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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.
| 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).
| 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.
| 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.
| New light on the search for low-metallicity galaxies - I. The N2 calibrator
We present a simple metallicity estimator based on the logarithmic [Nii]ratio, hereafter N2, which we envisage will become very useful forranking galaxies in a metallicity sequence from redshift survey-qualitydata even for moderately low spectral resolution. We have calibrated theN2 estimator using a compilation of Hii galaxies having accurate oxygenabundances, plus photoionization models covering a wide range ofabundances. The comparison of models and observations indicates thatboth primary and secondary nitrogen are important for the relevant rangeof metallicities. The N2 estimator follows a linear relation withlog(O/H) that holds for the whole abundance range covered by the sample,from approximately to twice the Solar value . We suggest that the ([Sii]ratio (hereafter S2) can also be used as a rough metallicity indicator.Because of its large scatter the S2 estimator will be useful only insystems with very low metallicity, where [Nii] λ 6584 is notdetected or in low-resolution spectra where [Nii] λ 6584 isblended with Hα .
| An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.
| At The Eyepiece: Galaxies - A Matter of Subtle Contrast
Not Available
| Stellar Abundances and Winds of A-Type Supergiant Stars in M33: First Results from the Keck HIRES Spectrograph
In this Letter, we report the first results of detailed analyses ofA-type supergiants in M33 from high-quality Keck HIRES echelle spectra.The two stars, designated 117-A and B-324, now constitute the mostdistant stars for which detailed abundances have been measured. We find117-A is metal-poor (roughly 1/10 solar), resembling an early Asupergiant in the SMC. B-324 has P Cygni--like profiles for most of themetal lines, but fitting only the photospheric component yields metalabundances similar to solar. These two stars are located at distinctlydifferent galactocentric distances in M33; comparing their abundancesshows that the metallicity gradient of this galaxy, previously reportedonly from H II--region surveys, is also qualitatively apparent in thesestars. Estimates of the stellar-wind parameters for these stars yieldmass-loss rates of ~2.5 x 10-8 Mȯ yr-1 for 117-A and ~1.2 x 10-5Mȯ yr-1 for B-324 from the H alpha line profiles. When we examinethe location of these stars in the wind momentum--luminosity plane, wefind that they are in excellent agreement with the loci of luminous bluestars in the Galaxy, LMC, and SMC, taking into account differences inmetallicity.
| Extinction characteristics of giant HII regions - Star-forming complexes in the galaxies M33, LMC, and NGC 2403
The discrepancies between the extinction of gas emission and that of thestarlight in giant HII regions, star-forming complexes in the galaxiesM33, LMC, and NGC 2403, were empirically investigated. The extinctionvalues were determined for 30 stars in eight associations in M33. Anempirical relation between the extinction of starlight and that of thegas emission in giant HII regions, star-forming complexes in thegalaxies under study, was obtained.
| 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.
| High chemical abundances in Virgo spiral galaxies?
Evidence is presented that spiral galaxies in the Virgo Cluster havesystematically higher interstellar abundances than comparable fieldgalaxies. This conclusion is based on spectra of H II regions in fiveVirgo spirals of type Sc. A possible explanation is that abundances inthe field spirals are strongly affected by infall of metal-poor gas orby radial inflow of gas from the outer H I disk. These processes areinhibited in the cluster environment, and the Virgo spirals may haveevolved more nearly in the manner of the closed box 'simple model' ofchemical evolution.
| 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.
| The molecular hydrogen content of NGC 604 and other M33 H II region complexes
Six bright H II regions in M33 have been searched for H2 line emission.Previously detected fluorescent H2 emission in the first-ranked M33 H IIregion, NGC 604, has been mapped, making it possible to estimate themass of its molecular cloud (of order 3 x 10 to the 6th solar masses).This result implies a ratio I(CO)/N(H2) lower by an order of magnitudethan that of galactic molecular clouds. The lack of shocked H2 in NGC604 implies that the ionized stellar-wind bubbles seen in the nebula arenot interacting strongly with the molecular material. The dynamics ofNGC 604 appear to be dominated by gravitational interaction.
