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Massive-Star Supernovae as Major Dust Factories
We present late-time optical and mid-infrared observations of the TypeII supernova 2003gd in the galaxy NGC 628. Mid-infrared excessesconsistent with cooling dust in the ejecta are observed 499 to 678 daysafter outburst and are accompanied by increasing optical extinction andgrowing asymmetries in the emission-line profiles. Radiative-transfermodels show that up to 0.02 solar masses of dust has formed within theejecta, beginning as early as 250 days after outburst. Theseobservations show that dust formation in supernova ejecta can beefficient and that massive-star supernovae could have been major dustproducers throughout the history of the universe.
| The Light Echo around Supernova 2003gd in Messier 74
We confirm the discovery of a light echo around the Type II-plateausupernova 2003gd in Messier 74 (NGC 628), seen in images obtained withthe High Resolution Channel of the Advanced Camera for Surveys on boardthe Hubble Space Telescope (HST) as part of a larger Snapshot program onthe late-time emission from supernovae. The analysis of the echo wepresent suggests that it is due to the SN light pulse scattered by asheet of dust grains located ~113 pc in front of the SN, and that thesegrains are not unlike those assumed to be in the diffuse Galacticinterstellar medium, both in composition and in size distribution. Theecho is less consistent with scattering off carbon-rich grains, and ifanything, the grains may be somewhat more silicate rich than theGalactic dust composition. The echo also appears to be more consistentwith a SN distance closer to 7 Mpc than to 9 Mpc. This further supportsthe conclusion we reached elsewhere that the initial mass for the SNprogenitor was relatively low (~8-9 Msolar). The HST shouldbe used to continue to monitor the echo in several bands, particularlyin the blue, to better constrain its origin.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute (STScI),which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555.
| SN 2004A: another Type II-P supernova with a red supergiant progenitor
We present a monitoring study of SN 2004A and probable discovery of aprogenitor star in pre-explosion Hubble Space Telescope (HST) images.The photometric and spectroscopic monitoring of SN 2004A show that itwas a normal Type II-P which was discovered in NGC 6207 about two weeksafter explosion. We compare SN 2004A to the similar Type II-P SN 1999emand estimate an explosion epoch of 2004 January 6. We also calculatethree new distances to NGC 6207 of 21.0 +/- 4.3,21.4 +/- 3.5 and 25.1+/- 1.7Mpc. The former was calculated using the Standard Candle Method(SCM) for SNeII-P, and the latter two from the brightest supergiantsmethod (BSM). We combine these three distances with existing kinematicdistances, to derive a mean value of 20.3 +/- 3.4Mpc. Using thisdistance, we estimate that the ejected nickel mass in the explosion is0.046+0.031-0.017Msolar. The progenitorof SN 2004A is identified in pre-explosion WFPC2 F814W images with amagnitude of mF814W = 24.3 +/- 0.3, but is below thedetection limit of the F606W images. We show that this was likely a redsupergiant (RSG) with a mass of9+3-2Msolar. The object is detected at4.7σ above the background noise. Even if this detection isspurious, the 5σ upper limit would give a robust upper mass limitof 12Msolar for a RSG progenitor. These initial masses arevery similar to those of two previously identified RSG progenitors ofthe Type II-P SNe 2004gd (8+4-2Msolar)and 2005cs (9+3-2Msolar).
| 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.
| Spurs and feathering in spiral galaxies
We present smoothed particle hydrodynamic (SPH) simulations of theresponse of gas discs to a spiral potential. These simulations show thatthe commonly observed spurs and feathering in spiral galaxies can beunderstood as being due to structures present in the spiral arms thatare sheared by the divergent orbits in a spiral potential. Thus, densemolecular cloud-like structures generate the perpendicular spurs as theyleave the spiral arms. Subsequent feathering occurs as spurs are furthersheared into weaker parallel structures as they approach the next spiralpassage. Self-gravity of the gas is not included in these simulations,stressing that these features are purely due to the hydrodynamics inspiral shocks. Instead, a necessary condition for this mechanism to workis that the gas need be relatively cold (1000 K or less) in order thatthe shock is sufficient to generate structure in the spiral arms, andsuch structure is not subsequently smoothed by the gas pressure.
