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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.
| 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.
| The Starburst-Interstellar Medium Interaction in NGC 1569. II. Small-Scale Examination of Nebular Emission, H II Region Size Distribution, and H II Region Luminosity Function
As the nearest dramatic example of a poststarburst galaxy driving agalactic wind, NGC 1569 is an ideal test environment to understand theimpact of ``feedback'' from massive star lives and deaths on thesurrounding interstellar medium. We present Hubble Space Telescope WideField Planetary Camera 2 narrowband imagery of NGC 1569 in an attempt tounderstand the underlying ionizing emission mechanisms on a 3 pc scaleand to generate a H II region size distribution and luminosity function.We use [O III]/Hβ and [S II]/Hα ratio maps to find thatnonphotoionizing mechanisms (e.g., shocks) are responsible for 10%+/-3%of the Hα emission, 2.5-3 times larger than results from similargalaxies. Note that our method of determining this result is differentfrom these past results, a point that we discuss further in this paper.The area of the nonphotoionized region is 10%-23% of the total. Ourresults for NGC 1569 indicate that these nonphotoionized areas do notlie in low surface brightness regions exclusively. A comparison withmultiwavelength point-source catalogs of NGC 1569 indicates that thedominant nonphotoionizing mechanisms are shocks from supernovae or windsfrom massive stars. To explain this large percentage of nonphotoionizedemission, we suggest that NGC 1569 is, indeed, in a poststarburst phase,as previous authors have claimed. We also derive slopes for the H IIregion luminosity function (-1.00+/-0.08) and size distribution(-3.02+/-0.27). The luminosity slope, although shallow, is similar toprevious work on this galaxy and other irregular galaxies. The sizedistribution slope is shallower than previous slopes found for irregulargalaxies, but our slope value fits into their confidence intervals, andvice versa. Within 4 pc of the 10-20 Myr old super star clusters A1, A2,and B, no bright H II regions exist to a luminosity limit of2.95×1036 ergs s-1, suggesting that thewinds and shocks have effectively terminated star formation in thissmall cavity. In the three annular regions around the super starclusters, both the H II region luminosity function and H II region sizedistribution are consistent with respect to one another and the galaxyas a whole. The H II region surface densities within the annuli remainthe same as the annuli are moved away from the super star clusters.These results indicate that feedback effects in NGC 1569 are confined tothe immediate vicinity of the most recent massive star formation eventon scales of ~1 pc.
| Tramp Classical Novae as Tracers of Intergalactic Stars
Simulations predict that collisions between galaxies must liberate starsinto intergalactic space. The stripping of a galaxy's stars by thepotential of a cluster in which it resides must also occur. Thisprediction is verified by the detections of classical novae, red giants,and planetary nebulae between the galaxies of the Virgo and FornaxClusters. These tracers suggest a tramp stellar component of 10%-40% ofthe cluster baryonic mass. I point out that classical novae can usefullyextend these results to the 250,000 Mpc3 of intergalacticspace outside of galaxy clusters surrounding the Local Group. This isbecause individual novae are well-understood standard candles, withlight curves and spectra that are distinct from all other astrophysicalphenomena. In addition, the frequency of nova outbursts in any givengalaxy is measured to be directly proportional to that galaxy's K-bandluminosity (and independent of its Hubble type). Thus, intergalacticnovae should be excellent tracers of the fraction of stars liberatedfrom galaxies over the past 13 Gyr. Pan-STARRS, the Large SynopticSurvey Telescope (LSST), and other large-area synoptic survey telescopeswill begin to regularly discover tramp classical novae out to 20-40 Mpcin the coming decade. I estimate the expected discovery rates withLSST-like surveys to be hundreds of intergalactic tramp novae per year,and suggest survey parameters to optimize detections of these tramps.
