Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

NGC 1741



Upload your image

DSS Images   Other Images

Related articles

A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.

Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.

Kinematics of Interstellar Gas in Nearby UV-selected Galaxies Measured with HST STIS Spectroscopy
We measure Doppler shifts of interstellar absorption lines in HST STISspectra of individual star clusters in nearby UV-selected galaxies.Values for systemic velocities, which are needed to quantify outflowspeeds, are taken from the literature and verified with stellar lines.We detect outflowing gas in 8 of 17 galaxies via low-ionization lines(e.g., C II, Si II, Al II), which trace cold and/or warm gas. Thestarbursts in our sample are intermediate in luminosity (and mass) todwarf galaxies and luminous infrared galaxies (LIRGs), and we confirmthat their outflow speeds (ranging from -100 to nearly -520 kms-1, with an accuracy of ~80 km s-1) areintermediate to those previously measured in dwarf starbursts and LIRGs.We do not detect the outflow in high-ionization lines (such as C IV orSi IV); higher quality data will be needed to empirically establish howvelocities vary with the ionization state of the outflow. We do verifythat the low-ionization UV lines and optical Na I doublet give roughlyconsistent outflow velocities, solidifying an important link betweenstudies of galactic winds at low and high redshift. To obtain a highersignal-to-noise ratio (S/N), we create a local average compositespectrum and compare it to the high-z Lyman break composite spectrum. Itis surprising that the low-ionization lines show similar outflowvelocities in the two samples. We attribute this to a combination ofweighting toward higher luminosities in the local composite, as well asboth samples being, on average, brighter than the ``turnover''luminosity in the v-SFR relation.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program GO-9036.

On the Determination of N and O Abundances in Low-Metallicity Systems
We show that in order to minimize the uncertainties in the N and Oabundances of low-mass, low-metallicity (O/H<=1/5 solar)emission-line galaxies, it is necessary to employ separateparameterizations for inferring Te(N+) andTe(O+) from Te(O+2). Inaddition, we show that for the above systems, the ionization correctionfactor (ICF) for obtaining N/O from N+/O+, wherethe latter is derived from optical emission-line flux ratios, is=1.08+/-0.09. These findings are based on state-of-the-art single-star HII region simulations, employing our own modeled stellar spectra asinput. Our models offer the advantage of having matching stellar andnebular abundances. In addition, they have O/H as low as 1/50 solar(lower than any past work), as well as log(N/O) and log(C/O) fixed atcharacteristic values of -1.46 and -0.7, respectively. The above resultswere used to rederive N and O abundances for a sample of 68 systems with12+log(O/H)<=8.1, whose dereddened emission-line strengths werecollected from the literature. The analysis of the log(N/O) versus12+log(O/H) diagram of the above systems shows that (1) the largestgroup of objects forms the well-known N/O plateau with a value for themean (and its statistical error) of-1.43+0.0084-0.0085, (2) the objects aredistributed within a range in log(N/O) of -1.54 to -1.27 in Gaussianfashion around the mean with a standard deviation ofσ=+0.071-0.084, and (3) a χ2analysis suggests that only a small amount of the observed scatter inlog(N/O) is intrinsic.

Ultraviolet-to-Far-Infrared Properties of Local Star-forming Galaxies
We present the results of a multiwavelength study of nearby galaxiesaimed at understanding the relation between the ultraviolet andfar-infrared emission in star-forming galaxies. The data set comprisesnew ultraviolet (from HST STIS), ground-based Hα, and radiocontinuum observations, together with archival infrared data (from IRASand ISO). The local galaxies are used as benchmarks for comparison ofthe infrared-to-ultraviolet properties with two populations ofhigh-redshift galaxies: the submillimeter star-forming galaxies detectedby SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). Inaddition, the long wavelength baseline covered by the present dataenables us to compare the star formation rates (SFRs) derived from theobserved ultraviolet, Hα, infrared, and radio luminosities and togauge the impact of dust opacity in the local galaxies. We also derive anew calibration for the nonthermal part of the radio SFR estimator,based on the comparison of 1.4 GHz measurements with a new estimator ofthe bolometric luminosity of the star-forming regions. We find that moreactively star-forming galaxies show higher dust opacities, which is inline with previous results. We find that the local star-forming galaxieshave a lower Fλ(205 μm)/Fλ(UV)ratio by 2-3 orders of magnitude than the submillimeter-selectedgalaxies and may have a similar or somewhat higherFλ(205 μm)/Fλ(UV) ratio thanLBGs. The Fλ(205 μm)/Fλ(UV) ratioof the local galaxy population may be influenced by the cool dustemission in the far-infrared heated by nonionizing stellar populations,which may be reduced or absent in the LBGs.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.

