to achieve 1%-2%photometry at 21 mag (corresponding to stars more massive than 20Msolar) and achieves <10% errors at U~B~V~R~I~23 mag.Although our typical seeing was only modest (0.8"-1.4", with median1.0") by some standards, we find excellent correspondence between ourcatalog sources and those we see in our Hubble Space Telescope ACS datafor OB48, a crowded region in M31. We compare our final photometry withthat of others and find good agreement with the CCD catalog of M31 starsby Magnier et al., although our study covers twice the area and goesabout 2 mag deeper. There is also excellent agreement with the CCD``DIRECT'' surveys of M31 and M33. The photographic studies of othersfare less well, particularly at the faint end in V, where accuratebackground subtraction is needed for good photometry. We providecross-references to the stars confirmed as members by spectroscopy andcompare the locations of these to the complete set in color-magnitudediagrams. While follow-up spectroscopy is needed for many projects, wedemonstrate the success of our photometry in being able to distinguishM31/M33 members from foreground Galactic stars. Finally, we present theresults of a single night of spectroscopy on the WIYN 3.5 m telescope,examining the brightest likely members of M31. The spectra identify 34newly confirmed members, including B-A supergiants, the earliest O starknown in M31, and two new luminous blue variable candidates whosespectra are similar to that of P Cygni.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555. These observations areassociated with program GO-9794.
| Diffuse Ionized Gas in Irregular Galaxies. I. GR 8 and ESO 245-G05
We have studied the spectral characteristics of the diffuse ionized gas(DIG) in two irregular galaxies with low metallicities and intermediatestar formation rates: ESO 245-G05 and GR 8. The [O III]/Hβ ratio inthese galaxies is higher than in the DIG of spiral galaxies but not ashigh as in other irregular galaxies previously studied, such as IC 10and NGC 6822. The [N II]/Hα and [S II]/Hα ratios have verysmall values, indicating the absence of shocks as the ionization sourcefor this gas. This ionization can be explained in both galaxies withphoton leakage from the H II regions as the only source. The percentageof photons that have escaped from the H II regions is small in ESO245-G05, only 35%, but varies from 35% up to 60% in GR 8. We alsoinvestigated whether the differences found between spiral and irregulargalaxies in the [O III]/Hβ and the [N II]/Hα ratios are dueto differences in the metal content between these types of galaxies.Although the number of galaxies studied is not very large, it can beconcluded that the [O III]/Hβ ratio is not related to the oxygencontent, while the situation is more ambiguous for the [N II]/Hαratio.
| The Anisotropic Distribution of M31 Satellite Galaxies: A Polar Great Plane of Early-type Companions
The highly anisotropic distribution and apparent alignment of theGalactic satellites in polar great planes begs the question of howcommon such distributions are. The satellite system of M31 is the onlynearby system for which we currently have sufficiently accuratedistances to study the three-dimensional satellite distribution. Wepresent the spatial distribution of the 15 currently known M31companions in a coordinate system centered on M31 and aligned with itsdisk. Through a detailed statistical analysis we show that the fullsatellite sample describes a plane that is inclined by -56° withrespect to the poles of M31 and has an rms height of 100 kpc. At 88% thestatistical significance of this plane is low, and it is unlikely tohave a physical meaning. We note that the great stellar stream foundnear Andromeda is inclined to this plane by 7°. Most of the M31satellites are found within <+/-40° of M31's disk; i.e., there islittle evidence for a Holmberg effect. If we confine our analysis toearly-type dwarfs, we find a best-fit polar plane within 5°-7°from the pole of M31. This polar great plane has a statisticalsignificance of 99.7% and includes all dSphs (except for And II), M32,NGC 147, and PegDIG. The rms distance of these galaxies from the polarplane is 16 kpc. The nearby spiral M33 has a distance of only ~3 kpcfrom this plane, which points toward the M81 group. We discuss theanisotropic distribution of M31's early-type companions in the frameworkof three scenarios, namely, as remnants of the breakup of a largerprogenitor, as a tracer of a prolate dark matter halo, and as a tracerof collapse along large-scale filaments. The first scenario requiresthat the breakup must have occurred at very early times and that thedwarfs continued to form stars thereafter to account for their stellarpopulation content and luminosity-metallicity relation. The thirdscenario seems to be plausible, especially when considering the apparentalignment of our potential satellite filament with several nearbygroups. The current data do not permit us to rule out any of thescenarios. Orbit information is needed to test the physical reality ofthe polar plane and of the different scenarios in more detail.
