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The Iron Abundance in Galactic Planetary Nebulae
We constrain the iron abundance in a sample of 33 low-ionizationGalactic planetary nebulae (PNe) using [Fe III] lines andcorrecting for the contribution of higher ionization states withionization correction factors that take into account uncertainties inthe atomic data. We find very low iron abundances in all the objects,suggesting that more than 90% of their iron atoms are condensed ontodust grains. This number is based on the solar iron abundance andimplies a lower limit on the dust-to-gas mass ratio, solely due to iron,of M dust/M gas >= 1.3 ×10–3 for our sample. The depletion factors of differentPNe cover about two orders of magnitude, probably reflecting differencesin the formation, growth, or destruction of their dust grains. However,we do not find any systematic difference between the gaseous ironabundances calculated for C-rich and O-rich PNe, suggesting similar irondepletion efficiencies in both environments. The iron abundances of oursample PNe are similar to those derived following the same procedure fora group of 10 Galactic H II regions. These high depletion factorsargue for high depletion efficiencies of refractory elements onto dustgrains both in molecular clouds and asymptotic giant branch stars, andlow dust destruction efficiencies both in interstellar and circumstellarionized gas.Partly based on observations made with the 2.1 m telescope atObservatorio Astronómico Nacional, San Pedro Mártir,Mexico.
| Spiral Density Wave Triggering of Star Formation in SA and SAB Galaxies
Azimuthal color (age) gradients across spiral arms are one of the mainpredictions of density wave theory; gradients are the result of starformation triggered by the spiral waves. In a sample of 13 spiralgalaxies of types A and AB, we find that ten of them present regionsthat match the theoretical predictions. By comparing the observedgradients with stellar population synthesis models, the pattern speedand the location of major resonances have been determined. The resonancepositions inferred from this analysis indicate that nine of the objectshave spiral arms that extend to the outer Lindblad resonance; for one ofthe galaxies, the spiral arms reach the corotation radius. The effectsof dust, and of stellar densities, velocities, and metallicities on thecolor gradients are also discussed.
| Dynamical Constraints on the Masses of the Nuclear Star Cluster and Black Hole in the Late-Type Spiral Galaxy NGC 3621
NGC 3621 is a late-type (Sd) spiral galaxy with an active nucleus,previously detected through mid-infrared [Ne V] line emission. ArchivalHubble Space Telescope (HST) images reveal that the galaxy contains abright and compact nuclear star cluster. We present a newhigh-resolution optical spectrum of this nuclear cluster, obtained withthe Echellette Spectrograph and Imager at the Keck Observatory. Thenucleus has a Seyfert 2 emission-line spectrum at optical wavelengths,supporting the hypothesis that a black hole (BH) is present. Theline-of-sight stellar velocity dispersion of the cluster isσsstarf = 43 ± 3 km s–1, one ofthe largest dispersions measured for any nuclear cluster in a late-typespiral galaxy. Combining this measurement with structural parametersmeasured from archival HST images, we carry out dynamical modeling basedon the Jeans equation for a spherical star cluster containing a centralpoint mass. The maximum BH mass consistent with the measured stellarvelocity dispersion is 3 × 106 Msun. If theBH mass is small compared with the cluster's stellar mass, then thedynamical models imply a total stellar mass of ~1 × 107Msun, which is consistent with rough estimates of the stellarmass based on photometric measurements from HST images. From thestructural decomposition of Two Micron All Sky Survey images, we find noclear evidence for a bulge in NGC 3621; the galaxy contains at most avery faint and inconspicuous pseudobulge component (MK gsim–17.6 mag). NGC 3621 provides one of the best demonstrations thatvery late-type spirals can host both active nuclei and nuclear starclusters, and that low-mass BHs can occur in disk galaxies even in theabsence of a substantial bulge.