The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5

Star formation and stellar populations in the Wolf-Rayet(?) luminous compact blue galaxy IRAS 08339+6517
Aims.IRAS 08339+6517 is a luminous infrared and Lyα-emittingstarburst galaxy that possesses a dwarf companion object at a projecteddistance of 56 kpc. An H I tidal tail has recently been detected betweenboth galaxies, suggesting that about 70% of the neutral gas has beenejected from them.Methods.We present deep broad-band optical images,together with narrow band Hα CCD images, and opticalintermediate-resolution spectroscopy of both galaxies.Results.The imagesreveal interaction features between both systems and strong Hαemission in the inner part of IRAS 08339+6517. The chemical compositionof the ionized gas of the galaxies is rather similar. The analysis oftheir kinematics also indicates interaction features and reveals anobject that could be a candidate tidal dwarf galaxy or a remnant of anearlier merger. Our data suggest that the H I tail has been mainlyformed from material stripped from the main galaxy. We find weakspectral features that could be attributed to the presence of Wolf-Rayetstars in this starburst galaxy and estimate an age of the most recentburst of around 4-6 Myr. A more evolved underlying stellar population,with a minimal age between 100-200 Myr, is also detected and fits anexponential intensity profile. A model which combines 85% young and 15%old populations can explain both the spectral energy distribution andthe H I Balmer and He I absorption lines presented in our spectrum. Thestar formation rate of the galaxy is consistently derived using severalcalibrations, giving a value of ~9.5 Mȯyr-1.Conclusions.IRAS 08339+6517 does satisfy the criteria ofa luminous compact blue galaxy, rare objects in the local universe butcommon at high redshifts, being a very interesting target for detailedstudies of galaxy evolution and formation.

Comparison of Star Clusters With and Without Wolf-Rayet Stars in Wolf-Rayet Galaxies
We compare the properties of young star clusters with and withoutWolf-Rayet (W-R) stars in W-R galaxies using optical, near-infraredimagery and optical spectroscopy. Our work identifies the clusters withW-R stars in these galaxies for the first time. With this information,comparisons of clusters with and without W-R stars are now possible,enabling us to understand the chemical and morphological impact ofmassive stars on their environment and to constrain the parameters formodeling these systems. We find that clusters with W-R stars (W-Rclusters) are systematically younger, bluer clusters. Knowing this agedifference between the two cluster sets, we use an evolutionary scenarioto interpret their other properties. Young clusters, typically W-Rclusters, have a Strömgren sphere-like gas configuration. They alsotend to have H-K colors redder than those of theoretical models. Weinterpret the H-K excess as a combination of thermal emission from hotdust, nebular emission, and molecular emission. Older clusters,typically clusters without W-R stars, have ionized gas in a superbubbleconfiguration caused by the prolonged influence of stellar winds andsupernovae. The H-K excess is generally absent for these clusters. Thenitrogen-to-oxygen abundance ratio (N/O) does not appear to increase asa function of age over the first 10 Myr. Systems without W-R stars doappear to have a significant, elevated N/O over systems with W-R starsin the metallicity range 12+log(O/H)=7.7-7.9. For the entire metallicityrange in our sample, this finding is only marginally significant. Weconcur with previous studies, which find no correlation between thesulfur-to-oxygen abundance ratio and metallicity.

The Stellar Content of Nearby Star-forming Galaxies. III. Unravelling the Nature of the Diffuse Ultraviolet Light
We investigate the nature of the diffuse intracluster ultraviolet lightseen in 12 local starburst galaxies, using long-slit ultravioletspectroscopy obtained with the Space Telescope Imaging Spectrograph(STIS) aboard the Hubble Space Telescope (HST). We take this faintintracluster light to be the field in each galaxy and compare itsspectroscopic signature with Starburst99 evolutionary synthesis modelsand with neighboring star clusters. Our main result is that the diffuseultraviolet light in 11 of the 12 starbursts lacks the strong O starwind features that are clearly visible in spectra of luminous clustersin the same galaxies. The difference in stellar features dominatingcluster and field spectra indicates that the field light comes primarilyfrom a different stellar population and not from scattering of UVphotons originating in the massive clusters. A cut along the spatialdirection of the UV spectra establishes that the field light is notsmooth but rather shows numerous ``bumps and wiggles.'' Roughly 30%-60%of these faint peaks seen in field regions of the closest (<4 Mpc)starbursts appear to be resolved, suggesting a contribution fromsuperpositions of stars and/or faint star clusters. Complementary WFPC2UVI imaging for the three nearest target galaxies, NGC 4214, NGC 4449,and NGC 5253, is used to obtain a broader picture and establish that allthree galaxies have a dispersed population of unresolved, luminous bluesources. Because the field spectra are dominated by B stars, we suggestthat the individual sources observed in the WFPC2 images are individualB stars (rather than O stars) or small star clusters. We considerseveral scenarios to understand the lack of observed massive stars inthe field and their implications for the origin of the field stellarpopulation. If the field stellar populations formed in situ, the fieldmust have either an IMF that is steeper than Salpeter (α~-3.0 to-3.5) or a Salpeter slope with an upper mass cutoff of 30-50Msolar. If star formation occurs primarily in star clusters,the field could be composed of older, faded clusters and/or a populationthat is coeval with the luminous clusters but lower in mass. We usethese benchmark populations to place constraints on the field stellarpopulation origin. Although the field probably includes stars ofdifferent ages, the UV light is dominated by the youngest stellarpopulations in the field. If the field is composed of older, dissolvingclusters, we estimate that star clusters (regardless of mass) need todissolve on timescales 7-10 Myr to create the field. If the field iscomposed of young clusters that fall below the detection limit ofindividual sources in our spectroscopy, they would have to be severalhundred solar masses or less, in order to be deficient in O stars,despite their extreme youth.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 NAS5-26555.