| Hαkinematics of the SINGS nearby galaxies survey - I*
This is the first part of an Hαkinematics follow-up survey of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for28galaxies are presented. The observations were done on three differenttelescopes with Fabry-Perot of New Technology for the Observatoire dumont Megantic (FaNTOmM), an integral field photon-counting spectrometer,installed in the respective focal reducer of each telescope. The datareduction was done through a newly built pipeline with the aim ofproducing the most homogenous data set possible. Adaptive spatialbinning was applied to the data cubes in order to get a constantsignal-to-noise ratio across the field of view. Radial velocity andmonochromatic maps were generated using a new algorithm, and thekinematical parameters were derived using tilted-ring models.
| Late-type galaxies observed with SAURON: two-dimensional stellar and emission-line kinematics of 18 spirals
We present the stellar and gas kinematics of a sample of 18 nearbylate-type spiral galaxies (Hubble types ranging from Sb to Sd), observedwith the integral-field spectrograph SAURON at the 4.2-m WilliamHerschel Telescope. SAURON covers the spectral range 4800-5380Å,allowing us to measure the Hβ, Fe, Mgb absorption features and theemission in the Hβ line and the [OIII]λλ4959,5007Å and [NI]λλ5198, 5200Å doublets over a 33× 41-arcsec2 field of view. The maps cover the nuclearregion of these late-type galaxies and in all cases include the entirebulge. In many cases the stellar kinematics suggests the presence of acold inner region, as visible from a central drop in the stellarvelocity dispersion. The ionized gas is almost ubiquitous and behaves ina complicated fashion: the gas velocity fields often display morefeatures than the stellar ones, including wiggles in the zero-velocitylines, irregular distributions, ring-like structures. The line ratio[OIII]/Hβ often takes on low values over most of the field,probably indicating a wide-spread star formation.
| 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.
| Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Hierarchical Star Formation in the Spiral Galaxy NGC 628
The distributions of size and luminosity for star-forming regions in thenearby spiral galaxy NGC 628 are studied over a wide range of scalesusing progressively blurred versions of an image from the Hubble SpaceTelescope Advanced Camera for Surveys. Four optical filters areconsidered for the central region, including Hα. Two filters areused for an outer region. The features in each blurred image are countedand measured using SExtractor. The cumulative size distribution is foundto be a power law in all passbands, with a slope of approximately -1.5over 1.8 orders of magnitude. The luminosity distribution isapproximately a power law as well, with a slope of approximately -1 forlogarithmic intervals of luminosity. The results suggest a scale-freenature for stellar aggregates in a galaxy disk. Fractal models of thindisks reproduce the projected size distribution and suggest a projectedmass distribution slope of approximately -0.5 for these extendedregions. This mass slope converts to the observed luminosity slope if weaccount for luminosity evolution and longer lifetimes in larger regions.
| The Optical/Near-Infrared Light Curves of SN 2002ap for the First 1.5 Years after Discovery
Late-time BVRIJHK photometry of the peculiar Type Ic SN 2002ap, takenbetween 2002 June 12 and 2003 August 29 with the MAGNUM Telescope, ispresented. The light-curve decline rate is derived in each band, and thecolor evolution is studied through comparison with nebular spectra andwith SN 1998bw. Using the photometry, the OIR bolometric light curve isbuilt, extending from before light maximum to day 580 after explosion.The light curve has a late-time shape strikingly similar to that of thehypernova SN 1998bw. The decline rate changes from 0.018 magday-1 between days 130 and 230 to 0.014 mag day-1between days 270 and 580. To reproduce the late-time light curve, adense core must be added to the one-dimensional hypernova model thatbest fits the early-time observations, bringing the ejecta mass from 2.5to 3 Msolar without much change in the kinetic energy, whichis 4×1051 ergs. This is similar to the case of otherhypernovae and suggests asymmetry. A large H-band bump developed in thespectral energy distribution after ~day 300, probably caused by strong[Si I] 1.646 and 1.608 μm emissions. The near-infrared fluxcontribution increased simultaneously from <30% to >50% at day580. The near-infrared light curves were compared with those of otherType Ib/c supernovae, among which SN 1983I seems similar to SN 2002apboth in the near-infrared and in the optical.
| The Extended H I Rotation Curve and Mass Distribution of M31
New H I observations of Messier 31 (M31) obtained with the Effelsbergand Green Bank 100 m telescopes make it possible to measure the rotationcurve of that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotationcurve is nearly flat at a velocity of ~226 km s-1. A model ofthe mass distribution shows that at the last observed velocity point,the minimum dark-to-luminous mass ratio is ~0.5 for a total mass of3.4×1011 Msolar at R<35 kpc. This can becompared to the estimated Milky Way mass of 4.9×1011Msolar for R<50 kpc.
| Identification of the Red Supergiant Progenitor of Supernova 2005cs: Do the Progenitors of Type II-P Supernovae Have Low Mass?