| An unbiased deep search for small-area molecular structures
Context: .Small-area molecular structures (SAMS) resembling clumpusculesproposed as candidates for baryonic dark matter, have been detected inan area where the shielding is too low for them to survive for a longtime. Aims: .I present the results of an unbiased deep search formolecular clumpuscules to study the frequency of occurence of suchstructures km s-1. Methods: .The area surroundingthese structures has been surveyed using the FCRAO 14m telescope in theCO (1to0) transition. The field covered is 20' by 20'. The resulting rmsof the data is only 0.04 K in a 0.127 wide channel. Additionally,high-angular resolution observations of the 13CO andC18O (J=1to0) transitions were obtained with the IRAMPlateau-de-Bure Interferometer. Results: .3 new SAMS have beendetected. The structures have very low intensities which makes itimpossible to detect them in large scale CO surveys conducted to map themolecular gas of the Milky Way. Their radial velocity is similar to thatof the surrounding HI gas. The clouds follow the same size-linewidthrelation as found for giant molecular clouds or Galactic cirrus clouds.The observations clearly show that most of the large linewidths observedat low angular resolution are caused by a large velocity differencebetween the clumps seen at highest angular resolution. The non-detectionof the structures in the high-angular resolution observations of the13CO and C18O (J=1to0) transitions shows that the12CO (J=1to0) transition must have a low optical depth. At anadopted distance of 100 pc the structures have masses of only Jupitermass or below. Conclusions: .The new observations show that SAMSmight be an abundant phenomenon in the interstellar medium but notrecognized as such due to their small size. If they are made of ordinaryinterstellar matter with solar metallicity they likely contribute onlylittle to the total interstellar mass.
| Subarcsecond radio observations of the dwarf starburst galaxy NGC 3077
We present the first subarcsecond radio observations of the nearby dwarfstarburst galaxy NGC 3077 obtained with the MERLIN interferometer. Wehave detected two resolved sources which are coincident with thepositions of two discrete X-ray sources detected by Chandra. One of theradio sources is associated with a supernova remnant (SNR) and theobserved radio flux is consistent with having a non-thermal origin. Theage of the SNRs of about 760 yr is between the average age of the SNRsdetected in M82 and those detected in the Milky Way and the LargeMagellanic Cloud. We use this detection to calculate a star formationrate (SFR) of 0.28 Msolar yr-1, which is similarto the SFR calculated by using far-infrared and millimetre observationsbut larger than the SFR given by optical recombination lines correctedfor extinction. The other compact radio source detected by MERLIN, whichis coincident with the position of an X-ray binary, has the propertiesof an HII region with a flux density of about 747 μJy, whichcorresponds to an ionizing flux of 6.8 × 1050s-1. A young massive stellar cluster with a mass of ~2× 105 Msolar detected by the Hubble SpaceTelescope could be responsible for the production of the ionizing flux.
| The initial mass distribution of the M82 star cluster system
We explore whether we can constrain the shape of the initial massdistribution of the star cluster population in M82's ~1-Gyr-oldpost-starburst region `B', in which the present-day cluster massfunction (CMF) is closely approximated by a lognormal distribution. Weconclude that the M82 B initial CMF must have had a mean mass very closeto that of the `equilibrium' CMF of Vesperini. Consequently, if thepresently observed M82 B CMF has remained approximately constant sinceits formation, as predicted, then the initial CMF must have beencharacterized by a mean mass that was only slightly larger than thepresent mean mass. From our detailed analysis of the expected evolutionof CMFs, we conclude that our observations of the M82 B CMF areinconsistent with a scenario in which the 1-Gyr-old cluster populationoriginated from an initial power-law mass distribution. Our conclusionis supported by arguments related to the initial density in M82 B, whichwould have been unphysically high if the present cluster population werethe remains of an initial power-law distribution.
| Dark galaxies or tidal debris? Kinematical clues to the origin of massive isolated HI clouds
An extended HI cloud (VIRGOHI21) with an HI mass of~108Msolar and no apparent optical counterpart wasrecently discovered in the Virgo cluster. In order to understand theorigin of physical properties of apparently isolated HI clouds likeVIRGOHI21, we numerically investigate physical properties of tidal HIdebris that were formed by galaxy-galaxy interactions in clusters ofgalaxies. Our hydrodynamical simulations demonstrate that tidal debriswith total HI masses of 108-109Msolarcan have (1) a wide spread of HI velocities (>200kms-1),(2) a small mass fraction of stars (~10 per cent), and (3) a mean B-bandsurface brightness of the stellar components fainter than 30magarcsec-2. These results suggest that VIRGOHI21, which liesat a projected distance of ~150 kpc from the one-armed, HI-rich spiralgalaxy M99 (NGC 4254), is tidal debris. We propose that the comparisonbetween the simulated and the observed velocity fields of HI cloudsallows us to better understand their nature and origin (e.g. whetherthey are just tidal debris or are `dark galaxies' that have HI gas onlyand are embedded within dark matter haloes).
