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HI content in galaxies in loose groups
Gas deficiency in cluster spirals is well known and ram-pressurestripping is considered the main gas removal mechanism. In some compactgroups too gas deficiency is reported. However, gas deficiency in loosegroups is not yet well established. Lower dispersion of the membervelocities and the lower density of the intragroup medium in small loosegroups favour tidal stripping as the main gas removal process in them.Recent releases of data from the HI Parkes All-Sky Survey (HIPASS) andcatalogues of nearby loose groups with associated diffuse X-ray emissionhave allowed us to test this notion. In this paper, we address thefollowing questions: (i) do galaxies in groups with diffuse X-rayemission statistically have lower gas content compared to the ones ingroups without diffuse X-ray emission? (ii) does HI deficiency vary withthe X-ray luminosity, LX, of the loose group in a systematicway? We find that (i) galaxies in groups with diffuse X-ray emission, onaverage, are HI deficient, and have lost more gas compared to those ingroups without X-ray emission; the latter are found not to havesignificant HI deficiency; (ii) no systematic dependence of the HIdeficiency with LX is found. Ram-pressure-assisted tidalstripping and evaporation by thermal conduction are the two possiblemechanisms to account for this excess gas loss.
| The effect of gravitational recoil on black holes forming in a hierarchical universe
Galactic bulges are known to harbour central black holes whose mass istightly correlated with the stellar mass and velocity dispersion of thebulge. In a hierarchical universe, galaxies are built up throughsuccessive mergers of subgalactic units, a process that is accompaniedby the amalgamation of bulges and the likely coalescence ofgalactocentric black holes. In these mergers, the beaming ofgravitational radiation during the plunge phase of the black holecollision can impart a linear momentum kick or `gravitational recoil' tothe remnant. If large enough, this kick will eject the remnant from thegalaxy entirely, and populate intergalactic space with wandering blackholes. Using a semi-analytic model of galaxy formation, we investigatethe effect of black hole ejections on the scatter of the relationbetween black hole and bulge mass. We find that while not being thedominant source of the measured scatter, they do provide a significantcontribution and may be used to set a constraint, vkick<~500kms-1, on the typical kick velocity, in agreement withvalues found from general relativistic calculations. Even for the moremodest kick velocities implied by these calculations, we find that asubstantial number of central black holes are ejected from theprogenitors of present-day galaxies, giving rise to a population ofwandering intrahalo and intergalactic black holes whose distribution weinvestigate in high-resolution N-body simulations of the Milk Way masshaloes. We find that intergalactic black holes make up only ~2-3 percent of the total galactic black hole mass but, within a halo, wanderingblack holes can contribute up to about half of the total black hole massorbiting the central galaxy. Intrahalo black holes offer a naturalexplanation for the compact X-ray sources often seen near the centres ofgalaxies and for the hyperluminous non-central X-ray source in M82.
| Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| The impact of the visibility of the [OIII]λ4363 line on the general properties of HII galaxies in the Local Universe
We present a statistical study of a very large sample of HII galaxiestaken from the literature. We focus on the differences in severalproperties between galaxies that show the auroral line[OIII]λ4363 and those that do not present this feature in theirspectra. It turns out that objects without this auroral line are moreluminous, are more metal-rich and present a lower ionization degree. Theunderlying population is found to be much more important for objectswithout the [OIII]λ4363 line, and the effective temperature ofthe ionizing star clusters of galaxies not showing the auroral line isprobably lower. We also study the subsample of HII galaxies whoseproperties most closely resemble the properties of theintermediate-redshift population of luminous compact blue galaxies(LCBGs). The objects from this subsample are more similar to the objectsnot showing the [OIII]λ4363 line. It might therefore be expectedthat the intermediate-redshift population of LCBGs is powered by verymassive, yet somewhat aged, star clusters. The oxygen abundance of LCBGswould be greater than the average oxygen abundance of local HIIgalaxies.