| Kinematics and composition of H II regions in spiral galaxies. I - M33
Moderate-dispersion, 0l.7 A/pixel spectra of H II regions in M33 weretaken, and the velocity as well as the excitation were measured byobserving the spectral region around H-beta. Velocity measurements for55 H II regions and excitation measurements for 42 regions arepresented. The velocity data are used to measure the systemic velocityat -172 + or - 6 km/s, the inclination at 56 + or - 1 deg, the positionangle of the major axis at 23 + or - 1 deg, the rotation curve of the HII regions, and their line-of-sight velocity dispersion at 9 + or - 4km/s. These data are used to derive the M/L ratio from radii of 0.5-3.7kpc. The excitation measurements are used to derive a metallicitygradient for M33 and to quantify the dispersion in the excitation valuesat a given radius.
| The chemical composition gradient across M 33
The abundance gradient in M 33 is studied on the basis of IPCS and CCDdata on emission lines in selected H II regions, using O II, O III, SII, and S III in the wavelength range 3700-9600 A to refine the oxygenabundances in the inner parts as well as to study the behavior of S/O.Spatially resolved observations in each H II region permit theionization structure to be studied and enable ionization parameters andstellar effective temperatures to be separately determined. The mainresults are: (1) the hardness of the ionizing radiation and theionization parameter increase outwards (with diminishing abundances)along the galactic radius; (2) the O/H gradient is steep in the innerregions, but much flatter in the outer regions; (3) N/O is constant overmost of the visible disk, but lower in an outer H II region; and (4) theS/H gradient is shallower than the O/H gradient exemplifying whatappears to be a universal trend for S/O to decrease with O/H in H IIregions. This latter trend is rather unexpected from the viewpoint ofnucleosynthesis theory.
| Correlations between integrated parameters and H-alpha velocity widths in giant extragalactic H II regions - A new appraisal
Investigations of relationships between diameters (or luminosities) andvelocity widths of H-alpha line profiles in giant extragalactic H IIregions (GEHR) have firmly established that these parameters arecorrelated. However, three independent studies on the subject disagreeon the slopes of these relations. It is shown that all measurements ofthe velocity width of integrated H-alpha profiles of GEHRs are entirelyconsistent. Discrepancies in the relations are explained by differentsamples and by the use of different parameters such as distances togalaxies and diameters of GEHRs. Assembling all observations of H-alphavelocity widths, new values of slopes and zero-points are derived forthe relations between luminosities (or diameters) and velocity widths.
| HII regions in M33. II - Radio continuum survey
A new WSRT survey of M33 in the continuum at 1.4 GHz is presented. Withan angular resolution of 25 x 49 arcsec HPBW and a rms noise of 0.2 mJyper beam area (at the field center), 112 radio sources with H-alphanebulosities have been identified. From a comparison of this survey dataand 5 GHz observations obtained with the VLA and WSRT, the spectralindices of 17 sources have been determined. Only three of these sourceshave nonthermal spectra. A compact source is discovered 1 arcmin southof the nucleus of M33. This source has a flat radio spectrum and noH-alpha counterpart. This source is either a heavily obscured compactHII region or, more likely, a source similar to the Crab Nebula.
| Turbulent gas motions in giant H II regions. II - The luminosity-velocity dispersion relation
The relation between luminosity L(H-alpha) and velocity dispersion sigmais reinvestigated in view of H-alpha and O III 5007-A forbidden linemeasurements of the velocity dispersion for 43 giant, extragalactic H IIregions. A correlation of L(H-alpha) that varies as sigma (H-alpha) tothe 6.6th power is obtained; O III-line velocity dispersions are notedto obey a similar relation, but are somewhat smaller, and the slope ofboth relations is steeper than expected for gravitationally boundsystems.