| From young massive star cluster to old globular: the LV-σ0 relationship as a diagnostic tool
We present a new analysis of the properties of the young massive starclusters (YMCs) forming profusely in intense starburst environments,which demonstrates that these objects are plausible progenitors of theold globular clusters (GCs) seen abundantly in the Local Group. Themethod is based on the tight relationship for old GCs between theirV-band luminosities, LV, and (central) velocity dispersions,σ0. We improve the significance of the relationship byincreasing the GC sample size and find that its functional form,LV/Lsolar~σ1.57+/-0.100(km s-1), is fully consistent with previous determinationsfor smaller Galactic and M31 GC samples. The tightness of therelationship for a GC sample drawn from environments as diverse as thosefound in the Local Group implies that its origin must be sought inintrinsic properties of the GC formation process itself. We evolve theluminosities of those YMCs in the local Universe which have velocitydispersion measurements to an age of 12 Gyr, adopting a variety ofinitial mass function (IMF) descriptions, and find that most YMCs willevolve to loci close to, or to slightly fainter luminosities than theimproved GC relationship. In the absence of significant externaldisturbances, this implies that these objects may potentially survive tobecome old GC-type objects over a Hubble time. The main advantage of ournew method is its simplicity. Whereas alternative methods, based ondynamical mass estimates, require one to obtain accurate size estimatesand to make further assumptions, the only observables required here arethe system's velocity dispersion and luminosity. The most importantfactor affecting the robustness of our conclusions is the adopted formof the IMF. We use the results of N-body simulations to confirm thatdynamical evolution of the clusters does not significantly alter ourconclusions about the likelihood of individual clusters surviving tolate times. Finally, we find that our youngest observed clusters areconsistent with having evolved from a relation of the form . Thisrelation may actually correspond to the origin of the GC fundamentalplane.
| A Chandra X-ray survey of nearby dwarf starburst galaxies - II. Starburst properties and outflows
We present a comprehensive comparison of the X-ray properties of asample of eight dwarf starburst galaxies observed with Chandra (IZw18,VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253 and He2-10). InPaperI, we presented in detail the data reduction and analysis of theindividual galaxies. For the unresolved X-ray sources, we find thefollowing: point sources are in general located close to bright HIIregions, rims of superbubbles or young stellar clusters. The number ofX-ray point sources appears to be a function of the current starformation (SF) rate and the blue luminosity of the hosts. UltraluminousX-ray sources (ULXs) are only found in those dwarf galaxies that arecurrently interacting. The power-law (PL) index of the combinedcumulative X-ray point-source luminosity function is α= 0.24 +/-0.06, shallower than that of more massive starburst galaxies (α=0.4 -0.8) and of non-starburst galaxies (α~ 1.2). For thosegalaxies showing extended X-ray emission (six out of the eightgalaxies), we derive the following: superwinds develop along thesteepest gradient of the HI distribution with volume densities of0.02-0.06cm-3, pressures of 1-3 ×105Kcm-3, thermal energies of 2-30 ×1054erg and hot gas masses of 2-20 ×106Msolar (~1 per cent of the HI masses). Onglobal scales, the distribution of the X-ray emission looks remarkablysimilar to that seen in Hα (comparing azimuthal averages);locally, however, their distribution is clearly distinct in many cases,which can be explained by the different emission mechanisms (forwardversus reverse shocks). Mass loading of order 1 to 5 is required toexplain the differences between the amount of hot gas and the modelledmass loss from massive stars. The metallicity of the dwarf galaxiescorrelates with the diffuse X-ray luminosity and anticorrelates with thecooling time of the hot gas. The diffuse X-ray luminosity is also afunction of the current star formation rate (SFR). The mechanicalluminosities of the developing superwinds are energetic enough toovercome the gravitational potentials of their host galaxies. Thisscenario is supported by the overpressures of the hot gas compared withthe ambient interstellar medium (ISM). Extended HI envelopes such astidal tails, however, may delay outflows on time-scales exceeding thoseof the cooling time of the hot gas.
