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Low-Luminosity Extragalactic Water Masers toward M82, M51, and NGC 4051
Subarcsecond observations using the Very Large Array (VLA) are presentedfor low-luminosity H2O maser emission in M82, M51, and NGC4051. New maser features have been detected within the M82 starburstcomplex. They are largely associated with star-forming activity such asoptically identified starburst-driven winds, H II regions, or the earlyphase of star formation in the galaxy. The H2O maser in M51consists of blueshifted and redshifted features relative to the systemicvelocity of the galaxy. The redshifted features are measured to thenorthwest of the nuclear radio source, while the location of theblueshifted counterpart is displaced by ~2" from the radio source. Asmall velocity gradient closely aligned with the radio jet is detectedfrom the redshifted features. The redshifted maser most likely amplifiesthe background radio continuum jet, while the blueshifted counterpartmarks off-nuclear star formation in the galaxy. All of the detectedmaser features in the narrow-line Seyfert 1 galaxy NGC 4051 remainunresolved by new VLA observations. Due to the low luminosity of themaser, the maser excitation is not directly related to the activegalactic nucleus.
| The proper motion of the Local Group galaxy IC 10
We have measured the proper motion of the Local Group galaxy IC 10 withthe Very Long Baseline Array by measuring the position of an H2O maserrelative to two background quasars over 4.3 years. After correction forthe rotation of the Milky Way and IC 10, we obtain a motion of-39±9 μas yr-1 toward the East and 31±8μas yr-1 toward the North. This corresponds to a totalspace velocity of 215 ± 42 km s-1 relative to theMilky Way for an assumed distance of 660±66 kpc. Assuming that IC10 and M 33, for which also a proper motion measurement exists, arebound to M 31, we calculate a lower limit for the mass of M 31 of 7.5× 1011 M_ȯ.
| Water masers in the Local Group of galaxies
We compare the number of detected 22 GHz H2O masers in the Local Groupgalaxies M 31, M 33, NGC 6822, IC 10, IC 1613, DDO 187, GR8, NGC 185,and the Magellanic Clouds with the water maser population of the MilkyWay. To accomplish this we searched for water maser emission in the twoLocal Group galaxies M 33 and NGC 6822 using the Very Large Array (VLA)and incorporated results from previous studies. We observed 62 Hiiregions in M 33 and 36 regions with Hα emission in NGC 6822.Detection limits are 0.0015 and 0.0008 L_ȯ for M 33 and NGC 6822,respectively (corresponding to 47 and 50 mJy in three channels with 0.7km s-1 width). M 33 hosts three water masers above ourdetection limit, while in NGC 6822 no maser source was detected. We findthat the water maser detection rates in the Local Group galaxies M 31, M33, NGC 6822, IC 1613, DDO 187, GR8, NGC 185, and the Magellanic Cloudsare consistent with expectations from the Galactic water masers if oneconsiders the different star formation rates of the galaxies. However,the galaxy IC 10 exhibits an overabundance of masers, which may resultfrom a compact central starburst.
| Mega-Masers and Galaxies
In the Galaxy, microwave radiation can be amplified in the interstellarmedium in the immediate neighborhood of young stellar objects, orcircumstellar envelopes around evolved stars, resulting in cosmic maseremission. Cosmic masers exist because, in contrast to terrestrialconditions, the interstellar gas density is very low so that levelpopulation in molecules is typically not in thermal equilibrium, andsometimes inverted. In the nuclear regions of external galaxies, thereexist very powerful OH ( 18 cm) and H2O ( 1.35 cm) cosmicmasers with line luminosities of 102 104Lȯ, 106 times more luminous than typicalGalactic maser sources. These are the "mega-masers," found inhigh-density molecular gas located within parsecs of active galacticnuclei in the case of H2O mega-masers, or within the central100 pc of nuclear star-burst regions in the case of OH mega-masers.H2O mega-masers are most frequently found in galactic nucleiwith Seyfert2 or LINER spectral characteristics, in spiral and someelliptical galaxies. OH mega-masers are found in ultra-luminous IRgalaxies (ULIRG) with the warmest IR colors, and importantly, the OHluminosity is observed to increase with the IR luminosity:LOH L1.2IR. Because of the extremelyhigh-surface brightness, H2O mega-maser emission can bemapped at sub-milli-arc-second resolution by Very Long BaselineInterferometry (VLBI), providing a powerful tool to probe spatial andkinematic distributions of molecular gas in distant galactic nuclei atscales below one parsec. An excellent example is the active galaxy, NGC4258, in which mapping of the H2O mega-maser emission hasprovided the first direct evidence in an active galactic nucleus for theexistence of a thin Keplerian accretion disk with turbulence, as well ashighly