| 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.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| A 62 Day X-Ray Periodicity and an X-Ray Flare from the Ultraluminous X-Ray Source in M82
In 240 days of X-ray monitoring of M82, we have discovered an X-rayperiodicity at 62.0+/-2.5 days with a peak-to-peak amplitudecorresponding to an isotropic luminosity of 2.4×1040ergs s-1 in M82 and an X-ray flare reaching a peak luminosityof 9.8×1040 ergs s-1. The periodicity andflare likely originate from the ultraluminous X-ray source (ULX) in M82,which has been identified as a possible intermediate-mass black hole. Wesuggest that the 62 day modulation is due to orbital motion within anX-ray binary with a Roche lobe overflowing companion star, which wouldimply that the average density of the companion star is near5×10-5 g cm-3 and is therefore a giant orsupergiant. Chandra observations just after the flare show an energyspectrum that is consistent with a power law with no evidence of athermal component or line emission. Radio observations made with the VLAduring the flare allow us to rule out a blazar identification for thesource and place strong constraints on relativistically beamed models ofthe X-ray emission. The Chandra observations reveal that a second X-raysource reached a flux of 4.4×10-12 ergs cm-2s-1 in the 0.3-7 keV band, which is dramatically higher thanany flux previously seen from this source and corresponds to anisotropic luminosity of 1.1×1040 ergs s-1.This source is a second ultraluminous X-ray source in M82 and may giverise to the QPOs detected from the central region of M82.
| Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations
We use very deep Spitzer MIPS 24 μm observations to examine thebolometric luminosities (Lbol) and UV extinction propertiesof more than 200 spectroscopically identified, optically selected(UnGR) z~2 galaxies, supplemented with near-IR-selected(``BzK'' and ``DRG'') and submillimeter galaxies at similar redshifts,in the GOODS-N field. Focusing on redshifts 1.51012 Lsolar, with a mean~=2×1011 Lsolar. Using24 μm observations as an independent probe of dust extinction, wefind that, as in the local universe, the obscurationLIR/L1600 is strongly dependent on Lboland ranges in value from <1 to ~1000 within the sample considered.However, the obscuration is generally ~10 times smaller at a givenLbol at z~2 than at z~0. We show that the values ofLIR and obscuration inferred from the UV spectral slopeβ generally agree well with the values inferred fromL5-8.5μm for Lbol<1012Lsolar. Using the specific SFRs of galaxies as a proxy forcold gas fraction, we find a wide range in the evolutionary state ofgalaxies at z~2, from galaxies that have just begun to form stars tothose that have already accumulated most of their stellar mass and areabout to become, or already are, passively evolving.Based, in part, on data obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation. Alsobased in part on observations made with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with NASA.
| 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 Polycyclic Aromatic Hydrocarbon Emission Deficit in Low-Metallicity Galaxies-A Spitzer View
Archival observations of 18 starburst galaxies that span a wide range inmetallicity reveal for the first time a correlation between the ratio ofemission-line fluxes of [Fe II] at 26 μm and [Ne II] at 12.8 μmand the 7.7 μm PAH strength, with the [Fe II]/[Ne II] flux ratiodecreasing with increasing PAH strength. We also find a strongcorrelation between the [Fe II]/[Ne II] flux ratio and the host galaxymetallicity, with the flux ratio decreasing with increasing metallicity.Since [Fe II] emission has been linked primarily to supernova shocks, weattribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies toenhanced supernova activity. We consider this to be a dominant mechanismfor PAH destruction, rather than grain destruction in photoionizedregions surrounding young massive stars. We also consider whether theextreme youth of the low-metallicity galaxies is responsible for thelack of PAH emission.
| IRS Spectra of Two Ultraluminous Infrared Galaxies at z = 1.3
We present low-resolution (6420) galaxies in the NDWFS. Its mid-infraredspectrum lacks emission features, but the broad 9.7 μm silicateabsorption band places this source at z~1.3. Optical spectroscopyconfirms a redshift of z=1.293+/-0.001. Given this redshift, SST24J142827.19+354127.71 has among the largest rest-frame 5 μmluminosities known. The similarity of its SED to those of knownAGN-dominated ULIRGs and its lack of either PAH features or largeamounts of cool dust indicate that the mid-infrared emission isdominated by an AGN rather than a starburst.