| The Dual-Axis Circumstellar Environment of the Type IIn Supernova 1997eg
We present multi-epoch spectral and spectropolarimetric observations ofthe Type IIn supernova (SN) 1997eg that indicate the presence of aflattened disklike concentration of circumstellar material surroundingaspherical ejecta, with which the disk is misaligned. The polarizationacross the broad Hα, Hβ, and He I λ5876 lines of SN1997eg forms closed loops when viewed in the Stokes q-u plane. Suchloops occur when the geometrical symmetry of one or both of the Stokesparameters across spectral lines is broken, in this case most likely byoccultation of the ejecta by the equatorial circumstellar matterconcentration. The polarization of the narrow Balmer lines possesses anintrinsic axis that differs by 12° from that of the elongated ejectaand probably indicates the orientation of the disklike circumstellarmaterial. The existence of two different axes of symmetry in SN 1997egsuggests that neither rotation of the progenitor nor the influence of acompanion star can be the sole mechanism creating a preferred axiswithin the supernova system. Our model supports the emerging hypothesisthat the progenitors of some Type IIn supernovae are luminous bluevariable stars, whose presupernova mass eruptions form the circumstellarshells that physically characterize the SN IIn subclass. Theseconclusions, which are independent of interstellar polarization effects,would have been unobservable with only a single epoch ofspectropolarimetry.
| Deep Mixing and Metallicity: Carbon Depletion in Globular Cluster Giants
We present the results of an observational study of the efficiency ofdeep mixing in globular cluster red giants as a function of stellarmetallicity. We determine [C/Fe] abundances based on low-resolutionspectra taken with the Kast spectrograph on the 3 m Shane telescope atLick Observatory. Spectra centered on the 4300 Å CH absorptionband were taken for 42 bright red giants in 11 Galactic globularclusters ranging in metallicity from M92 ([Fe/H] = 2.29) to NGC 6712([Fe/H] = 1.01). Carbon abundances were derived by comparing values ofthe CH-band strength index S 2(CH) measured from the datawith values measured from a large grid of SSG synthetic spectra.Present-day abundances are combined with theoretical calculations of thetime since the onset of mixing, which is also a function of stellarmetallicity, to calculate the carbon depletion rate across ourmetallicity range. We find that the carbon depletion rate is twice ashigh at a metallicity of [Fe/H] = 2.3 than at [Fe/H] = 1.3, which is aresult qualitatively predicted by some theoretical explanations of thedeep mixing process.
| Is the Optically Unidentified Radio Source FIRST J121839.7+295325 a Dark Lens?
We present evidence that the optically unidentified radio source FIRSTJ121839.7+295325 may be strongly lensing a background galaxy. Weestimate the redshift of the assumed gravitational arc, discovered inparallel imaging with HST, from MMT-Blue Channel spectroscopy to bezarc=2.48+0.14-0.05. We present lensmodels with an Einstein radius of RE=1.3'' whichcontains a mass of Mdyn=1012+/-0.5Msolar, where the uncertainty reflects the range of possiblelens redshifts. The putative lens is not detected toJlim=22.0 mag and Hlim=20.7 mag in our MMT-SWIRCimaging. Using the flux limits from WFPC2 and SWIRC, we estimate thatthe dynamical mass-to-light ratio of J121839.7+295325 isMdyn/LB>~10 MsolarL-1solar for AV=1 mag, and this lowerlimit could be as high as 30 MsolarL-1solar for AV=0 mag. Since the radiosource is optically unidentified (Vlim=25.5 mag) and has aradio flux of S1.4GHz=33 mJy, it is likely a massiveearly-type galaxy which hosts a radio-loud AGN at0.8<~zradio<~1.5. However, the present data cannotuniquely determine the mass-to-light ratio of the lensing galaxy, andhence the possibility that this system may be a reasonably dark lens isnot ruled out.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| Active Galactic Nuclei in Four Metal-poor Dwarf Emission-Line Galaxies
We present 3.5 m Apache Point Observatory second-epoch spectra of fourlow-metallicity emission-line dwarf galaxies discovered serendipitouslyin Data Release 5 of the Sloan Digital Sky Survey (SDSS) to haveextraordinarily large broad Hα luminosities, ranging from3×1041 to 2×1042 erg s-1.The oxygen abundance in these galaxies is very low, varying in the range12+logO/H=7.36-7.99. Such extraordinarily high broad Hαluminosities cannot be accounted for by massive stars at differentstages of their evolution. By comparing these with the first-epoch SDSSspectra, we find that the broad Hα luminosities have remainedconstant over a period of 3-7 yr, which probably excludes Type IInsupernovae as a possible mechanism of broad emission. The emission mostlikely comes from accretion disks around intermediate-mass black holes,with lower mass limits in the range~5×105-3×106 Msolar. Ifthis is the case, these four objects form a new class of very lowmetallicity active galactic nuclei (AGNs) that have been elusive untilnow. The absence of the strong high-ionization lines [Ne V] λ3426and He II λ4686 can be understood if the nonthermal radiationcontributes less than ~10% of the total ionizing radiation.