| A Chandra X-ray survey of nearby dwarf starburst galaxies - I. Data reduction and results
We present an analysis of Chandra X-ray observations of a sample ofeight dwarf starburst galaxies (IZw18, VIIZw403, NGC1569, NGC3077,NGC4214, NGC4449, NGC5253 and He2-10). Extended, diffuse X-ray emissionis detected in all but two of the objects. Unresolved sources were foundwithin all dwarf galaxies (total: 55 sources). These point sources arewell fitted by power-law (PL), thermal plasma (TP) or blackbody (BB)models. 10 of the point sources exceed an X-ray luminosity of1039 erg s-1 (ultraluminous X-ray sources, ULXs).In those galaxies where diffuse X-ray emission is detected, thisemission (with X-ray luminosities ranging from 4 × 1038to 2 × 1040 erg s-1) contains most (60-80per cent) of the X-ray photons. This diffuse emission can be well fittedby MEKAL one-temperature TP models once the contribution from theunresolved point sources is subtracted properly. The diffuse X-raycomponent is significantly extended, reaching as far as 0.5-5kpc intothe outskirts of their hosts. Azimuthally averaged X-ray surfacebrightness profiles are well approximated by exponential functions.Temperatures of various regions within the galaxies range from 1.6 to7.6 × 106K. With few exceptions, temperatures of thehot gas are remarkably uniform, hovering around 2-3 ×106K. Temperatures of the coronal gas in the outer regionsare in general ~2-3 times lower than those found in the central regions.Fits to the diffuse emission do not allow strong constraints to be puton the metallicities of the emitting plasmas. However, the derivedmetallicities are compatible with those determined from their HIIregions. An α/Fe ratio of ~2 is indicated for the hot gas withinat least three objects (NGC1569, NGC4449 and He2-10). Shadowing of thediffuse X-ray emission by the cooler disc gas is used to constrain theorientation of the galaxies.
| XMM-Newton observations of the interacting galaxy pairs NGC 7771/0 and NGC 2342/1
We present XMM-Newton X-ray observations of the interacting galaxy pairsNGC 7771/7770 and NGC 2342/2341. In NGC 7771, for the first time we areable to resolve the X-ray emission into a bright central source plus twobright (LX > 1040 erg s-1)ultraluminous X-ray sources (ULXs) located either end of the bar. In thebright central source (LX~ 1041 ergs-1), the soft emission is well-modelled by a two-temperaturethermal plasma with kT= 0.4/0.7 keV. The hard emission is modelled witha flat absorbed power-law (Γ~ 1.7, NH~ 1022cm-2), and this together with a low-significance (1.7σ)~ 300 eV equivalent width emission line at ~6 keV are the firstindications that NGC 7771 may host a low-luminosity AGN. For the barULXs, a power-law fit to X-1 is improved at the 2.5σ level withthe addition of a thermal plasma component (kT~ 0.3 keV), while X-2 isimproved only at the 1.3σ level with the addition of a discblackbody component with Tin~ 0.2 keV. Both sources arevariable on short time-scales implying that their emission is dominatedby single accreting X-ray binaries (XRBs). The three remaining galaxies,NGC 7770, NGC 2342 and NGC 2341, have observed X-ray luminosities of0.2, 1.8 and 0.9 × 1041 erg s-1,respectively (0.3-10 keV). Their integrated spectra are alsowell-modelled by multi-temperature thermal plasma components with kT=0.2-0.7 keV, plus power-law continua with slopes of Γ= 1.8-2.3that are likely to represent the integrated emission of populations ofXRBs as observed in other nearby merger systems. A comparison with otherisolated, interacting and merging systems shows that all four galaxiesfollow the established correlations for starburst galaxies betweenX-ray, far-infrared and radio luminosities, demonstrating that theirX-ray outputs are dominated by their starburst components.
| X-Ray Nature of the LINER Nuclear Sources
We have investigated the nature of the energy source of 36 LINERs withChandra X-ray observations selected from the catalogue by Carrillo etal. (1999). In most galaxies a nuclear compact source has been detectedin the hard band (2-8 KeV). However they show a rather irregularmorphology embedded in diffuse X-ray emission for lower energies (0.3-2KeV). In this work we report the spectral analysis of the nuclearsource. Color-color diagrams allow us to determine the dominantmechanism in them. Synthetic colors have been computed for a power-law ,thermal emission and a combination of both. The results suggest a nonthermal nature in most of the LINER galaxies observed.