| First Detection of PAHs and Warm Molecular Hydrogen in Tidal Dwarf Galaxies
We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope.We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2,7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of~100 carbon atoms with an equal mixture of neutral and ionized PAHs. TheTDGs have a dominant warm (~140 K) dust component in marked contrast tothe cooler (40-60 K) dust found in starburst galaxies. For the firsttime we detect the low-J rotational lines from molecular hydrogen.Adopting LTE, there is ~105 Msolar of ~400 K gas,which is <0.1% of the cold gas mass inferred from 12CO(1-0) measurements. The combination of one-third solar metallicity witha recent (<5 million year) episode of star formation is reflected inthe S and Ne ratios. The excitation is higher than typical values forstarburst galaxies and similar to that found in BCDs. Using the InfraredArray Camera, we identify an additional 13 PAH-rich candidate TDGs.These sources occupy a distinct region of IRAC color space with[3.6]-[4.5]<0.4 and [4.5]-[8.0]>3.2. Their disturbed morphologiessuggest past merger events between companions; for example, NGC 5291 Shas a projected 11 kpc tail. NGC 5291 N and S have stellar masses of(1.5 and 3.0)×108 Msolar, which iscomparable to BCDs, although still roughly 10% of the LMC's stellarmass. The candidate TDGs are an order of magnitude less massive. Thissystem appears to be a remarkable TDG nursery.
| Mid-Infrared Properties of Low-Metallicity Blue Compact Dwarf Galaxies from the Spitzer Infrared Spectrograph
We present a Spitzer-based mid-infrared (MIR) study of a large sample ofblue compact dwarfs (BCDs) using the Infrared Spectrograph (IRS),including the first MIR spectrum of I Zw 18, the archetype for the BCDclass and among the most metal-poor galaxies known. We show the spectraof polycyclic aromatic hydrocarbon (PAH) emission in a low-metallicityenvironment. We find that the equivalent widths (EWs) of PAHs at 6.2,7.7, 8.6, and 11.2 μm are generally weaker in BCDs than in typicalstarburst galaxies and that the fine-structure line ratio, [Ne III]/[NeII], has a weak anticorrelation with the PAH EW. A much strongeranticorrelation is shown between the PAH EW and the product of the [NeIII]/[Ne II] ratio and the UV luminosity density divided by themetallicity. We conclude that the PAH EW in metal-poor high-excitationenvironments is determined by a combination of PAH formation anddestruction effects.
| Silicate Emission in the Spitzer IRS Spectrum of FSC 10214+4724
We present the first mid-infrared (MIR) spectrum of the z=2.2856ultraluminous infrared galaxy FSC 10214+4724, obtained with the InfraredSpectrograph (IRS) on board the Spitzer Space Telescope. The spectrumspans a rest-wavelength range of 2.3-11.5 μm, covering a number ofkey diagnostic emission and absorption features. The most prominentfeature in the IRS spectrum is the silicate emission at rest-frame ~10μm. We also detect an unresolved emission line at a rest wavelengthof 7.65 μm that we identify with [Ne VI], and a slightly resolvedfeature at 5.6 μm identified as a blend of [Mg VII] and [Mg V]. Thereare no strong PAH emission features in the FSC 10214+4724 spectrum. Weplace a limit of 0.1 μm on the equivalent width of 6.2 μm PAHemission but see no evidence of a corresponding 7.7 μm feature.Semiempirical fits to the spectral energy distribution suggest that ~45%of the bolometric luminosity arises from cold (~50 K) dust, half arisesfrom warm (190 K) dust, and the remainder, ~5%, originates from hot(~640 K) dust. The hot dust is required to fit the blue end of the steepMIR spectrum. The combination of a red continuum, strong silicateemission, little or no PAH emission, and no silicate absorption makesFSC 10214+4724 unlike most other ULIRGs or AGNs observed thus far withthe IRS. These apparently contradictory properties may be explained byan AGN that is highly magnified by the lens, masking a (dominant)overlying starburst with unusually weak PAH emission.