compelling evidence for the existence of a massive black hole.The NGC 4258 mega-maser has also provided a geometric distancedetermination of extremely high precision. H2O mega-maseremission is also found to arise from postshocked gas from the impact ofnuclear jets or outflows on the surrounding molecular clouds.High-resolution observations have shown that OH mega-masers originatefrom the molecular gas medium in 100-pc scale nuclear star-burstregions. It is proposed that such extreme star-burst regions, withextensive high-density gas bathed in a very high far-IR radiation field,are conducive to the formation of a very large number of OH masersources that collectively produce the OH mega-maser emission. In theearly Universe, galaxies or mergers could go through a very luminousphase, powered by intensive star-bursts and AGN formation, and couldhave extremely large OH and H2O maser luminosities, possiblyproducing giga-masers. With the increasing sensitivity of new telescopesand receivers, surveys and high-resolution studies of mega-masers andgiga-masers will be very important tracers and high-resolution probes ofactive galactic nuclei, dust embedded star-bursts in the earliestgalaxies and galaxy mergers in the epoch of very active star formationat z 2 and beyond. Distance determination of giga-masers at z 1 2can provide on independent measure of how fast the universe isexpanding.
| 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.
| Microarcsecond astrometry using the SKA
The sensitivity and versatility of SKA will provide microarcsecastrometric precision and high quality milliarcsec-resolution images bysimultaneously detecting calibrator sources near the target source. Toreach these goals, we suggest that the long-baseline component of SKAcontains at least 25% of the total collecting area in a region between1000 and 5000 km from the core SKA. We also suggest a minimum of 60elements in the long-baseline component of SKA to provide the necessary(u v) coverage. For simultaneous all-sky observations, which provideabsolute astrometric and geodetic parameters, we suggest using 10independent subarrays each composed of at least six long-baselineelements correlated with the core SKA. We discuss many anticipated SKAlong-baseline astrometric experiments: determination of distance, propermotion and orbital motion of thousands of stellar objects; planetarymotion detections; mass determination of degenerate stars using theirkinetics; calibration of the universal distance scale from 10 to107 pc; the core and inner-jet interactions of AGN. With anincrease by a factor of 10 in absolute astrometric accuracy usingsimultaneous all sky observations, the fundamental quasar referenceframe can be defined to <10 μas and tied to the solar-systemdynamic frame to this accuracy. Parameters associated with the earthrotation and orientation, nutation, and geophysical parameters, can beaccurately monitored. Tests of fundamental physics include: solar andJovian deflection experiments, the sky frame accuracy needed tointerpret the gravity wave/pulsar-timing experiment, accurate monitoringof spacecraft orbits that impact solar system dynamics.
| The IC 133 Water Vapor Maser in the Galaxy M33: A Geometric Distance
We report on the results of a 14 yr long VLBI study of proper motions inthe IC 133 H2O maser source in the galaxy M33. The method ofordered motion parallax was used to model the three-dimensionalstructure and dynamics of IC 133 and to obtain a distance estimate,800+/-180 kpc. Our technique for determining the distance to M33 isindependent of calibrations common to other distance indicators, such asCepheid period-luminosity relations, and therefore provides an importantcheck for previous distance determinations.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Superassociations and Stellar Complexes in Galaxies
The basic characteristics of stellar complexes and superassociations, aswell as the differences between these kinds of gigantic groups of youngstars, are discussed. The main difference is that superassociations arethe result of induced (triggered) star formation, while the stars andclusters in stellar complexes develop as a result of the spontaneousprocesses typical of galactic gaseous disks.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| The spiral galaxy M 33 mapped in the FIR by ISOPHOT. A spatially resolved study of the warm and cold dust