Hubble Space Telescope Observations of Star-forming Regions in NGC 3994/3995
We obtained ultraviolet and optical images of star-forming regions inthe interacting galaxy system NGC 3994/3995 using the Space TelescopeImaging Spectrograph on the Hubble Space Telescope. We describe thecharacteristics of 56 detected knots, a majority of which are youngerthan 20 Myr old. There is no discernible pattern of knot age withrespect to position within the galaxies. The knots have masses and radiiranging from 7.2×103 to 4.4×106Msolar and from 4 to 23 pc, respectively. Using aconservative criterion, we find that ~15% of these knots may beproto-globular clusters; the percentage of proto-globular clusters maybe as high as ~70%. The UV flux distribution of the knots in NGC 3995can be fitted with a power law with α=-0.72+/-0.11, with noturnover detected brightward of the completeness limit.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.

Dust properties of UV bright galaxies at z ~ 2
We investigate the properties of the extinction curve in the rest-frameUV for a sample of 34 UV-luminous galaxies at 2 < z < 2.5,selected from the FORS Deep Field (FDF) spectroscopic survey. A newparametric description of the rest-frame UV spectral energy distributionis adopted; its sensitivity to properties of the stellar populations orof dust attenuation is established with the use of models. The latterare computed by combining composite stellar population models andcalculations of radiative transfer of the stellar and scatteredradiation through the dusty interstellar medium (ISM) for a dust/starsconfiguration describing dust attenuation in local starbursts. In thefavoured configuration the stars are enveloped by a shell with atwo-phase, clumpy, dusty ISM. The distribution of the z ˜ 2UV-luminous FDF galaxies in several diagnostic diagrams shows that theirextinction curves range between those typical of the Small and LargeMagellanic Clouds (SMC and LMC, respectively). For the majority ofstrongly reddened objects having a UV continuum slope β > -0.4 asignificant 2175 Å absorption feature (or "UV bump") is inferred,indicating an LMC-like extinction curve. On the other hand, the UVcontinua of the least reddened objects are mostly consistent withSMC-like extinction curves, lacking a significant UV bump, as for thesample of local starbursts investigated by Calzetti and collaborators.Furthermore, the most opaque (⠘ 0) and, thus (for ourmodels), dustiest UV-luminous FDF galaxies tend to be among the mostmetal-rich, most massive, and largest systems at z ˜ 2, indicating< Z > ˜ 0.5 {-} 1 Zȯ, < Mstars> ˜ 6 × 1010 Mȯ, and ˜ 4 kpc, respectively. The presence of the UVbump does not seem to depend on the total metallicity, as given by theequivalent width (EW) of the C IV doublet. Conversely, it seems to beassociated with a large average EW of the six most prominentinterstellar low-ionisation absorption lines falling in the FORSspectra. The average EW of these saturated lines offers a proxy for theISM topology. We interpret these results as the evidence for adifference in the properties of the dusty ISM among the most evolvedUV-luminous, massive galaxies at z ˜ 2.

ISO observations of the Wolf-Rayet galaxies NGC 5430, NGC 6764, Mrk 309 and VII Zw 19
Observations of four WR galaxies (NGC 5430, NGC 6764, Mrk 309 and VII Zw19) using the Infrared Space Observatory are presented here. ISOCAM mapsof NGC 5430, Mrk 309 and NGC 6764 revealed the location of starformation regions in each of these galaxies. ISOPHOT spectralobservations from 4 to 12 μm detected the ubiquitous PAH bands in thenuclei of the targets and several of the disk star forming regions,while LWS spectroscopy detected [O I] and [C II] emission lines from twogalaxies, NGC 5430 and NGC 6764. Using a combination of ISO and IRASflux densities, a dust model based on the sum of modified blackbodycomponents was successfully fitted to the available data. These modelswere then used to calculate new values for the total IR luminosities foreach galaxy, the size of the various dust populations, and the globalSFR. The derived flux ratios, the SFRs, the high L(PAH)/L(40-120 μm)and F(PAH 7.7 μm)/F(7.7 μm continuum) values suggest that most ofthese galaxies are home to only a compact burst of star formation. Theexception is NGC 6764, whose F(PAH 7.7 μm)/F(7.7 μm continuum)value of 1.22 is consistent with the presence of an AGN, yet theL(PAH)/L(40-120 μm) is more in line with a starburst, a finding inline with a compact low-luminosity AGN dominated by the starburst.