| Late-Time Radio Observations of 68 Type Ibc Supernovae: Strong Constraints on Off-Axis Gamma-Ray Bursts
We present late-time radio observations of 68 local Type Ibc supernovae,including six events with broad optical absorption lines(``hypernovae''). None of these objects exhibit radio emissionattributable to off-axis gamma-ray burst jets spreading into our line ofsight. Comparison with our afterglow models reveals the followingconclusions. (1) Less than ~10% of Type Ibc supernovae are associatedwith typical gamma-ray bursts initially directed away from our line ofsight; this places an empirical constraint on the GRB beaming factor of<~104, corresponding toan average jet opening angle, θj>~0.8d. (2) Thisholds in particular for the broad-lined supernovae (SNe 1997dq, 1997ef,1998ey, 2002ap, 2002bl, and 2003jd), which have been argued to host GRBjets. Our observations reveal no evidence for typical (or evensubenergetic) GRBs and rule out the scenario in which every broad-linedSN harbors a GRB at the 84% confidence level. Their large photosphericvelocities and asymmetric ejecta (inferred from spectropolarimetry andnebular spectroscopy) appear to be characteristic of the nonrelativisticSN explosion and do not necessarily imply the existence of associatedGRB jets.
| Redshifts from Spitzer Spectra for Optically Faint Radio-selected Infrared Sources
Spectra have been obtained with the Infrared Spectrograph on the SpitzerSpace Telescope for 18 optically faint sources (R>~23.9 mag) havingfν(24 μm)>1.0 mJy and having radio detections at 20cm to a limit of 115 μJy. The sources are within the Spitzer FirstLook Survey. Redshifts are determined for 14 sources from strongsilicate absorption features (12 sources) or strong PAH emissionfeatures (two sources), with median redshift of 2.1. Results confirmthat optically faint sources of ~1 mJy at 24 μm are typically atredshifts z~2, verifying the high efficiency in selecting high-redshiftsources based on extreme infrared-to-optical flux ratio, and indicatethat 24 μm sources that also have radio counterparts are notsystematically different than samples chosen only by theirinfrared-to-optical flux ratios. Using the parameterq=log[fν(24 μm)/fν(20 cm)], 17 of the 18sources observed have values of 0~1.0 mJy are predominately AGNs andrepresent the upper end of the luminosity function of dusty sources atz~2. Based on the characteristics of the sources observed so far, wepredict that the nature of sources selected at 24 μm will change forfν(24 μm)<~0.5 mJy to sources dominated primarily bystarbursts.
| Stellar Abundances and Molecular Hydrogen in High-Redshift Galaxies: The Far-Ultraviolet View
FUSE spectra of star-forming regions in nearby galaxies are compared tocomposite spectra of Lyman break galaxies (LBGs), binned by strength ofLyα emission and by mid-UV luminosity. Several far-UV spectralfeatures, including lines dominated by stellar wind and by photosphericcomponents, are very sensitive to stellar abundances. Their measurementin LBGs is compromised by the strong interstellar absorption features,allowing in some cases only upper limits to be determined. The derived Cand N abundances in the LBGs are no higher than half solar (scaled tooxygen abundance for comparison with emission-line analyses),independent of the strength of Lyα emission. P V absorptionindicates abundances as low as 0.1 solar, with an upper limit near 0.4solar in the reddest and weakest emission galaxies. Unresolvedinterstellar absorption components would further lower the derivedabundances. Trends of line strength and derived abundances are strongerwith mid-UV luminosity than with Lyα strength. H2absorption in the Lyman and Werner bands is very weak in the LBGs.Template H2 absorption spectra convolved to the appropriateresolution show that strict upper limitsN(H2)<1018 cm-2 apply in all cases,with more stringent values appropriate for the stronger emissioncomposites and for mixes of H2 level populations like thoseon Milky Way sight lines. Since the UV-bright regions are likely to bewidespread in these galaxies, these results rule out massive diffusereservoirs of primordial H2 and suggest that the dust-to-gasratio is already fairly large at z~3.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by The JohnsHopkins University under NASA contract NAS5-32985.