| [CII] 158 μm emission and metallicity in photon dominated regions
We study the effects of a metallicity variation on the thermal balanceand [CII] fine-structure line strengths in interstellar photon dominatedregions (PDRs). We find that a reduction in the dust-to-gas ratio andthe abundance of heavy elements in the gas phase changes the heatbalance of the gas in PDRs. The surface temperature of PDRs decreases asthe metallicity decreases except for high density (n>106cm-3) clouds exposed to weak (χ< 100) FUV fields wherevibrational H2-deexcitation heating dominates over photoelectric heatingof the gas. We incorporate the metallicity dependence in our KOSMA-τPDR model to study the metallicity dependence of [CII]/CO line ratios inlow metallicity galaxies. We find that the main trend in the variationof the observed CII/CO ratio with metallicity is well reproduced by asingle spherical clump, and does not necessarily require an ensemble ofclumps as in the semi-analytical model presented by Bolatto et al.(1999).
| Photometric survey of the polar ring galaxy NGC 6822
Context: .We have previously established, from a carbon star survey,that the Local Group dwarf irregular galaxy NGC 6822 is much larger thanits central bright core. Aims: . Four MegaCam fields are acquiredto survey a 2°× 2° area centred on NGC 6822 to fullydetermine its extent and map its stellar populations. Methods:.Photometry of over one million stars is obtained in the SDSS g', r', i'to three magnitudes below the TRGB. RGB stars, selected from theirmagnitudes and colours, are used to map the NGC 6822 stellardistribution up to a distance of 60 arcmin. Results: .We map thereddening over the whole area. We establish that the stellar outerstructure of NGC 6822 is elliptical in shape, with ɛ=0.36 and amajor-axis PA = 65°, contrasting with the orientation of the HIdisk. The density enhancement can be seen up to a semi-major axis of 36'making NGC 6822 as big as the Small Magellanic Cloud. We fit twoexponentials to the surface density profile of the spheroid, andidentify a bulge with a scale length of 3.85' and an outer spheroid witha scale length of 10.0'. We find intermediate-age C stars up to 40'while demonstrating that the SDSS filters are unsuitable to identifyextragalactic C stars. Conclusions: .NGC 6822 is a unique LocalGroup galaxy with shape and structure suggesting a polar ringconfiguration. Radial velocities of carbon stars have indeeddemonstrated that there are two kinematical systems in NGC 6822.
| 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.
| Reduced Wolf-Rayet line luminosities at low metallicity
New NTT/EMMI spectrophotometry of single WN2-5 stars in the Small andLarge Magellanic Clouds are presented, from which He ii λ4686line luminosities have been derived, and compared with observations ofother Magellanic Cloud Wolf-Rayet stars. SMC WN3-4 stars possess lineluminosities which are a factor of 4 times lower than LMC counterparts,incorporating several binary SMC WN3-4 stars from the literature.Similar results are found for WN5-6 stars, despite reduced statistics,incorporating observations of single LMC WN5-9 stars from theliterature. C iv λ5808 line luminosities of carbon sequence WRstars in the SMC and IC 1613 (both WO subtypes) from the recentliterature are a factor of 3 lower than LMC WC stars from Mt Stromlo/DBSspectrophotometry, although similar results are also obtained for thesole LMC WO star. We demonstrate how reduced line luminosities at lowmetallicity follow naturally if WR winds are metallicity-dependent, asrecent empirical and theoretical results suggest. We apply massloss-metallicity scalings to atmospheric non-LTE models of Milky Way andLMC WR stars to predict the wind signatures of WR stars in themetal-poor star forming WR galaxy I Zw 18. WN He ii λ4686 lineluminosities are 7-20 times lower than in metal-rich counterparts ofidentical bolometric luminosity, whilst WC C iv λ5808 lineluminosities are 3-6 times lower. Significant He+ Lymancontinuum fluxes are predicted for metal-poor early-type WR stars.Consequently, our results suggest a larger population of WR stars in IZw 18 than is presently assumed, particularly for WN stars, potentiallyposing a severe challenge to evolutionary models at very lowmetallicity. Finally, reduced wind strengths from WR stars at lowmetallicities impacts upon the immediate circumstellar environment oflong duration GRB afterglows, particularly since the host galaxies ofhigh-redshift GRBs tend to be metal-poor.
| Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
| The C star population of DDO 190
We have carried out deep R, I, CN, TiO observations of the dwarfirregular galaxy DDO 190. We confirm the existence of anintermediate-age population around this galaxy. The identification of 47carbon stars seen up to 5 arcmin from the centre of the galaxy impliesthat the population distribution of DDO 190 is similar to those found insome other Local Group dIrr galaxies. An estimate of the metallicity,[Fe/H] = -1.55 ± 0.12, is obtained based on the observed C/Mratio. From the analysis of star counts, corrected for the radialvariation of the incompleteness level, we determine a scale-lengthα = 40 ± 5'', in agreement with the recent literature.
| The Geometric Distance and Proper Motion of the Triangulum Galaxy (M33)
We measured the angular rotation and proper motion of the TriangulumGalaxy (M33) with the Very Long Baseline Array by observing twoH2O masers on opposite sides of the galaxy. By comparing theangular rotation rate with the inclination and rotation speed, weobtained a distance of 730 +/- 168 kiloparsecs. This distance isconsistent with the most recent Cepheid distance measurement. M33 ismoving with a velocity of 190 +/- 59 kilometers per second relative tothe Milky Way. These measurements promise a method to determinedynamical models for the Local Group and the mass and dark-matter halosof M31, M33, and the Milky Way.
| The Diffuse Ionized Gas in the large telescopes era
In this workshop we summarize the ``state of the art'' of the DiffuseIonized Gas. We present all the possible situations which can produceionization outside an H II region, as well as some of the observationsthat can be performed with the GTC instrumentation and how relevant theycan be in the undestanding of the ionization mechanisms of the DIG.
| Imaging resources for the GTC: the Local Group Census
The Local Group Census is a narrowband imaging survey aimed atcataloguing the emission-line populations in the galaxies of the LocalGroup. Data, which were obtained using the Wide Field Camera of the 2.5mIsaac Newton Telescope, are available to the whole astronomicalcommunity, resulting in a valuable imaging resource for follow-upspectroscopy with the GTC.
| A Dynamical Model for the Orbit of the Andromeda Galaxy M31 and the Origin of the Local Group of Galaxies
We propose a new model for the origin and evolution of the Local Groupof Galaxies (LGG) that naturally explains the formation of theMagellanic Clouds and their large orbital angular momenta around theGalaxy. The basic idea is that an off-center hydrodynamical collisionoccurred some 10Gyr ago between the primordial Andromeda galaxy (M31)and a similar Galaxy, and compressed the halo gas to form the LGG dwarfgalaxies, including the Magellanic Clouds. New-born dwarf galaxies canbe expected to locate on the orbital plane of these two massivegalaxies. We reexamined the two-dimensional sky distribution of the LGGmembers, and confirmed an early idea that they align along two similargreat circles. The planes of these circles are approximately normal tothe line joining the present position of the Sun and the galacticcenter. We made a distribution map of these objects, and found awell-defined plane of finite thickness. Thus we could determine theorbital elements of M31 relative to the Galaxy by reproducing thewell-studied dynamics of the LMC and the SMC around the Galaxy. Theexpected proper motion of M31 is (μl, μb) =(38 ± 16 μas yr-1, -49 ± 5 μasyr-1).
| 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.
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