| 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 Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| Halos of Spiral Galaxies. III. Metallicity Distributions
We report results of a campaign to image the stellar populations in thehalos of highly inclined spiral galaxies, with the fields roughly 10 kpc(projected) from the nuclei. We use the F814W (I) and F606W (V) filtersin the Wide Field Planetary Camera 2 on board the Hubble SpaceTelescope. We unambiguously resolve the stellar halos 1 to 2 mag fainterthan the tip of the red giant branch. Extended halo populations aredetected in all galaxies. The color-magnitude diagrams appear to becompletely dominated by giant branch stars, with no evidence for thepresence of young stellar populations in any of the fields. Themetallicity distribution function for the galaxy sample is derived frominterpolation within an extensive grid of red giant branch loci. Theseloci are derived from theoretical sequences that are calibrated usingthe Galactic globular clusters and from empirical sequences formetal-rich stellar populations. We find that the metallicitydistribution functions are dominated by metal-rich populations, with atail extending toward the metal-poor end. To first order, the overallshapes of the metallicity distribution functions are similar to what ispredicted by a simple, single-component model of chemical evolution withthe effective yields increasing with galaxy luminosity. However,metallicity distributions significantly narrower than the simple modelare observed for a few of the most luminous galaxies in the sample. Thediscrepancies are similar to those previously observed for NGC 5128, thehalo of M31, and the Galactic bulge. Our observations can be used tohelp distinguish between models for the formation of spiral galaxies. Itappears that more luminous spiral galaxies also have more metal-richstellar halos. The increasingly significant departures from theclosed-box model for the more luminous galaxies indicate that aparameter in addition to a single yield is required to describe chemicalevolution. This parameter, which could be related to gas infall oroutflow either in situ or in progenitor dwarf galaxies that later mergeto form the stellar halo, tends to make the metallicity distributionsnarrower at high metallicity.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Intergalactic Stellar Distributions in the Interacting M81/M82 Galaxy Group
Previous H I observations of the M81/M82/NGC 3077 galaxy group clearlyshow a widespread H I distribution within this galaxy group. While thegas is vulnerable to tidal disruption from a galaxy encounter, are therealso stars embedded in this H I distribution? Our deep, 1deg2 exposures of the M81/M82 group in 10 optical bands usingthe Beijing-Arizona-Taipei-Connecticut (BATC) filter set clearly revealwidespread stellar distributions that coincide with the atomic hydrogenclouds-considered to be the relics of the merging process of thegalaxies-splayed over the region. The spectral energy distributions ofthe stellar groups to the east and west of M81 (including the ``ArpLoop'') are similar to that measured at the southeast edge of theoptical disk of M82. This similarity in stellar radiation, combined withthe observed peculiar rotational velocity of M82, suggests that thediffuse stellar population in the intergalactic space around M81 ispossibly a relic of the tidally disrupted disk of M82 during the lastclose encounter. Alternately, the stars could have formed in situ in theH I as it was drawn out of the galaxies. Recent measurements ofdistances to and radial velocities of M81 (3.63 Mpc and 48 kms-1, respectively) and M82 (3.9 Mpc and 296 kms-1) lend further support to the notion of a close passagebetween these two galaxies several hundred million years ago.
| The Discovery of Spiral Arms in the Starburst Galaxy M82
We report the discovery of two symmetric spiral arms in thenear-infrared (NIR) images of the starburst galaxy M82. The spiral armsare recovered when an axisymmetric exponential disk is subtracted fromthe NIR images. The arms emerge from the ends of the NIR bar and can betraced up to 3 disk scale lengths. The winding of the arms is consistentwith an m=2 logarithmic spiral mode of pitch angle 14°. The arms arebluer than the disk in spite of their detection on the NIR images. Ifthe northern side of the galaxy is nearer to us, as is normally assumed,the observed sense of rotation implies trailing arms. The nearly edge-onorientation, high disk surface brightness, and presence of a complexnetwork of dusty filaments in the optical images are responsible for thelack of detection of the arms in previous studies.
| Metallicity Effects on Mid-Infrared Colors and the 8 μm PAH Emission in Galaxies
We examine colors from 3.6 to 24 μm as a function of metallicity(O/H) for a sample of 34 galaxies. The galaxies range over 2 orders ofmagnitude in metallicity. They display an abrupt shift in the 8μm-to-24 μm color for metallicities between one-third andone-fifth of the solar value. The mean 8-to-24 μm flux density ratiobelow and above 12+log(O/H)=8.2 is 0.08+/-0.04 and 0.70+/-0.53,respectively. We use mid-IR colors and spectroscopy to demonstrate thatthe shift is primarily due to a decrease in the 8 μm flux density, asopposed to an increase in the 24 μm flux density. This result is mostsimply interpreted as being due to a weakening at low metallicity of themid-IR emission bands usually attributed to PAHs (polycyclic aromatichydrocarbons) relative to the small-grain dust emission. However,existing empirical spectral energy distribution models cannot accountfor the observed short-wavelength (below 8 μm) colors of thelow-metallicity galaxies merely by reducing the strength of the PAHfeatures; some other emission source (e.g., hot dust) is required.