The stars that end their lives as supernovae (SNe) have been directlyobserved in only a handful of cases, mainly because of the extremedifficulty of identifying them in images obtained prior to the SNexplosions. Here we report the identification of the progenitor for therecent Type II-plateau (core collapse) SN 2005cs in pre-explosionarchival images of the Whirlpool Galaxy (M51) obtained with the HubbleSpace Telescope (HST) Advanced Camera for Surveys (ACS). Fromhigh-quality ground-based images of the SN obtained with theCanada-France-Hawaii Telescope, we precisely determine the position ofthe SN and are able to isolate the SN progenitor to within 0.04" in theHST ACS optical images. We further pinpoint the SN location to within0.005" from HST ACS ultraviolet images of the SN, confirming ourprogenitor identification. From photometry of the SN progenitor obtainedwith the pre-SN ACS images, and also from limits to its brightness inpre-SN HST NICMOS images, we infer that the progenitor is a redsupergiant star of spectral type K3-M4 with initial mass 10+/-3Msolar. We also discuss the implications of the SN 2005csprogenitor identification and its mass estimate. There is an emergingtrend that the most common Type II-plateau SNe originate from low-masssupergiants (8-20 Msolar).
| 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.
| The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium
We investigate the relationship between the velocity dispersion of thegas and the supernova (SN) rate and feedback efficiency withthree-dimensional numerical simulations of SN-driven turbulence in theinterstellar medium (ISM). Our simulations aim to explore the constancyof the velocity dispersion profiles in the outer parts of galactic disksat ~6-8 km s-1 and the transition to the starburst regime,i.e., high star formation rates (SFRs) associated with high velocitydispersions. With our fiducial value of the SN feedback efficiency(i.e., ε=0.25, corresponding to an injected energy per SN of0.25×1051 ergs), our results show that (1) SN drivingleads to constant velocity dispersions of σ~6 km s-1for the total gas and σHI~3 km s-1 for the HI gas, independent of the SN rate, for values of the rate between 0.01and 0.5 the Galactic value (ηG) (2) the position of thetransition to the starburst regime (i.e., location of sharp increase inthe velocity dispersion) at around SFR/area~=5×10-3 to10-2 Msolar yr-1 kpc-2observed in the simulations is in good agreement with the transition tothe starburst regime in the observations (e.g., NGC 628 and NGC 6949);(3) for the high SN rates, no H I gas is present in the simulations box;however, for the total gas velocity dispersion, there is good agreementbetween the models and the observations; (4) at the intermediate SNrates (η/ηG~0.5-1), taking into account the thermalbroadening of the H I line helps reach a good agreement in that regimebetween the models and the observations; and (5) forη/ηG<0.5, σ and σHI fallbelow the observed values by a factor of ~2. However, a set ofsimulations with different values of ɛ indicates that, forlarger values of the SN feedback efficiencies, velocity dispersions ofthe H I gas of the order of 5-6 km s-1 can be obtained, incloser agreement with the observations. The fact that forη/ηG<0.5, the H I gas velocity dispersions are afactor of ~2 smaller than the observed values could result from the factthat we might have underestimated the SN feedback efficiency. On theother hand, it might also be an indication that other physical processescouple to the stellar feedback in order to produce the observed level ofturbulence in galactic disks.
| 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.