| Absolute Magnitude Distributions and Light Curves of Stripped-Envelope Supernovae
The absolute visual magnitudes of three Type IIb, 11 Type Ib, and 13Type Ic supernovae (collectively known as stripped-envelope supernovae)are studied by collecting data on the apparent magnitude, distance, andinterstellar extinction of each event. Weighted and unweighted meanabsolute magnitudes of the combined sample, as well as various subsetsof the sample, are reported. The limited sample size and theconsiderable uncertainties, especially those associated with extinctionin the host galaxies, prevent firm conclusions regarding differencesbetween the absolute magnitudes of supernovae of Types Ib and Ic, andregarding the existence of separate groups of overluminous andnormal-luminosity stripped-envelope supernovae. The spectroscopiccharacteristics of the events of the sample are considered. Three of thefour overluminous events are known to have had unusual spectra. Most butnot all of the normal-luminosity events have had typical spectra. Thelight curves of stripped-envelope supernovae are collected and compared.Because SN 1994I in M51 was very well observed, it often is regarded asthe prototypical Type Ic supernova, but it has the fastest light curvein the sample. Light curves are modeled by means of a simple analyticaltechnique that, combined with a constraint on E/M from spectroscopy,yields internally consistent values of ejected mass, kinetic energy, andnickel mass.
| A Chandra ACIS Study of 30 Doradus. I. Superbubbles and Supernova Remnants
We present an X-ray tour of diffuse emission in the 30 Doradusstar-forming complex in the Large Magellanic Cloud using high spatialresolution X-ray images and spatially resolved spectra obtained with theAdvanced CCD Imaging Spectrometer on board the Chandra X-RayObservatory. The dominant X-ray feature of the 30 Doradus nebula is theintricate network of diffuse emission generated by interacting stellarwinds and supernovae working together to create vast superbubbles filledwith hot plasma. We construct maps of the region showing variations inplasma temperature (T=3-9 million degrees), absorption[NH=(1-6)×1021 cm-2], andabsorption-corrected X-ray surface brightness[SX=(3-126)×1031 ergs s-1pc-2]. Enhanced images reveal the pulsar wind nebula in thecomposite supernova remnant N157B, and the Chandra data show spectralevolution from nonthermal synchrotron emission in the N157B core to athermal plasma in its outer regions. In a companion paper we show thatR136, the central massive star cluster, is resolved at the arcsecondlevel into almost 100 X-ray sources. Through X-ray studies of 30 Doradusthe complete life cycle of such a massive stellar cluster can berevealed.
| Examining the Seyfert-Starburst Connection with Arcsecond-Resolution Radio Continuum Observations
We compare the arcsecond-scale circumnuclear radio continuum propertiesof five Seyfert and five starburst galaxies, concentrating on the searchfor any structures that could imply a spatial or causal connectionbetween the nuclear activity and a circumnuclear starburst ring. Noevidence is found in the radio emission for a link between thetriggering or feeding of nuclear activity and the properties ofcircumnuclear star formation. Conversely, there is no clear evidence ofnuclear outflows or jets triggering activity in the circumnuclear ringsof star formation. Interestingly, the difference in the angle betweenthe apparent orientation of the most elongated radio emission and theorientation of the major axis of the galaxy is on average larger inSeyfert galaxies than in starburst galaxies, and Seyfert galaxies appearto have a larger physical size scale of the circumnuclear radiocontinuum emission. The concentration, asymmetry, and clumpinessparameters of radio continuum emission in Seyfert galaxies andstarbursts are comparable, as are the radial profiles of radio continuumand near-infrared line emission. The circumnuclear star formation andsupernova rates do not depend on the level of nuclear activity. Theradio emission usually traces the near-infrared Brγ andH2 1-0 S(1) line emission on large spatial scales, butlocally their distributions are different, most likely because of theeffects of varying local magnetic fields and dust absorption andscattering.