| A Chandra X-Ray Investigation of the Violent Interstellar Medium: From Dwarf Starbursts to Ultraluminous Infrared Galaxies
We have analyzed observations with the Chandra X-Ray Observatory of thediffuse emission by hot gas in seven dwarf starburst galaxies, sixedge-on starburst galaxies, and nine ultraluminous infrared galaxies.These systems cover ranges of ~104 in X-ray luminosity, andseveral thousand in star formation rate and K-band luminosity (a proxyfor stellar mass). Despite this range in fundamental parameters, we findthat the properties of the diffuse X-ray emission are very similar inall three classes of starburst galaxies. The spectrum of the diffuseemission is well fitted by thermal emission from gas with kT~0.25-0.8keV and with several times solar abundance ratios of α-elements toFe. The ratio of the thermal X-ray to far-infrared luminosity is roughlyconstant, as is the characteristic surface brightness of the diffuseX-ray emission. The size of the diffuse X-ray source increasessystematically with both far-infrared and K-band luminosity. All threeclasses show strong morphological relationships between the regions ofhot gas probed by the diffuse X-ray emission and the warm gas probed byoptical line emission. These findings suggest that the same physicalmechanism is producing the diffuse X-ray emission in the three types ofstarbursts. These results are consistent with that mechanism beingshocks driven by a galactic ``superwind,'' which is powered by thekinetic energy collectively supplied by stellar winds and supernovae inthe starburst.
| VLA Imaging of the Intriguing H I Cloud HIJASS J1021+6842 in the M81 Group
We present VLA H I 21 cm observations of HIJASS J1021+6842, which hasbeen discovered in the direction of the M81 Group. Our synthesis imagingreveals that the H I is distributed over a larger angular extent andvelocity range than the single-dish discovery observations. Assumingthat HIJASS J1021+6842 is at the distance of the M81 Group, we detect1.5×108 Msolar of H I distributed over asmuch as 30 kpc, i.e., substantially larger than the biggest dwarfgalaxies in the same group. At the depth of our imaging, the H I appearsto be confined to at least seven clouds. Peak H I column densities are~1.8×1020 atoms cm-2, which is well belowthe canonical star formation threshold of ~1021 atomscm-2 and therefore consistent with the fact that no opticalcounterpart has as yet been identified. A gradient in velocity isobserved across the extent of the detected H I; assuming that the objectis gravitationally bound we derive a dynamical mass of7×109 Msolar and a dark-to-luminous massratio of >10. Alternatively, a tidal origin may also result in theobserved velocity gradient, which would lead to a considerably lowerdynamical mass. Given the above properties and the absence of evidenceof a stellar population, HIJASS J1021+6842 is unique amongst the othersystems in the M81 Group.
| The Disk and Extraplanar Regions of NGC 55
The stellar content of the nearby SB(s)m galaxy NGC 55 is investigatedusing images obtained with the Gemini South and Canada-France-Hawaiitelescopes. The (K, H-K) and (K, J-K) color-magnitude diagrams (CMDs) ofstars near the plane of the disk reveal signatures of large-scale starformation during recent and intermediate epochs in the form of redsupergiants (RSGs) with MK=-11.5, and an asymptotic giantbranch (AGB) that peaks near MK=-10. Comparisons with stellarevolution models suggest that the brightest RSGs have an age near 8 Myr.A well-defined plume, which stellar evolution models suggest containsstars with masses near the RSG-AGB transition, is detected in CMDsconstructed both from infrared and visible wavelength observations. Itis concluded that star formation in the thin disk of NGC 55 has occurredat a significant rate for at least the past 0.1-0.2 Gyr, and this isconsistent with other indicators. The near-infrared spectral energydistribution of the integrated light near the center of the galaxy isconsistent with that in other Magellanic irregular galaxies, indicatingthat the star-forming history of NGC 55, when averaged over timescalesof 0.1-1 Gyr, has likely not been peculiar when compared with otherlate-type systems. Evidence is also presented that the disk contains alarge population of old [log(tyr)~10] stars, and it is arguedthat a stable disk has been in place in NGC 55 for a significantfraction of the age of the universe. At projected distances in excess of2 kpc off of the disk plane, the brightest AGB stars have ages10+3-2 Gyr. Thus, despite indications that dustand gas are present in the envelope surrounding the NGC 55 disk, the AGBcontent suggests that recently formed stars do not occur in largenumbers in the extraplanar region. The (r'-i')colors of the RGB in the extraplanar region are consistent with [Fe/H]between -2.2 and -0.7, with the majority of stars having [Fe/H]>-1.2,and the mean metallicity inferred from the RGB color does not changewith distance above the disk plane. Thus, the stellar component in theextraplanar envelope is well mixed, at least in terms of metallicity.The mean metallicity of RGB stars is in excellent agreement with thatmeasured in the extraplanar H II regions EHR 1 and 2, suggesting thatthe age-metallicity relation in this part of NGC 55 has been flat for atleast a few Gyr. Finally, the RGB tip occurs near i'=23.1 inthe extraplanar region, and a distance modulus of 26.5 is computed fromthis feature.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council of Canada (Canada), CONICYT (Chile), theAustralian Research Council (Australia), CNPq (Brazil), and CONICET(Argentina).This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| Subaru Imaging and Spectroscopy of Globular Cluster Candidates around M82