Discovery of a high-redshift Einstein ring
We report the discovery of a partial Einstein ring of radius 1.48 arcsecproduced by a massive (and seemingly isolated) elliptical galaxy. Thespectroscopic follow-up at the VLT reveals a 2Lstar galaxy atz=0.986, which is lensing a post-starburst galaxy at z=3.773. Thisunique configuration yields a very precise measure of the mass of thelens within the Einstein radius, (8.3 ± 0.4) ×1011h70-1Mȯ . Thefundamental plane relation indicates an evolution rate of dlog(M/L)B / dz = -0.57±0.04, similar to other massiveellipticals at this redshift. The source galaxy shows stronginterstellar absorption lines indicative of large gas-phasemetallicities, with fading stellar populations after a burst. Higherresolution spectra and imaging will allow the detailed study of anunbiased representative of the galaxy population when the universe wasjust 12% of its current age.

A sample of X-ray emitting normal galaxies from the BMW-HRI Catalogue
We obtained a sample of 143 normal galaxies with X-ray luminosity in therange 1038{-}1043 erg s-1 from thecross-correlation of the ROSAT HRI Brera Multi-scale Wavelet (BMW-HRI)Catalogue with the Lyon-Meudon Extragalactic Database (LEDA). We findthat the average X-ray properties of this sample are in good agreementwith those of other samples of galaxies in the literature. We selected acomplete flux limited serendipitous sample of 32 galaxies from which wederived the log N-log S distribution of normal galaxies in the fluxrange 1.1{-} 110 × 10-14 erg cm-2s-1. The resulting distribution is consistent with theEuclidean -1.5 slope. Comparisons with other samples, such as theExtended Medium Sensitivity Survey, the ROSAT All Sky Survey, theXMM-Newton/2dF survey, and the Chandra Deep Field Survey indicate thatthe log N -log S distribution of normal galaxies is consistent with aEuclidean slope over a flux range of about 6 decades.

The evolution of HCG 31: Optical and high-resolution HI study
Here we present the results of our new optical imaging and spectroscopicstudy and the analysis of new high-resolution HI images of the HicksonCompact Group HCG 31. Taking advantage of the improved sensitivity andangular resolution of the new optical and HI images, we have identifiedan extensive complex of stellar and HI tidal features and theirkinematics. Our HI study show that H31A and C are not an advanced mergersince their velocity fields can be still separated and have almostorthogonal orientations. All of the current sites of ongoing activestar formation are shown to be associated with the highest columndensity peaks traced in HI. A new companion A0500-0434 located 240 kpcsouth of the group center is also discovered in HI. A detailed scenariofor the tidal interactions involved and the origins of the individualtidal features are constructed using the morphology and kinematics ofthe tidal features. The derived dynamical mass for the entire group isabout 2× 1011 Mȯ, which is a few timeslarger than the sum of the masses of the individual group galaxies. Theultimate fate of the group is that HCG 31 is probably on its way to forma single HI cloud group containing all galaxies.Based on observations made with the VLA operated by the National RadioAstronomy Observatory (the National Radio Astronomy Observatory is afacility of the National Science Foundation operated under cooperativeagreement by Associated Universities, Inc.) and on data taken usingALFOSC, which is owned by the Instituto de Astrofísica deAndalucía (IAA) and operated at the Nordic Optical Telescopeunder agreement between IAA and the NBIfA of the AstronomicalObservatory of Copenhagen.

Infrared mergers and infrared quasi-stellar objects with galactic winds - II. NGC5514: two extranuclear starbursts with LINER properties and a supergiant bubble in the rupture phase
A study of the morphology, kinematics and ionization structure of theinfrared (IR) merger NGC5514 is presented. This study is based mainly onINTEGRAL two-dimensional (2D) spectroscopy (obtained at the 4.2-mWilliam Herschel Telescope, WHT), plus optical and near-IR images. Clearevidence of two extranuclear starbursts with young outflows (OFs) andlow-ionization nuclear emission region (LINER) activity are reported.One of these OFs has generated a supergiant bubble and the other isassociated with an extended complex of HII regions.In the galactic bubble it was found that: (i) the [SII], Hα,[NII], [OI] and [OIII] emission-line maps show a bubble with a distortedellipsoidal shape, with major and minor axes of ~6.5 kpc [13.6 arcsec;at position angle (PA) = 120°+/- 10°] and ~4.5 kpc (9.6 arcsec);(ii) these maps depict four main knots, a very strong one and threeothers more compact and located at the border; (iii) the centre of thebubble is located at ~4.1 kpc (8.5 arcsec) to the west of the mainnucleus; (iv) the WHT spectra show, in this area, two strong components:blue and red emission-line systems, probably associated with emissionfrom the near and far side of the external shell, for which the mean OFvelocities were measured as VOFblue= (-320 +/- 20)kms-1 and VOFred= (+265 +/- 25) kms-1(v) these two components depict LINER properties, probably associatedwith large-scale OF + shocks; (vi) at the east border, the kinematics ofthe ionized gas and the [SII] emission-line maps show an extendedejection of 4 kpc aligned with the PA of the major axis; (vii) threeother ejections were found, two of them perpendicular to the extendedone. Each ejection starts in one of the knots. These results suggestthat the bubble is in the rupture phase.For the complex of giant HII regions it was found that: (i) theHα, [NII] and [SII] emission-line maps show a compact strongemission area (peaking at ~810 pc ~1.7 arcsec, to the east of the secondnucleus) and faint extended emission with an elongated shape, and majorand minor axes of ~5.1 kpc (10.8 arcsec; at PA ~20°) and ~2.9 kpc(6.0 arcsec); (ii) inside this complex, the spectra show HII region andtransition LINER/HII characteristics; (iii) at the border of thisextended HII area the spectra have outflow components and LINERproperties.INTEGRAL 2D [NII], Hα, [SII] and [OIII] velocity fields (VFs) arepresented. These VF maps show results consistent with an expansion ofthe bubble, plus four ejections of ionized gas. The U, B, V, I, J, H andKS images show a pre-merger morphology, from which faintfilaments of emission emerge, centred on the bubble. The ionizationstructure and the physical conditions were analysed using the following2D emission-line ratio and width maps: [SII]/Hα, [NII]/Hα,[OI]/Hα, [OIII]/Hβ and FWHM-[NII]. In the region of thebubble, 100 per cent of the [NII]/Hα and [SII]/Hα ratiosshow very high values (>0.8) consistent with LINER processesassociated with high-velocity shocks. These new results support theprevious proposition that extreme nuclear and `extranuclear' starburstswith galactic winds + shocks play an important role in the evolution ofIR mergers/quasi-stellar objects.