| A spectroscopic study of the blue stragglers in M67
Based on spectrophotometric observations from the Guillermo HaroObservatory (Cananea, Mexico), a study of the spectral properties of thecomplete sample of 24 blue straggler stars (BSs) in the old Galacticopen cluster M67 (NGC 2682) is presented. All spectra, calibrated usingspectral standards, were recalibrated by means of photometric magnitudesin the Beijing-Arizona-Taipei-Connecticut system, which includes fluxesin 11 bands covering ~3500-10000 Å. The set of parameters wasobtained using two complementary approaches that rely on a comparison ofthe spectra with (i) an empirical sample of stars with well-establishedspectral types and (ii) a theoretical grid of optical spectra computedat both low and high resolution. The overall results indicate that theBSs in M67 span a wide range in Teff(~ 5600 -12600 K) andsurface gravities that are fully compatible with those expected formain-sequence objects (log g = 3.5 -5.0 dex).
| The Stellar Populations of Stripped Spiral Galaxies in the Virgo Cluster
We present an analysis of the stellar populations of the gas-strippedouter disks of ten Virgo cluster spiral galaxies, utilizing SparsePakintegral field spectroscopy on the WIYN 3.5 m telescope and GalaxyEvolution Explorer (GALEX) UV photometry. The galaxies in our sampleshow evidence for being gas-stripped spiral galaxies, with starformation within a truncation radius and a passive population beyond thetruncation radius. We find that all of the galaxies with spatiallytruncated star formation have outer-disk stellar populations consistentwith star formation ending within the last 500 Myr. The synthesis ofoptical spectroscopy and GALEX observations demonstrate that starformation was relatively constant until the quenching time, after whichthe galaxies passively evolved. Large starbursts at the time ofquenching are excluded for all galaxies, but there is evidence of amodest starburst in at least one galaxy. For approximately half of ourgalaxies, the timescales derived from our observations are consistentwith galaxies being stripped in or near the cluster core, where simpleram-pressure estimates can explain the observed stripping. However, theother half of our sample galaxies were clearly stripped outside thecluster core. Such galaxies provide evidence that the intra-clustermedium is not static and smooth. For three of the most recently strippedgalaxies, there are estimates for the stripping timescales from detailedgas-stripping simulations. For all three of these galaxies, ourstripping timescales agree with those from the gas-strippingsimulations, suggesting that star formation is quenched near the time ofpeak pressure. While the stripping of star-forming gas in the outer diskcreates a passive population in our sample of galaxies, there is stillnormal star formation in the center of these galaxies. It may be thatVirgo is not massive enough to completely strip these spiral galaxiesand, in a more dynamically active cluster or a cluster with a higherdensity intracluster medium, such a process would lead to passivespirals and/or S0s.
| Clusters of Galaxies in the First Half of the Universe from the IRAC Shallow Survey
We have identified 335 galaxy cluster and group candidates, 106 of whichare at z>1, using a 4.5 μm-selected sample of objects from a 7.25deg2 region in the Spitzer Infrared Array Camera (IRAC)Shallow Survey. Clusters were identified as three-dimensionaloverdensities using a wavelet algorithm, based on photometric redshiftprobability distributions derived from IRAC and NOAO Deep Wide-FieldSurvey data. We estimate only ~10% of the detections are spurious. Todate 12 of the z>1 candidates have been confirmed spectroscopically,at redshifts from 1.06 to 1.41. Velocity dispersions of ~750 kms-1 for two of these argue for total cluster masses wellabove 1014 Msolar, as does the mass estimated fromthe rest-frame near-infrared stellar luminosity. Although not selectedto contain a red sequence, some evidence for red sequences is present inthe spectroscopically confirmed clusters, and brighter galaxies aresystematically redder than the mean galaxy color in clusters at allredshifts. The mean I-[3.6] color for cluster galaxies up to z~1 is wellmatched by a passively evolving model in which stars are formed in a 0.1Gyr burst starting at redshift zf=3. At z>1, a wider rangeof formation histories is needed, but higher formation redshifts (i.e.,zf>3) are favored for most clusters.