We carried out a search for globular clusters in M82 by taking advantageof the relatively high resolution imaging and deep spectroscopiccapabilities provided by the Faint Object Camera And Spectograph (FOCAS)on the Subaru Telescope. Since M82 is a late-type galaxy, its globularcluster system provides a basis for comparison with the globular clustersystem of our Galaxy or that of M31. The archetypal starburst galaxy M82also constitutes an ideal target for studying the relation betweengalaxy interactions, starbursts, and globular cluster formation. First,we performed an imaging survey to pick up globular cluster candidatesusing the color-magnitude selection method. Follow-up spectroscopicobservations were carried out for 29 globular cluster candidates usingthe multiobject spectroscopic mode of FOCAS. In the central 6' diameterregion of M82, we found two bona fide globular clusters and three youngstar clusters in our first attempt at optical searching for globularclusters in M82. The estimated age of young star clusters is <~1 Gyr,which is consistent with or lower than the age derived by infraredphotometry. Two bona fide globular clusters should have formed at theepoch of M82's formation. Some of the young star clusters identified inM82 are likely to have been produced during the tidal-interactionepisode with M81.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback from Momentum-driven Winds
We investigate large-scale galactic winds driven by momentum deposition.Momentum injection is provided by (1) radiation pressure produced by thecontinuum absorption and scattering of photons on dust grains and (2)supernovae (momentum injection by supernovae is important even if thesupernova energy is radiated away). Radiation can be produced by astarburst or active galactic nucleus (AGN) activity. We argue thatmomentum-driven winds are an efficient mechanism for feedback during theformation of galaxies. We show that above a limiting luminosity,momentum deposition from star formation can expel a significant fractionof the gas in a galaxy. The limiting, Eddington-like luminosity isLM~=(4fgc/G)σ4, where σ isthe galaxy velocity dispersion and fg is the gas fraction;the subscript M refers to momentum driving. A starburst that attainsLM moderates its star formation rate and its luminosity doesnot increase significantly further. We argue that elliptical galaxiesattain this limit during their growth at z>~1 and that this is theorigin of the Faber-Jackson relation. We show that Lyman break galaxiesand ultraluminous infrared galaxies have luminosities nearLM. Since these starbursting galaxies account for asignificant fraction of the star formation at z>~1, this supports ourhypothesis that much of the observed stellar mass in early-type galaxieswas formed during Eddington-limited star formation. Star formation isunlikely to efficiently remove gas from very small scales in galacticnuclei, i.e., scales much smaller than that of a nuclear starburst. Thisgas is available to fuel a central black hole (BH). We argue that a BHclears gas out of its galactic nucleus when the luminosity of the BHitself reaches ~LM. This shuts off the fuel supply to the BHand may also terminate star formation in the surrounding galaxy. As aresult, the BH mass is fixed to beMBH~=(fgκes/πG2)σ4,where κes is the electron scattering opacity. Thislimit is in accord with the observed MBH-σ relation.
| The Local Group and Other Neighboring Galaxy Groups
Over the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ``ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.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,Inc., under NASA contract NAS 5-26555.
| Molecular gas in compact galaxies
New observations of eleven compact galaxies in the 12CO J =2{-}1 and J = 3{-}2 transitions are presented. From these observationsand literature data accurate line ratios in matched beams have beenconstructed, allowing the modelling of physical parameters. Matching asingle gas component to observed line ratios tends to produce physicallyunrealistic results, and is often not possible at all. Much betterresults are obtained by modelling two distinct gas components. In mostobserved galaxies, the molecular gas is warm (Tk = 50{-}150K) and at least partially dense (n(H2) ≥ 3000cm-3). Most of the gas-phase carbon in these galaxies is inatomic form; only a small fraction ( 5%) is in carbon monoxide.Beam-averaged CO column densities are low (of the order of1016 cm-2). However, molecular hydrogen columndensities are high (of the order of 1022 cm-2)confirming large CO-to- H2 conversion factors (typically X =1021{-}1022 cm-2/ {K kms-1}) found for low-metallicity environments by othermethods. From CO spectroscopy, three different types of molecularenvironment may be distinguished in compact galaxies. Type I (highrotational and isotopic ratios) corresponds to hot and dense molecularclouds dominated by star-forming regions. Type II has lower ratios,similar to the mean found for infrared-luminous galaxies in general, andcorresponds to environments engaged in, but not dominated by,star-forming activity. Type III, characterized by low 12CO(2-1)/(1-0) ratios, corresponds to mostly inactive environments ofrelatively low density.