The Iron Abundance in Blue Compact Galaxies
We determine the Fe abundances for several blue compact andemission-line galaxies. The O/Fe abundance ratios are found to increasewith the metallicity of the galaxy, and to decrease with its degree ofionization. These correlations are interpreted as reflecting a decreasefor lower metallicities of the depletion of Fe atoms on to dust grains,probably arising from the harder radiation fields prevailing at lowmetallicities.

Massive Star Formation and Tidal Structures in HCG 31
We present new broadband optical and near-infrared CCD imaging togetherwith deep optical intermediate-resolution spectroscopy of the HicksonCompact Group 31. We analyze the morphology and colors of the stellarpopulations of the galaxies, as well as the kinematics, physicalconditions, and chemical composition of the ionized gas in order to geta more complete view on the origin and evolution of the system. Weestimate the ages of the most recent star formation bursts of thesystem, finding an excellent consistency among the values obtained withdifferent indicators and starburst models. We find that member F hoststhe youngest starburst of the group, showing a substantial population ofWolf-Rayet stars. The chemical abundances are fairly similar in all themembers of the group despite their very different absolute magnitudes.We argue that the use of traditional metallicity-luminosity relationsbased on the absolute B-magnitude is not appropriate for dwarf starburstgalaxies, because their luminosity is dominated by the transientcontribution of the starburst to the blue luminosity. We think thatmembers E and F of the group are candidate tidal dwarf galaxies becauseof their high metallicity, their kinematics, and the absence ofunderlying old stellar populations. Finally, we propose that HCG 31 issuffering several almost simultaneous interaction processes. The mostrelevant of these processes are (1) the merging of members A and C,which would have produced two optical tidal tails, and (2) a fly-byencounter between G and the A+C complex, which would have produced an HI tidal tail from the stripping of the external gas of A+C, from whichmembers F and E have originated.ID="FN1"> 1Based on observations made with several telescopesoperated on the islands of La Palma and Tenerife by the Isaac NewtonGroup of Telescopes, Nordic Optical Telescope and Instituto deAstrofísica de Canarias in the Spanish observatories of Roque deLos Muchachos and Teide of the Instituto de Astrofísica deCanarias.

Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).

The Compact Group of Galaxies HCG 31 in an Early Phase of Merging
We have obtained high spectral resolution (R=45,900) Fabry-Perotvelocity maps of the Hickson compact group HCG 31 in order to revisitthe important problem of the merger nature of the central object A+C andto derive the internal kinematics of the candidate tidal dwarf galaxiesin this group. Our main findings are as follows: (1) double kinematiccomponents are present throughout the main body of A+C, which stronglysuggests that this complex is an ongoing merger; (2) regions A2 and E,to the east and south of complex A+C, present rotation patterns withvelocity amplitudes of ~25 km s-1, and they counterrotatewith respect to A+C; and (3) region F, which was previously thought tobe the best example of a tidal dwarf galaxy in HCG 31, presents norotation and negligible internal velocity dispersion, as is also thecase for region A1. HCG 31 presents an undergoing merger in its center(A+C), and it is likely that it has suffered additional perturbationsdue to interactions with the nearby galaxies B, G, and Q.Based on observations collected at the European Southern Observatory andGemini North Telescope (project GN-2003-Q-12).