| An Offset Seyfert 2 Nucleus in the Minor Merger System NGC 3341
We present the discovery of a triplet of emission-line nuclei in thedisturbed disk galaxy NGC 3341, based on archival data from the SloanDigital Sky Survey and new observations from the Keck Observatory. Thisgalaxy contains two offset nuclei within or projected against its disk,at projected distances of 5.1 and 8.4 kpc from its primary nucleus andat radial velocity separation of less than 200 km s-1 fromthe primary. These appear to be either dwarf ellipticals or the bulgesof low-mass spirals whose disks have already been stripped off whilemerging into the primary galaxy. The inner offset nucleus has a Seyfert2 spectrum and a stellar velocity dispersion of 70 +/- 7 kms-1. The outer offset nucleus has very weak emission linesconsistent with a LINER classification, and the primary nucleus has anemission-line spectrum close to the boundary between LINER/H IIcomposite systems and H II nuclei; both may contain accreting massiveblack holes, but the optical classifications alone are ambiguous. Thedetection of an offset active nucleus in NGC 3341 provides a strongsuggestion that black hole accretion episodes during minor mergers canbe triggered in the nuclei of dwarf secondary galaxies as well as in theprimary.
| Radiative levitation: a likely explanation for pulsations in the unique hot O subdwarf star SDSS J160043.6+074802.9
Context: SDSS J160043.6+074802.9 (J1600+0748 for short) is the only hotsdO star for which unambiguous multiperiodic luminosity variations havebeen reported so far. These rapid variations, with periods in the rangefrom ~60 s to ~120 s, are best qualitatively explained in terms ofpulsational instabilities, but the exact nature of the driving mechanismhas remained a puzzle. Aims: Our primary goal is to examinequantitatively how pulsation modes can be excited in an object such asJ1600+0748. Given the failure of uniform-metallicity models as welldocumented in the recent Ph.D. thesis of C.Rodríguez-López, we consider the effects of radiativelevitation on iron as a means to boost the efficiency of theopacity-driving mechanism in models of J1600+0748. Methods: We combinehigh sensitivity time-averaged optical spectroscopy and fullnonadiabatic calculations to carry out our study. In the first instance,this is used to estimate the location of J1600+0748 in the log g-T_effplane. Given this essential input, we pulsate stellar models consistentwith these atmospheric parameters. We construct both uniform-metallicitymodels and structures in which the iron abundance is specified by thecondition of diffusive equilibrium between gravitational settling andradiative levitation. Results: On the basis of NTLE H/He syntheticspectra, we find that the target star has the following atmosphericparameters: log g = 5.93 ± 0.11, T_eff = 71 070 ± 2725 K,and log N(He)/N(H) = -0.85 ± 0.08. This takes into account ourdeconvolution of the spectrum of J1600+0748 as it is polluted by thelight of a main sequence companion. We confirm that uniform-metallicitystellar models with Z in the range from 0.02 to 0.10 cannot excitepulsation modes of the kind observed. On the other hand, we find thatthe inclusion of radiative levitation, as we implemented it, leads topulsational instabilities in a period range that overlaps with, althoughit is narrower than, the observed range in J1600+0748. The excited modescorrespond to low-order, low-degree p-modes. Conclusions: We infer thatradiative levitation is a likely essential ingredient in the excitationphysics at work in J1600+0748.
| Discovery of a massive variable star with Z = Zsolar/36 in the galaxy DDO 68
The Local Volume dwarf galaxy DDO 68, from the spectroscopy of its twobrightest HII regions (Knots 1 and 2), was designated as the second mostmetal-poor star-forming galaxy [12 + log(O/H) = 7.14]. In the repeatedspectral observations in 2008 January with the 6-m telescope (BTA) ofthe HII region Knot 3 [having 12 + log(O/H) = 7.10 +/- 0.06], we find astrong evidence of a transient event related to a massive starevolution. From the follow-up observation with the higher spectralresolution in 2008 February, we confirm this phenomenon, and giveparameters of its emission-line spectrum comprising of Balmer HI and HeIlines. The luminosities of the strongest transient lines (Hα,Hβ) are of a few 1036 erg s-1. We alsodetected an additional continuum component in the new spectrum of Knot3, which displays the spectral energy distribution raising toultraviolet. The estimate of the flux of this continuum leads us to itsabsolute V-band magnitude of ~ -7.1. Based on the spectral properties ofthis transient component, we suggest that it is related to an evolvedmassive star of luminous blue variable type with Z =Zsolar/36. We briefly discuss observational constraints onparameters of this unique (in the aspect of the record low metallicityof the progenitor massive star) event and propose several lines of itsstudy.