| Remarkable Disk and Off-Nuclear Starburst Activity in the Tadpole Galaxy as revealed by the Spitzer Space Telescope
We present ground-based optical and Spitzer Space Telescope infraredimaging observations of the interacting galaxy UGC 10214, the Tadpolegalaxy (z=0.0310), focusing on the star formation activity in thenuclear, disk, spiral arms, and tidal tail regions. The ground-basedoptical data set spans a wavelength range between 0.3 and 0.8 μm, thenear-IR data set spans 1-2.2 μm, and the Spitzer IR data set spans3-70 μm. The major findings of this study are that the Tadpole isactively forming stars in the main disk outside of the nucleus and inthe tidal plume, with an estimated mean star formation rate of ~2-4Msolar yr-1. The most prominent sites of mid-IRemission define a ``ring'' morphology that, combined with the overallmorphology of the system, suggests the interaction may belong to therare class of off-center collisional ring systems that form bothshock-induced rings of star formation and tidal plumes. In starkcontrast to the disk star formation, the nuclear emission is solelypowered by older stars, with little evidence for ongoing star formationat the center of the Tadpole. Extranuclear star formation accounts for>50% of the total star formation in the disk and spiral arms,featuring infrared-bright ``hot spots'' that exhibit strong polycyclicaromatic hydrocarbon (PAH) emission, the band strength of which iscomparable to that of late-type star-forming disk galaxies. The tidaltail, which extends 2' (~75 kpc) into the intergalactic medium, ispopulated by supermassive star clusters, M~106Msolar, likely triggered by the galaxy-galaxy interactionthat has distorted UGC 10214 into its current ``tadpole'' shape. TheTadpole is therefore an example of an off-nuclear or tidal-tailstarburst, with several large sites of massive star formation in thedisk and in the plume, including the most prominent Hubble SpaceTelescope-revealed cluster, J160616.85+552640.6. The clusters exhibitremarkable IR properties, including exceptionally strong 24 μmemission relative to the underlying starlight, hot dust continuum, andPAH emission, with an estimated current star formation rate of ~0.1-0.4Msolar yr-1, representing >10% of the totalstar formation in the system. We estimate the mass of the largestcluster to be ~(1.4-1.6)×106 Msolar based onthe g'-band (0.5 μm) and near-IR (2.2 μm) integrated fluxes incombination with an assumed mass-to-light ratio appropriate to youngclusters, or large enough to be classified as a nascent dwarf galaxy orglobular cluster.
| Spectroscopic properties and dynamical evolution of the merging system AM 1003-435
Aims.We study the system AM 1003-435, which iscomposed of two strong interacting galaxies. Methods: .We obtainedlong-slit optical spectra of twelve zones of the system, and performednumerical simulations of the encounter between the components followingthe evolution of their stellar and gaseous contents. Results: .Thespectrum of the NW nucleus is typical of a starburst, while that of theSE one shows weak emission lines. The highest values of the oxygen andnitrogen abundances are at the NW nucleus. Its derived Hα andHα + [N II] equivalent widths indicate very intense starformation, in accord to its starburst nature. Indicative ages of thestarbursts in the nuclei were obtained. The morphological types of bothcomponents derived from their spectral characteristics are in agreementwith previous determinations based on photometric parameters. About 70%of the measured Hα luminosity would correspond to the NW componentcontribution. The IR luminosity of the system is not high (L_IR <1011~Lȯ). The estimated star formation ratefor AM 1003-435 indicates that its activity is alsomoderate. The IR radiation, if it has the same origin as the Hαemission, would arise mostly from the NW component. The resultingabundances, burst ages, and masses suggest that the starburst in the SEcomponent, the minor one, started earlier than that of the NW one, anddid so in a medium poorer in weighted elements. On the other hand, froma set of N-body simulations of the encounter between both components itwas found the time of the perigalacticum, to be comparable to the burstage derived for the SE component, and the star formation in bothgalaxies would have begun after the perigalacticum. It was alsoestimated that the central bodies will merge in about 1 Gyr.