Metal Enrichment in Near-Infrared Luminous Galaxies at z~2: Signatures of Proto-elliptical Galaxies?
We present the analysis of the co-added rest-frame UV spectrum (1200Å

NGC 3125-1: The Most Extreme Wolf-Rayet Star Cluster Known in the Local Universe
We use Space Telescope Imaging Spectrograph long-slit ultravioletspectroscopy of local starburst galaxies to study the massive starcontent of a representative sample of super-star clusters, with aprimary focus on their Wolf-Rayet (W-R) content as measured from the HeII λ1640 emission feature. The goals of this work are threefold.First, we quantify the W-R and O-star content for selected massive youngstar clusters. These results are compared with similar estimates madefrom optical spectroscopy available in the literature. We conclude thatthe He II λ4686 equivalent width is a poor diagnostic measure ofthe true W-R content. Second, we present the strongest known He IIλ1640 emission feature in a local starburst galaxy. This featureis clearly of stellar origin in the massive cluster NGC 3125-1, as it isbroadened (~1000 km s-1). Strong N IV λ1488 and N Vλ1720 emission lines commonly found in the spectra of individualW-R stars of WN subtype are also observed in the spectrum of NGC 3125-1.Finally, we create empirical spectral templates to gain a basicunderstanding of the recently observed strong He II λ1640 featureseen in Lyman break galaxies (LBGs) at redshifts z~3. The UV fieldobserved in local starbursts provides a good overall match to thecontinuum and weak photospheric features in LBGs in the spectral rangeλλ1300-1700 but cannot reproduce the He II λ1640emission seen in the composite LBG sample of Shapley et al. Anadditional (ad hoc) 10%-15% contribution from ``extreme'' W-R clusterssimilar to NGC 3125-1 on top of the field provides a good match to thestrength of this feature.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 NAS5-26555.

Systematic Effects and a New Determination of the Primordial Abundance of 4He and dY/dZ from Observations of Blue Compact Galaxies
We use spectroscopic observations of a sample of 82 H II regions in 76blue compact galaxies to determine the primordial helium abundanceYp and the slope dY/dZ from the Y-O/H linear regression. Toimprove the accuracy of the dY/dZ measurement, we have included newspectrophotometric observations of 33 H II regions that span a largemetallicity range, with oxygen abundance 12+log(O/H) varying between7.43 and 8.30 (Zsolar/30<=Z<=Zsolar/4). Mostof the new galaxies were selected from the First Byurakan, theHamburg/SAO, and the University of Michigan objective prism surveys. Fora subsample of seven H II regions, we derive the He mass fraction takinginto account known systematic effects, including collisional andfluorescent enhancements of He I emission lines, collisional excitationof hydrogen emission, underlying stellar He I absorption, and thedifference between the temperatures Te(He II) in theHe+ zone and Te(O III) derived from thecollisionally excited [O III] lines. We find that the net result of allthe systematic effects combined is small, changing the He mass fractionby less than 0.6%. By extrapolating the Y versus O/H linear regressionto O/H=0 for seven H II regions of this subsample, we obtainYp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds todY/dZ=3.7+/-1.2, assuming the oxygen mass fraction to be O=0.66Z. In theframework of the standard big bang nucleosynthesis theory, thisYp corresponds toΩbh2=0.012+0.003-0.002,where h is the Hubble constant in units of 100 km s-1Mpc-1. This is smaller at the 2 σ level than the valueobtained from recent deuterium abundance and microwave backgroundradiation measurements. The linear regression slope dY/dO=4.3+/-0.7(corresponding to dY/dZ=2.8+/-0.5) for the whole sample of 82 H IIregions is similar to that derived for the subsample of seven H IIregions, although it has a considerably smaller uncertainty.

The Relation between Galaxy Activity and the Dynamics of Compact Groups of Galaxies
Using a sample of 91 galaxies distributed over 27 compact groups (CGs)of galaxies, we define an index that allows us to quantify their levelof activity due to an active galactic nucleus (AGN) or star formation.By combining the mean activity index with the mean morphological type ofthe galaxies in a group, we are able to quantify the evolutionary stateof the groups. We find that they span an evolutionary sequence thatcorrelates with the spatial configuration of the galaxies in the CG. Wedistinguish three main configuration types: A, B, and C. Type A CGs showpredominantly low velocity dispersions and are rich in late-type spiralsthat show active star formation or harbor an AGN. Type B groups haveintermediate velocity dispersions and contain a large fraction ofinteracting or merging galaxies. Type C comprises CGs with high velocitydispersions, which are dominated by elliptical galaxies that show noactivity. We suggest that evolution proceeds A==>B==>C. Mappingthe groups with different evolution levels in a diagram of radius versusvelocity dispersion does not reveal the pattern expected based on theconventional fast merger model for CGs, which predicts a direct relationbetween these two parameters. Instead, we observe a trend contrary toexpectation: the evolutionary state of a group increases with velocitydispersion. This trend seems to be related to the masses of thestructures in which CGs are embedded. In general, the evolutionary stateof a group increases with the mass of the structure. This suggestseither that galaxies evolve more rapidly in massive structures or thatthe formation of CGs embedded in massive structures predated theformation of CGs associated with lower mass systems. Our observationsare consistent with the structure formation predicted by the CDM model(or ΛCDM), only if the formation of galaxies is a biased process.