The Sc galaxy M 33 has been mapped with ISOPHOT in the far-infrared, at60, 100, and 170 mu m. The spatial resolution of these FIR maps allowsthe separation of spiral arms and interarm regions and the isolation ofa large number of star-forming regions. The spectral energy distributionin the FIR indicates a superposition of two components, a warm oneoriginating from dust at ~ 45 K, and a cold one, at ~ 16 K. The warmcomponent is concentrated towards the spiral arms and the star-formingregions, and is likely heated by the UV radiation from OB stars. Thecold component is more smoothly distributed over the disk, and heated bythe diffuse interstellar radiation. For the about 60 star-formingregions detected the Hα /FIR flux ratio increases significantlywith the distance from the galaxy center, probably due to decreasingextinction. An anti-correlation of FHa/ F60 withF170 suggests the intrinsic extinction to be related to thecold dust surface brightness according to AV/ S170~ 0.03 mag MJy-1 sr. For the total galaxy the star formationrate (SFR) derived from the FIR is in agreement with that derived fromthe de-extincted Hα emission. For individual star-formingregions, a consistency between SFRs derived from the optical and fromthe FIR requires only a fraction of the UV radiation to be absorbedlocally. The individual star-forming regions also show a local radio-FIRcorrelation. This local correlation is, however, due to quite differentcomponents than to those that lead to the well-known global radio-FIRcorrelation for entire galaxies.Based on observations with the Infrared Space Observatory ISO. ISO is anESA project with instruments funded by ESA Member States (especially thePI countries France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
| Infrared Space Observatory Long-Wavelength Spectrometer Spectroscopy of Star-forming Regions in M33
We present Infrared Space Observatory (ISO) Long-Wavelength Spectrometer(LWS) far-infrared (FIR) spectra of the nucleus and six giant H IIregions in M33 (NGC 595, IC 142, NGC 592, NGC 604, NGC 588, and IC 133).The seven fine-structure lines observed in the FIR are used to model theH II and photodissociation regions (PDRs). There is no observed trend inthe FIR properties, observed with the LWS, as a function of galacticradius or metallicity. The cold neutral medium (CNM) is the mainreservoir for the atomic gas, containing between 60% and 95% of the gas.The FIRLWS spectral energy distribution can be fitted with asingle-temperature graybody spectrum with a temperature in the range35K<=T<=49 K. The [C II] 158 μm line flux is 0.2%-0.7%FIRLWS, which is typical of values seen (0.1%-1% FIR) in thenuclei of star-forming galaxies. The [C II]/FIRLWS ratiopeaks at the nucleus and is fairly constant across the rest of thesample. Massive star formation is traced by the intensity of the [O III]88 μm line. The emission from the observed FIR lines that arisesolely from H II regions can be modeled as a single component with agiven oxygen and nitrogen abundance, effective temperature, density, andionizing flux. There is no need for an extended low-density component(ELDWIM). Apart from NGC 604 and NGC 595, the fractional [C II] emissionthat arises from the H II regions and/or PDRs is not well constrained,but typically 5%-50% arises in the H II regions, 10%-35% from the CNM,and the bulk of the emission (40%-90%) in the PDRs. The average PDR inthis sample has a gas density ~103.1 cm-3, anaverage incident far-ultraviolet flux (in units of the localinterstellar value) 0>=102.4, a gas temperatureT~200 K, and an AV~10 through the clouds. NGC 604 has 40% ofthe atomic gas residing in the PDRs, while the rest have a much smallerfraction, ~5%-15%. The PDRs are similar to those found in otherstar-forming galaxies such as Centaurus A. 0> is at thelower end of the range observed in samples of spiral and starburstgalaxies (2.2<=logG0<=5), and sits comfortablyin the middle of the observed range (1.8<=logn<=4.2).
| Multicolor Photometry of 145 of the H II Regions in M33
This paper is the first in a series presenting CCD multicolor photometryfor 145 H II regions, selected from 369 candidate regions fromBoulesteix et al., in the nearby spiral galaxy M33. The observations,which covered the whole area of M33, were carried out with the BeijingAstronomical Observatory 60/90 cm Schmidt telescope, in 13intermediate-band filters, covering a range of wavelengths from 3800 to10000 Å. This procedure provides a series of maps that can beconverted into a multicolor map of M33, in pixels of 1.7"×1.7".Using aperture photometry we obtain the spectral energy distributions(SEDs) for these H II regions. We also give their identification charts.Using the relationship between the Beijing-Arizona-Taiwan-Connecticutintermediate-band system used for the observations and the UBVRIbroadband system, the magnitudes in the B and V bands are then derived.Histograms of the magnitudes in V and in B-V are plotted, and thecolor-magnitude diagram is also given. The distribution of magnitudes inthe V band shows that the apparent magnitude of almost all the regionsis brighter than 18, corresponding to an absolute magnitude of -6.62 foran assumed distance modulus of 24.62, which corresponds to a singlemain-sequence O5 star, while the distribution of color shows that thesample is blue, with a mode close to -0.05, as would be expected from arange of typical young clusters.