| Low-Luminosity Active Galaxies and Their Central Black Holes
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| Thermal and non-thermal components of the interstellar medium at sub-kiloparsec scales in galaxies
Aims. We present new radio continuum observations of ten BIMA SONGgalaxies, taken at 1.4 GHz with the Very Large Array. These observationsallow us to extend the study of the relationships between the radiocontinuum (RC) and CO emission to 22 CO luminous galaxies for whichsingle dish CO images have been added to interferometric data. NewSpitzer infrared (IR) images of six of these galaxies have beenreleased. The analysis of these high resolution images allowed us toprobe the RC-IR-CO correlations down to linear scales of a few hundredpc. Methods: .We compare the point-by-point RC, CO and mid-IRintensities across entire galaxy disks, producing radial profiles andspatially resolved images of the RC/CO and RC/mid-IR ratios.Results: .For the 22 galaxies analysed, the RC-CO correlation on scalesfrom ~10 kpc down to ~100 pc is nearly linear and has a scatter of afactor of two, i.e. comparable to that of the global correlations. Thereis no evidence for any severe degradation of the scatter below the kpcscale. This also applies to the six galaxies for which high-resolutionmid-IR data are available. In the case of NGC 5194,we find that the non-thermal radio spectral index is correlated with theRC/FIR ratio. Conclusions: .The scatter of the point-by-pointcorrelations does not increase significantly with spatial resolution. Wethus conclude that we have not yet probed the physical scales at whichthe correlations break down. However, we observe local deviations fromthe correlations in regions with a high star formation rate, such as thespiral arms, where we observe a flat radio spectrum and a low RC/FIRratio. In the intra-arm regions and in the peripheral regions of thedisk, the RC/FIR is generally higher and it is characterized by asteepening of the radio spectrum.
| A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources
Aims.We search for ultra luminous X-ray source (ULXs) radio counterpartslocated in nearby galaxies in order to constrain their physicalnature. Methods: .Our work is based on a systematiccross-identification of the most recent and extensive available ULXcatalogues and archival radio data. Results: .A catalogue of 70positional coincidences is reported. Most of them are located within thegalaxy nucleus. Among them, we find 11 new cases of non-nuclear ULXsources with possibly associated radio emission.
| Outer structure of the Galactic warp and flare: explaining the Canis Major over-density
Aims.In this paper we derive the structure of the Galactic stellar warpand flare. Methods: .We use 2MASS red clump and red giant stars,selected at mean and fixed heliocentric distances ofRȯ≃3, 7 and 17 kpc. Results: .Our resultscan be summarized as follows: (i) a clear stellar warp signature isderived for the 3 selected rings, proving that the warp starts alreadywithin the solar circle; (ii) the derived stellar warp is consistent(both in amplitude and phase-angle) with that for the Galacticinterstellar dust and neutral atomic hydrogen; (iii) the consistency andregularity of the stellar-gaseous warp is traced out to aboutR_GC20 kpc; (iv) the Sun seems not to fall on the line of nodes.The stellar warp phase-angle orientation (φ15°) is closeto the orientation angle of the Galactic bar and this, most importantly,produces an asymmetric warp for the inner Rȯ≃3 and7 kpc rings; (v) a Northern/Southern warp symmetry is observed only forthe ring at Rȯ≃17 kpc, at which the dependency onφ is weakened; (vi) treating a mixture of thin and thick diskstellar populations, we trace the variation with R_GC of the diskthickness (flaring) and derive an almost constant scale-height (~0.65kpc) within R_GC15 kpc. Further out, the disk flaring increasegradually reaching a mean scale-height of ~1.5 kpc at R_GC23 kpc;(vii) the derived outer disk warping and flaring provide further robustevidence that there is no disk radial truncation at R_GC14 kpc. Conclusions: .In the particular case of the Canis Major (CMa)over-density we confirm its coincidence with the Southern stellarmaximum warp occurring near l240° (forRȯ≃7 kpc) which brings down the Milky Waymid-plane by ~3° in this direction. The regularity and consistencyof the stellar, gaseous and dust warp argues strongly against a recentmerger scenario for Canis Major. We present evidence to conclude thatall observed parameters (e.g. number density, radial velocities, propermotion etc) of CMa are consistent with it being a normal Milky Wayouter-disk population, thereby leaving no justification for more complexinterpretations of its origin. The present analysis or outer diskstructure does not provide a conclusive test of the structure or originof the Monoceros Ring. Nevertheless, we show that a warped flared MilkyWay contributes significantly at the locations of the Monoceros Ring.Comparison of outer Milky Way H I and CO properties with those of othergalaxies favors the suggestion that complex structures close to planarin outer disks are common, and are a natural aspect of warped andflaring disks.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| ion{H}{i} study of the warped spiral galaxy NGC 5055: a disk/dark matter halo offset?