HS 0837+4717 - a metal-deficient blue compact galaxy with large nitrogen excess
We present the results of high S/N long-slit spectroscopy with theMultiple Mirror and the SAO 6-m telescopes, optical imaging with theWise 1-m telescope and H I observations with the Nançay RadioTelescope of the very metal-deficient (12+ log (O/H)=7.64) luminous(MB= -18.1m) blue compact galaxy (BCG) HS 0837+4717. The bluebump near λ4650, characteristic of WR stars is detected in thecentral supergiant H II region, as well as the barely seen red bump atλ5808. The derived number of WR stars in the region of thecurrent starburst is ˜1000. Evidence for fast motions in this regionis also seen as broad, low-contrast components in the Hα, Hβand strong [O III] lines λλ4959,5007. While the extinctionof the narrow emission lines from the supergiant H II region is low, thevery large Balmer decrement of the broad components suggests that thepart of current starburst is highly obscured by dust. Abundance ratiosX/O for X=Ne, Ar, S, Fe and Cl in the supergiant H II region are in goodagreement with the mean values of other very metal-deficient BCGs.Nitrogen, however, is overabundant by a factor of ˜6. This impliesan unusually efficient N enrichment in HS 0837+4717, and probably, anon-typical evolution scenario. The Hα-line position-velocity(P-V) diagrams for directions approximately along the major and minoraxes reveal disturbed motions of the ionized gas, mainly in peripheralregions. The SW part of the major axis P-V diagram looks like a rotationcurve, with the velocity amplitude Vrot ˜50-70 kms-1 at r =4.3 kpc. Its NE part displays, however, strongdeviations, indicating either counter-rotation, or a strongoutflow/supershell. If it is considered as indicating a shell-likefeature its velocity amplitude of ˜70 km s-1 (relative tothe extrapolated rotation curve), and the apparent extent of ˜4arcsec (3.3 kpc) imply a dynamical age of ˜14 Myr and the fullenergetic equivalent of ˜2.3× 104 SNe. The latterindicates continuing starbursts during at least this time interval. Thelong-slit spectra reveal a complex morphology for this galaxy. Itconsists of two compact regions at a distance of ˜2 kpc. Theircontinuum flux differs by a factor of four. The brightest one is relatedto the current starburst with the age of ˜3.7 Myr. The slightlyredder fainter component could be an older starburst (˜25 Myr). TheWise 1-m telescope UBVR integrated photometry reveals a high opticalluminosity for this BCG, and the unusual (B-V) and (V-R) colours. Themorphology of HS 0837+4717 is highly disturbed, with two small tailsemerging to NNW and SSE. Such a disturbed overall morphology, a``double-nucleus" structure, significantly disturbed velocities ofionized gas, together with the very high power of the starburst suggestsa possible explanation of the object as a recent merger. We compare theproperties of this BCG and of similar objects known in the literature,and conclude that their high nitrogen excess is most probably related tothe short phase of a powerful starburst when many WR stars contribute tothe enrichment of ISM.Tables 2-4 are also available in electronic form at the CDS viaanonymous ftp tp cdsarc.u-strasbg.fr ( or via http: / /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/A+A/419/469

Starbursts in barred spiral galaxies. VI. HI observations and the K-band Tully-Fisher relation
This paper reports a study of the effect of a bar on the neutralhydrogen (HI) content of starburst and Seyfert galaxies. We also makecomparisons with a sample of ``normal'' galaxies and investigate howwell starburst and Seyfert galaxies follow the fundamental scalingTully-Fisher (TF) relation defined for normal galaxies. 111 Markarian(Mrk) IRAS galaxies were observed with the Nançay radiotelescope,and HI data were obtained for 80 galaxies, of which 64 are newdetections. We determined the (20 and 50%) linewidths, the maximumvelocity of rotation and total HI flux for each galaxy. Thesemeasurements are complemented by data from the literature to form asample of Mrk IRAS (74% starburst, 23% Seyfert and 3% unknown) galaxiescontaining 105 unbarred and 113 barred ones. Barred galaxies have lowertotal and bias-corrected HI masses than unbarred galaxies, and this istrue for both Mrk IRAS and normal galaxies. This robust result suggeststhat bars funnel the HI gas toward the center of the galaxy where itbecomes molecular before forming new stars. The Mrk IRAS galaxies havehigher bias-corrected HI masses than normal galaxies. They also showsignificant departures from the TF relation, both in the B and K bands.The most deviant points from the TF relation tend to have a strongfar-infrared luminosity and a low oxygen abundance. These resultssuggest that a fraction of our Mrk IRAS galaxies are still in theprocess of formation, and that their neutral HI gas, partly of externalorigin, has not yet reached a stationary state.Based on observations obtained at the large radiotelescope ofObservatoire de Nançay, operated by Observatoire de Paris.Tables 5 and 6 are only (and Table 4 also) available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/515