| Observations of Galaxies with the Midcourse Space Experiment
We have imaged eight nearby spiral galaxies with the SPIRIT III infraredtelescope on the Midcourse Space Experiment (MSX) satellite in themid-infrared at 18" resolution at 8.3, 12.1, 14.7, and 21.3 μm. Eachof the eight shows interesting structure not previously detected witholder, lower resolution infrared data sets, such as a resolved nucleusor spiral structure. The MSX data are compared with existing data setsat ultraviolet, optical, and infrared wavelengths, including recentobservations from the Infrared Space Observatory. The infraredstructures in M83 and NGC 5055 show a striking similarity to theultraviolet emission but are less similar to the optical emission.Several point sources with no identified counterparts at otherwavelengths are found near M31, NGC 4945, M83, and M101. Over 200previously known objects are also detected at 8 μm.
| Observational study on extragalactic H2O megamaser sources and associated molecular emission lines
Since the first extragalactic H2O megamaser source toward theactive galactic nuclei NGC 4945 was discovered, 19 sources have beenfound. The observational study of extragalactic H2O megamasersources and molecular emission lines associated with AGNs is a veryefficient tool to investigate characteristics of central sources in AGNsand circumnuclear tori of gas and dust. Current researches of searching,observation and theory on extragalactic H2O megamaser sourcesand associated molecular emission lines are mainly reviewed in thispaper.
| The association between water kilomasers and compact radio sources in the starburst galaxy NGC 2146
We report the detection of 22 GHz water vapor emission toward thestarburst galaxy NGC 2146, made using the Effelsberg 100-m telescope.Interferometric observations with the Very Large Array (VLA) show that apart of the emission originates from two prominent sites of starformation that are associated with compact radio continuum sources,likely ultra-compact HII regions. It is concluded that the emissionarises from the most luminous and distant H2O ``kilomasers''detected so far. Our data increase the number of water maser detectionsin northern galaxies (Dec >-30deg) with IRAS point sourcefluxes S100μm > 50 Jy to 18%.
| Neon Abundances in the H II Regions of M33
We present neon abundances for 25 H II regions of M33, measured fromline profiles of the mid-infrared transitions of [Ne II] and [Ne III]taken with the Infrared Space Observatory Short-Wavelength Spectrometer.The distribution of neon abundances as a function of galactocentricradius is best described as a step, -0.15 dex relative to the solar neonabundance from 0.7 to 4.0 kpc and -0.35 dex from 4.0 to 6.7 kpc, withestimated intrinsic scatter of 0.07 dex. The nearly flat neon abundancedistribution differs from the steep oxygen abundance gradient found byprevious investigators. Unless the oxygen abundance determinations arewrong, the chemical evolution of the galaxy has been radially dependent.
| The Excitation and Metallicity of Galactic H II Regions from Infrared Space Observatory SWS Observations of Mid-Infrared Fine-Structure Lines
We present mid-infrared Infrared Space Observatory Short-WavelengthSpectrometer (ISO-SWS) observations of the fine-structure emissionslines [Ne II] 12.8 μm, [Ne III] 15.6 μm, [Ne III] 36.0 μm, [ArII] 6.99 μm, [Ar III] 8.99 μm, [S III] 18.7 μm, [S III] 33.5μm, and [S IV] 10.5 μm and the recombination lines Brα andBrβ in a sample of 112 Galactic H II regions and 37 nearbyextra-Galactic H II regions in the LMC, SMC, and M33. We selected oursources from archival ISO-SWS data as those showing prominent [Ne II]12.8 μm or [Ne III] 15.6 μm emissions. The Galactic sources have awide range in galactocentric distance (0kpc<~Rgal<~18kpc), which enables us to study excitation and metallicity variationsover large Galactic scales. We detect a steep rise in the [Ne III] 15.6μm/[Ne II] 12.8 μm, [Ar III] 8.99 μm/[Ar II] 6.99 μm, and [SIV] 10.5 μm/[S III] 33.5 μm excitation ratios from the innerGalaxy outward, and a moderate decrease in metallicity, from ~2Zsolar in the inner Galaxy to ~1 Zsolar in theouter disk. The extra-Galactic sources in our sample show low gasdensity, low metallicity, and high excitation. We find a goodcorrelation between [Ne III] 15.6 μm/[Ne II] 12.8 μm and [Ar III]8.99 μm/[Ar II] 6.99 μm excitation ratios in our sample. Theobserved correlation is well reproduced by theoretical nebular modelsthat incorporate new-generation wind-driven non-LTE model stellaratmospheres for the photoionizing stars. In particular, the non-LTEatmospheres can account for the production of [Ne III] emission in the HII regions. We have computed self-consistent nebular and stellaratmosphere models for a range of metallicities (0.5-2Zsolar). We conclude that the increase in nebular excitationwith galactocentric radius is due to an increase in stellar effectivetemperature (as opposed to a hardening of the stellar spectral energydistributions due to the metallicity gradient). We estimate anintegrated [Ne III] 15.6 μm/[Ne II] 12.8 μm ratio for the Galaxyof 0.8, which puts it well inside the range of values for starburstgalaxies. The good fit between observations and our models support theconclusion of Thornley and coworkers that the low [Ne III] 15.6μm/[Ne II] 12.8 μm ratios observed in extra-Galactic sources aredue to global aging effects. Based on observations with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom)with the participation of ISAS and NASA.