| H II regions in M33. III - Physical properties
The properties of the radio H II regions in M33 are investigatedutilizing a calibrated H-alpha survey of a large number of H II regionsin the inner part of M33 (Donas, 1977) and a forbidden O III/H-betasurvey obtained by Boulesteix et al. (1981). The luminosity function ofthe H II region in the inner part of M33, the extinction in the H IIregion, and the dust content in these nebulae are determined. Theassociation of H II regions with H I concentrations is brieflydiscussed.
| Age determination of extragalactic H II regions
The H II region evolution models of Copetti et al. (1984) were comparedwith observational data of H II regions in the Magellanic Clouds, M 33,M 101 and of 'isolated extragalactic H II regions'. IMF with chi = 3 or2.5 are inconsistent with a large number of H II regions. The moreuniform age distribution of isolated extragalactic H II regions obtainedthrough an IMF with chi = 2 suggests that this value is more realisticthan chi = 1 or 1.5. The H II region age estimates indicate a burst ofstar formation about 5.5 + or - 1.0 10 to the -6th yr ago in the LMC andabout 2.3 + or - 0.9 x 10 to the 6th yr ago in the SMC. The observedforbidden O III/H-beta gradient in M 33 and M 101 must be caused bycolor temperature variation of the radiation ionizing the H II regions.
| Chemical compositions of H II regions in the Triangulum spiral, M33
Measurements of 12 H II regions secured with the Robinson-Wampler ImageTube Scanner at the Shane Telescope, Lick Observatory cover the spectralrange 3700-7600 A. The distances of these regions from the nucleus rangefrom 1 to 6 kpc. These data are analysed to establish plasma diagnosticsand chemical compositions. In a manner similar to that previouslyemployed for studies of the Magellanic Clouds, theoretical models areused as interpolation devices to establish ionization correction factorsfor S, Cl and Ar. Except for helium, the N(element)/N(H) ratios fall offwith increasing radial distance with closely similar rates.Consequently, the ratios not only of nitrogen, but also neon, sulphurand argon, with respect to oxygen, remain essentially constant. Thefollowing log N(element)/N(oxygen) are found: N = -1.25, Ne = -1.5, Cl =-3.6 and Ar = -2.4.
| An optical study of M 33. I - Morphology of the gas
A general catalogue of 369 distinct H II regions has been compiled fromwide-field photographs obtained with a focal reducer and narrow-bandinterference filters. Ring-like emission nebulae have been observed asfar as 35 min of arc from the nucleus. It is proposed that these ringsrepresent a late stage in the life of expanding ionized regions. Generalhistograms related to the distribution of H II regions, intensities anddiameters are plotted. The number surface density of the faint regionsis rather constant over the whole galaxy, while for the strongerregions, it is distinctly greater in the central part. A most-probablevalue of 13 sec of arc is found for the apparent diameter of the H IIregions. A detailed spatial comparison is made with radio continuumsources, Wolf-Rayet stars, H I clouds, and clusters of hot stars. Spiralstructure is drawn based on the H II and H I observations. A very goodcorrelation is found between H II arms and the distribution of H Iclouds. Outer extensions of H emission are related to the spiralstructure.
| Radio observations of H II regions in external galaxies. I. H II regions in M33.
Abstract image available at:http://adsabs.harvard.edu/abs/1974A&A....32..363I
| Evidence for Composition Gradients across the Disks of Spiral Galaxies
Abstract image available at:http://adsabs.harvard.edu/abs/1971ApJ...168..327S
| The Spectra of the Emission Nebulosities in Messier 33.
Abstract image available at:http://adsabs.harvard.edu/abs/1942ApJ....95...52A
| The Rotation of the Spiral Nebula Messier 33.
Abstract image available at:http://adsabs.harvard.edu/abs/1942ApJ....95....5M
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