We present a study of the ion{H}{i} distribution and dynamics of thenearby spiral galaxy NGC 5055 based on observations with the WesterborkSynthesis Radio Telescope. The gaseous disk of NGC 5055 extends out toabout 40 kpc, equal to 3.5 R25, and shows a pronounced warpthat starts at the end of the bright optical disk (R25= 11.6kpc). This very extended warp has large-scale symmetry, which along withthe rotation period of its outer parts (≃1.5 Gyr at 40 kpc),suggests a long-lived phenomenon. The rotation curve rises steeply inthe central parts up to the maximum velocity (vmax≃ 206km s-1). Beyond the bright stellar disk (R25), itshows a decline of about 25 km s-1 and then remains flat outto the last measured point. The standard analysis with luminous and darkmatter components shows the dynamical importance of the disk. The bestfit to the rotation curve is obtained with a "maximum disk". Lesssatisfactory fits with lighter disks help to set a firm lower limit of1.4 to the mass-to-light ratio in F band of the disk. Such a "minimumdisk" contributes about 60% of the observed maximum rotational velocity.NGC 5055 shows remarkable overall regularity and symmetry. A mildlopsidedness is noticeable, however, both in the distribution andkinematics of the gas. The tilted ring analysis of the velocity fieldled us to adopt different values for the kinematical centre and for thesystemic velocity for the inner and the outer parts of the system. Thishas produced a remarkable result: the kinematical and geometricalasymmetries disappear, both at the same time. These results point at twodifferent dynamical regimes: an inner region dominated by the stellardisk and an outer one, dominated by a dark matter halo offset withrespect to the disk.
| Classifications of the Host Galaxies of Supernovae, Set III
A homogeneous sample comprising host galaxies of 604 recent supernovae,including 212 objects discovered primarily in 2003 and 2004, has beenclassified on the David Dunlap Observatory system. Most SN 1991bg-likeSNe Ia occur in E and E/Sa galaxies, whereas the majority of SN1991T-like SNe Ia occur in intermediate-type galaxies. This differenceis significant at the 99.9% level. As expected, all types of SNe II arerare in early-type galaxies, whereas normal SNe Ia occur in all Hubbletypes. This difference is significant at the 99.99% level. A smallnumber of SNe II in E galaxies might be due to galaxy classificationerrors or to a small young-population component in these mainly oldobjects. No significant difference is found between the distributionsover the Hubble type of SNe Ibc and SNe II. This confirms that both ofthese types of objects have similar (massive) progenitors. The presentdata show that in order to understand the dependence of supernova typeon host-galaxy population, it is more important to obtain accuratemorphological classifications than it is to increase the size of thedata sample.
| 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.
| A study of the Type II-P supernova 2003gd in M74
We present photometric and spectroscopic data of the Type II-P supernova(SN II-P) 2003gd, which was discovered in M74 close to the end of itsplateau phase. SN 2003gd is the first Type II supernova (SN) to have adirectly confirmed red supergiant (RSG) progenitor. We compare SN 2003gdto SN 1999em, a similar SN II-P, and estimate an explosion date of 2003March 18. We determine a reddening towards the SN of E(B-V) = 0.14 +/-0.06, using three different methods. We also calculate three newdistances to M74 of 9.6 +/- 2.8, 7.7 +/- 1.7 and 9.6 +/- 2.2Mpc. Theformer was estimated using the standard candle method (SCM), for Type IIsupernovae (SNe II), and the latter two using the brightest supergiantsmethod (BSM). When combined with existing kinematic and BSM distanceestimates, we derive a mean value of 9.3 +/- 1.8Mpc. SN 2003gd was foundto have a lower tail luminosity compared with other normal Type II-Psupernovae (SNe II-P) bringing into question the nature of this SN. Wepresent a discussion concluding that this is a normal SN II-P, which isconsistent with the observed progenitor mass of8+4-2 Msolar.
| A grid of chemical evolution models as a tool to interpret spiral and irregular galaxies data
We present a generalization of the multiphase chemical evolution model(CEM) applied to a wide set of theoretical galaxies with differentmasses and evolutionary rates. This generalized set of models has beencomputed using the so-called universal rotation curve from Persic,Salucci & Steel to calculate the radial mass distribution of 44theoretical protogalaxies. This distribution is a fundamental inputwhich, besides its own effect on the galaxy evolution, defines thecharacteristic collapse time-scale or gas infall rate on to the disc. Wehave adopted 10 sets of values, between 0 and 1, for the molecular cloudand star formation efficiencies, as corresponding to their probabilitynature, for each one of the radial distributions of total mass. Thus, wehave constructed a biparametric grid of models, depending on thoseefficiency sets and on the rotation velocity, whose results are valid inprinciple for any spiral or irregular galaxy. The model results providethe time-evolution of different regions of the disc and the halo alonggalactocentric distance, measured by the gas (atomic and molecular) andstellar masses, the star formation rate (SFR) and chemical abundances of14 elements, for a total of 440 models. This grid may be used toestimate the evolution of a given galaxy for which only present timeinformation, such as radial distributions of elemental abundances, gasdensities and/or star formation, which are the usual observationalconstraints of chemical evolution models (CEMs), is available.