| Lyman Continuum Extinction by Dust in H II Regions of Galaxies
We examine Lyman continuum extinction (LCE) in H II regions by comparinginfrared fluxes of 49 H II regions in the Galaxy, M31, M33, and theLarge Megellanic Cloud with estimated production rates of Lymancontinuum photons. A typical fraction of Lyman continuum photons thatcontribute to hydrogen ionization in the H II regions of three spiralgalaxies is <~50%. The fraction may become smaller as the metallicity(or dust-to-gas ratio) increases. We examine the LCE effect on estimatedstar formation rates of galaxies. The correction factor for the Galacticdust-to-gas ratio is 2-5.
| An X-Ray Emitting Supernova Remnant Candidate, M 33 X-4
Based on ASCA observations of the Local-Group spiral galaxy M 33, areport is made on the X-ray properties of luminous X-ray source calledX-4, which is located in a giant H II region in M 33. The 0.8-8.0keVspectrum of X-4 is described by a two-component model, consisting of athin thermal plasma emission with a temperature of ~ 1.0 keV, and apower law emission with a photon index of ~ 2.0. The two spectralcomponents cross over at ~ 3.0 keV. These spectral properties aresimilar to those of a young supernova remnant (SNR), except that the0.8-8.0keV luminosity of M 33 X-4, ~ 1.0 × 1038 ergs-1, is considerably higher than those of the SNRs in theMilky Way. There is also evidence of excess absorption, or anover-abundance of α-elements relative to iron. If this source isan SNR, the hard component is thought to be synchrotron radiation fromhigh-energy particles, produced either by shock acceleration, like in SN1006, or by a central neutron star, like in the Crab Nebula.
| BeppoSAX spectroscopy of the luminous X-ray sources in M 33
The nearby galaxy M 33 was observed by the imaging X-ray instrumentson-board BeppoSAX. Two observations at different phases of the 105.9 dayintensity cycle of the luminous central source X-8 failed to reveal theexpected modulation, suggesting that it is probably transitory. Similarbehavior has been observed from several X-ray binary sources. Thisstrengthens somewhat the idea that M 33 X-8 is a black hole accretingfrom a binary companion. The 0.2-10 keV spectrum of M 33 X-8 can best bemodeled by an absorbed power-law with a photon index, alpha , of 1.89+/- 0.40 0.79 and a disk-blackbody with atemperature, kT, of 1.10 +/- 0.05 keV and a projected inner-disk radiusof 55.4 +/- 6.0 7.7 km. This spectral shape is ingood agreement with earlier ASCA results. The 2-10 keV spectra of M 33X-4, X-5, X-7, X-9 and X-10 are all consistent with power-law orbremsstrahlung models, whilst that of X-6 appears to be significantlymore complex and may be reasonably well modeled with a disk-blackbodywith kT = 1.7 +/- 0.2 keV and a projected inner-disk radius of 7 +/- 2km. The spectrum of M 33 X-9 is rather hard with alpha = 1.3. Comparedto earlier Einstein and ROSAT observations, M 33 X-7 and X-9 varied inintensity, M 33 X-4, X-6, and X-10 may have varied and M 33 X-5 remainedconstant.
| H(alpha), Far-Infrared and Thermal Radio Continuum Emission Within the Late-Type Spiral Galaxy M33
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113..236D
| An analysis of the VLBI relative-position map of H2O masers in the HII-region complex IC 133 of the spiral galaxy M33.