| The Hα Galaxy Survey . III. Constraints on supernova progenitors from spatial correlations with Hα emission
Aims.We attempt to constrain progenitors of the different types ofsupernovae from their spatial distributions relative to star formationregions in their host galaxies, as traced by Hα + [Nii] lineemission. Methods: .We analyse 63 supernovae which have occurredwithin galaxies from our Hα survey of the local Universe. Threestatistical tests are used, based on pixel statistics, Hα radialgrowth curves, and total galaxy emission-line fluxes. Results:.Many type II supernovae come from regions of low or zero emission lineflux, and more than would be expected if the latter accurately traceshigh-mass star formation. We interpret this excess as a 40% "Runaway"fraction in the progenitor stars. Supernovae of types Ib and Ic doappear to trace star formation activity, with a much higher fractioncoming from the centres of bright star formation regions than is thecase for the type II supernovae. Type Ia supernovae overall show a weakcorrelation with locations of current star formation, but there isevidence that a significant minority, up to about 40%, may be linked tothe young stellar population. The radial distribution of allcore-collapse supernovae (types Ib, Ic and II) closely follows that ofthe line emission and hence star formation in their host galaxies, apartfrom a central deficiency which is less marked for supernovae of typesIb and Ic than for those of type II. Core-collapse supernova ratesoverall are consistent with being proportional to galaxy totalluminosities and star formation rates; however, within this total thetype Ib and Ic supernovae show a moderate bias towards more luminoushost galaxies, and type II supernovae a slight bias towardslower-luminosity hosts.
| Hydrogen and helium traces in type Ib-c supernovae
Aims.To investigate the spectroscopic properties of a selected opticalphotospheric spectra of core collapse supernovae (CCSNe). Specialattention is devoted to traces of hydrogen at early phases. The impacton the physics and nature of their progenitors is emphasized.Methods: .The CCSNe-sample spectra are analyzed with the parameterizedsupernova synthetic spectrum code "SYNOW" adopting some simplifyingapproximations. Results: .The generated spectra are found to matchthe observed ones reasonably well, including a list of only 23 candidateions. Guided by SN Ib 1990I, the observed trough near 6300 Å isattributed to Hα in almost all type Ib events, although in someobjects it becomes too weak to be discernible, especially at laterphases. Alternative line identifications are discussed. Differences inthe way hydrogen manifests its presence within CCSNe are highlighted. Intype Ib SNe, the Hα contrast velocity (i.e. line velocity minusthe photospheric velocity) seems to increase with time at early epochs,reaching values as high as 8000 km s-1 around 15-20 daysafter maximum and then remains almost constant. The derived photosphericvelocities, indicate a lower velocity for type II SNe 1987A and 1999emas compared to SN Ic 1994I and SN IIb 1993J, while type Ib eventsdisplay a somewhat larger variation. The scatter, around day 20, ismeasured to be ~5000 km s-1. Following two simple approaches,rough estimates of ejecta and hydrogen masses are given. A mass ofhydrogen of approximately 0.02 M_ȯ is obtained for SN 1990I, whileSNe 1983N and 2000H ejected ~0.008 M_ȯ and ~0.08 M_ȯ ofhydrogen, respectively. SN 1993J has a higher hydrogen mass, ~0.7M_ȯ with a large uncertainty. A low mass and thin hydrogen layerwith very high ejection velocities above the helium shell, is thus themost likely scenario for type Ib SNe. Some interesting and curiousissues relating to oxygen lines suggest future investigations.