| XMM-Newton observations of the interacting galaxy pairs NGC 7771/0 and NGC 2342/1
We present XMM-Newton X-ray observations of the interacting galaxy pairsNGC 7771/7770 and NGC 2342/2341. In NGC 7771, for the first time we areable to resolve the X-ray emission into a bright central source plus twobright (LX > 1040 erg s-1)ultraluminous X-ray sources (ULXs) located either end of the bar. In thebright central source (LX~ 1041 ergs-1), the soft emission is well-modelled by a two-temperaturethermal plasma with kT= 0.4/0.7 keV. The hard emission is modelled witha flat absorbed power-law (Γ~ 1.7, NH~ 1022cm-2), and this together with a low-significance (1.7σ)~ 300 eV equivalent width emission line at ~6 keV are the firstindications that NGC 7771 may host a low-luminosity AGN. For the barULXs, a power-law fit to X-1 is improved at the 2.5σ level withthe addition of a thermal plasma component (kT~ 0.3 keV), while X-2 isimproved only at the 1.3σ level with the addition of a discblackbody component with Tin~ 0.2 keV. Both sources arevariable on short time-scales implying that their emission is dominatedby single accreting X-ray binaries (XRBs). The three remaining galaxies,NGC 7770, NGC 2342 and NGC 2341, have observed X-ray luminosities of0.2, 1.8 and 0.9 × 1041 erg s-1,respectively (0.3-10 keV). Their integrated spectra are alsowell-modelled by multi-temperature thermal plasma components with kT=0.2-0.7 keV, plus power-law continua with slopes of Γ= 1.8-2.3that are likely to represent the integrated emission of populations ofXRBs as observed in other nearby merger systems. A comparison with otherisolated, interacting and merging systems shows that all four galaxiesfollow the established correlations for starburst galaxies betweenX-ray, far-infrared and radio luminosities, demonstrating that theirX-ray outputs are dominated by their starburst components.
| The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties
In a recently completed survey of the stellar population properties oflow-ionization nuclear emission-line regions (LINERs) and LINER/HIItransition objects (TOs), we have identified a numerous class ofgalactic nuclei which stand out because of their conspicuous108-9 yr populations, traced by high-order Balmer absorptionlines and other stellar indices. These objects are called `young-TOs',because they all have TO-like emission-line ratios. In this paper weextend this previous work, which concentrated on the nuclear properties,by investigating the radial variations of spectral properties inlow-luminosity active galactic nuclei (LLAGNs). Our analysis is based onhigh signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distancesof typically up to 850 pc from the nucleus with a resolution of ~100 pc(~1 arcsec) and S/N ~ 30. Stellar population gradients are mapped by theradial profiles of absorption-line equivalent widths and continuumcolours along the slit. These variations are further analysed by meansof a decomposition of each spectrum in terms of template galaxiesrepresentative of very young (<=107 yr), intermediate age(108-9 yr) and old (1010 yr) stellar populations.This study reveals that young-TOs also differ from old-TOs andold-LINERs in terms of the spatial distributions of their stellarpopulations and dust. Specifically, our main findings are as follows.(i) Significant stellar population gradients are found almostexclusively in young-TOs. (ii) The intermediate age population ofyoung-TOs, although heavily concentrated in the nucleus, reachesdistances of up to a few hundred pc from the nucleus. Nevertheless, thehalf width at half-maximum of its brightness profile is more typically100 pc or less. (iii) Objects with predominantly old stellar populationspresent spatially homogeneous spectra, be they LINERs or TOs. (iv)Young-TOs have much more dust in their central regions than otherLLAGNs. (v) The B-band luminosities of the central <~1 Gyr populationin young-TOs are within an order of magnitude of MB=-15,implying masses of the order of ~107-108Msolar. This population was 10-100 times more luminous in itsformation epoch, at which time young massive stars would have completelyoutshone any active nucleus, unless the AGN too was brighter in thepast.
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