Not Available
| Molecular Tori and H 2O Megamaser Variability
Five prominent extragalactic water masers have been observed at roughlymonthly intervals from 1984 until 1993 to study their temporalcharacteristics. The observing program at Haystack Observatory comprisedfive sources: NGC 1068, NGC 3079, NGC 4258, the irregular galaxy IC 10,and IC 133, an H II region complex in M33. The data on two sources, NGC4258 and IC 10SE, have been presented earlier. All galaxies show strongvariability in line strength, line velocity, and line width. Theobserved variability characteristics support a model with invertedmolecular gas located in front of the nuclear source that experiences avariable pump rate and in part amplifies the background continuum. Theextragalactic sources have been classified as class I for H II regionemission sources in nearby galaxies or class II for mostly powerfulemissions from galactic nuclei. In analogy with NGC 4258, the spectralfeatures of class II sources NGC 1068 and NGC 3079 display a drift rateconsistent with a compact molecular torus for the emission regIon.
| High-resolution X-ray imaging of M33
The nearby spiral galaxy M33 was observed for 35 ks with thehigh-resolution imager on ROSAT in 1992 January and August. We find 27X-ray sources more luminous than 6 x 1036 ergs/s within 17.5arcmin of the nucleus, of which 12 were discovered by the EinsteinObservatory. Three of the ROSAT sources are coincident with giant H IIregions, and seven are coincident or nearly coincident with supernovaremnants. There is evidence for long-term variability between theEinstein and ROSAT observations in three sources, one of which was notdetected in the ROSAT observations. Aside from an eclipsing binary X-raysource discovered by the Einstein Observatory, there is no evidence forvariability within the ROSAT observations. Of the 120 cataloged holes inthe H I layer in the field of view, two have significant X-ray emission.One is coincident with the giant H II region IC 133 while the otherappears to be due to the chance superposition of a strong X-ray pointsource. Such holes are thought to be created by energetic stellar windsand supernovae from massive stars in young associations. We detectdiffuse X-ray emission with a luminosity of about 1039ergs/s, which may trace the spiral arms within 10 arcmin of the nucleus.
| High-velocity H I and superbubbles in nearby disk galaxies.
Not Available
| Microarcsecond proper motions of extragalactic water vapor masers in M33
A second-epoch spectral line VLBI synthesis map of the H2O maserassociated with the H II region IC 133 in the galaxy M33 is presented.Thirty-two spatially distinct maser features were identified, and asecond center of maser activity within the IC 133 complex, IC 133 West,which is displaced about 0.3 arcsec from IC 133 Main, was discovered. Acomparison of the two available maps of IC 133 is used to estimate theright ascension components of proper motion over a period of 479 d forfive maser features to accuracies of between 7 and 16 micro-as. Thedispersions in transverse and radial velocities for the maser featuresare consistent with the accepted distance to M33 of 720 kpc, where thedata admit a fractional uncertainty in distance of 50 percent.
| The ultraviolet color gradient in the late-type spiral galaxy M33
The ultraviolet surface brightness and color distributions for thelate-type spiral galaxy M33 are derived from images at 1520 and 2490 Athat were obtained by the Ultraviolet Imaging Telescope (UIT) on theAstro Spacelab mission. Although the surface brightness shows a generaldecline with radius, the dominant spiral arms cause significantdeviations from an exponential fit. Colors in elliptical annuli become0.2-0.3 mag bluer with increasing radius. Our measures of individual HII regions and sections of spiral arms follow this same trend. Interarmregions are redder than the arms. A metallicity gradient affecting onlythe colors of stars is inadequate in accounting for the observed colorgradient. A plausible explanation invokes a combination of an LMC-typereddening curve and a radial gradient in the internal reddening in M33.
| Astronomical masers
Recent research related to astronomical masers is reviewed. First,attention is given to phenomenology, including observations and modelingof galactic and extragalactic maser sources. The discussion then focuseson the developments concerning the physical properties of maserradiation. Finally, the use of masers as general tools for the study ofastronomical environments where the radiation is produced and where itpropagates, is discussed.
| The stellar content of associations and star complexes in M33
Photographic UBV photometry is used to map stellar associations and starcomplexes in M33 to characterize the galaxy and compare the results toother nearby galaxies. The appearance of the associations is revisedaccording to the images and a cluster analysis, which show themclustering within the classical OB associations. A shock wave across thesouthern spiral arm can be tested observationally by the stellarassociations, and the central region, contrasting the outer region, isfound to be rich in WR, O, and luminous stars. The difference betweenderived association ages and expansion ages suggests the existence oftwo or more generations of star formation in the old associations.
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