| Thick disks and halos of spiral galaxies M 81, NGC 55 and NGC 300
By using images from the HST/WFPC2/ACS archive, we have analyzed thespatial distribution of the AGB and RGB stars along the galactocentricradius of nearby spiral galaxies M 81, NGC 300 and NGC 55. Examiningcolor-magnitude diagrams and stellar luminosity functions, we gauge thestellar contents of the surroundings of the three galaxies. The redgiant population (RGB) identified at large galactocentric radii yields adistance of 3.85±0.08 Mpc for M 81, 2.12±0.10 Mpc for NGC55, and 2.00±0.13 Mpc for NGC 300, and a mean stellar metallicityof -0.65, -1.25, and -0.87 respectively. We find that there are twonumber density gradients of RGB stars along the radius, which correspondto the thick disk and halo components of the galaxies. We confirm thepresence of a metallicity gradient of evolved stars in these galaxies,based on the systematic changes of the color distribution of red giantstars. These results imply that the thick disk might be a generalfeature of spiral galaxies, and endorse a further investigation of theouter stellar edges of nearby spirals, which is critical in constrainingthe origin and evolution of galaxies.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Properties and environment of the molecular complex near Holmberg IX
This paper is aimed at providing new insight into the nature and originof the molecular complex situated near the line of sight toward HolmbergIX in the M 81 group of galaxies. The first high resolution CO maps ofthe complex as well as single dish 13CO(1-0),12CO(3-2) and millimeter continuum observations and theresults of a survey of 12CO in the region are presented.These data together with the available HI, optical and X-rayobservations are analyzed to study the properties and environment of thecomplex. We confirm there is no unobscured massive star formation insidethe complex, and from the millimeter constraint on the extinction itmust have a low star formation rate or be forming only low mass stars.According to the CO line ratios the abundances and physical conditionscould be similar to that of cold gas in spirals. We find from itsdynamics (no rotation) and its mass (2-6 million solar masses) that itresembles a massive GMC. Also, re-inspecting N-body simulations of the M81 group and the H I data we find that it might be located inside theextreme outer disk of M 81 and be cospatial with the H I feature knownas Concentration I. The negative result of the CO survey suggests thatthe complex is unique in this region and calls for a peculiar localformation process. We find that the distribution of the CO emission inthe data cube is asymmetrical in a way similar to a cometary object. Theoptical observations of the nearby supershell MH9/10 suggest theexistence of an outflow toward the complex. We consider the possibilitythat the molecular complex is related to this hypothetical outflow.Based on observations carried out with the IRAM Plateau de BureInterferometer and 30 m telescope, the 10 m Heinrich-Hertz-Telescope(HHT), and the NRAO 12 m telecsope. IRAM (Institut de Radio-AstronomieMillimétrique) is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain). The HHT was operated by the Submillimeter TelescopeObservatory on behalf of Steward Observatory and the Max-Planck-Institutfür Radioastronomie. The NRAO (National Radio AstronomyObservatory) is a facility of the National Science Foundation, operatedunder cooperative agreement by Associated Universities, Inc.Guest User, Canadian Astronomy Data Centre, that is operated by theDominion Astrophysical Observatory for the National Research Council ofCanada's Herzberg Institute of Astrophysics.
| Deep Near-Infrared Imaging of a Field in the Outer Disk of M82 with the Altair Adaptive Optics System on Gemini-North
Deep H and K' images recorded with the Altair adaptive opticssystem and Near-Infrared Imager on Gemini-North are used to probe thered stellar content in a field with a projected distance of 1 kpc abovethe disk plane of the starburst galaxy M82. The data have an angularresolution of 0.08" FWHM, and individual asymptotic giant branch (AGB)and red giant branch (RGB) stars are resolved. The AGB extends to atleast 1.7 mag in K above the RGB tip, which occurs at K=21.7. Therelative numbers of bright AGB stars and RGB stars are consistent withstellar evolution models, and one of the brightest AGB stars has an H-Kcolor and K brightness that is consistent with it being a C star. Thebrightnesses of the AGB stars suggest that they formed duringintermediate epochs, possibly after the last major interaction with M81.Therefore, star formation in M82 during intermediate epochs may not havebeen restricted to the plane of the disk.
| 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.
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