On the formation of star clusters in the merger NGC 6240
We identified star clusters in archived Hubble Space Telescope/WideField Planetary Camera 2 (HST/WFPC2) images of the merger andultraluminous infrared galaxy NGC 6240, with the aim of investigatingwhether star cluster properties (luminosity, age and mass) in such anextreme environment differ from those of clusters in less luminousstarburst galaxies. We found 54 star clusters in all of the F450W, F547Mand F814W exposures, of which 41 are located in the main body of NGC6240 and 13 are located in the galactic tails. Given that only twocolours are available to derive two independent variables (clusterreddening and age), we adopted an ad hoc procedure to derive clusterparameters statistically under the assumption that the clustermetallicity is like that in the Large Magellanic Cloud. The colours ofeach cluster are fitted to STARBURST99 models of fixed mass and variableages and reddenings. All cluster reddening and age solutions withχ2 < 1 are considered to be consistent with the data.Masses are derived by scaling the luminosity of the models withbest-fitting χ2 < 1 by the observed V luminosity,after correction for reddening and distance. Therefore, each cluster isdescribed by a range of reddening values, ages and masses; for each ofthese parameters, we derive probability functions. Thus we infer thatthe most probable age of the observed clusters is between 5 and 13 Myrand their most probable mass is about (1-2) × 105Msolar. A low probability exists for clusters as massive as108 Msolar, as well as for the trend that the meancluster mass increases towards the double nuclei of NGC 6240. Comparisonwith star clusters in starburst galaxies seems to point to more massiveclusters being formed in more massive galaxies and gas-rich mergers,while the overall cluster mass distribution might be relativelyindependent of the details of the associated starburst where dense,massive clusters preferentially form.

The PDS versus Markarian starburst galaxies: comparing strong and weak IRAS emitter at 12 and 25 μm in the nearby Universe
The characteristics of the starburst galaxies from the Pico dos Diassurvey (PDS) are compared with those of the nearby ultraviolet (UV)bright Markarian starburst galaxies, having the same limit in redshift(vh < 7500 km s-1) and absolute B magnitude(MB < -18). An important difference is found: theMarkarian galaxies are generally undetected at 12 and 25 μm in IRAS.This is consistent with the UV excess shown by these galaxies andsuggests that the youngest star-forming regions dominating thesegalaxies are relatively free of dust.The far-infrared selection criteria for the PDS are shown to introduce astrong bias towards massive (luminous) and large size late-type spiralgalaxies. This is contrary to the Markarian galaxies, which are found tobe remarkably rich in smaller size early-type galaxies. These resultssuggest that only late-type spirals with a large and massive disc arestrong emitters at 12 and 25 μm in IRAS in the nearby Universe.The Markarian and PDS starburst galaxies are shown to share the sameenvironment. This rules out an explanation of the differences observedin terms of external parameters. These differences may be explained byassuming two different levels of evolution, the Markarian being lessevolved than the PDS galaxies. This interpretation is fully consistentwith the disc formation hypothesis proposed by Coziol et al. to explainthe special properties of the Markarian SBNG.

The ionized gas in and around the Wolf-Rayet galaxy NGC 1741
Interaction can be the main mechanism that triggers star formation inWolf-Rayet galaxies. The compact group HCG 31 has one of this objects,NGC 1741. We present new observations of this compact interactinggroup, in order to analyse the physical conditions and chemicalcomposition of the ionized gas, as well as to estimate the ages of thestarbursts.

COLA. II. Radio and Spectroscopic Diagnostics of Nuclear Activity in Galaxies
We present optical spectroscopic observations of 93 galaxies taken fromthe infrared-selected COLA (compact objects in low-power AGNs) sample.These are all galaxies for which we have previously obtainedlow-resolution radio observations and high-resolution (<0.05")Australian Long Baseline Array snapshots. The sample spans the range offar-IR luminosities from normal galaxies to luminous infrared galaxiesand contains a significant number of galaxies involved in galaxy-galaxyinteractions. Of the galaxies observed, 78 (84%) exhibit emission linesindicating that they are either AGNs or actively forming stars(starburst galaxies). Using a newly developed, theoretically based,optical emission line scheme to classify the spectra, we find that 15%of the emission-line galaxies are Seyfert galaxies, 77% are starbursts,and the rest are either borderline AGN/starburst or show ambiguouscharacteristics. We find little evidence for an increase in the fractionof AGNs in the sample as a function of far-IR (FIR) luminosity, incontrast to previous studies, but our sample covers only a small rangein infrared luminosity(1010.5Lsolar<=LFIR<=1011.7 Lsolar), and thus a weak trend may be masked. Instead,as the infrared luminosity increases, so does the fraction of metal-richstarbursts, objects that on more traditional diagnostic diagrams wouldhave been classified as weak, low-ionization, narrow emission lineregions. As a whole the Seyfert galaxies exhibit a small, butstatistically significant, radio excess on the radio-FIR correlationcompared to the galaxies classified as starbursts. Compact (<0.05")radio cores are detected in 55% of the Seyfert galaxies, and thesegalaxies exhibit a significantly larger radio excess than the Seyfertgalaxies in which compact cores were not detected. Our results indicatethat there may be two distinct populations of Seyfert galaxies,``radio-excess'' Seyfert galaxies, which exhibit extended radiostructures and compact radio cores, and ``radio-quiet'' Seyfertgalaxies, in which the majority of the radio emission can be attributedto star formation in the host galaxy. No significant difference is seenbetween the IR and optical spectroscopic properties of Seyfert galaxieswith and without radio cores.

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Right ascension:05h01m38.30s
Aparent dimensions:1.259′ × 0.646′

Catalogs and designations:
Proper Names   (Edit)
NGC 2000.0NGC 1741

→ Request more catalogs and designations from VizieR