| Evidence for a Companion to BM Gem, a Silicate Carbon Star
Balmer and Paschen continuum emission, as well as Balmer series lines ofP Cygni-type profile from Hγ through H23, are revealedin the violet spectra of BM Gem, a carbon star associated with anoxygen-rich circumstellar shell (``silicate carbon star''). Theblueshifted absorption in the Balmer lines indicates the presence of anoutflow, the line-of-sight velocity of which is at least 400 kms-1. The Balmer lines show a significant change in profileover a period of 75 days. We argue that the observed unusual features inBM Gem are strong evidence for the presence of a companion, which shouldform an accretion disk that gives rise to both an ionized gas region anda high-velocity variable outflow. The estimated luminosity of ~0.2(0.03-0.6) Lsolar for the ionized gas can be maintained by amass accretion rate for a dwarf companion of ~10-8Msolar yr-1, while ~10-10Msolar yr-1 is sufficient for accretion to a whitedwarf companion. These accretion rates are feasible for some detachedbinary configurations on the basis of the Bondi-Hoyle-type accretionprocess. Therefore, we conclude that the carbon star BM Gem is in adetached binary system with a companion of low mass and low luminosity.However, we are unable to determine whether this companion object is adwarf or a white dwarf, although the gas outflow velocity of 400 kms-1, as well as the nondetection in the X-ray survey, favorits identity as a dwarf star. The upper limits for binary separation are210 and 930 AU for a dwarf and a white dwarf, respectively, in the caseof circular orbit. We also note that the observed features of BM Gemmimic those of Mira (ο Cet), which may suggest actual similaritiesin their binary configurations and circumstellar structures.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| The Overdense Environment of a Large Lyα Nebula at z ~ 2.7
Large nebulae (>~50 kpc) emitting strongly in Lyα (also knownas Lyα ``blobs'') are likely signposts of ongoing massive galaxyformation. The relative rarity of these sources and their discovery inwell-studied galaxy overdensities suggest that they may be associatedwith regions of high galaxy density. One of the largest Lyαnebulae, discovered at a redshift of z~2.7 via its strong mid-infraredemission, provides an unbiased test of this association. We have carriedout a deep intermediate-band imaging survey for Lyα-emittinggalaxies (LAEs) within a 30' × 26' field of view around thisLyα nebula. This is the first study of the environment of aLyα nebula found without a priori knowledge of its surroundings.We find that the nebula is located in an overdense region, at least 20× 50 h-170 comoving Mpc in size, showing afactor of ~3 LAE number density enhancement relative to the edge of thefield. Given the predicted number of such overdensities, we rule out thepossibility of a chance coincidence at the <~1% level. This study, inconjunction with previous work, provides strong confirmation of theassociation between the largest Lyα nebulae and overdense regionsof the universe.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.Based in part on observations obtained at Kitt Peak NationalObservatory, a division of the National Optical Astronomy Observatory,which is operated by the Association of Universities for Research inAstronomy, Inc., under cooperative agreement with the National ScienceFoundation.