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| Molecular hydrogen and [FeII] in active galactic nuclei - II. Results for Seyfert 2 galaxies
Near-infrared spectroscopy is used to study the kinematics andexcitation mechanisms of H2 and [FeII] lines in a sampledominated by Seyfert 2 galaxies. The spectra simultaneously cover theJHK bands, allowing us to compare line fluxes emitted in the interval0.8-2.4 μm and avoiding aperture and seeing effects. TheH2 lines are systematically narrower than the narrow-lineregion lines, suggesting that, very likely, the H2 does notoriginate from the same parcel of gas that forms the narrow-line region.Emission-line ratios between H2 lines favour thermalexcitation mechanisms for the molecular gas in active galactic nuclei.It was found that non-thermal excitation contributes, at most, 30 percent of the observed H2. Thermal excitation is also confirmedby the rather similar vibrational and rotational temperatures in theobjects (~2000 K). The mass of hot H2 ranges from102 to 103Msolar, with nearly half ofobjects showing values of <500 Msolar. It shows that thefraction of molecular mass present in the nuclear region and emitting inthe near-infrared is a very small fraction of the warm molecular masspresent in the centre. A diagnostic diagram composed of the line ratiosH2/Brγ and [FeII]/Paβ proves to be a useful toolin the near-infrared for separating emission-line objects by theirdegree of nuclear activity. We found that active galactic nuclei arecharacterized by H2 2.121 μm/Brγ and [FeII] 1.257μm/Paβ flux ratios between 0.6 and 2. Starburst/HII galaxiesdisplay line ratios <0.6 while low-ionization nuclear emission-lineregions are characterized by values larger than 2 in either ratio.
| A comparative analysis of empirical calibrators for nebular metallicity
We present a new analysis of the main empirical calibrators of oxygenabundance for ionized gas nebulae. With that aim we have compiled anextensive sample of objects with emission-line data including thenear-infrared [SIII] lines and the weak auroral lines which allow forthe determination of the gas electron temperature. For all the objectsthe oxygen abundances have been derived in a homogenous way, using themost recent sets of atomic coefficients and taking into the account theeffect of particle density on the temperature of O+. Theresiduals between directly and empirically derived abundances as afunction of abundance have been studied. A grid of photoionizationmodels, covering the range of physical properties of the gas, has beenused to explain the origin of the uncertainties affecting each abundancecalibrator. The range of validity for each abundance parameter has beenidentified and its average uncertainty has been quantified.
| Chandra observations of the interacting galaxies NGC 3395/3396 (Arp 270)
In this paper we present the results of a 20-ks high-resolution ChandraX-ray observation of the peculiar galaxy pair NGC 3395/3396, a system ata very early stage of merging, and less evolved than the famous Antennaeand Mice merging systems. Previously unpublished ROSAT High-ResolutionImager data are also presented. The point-source population and the hotdiffuse gas in this system are investigated and compared with othermerging galaxy pairs.16 X-ray point sources are detected in Arp 270, seven of which areclassified as ultraluminous X-ray sources (ULXs, LX>=1039 erg s-1). From spectral fits and the age ofthe system it seems likely that these are predominantly high-mass X-raybinaries. The diffuse gas emits at a global temperature of ~0.5 keV,consistent with temperatures observed in other interacting systems, andwe see no evidence of the starburst-driven hot gaseous outflows seen inmore evolved systems such as The Mice and The Antennae. It is likelythat these features are absent from Arp 270 as the gas has hadinsufficient time to break out of the galaxy discs. 32 per cent of theluminosity of Arp 270 arises from the diffuse gas in the system, this islow when compared with later stage merging systems and gives furthercredence that this is an early-stage merger.Comparing the ULX population of Arp 270 to other merging systems, wederive a relationship between the star formation rate of the system,indicated by LFIR, and the number [N(ULX)] and luminosity(LULX) of its ULX population. We find N(ULX)~L0.18FIR andLULX~L0.54FIR. These relationships,coupled with the relation of the point-source X-ray luminosity(LXP) to LK and LFIR+UV (Colbert et al.2003), indicate that the ULX sources in an interacting system havecontributions from both the old and young stellar populations.
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
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