| The Host Galaxy and the Extended Emission-Line Region of the Radio Galaxy 3C 79
We present extensive ground-based spectroscopy and HST imaging of 3C 79,an FR II radio galaxy associated with a luminous extended emission-lineregion (EELR). Surface brightness modeling of an emission-line-free HSTR-band image reveals that the host galaxy is a massive elliptical with acompact companion 0.8" away and 4 mag fainter. The host galaxy spectrumis best described by an intermediate-age (1.3 Gyr) stellar population(4% by mass), superimposed on a 10 Gyr old population and a power law(αλ=-1.8) the stellar populations are consistentwith supersolar metallicities, with the best fit given by the 2.5Zsolar models. We derive a dynamical mass of4×1011 Msolar within the effective radiusfrom the velocity dispersion. The EELR spectra clearly indicate that theEELR is photoionized by the hidden central engine. Photoionizationmodeling shows evidence that the gas metallicity in both the EELR andthe nuclear narrow-line region is mildly subsolar (0.3-0.7Zsolar), significantly lower than the supersolarmetallicities deduced from typical active galactic nuclei in the SloanDigital Sky Survey. The more luminous filaments in the EELR exhibit avelocity field consistent with a common disk rotation. Fainter clouds,however, show high approaching velocities that are uncoupled from thisapparent disk rotation. The striking similarities between this EELR andthe EELRs around steep-spectrum radio-loud quasars provide furtherevidence for the orientation-dependent unification schemes. Themetal-poor gas is almost certainly not native to the massive hostgalaxy. We suggest that the close companion galaxy could be the tidallystripped bulge of a late-type galaxy that is merging with the hostgalaxy. The interstellar medium of such a galaxy is probably the sourcefor the low-metallicity gas in 3C 79.Based in part on observations obtained at the Gemini Observatory, whichis operated by the Association of Universities for Research inAstronomy, Inc., under a cooperative agreement with the NSF on behalf ofthe Gemini partnership: the National Science Foundation (United States),the Particle Physics and Astronomy Research Council (United Kingdom),the National Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil) and CONICET (Argentina).Gemini Program ID: GN-2006B-C-3. Some of the data presented herein wereobtained at the W. M. Keck Observatory, which is operated as ascientific partnership among the California Institute of Technology, theUniversity of California and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W. M. Keck Foundation. Based also in part onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the Data Archive at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| Reionization and Galaxy Evolution Probed by z = 7 Lyα Emitters
We have performed narrowband NB973 (bandwidth 200 Å centered at9755 Å) imaging of the Subaru Deep Field (SDF) and found two z=7Lyα emitter (LAE) candidates down to NB973 =24.9. Carrying outdeep follow-up spectroscopy, we identified one of them as a real z=6.96LAE. This has established a new redshift record, showing that galaxyformation was in progress just 750 Myr after the big bang. Meanwhile,the Lyα line luminosity function of LAEs is known to decline fromz=5.7 to 6.6 in the SDF; L* at z=6.6 is 40%-60% that atz=5.7. We also confirm that the number density of z=7 LAEs is only 17%of the density at z=6.6 comparing the latest SDF LAE samples. Thisseries of significant decreases in LAE density with increasing redshiftcould be the result of galaxy evolution during these epochs. However,using the UV continuum luminosity functions of LAEs and Lyman breakgalaxies, and a LAE evolution model based on hierarchical clustering, wefind that galaxy evolution alone cannot entirely explain the decrease indensity. This extra density deficit might reflect the attenuation of theLyα photons from LAEs by the neutral hydrogen possibly left at thelast stage of cosmic reionization at z~6-7.Based on data collected at Subaru Telescope, which is operated byNational Astronomical Observatory of Japan.
| Optical Spectroscopy of Type ia Supernovae
We present 432 low-dispersion optical spectra of 32 Type Ia supernovae(SNe Ia) that also have well-calibrated light curves. The coverageranges from 6 epochs to 36 epochs of spectroscopy. Most of the data wereobtained with the 1.5 m Tillinghast telescope at the F. L. WhippleObservatory with typical wavelength coverage of 3700-7400 Å and aresolution of ~7 Å. The earliest spectra are 13 days before B-bandmaximum; two-thirds of the SNe were observed before maximum brightness.Coverage for some SNe continues almost to the nebular phase. Theconsistency of the method of observation and the technique of reductionmakes this an ideal data set for studying the spectroscopic diversity ofSNe Ia.Based in part on observations obtained at the F. L. Whipple Observatory,which is operated by the Smithsonian Astrophysical Observatory, and theMMT Observatory, a joint facility of the Smithsonian Institution and theUniversity of Arizona.
| The underluminous Type Ia supernova 2005bl and the class of objects similar to SN 1991bg
Optical observations of the Type Ia supernova (SN Ia) 2005bl in NGC4070, obtained from -6 to +66d with respect to the B-band maximum, arepresented. The photometric evolution is characterized by rapidlydeclining light curves [Δm15(B)true = 1.93]and red colours at peak and soon thereafter. With MB,max =-17.24 the SN is an underluminous SN Ia, similar to the peculiar SNe1991bg and 1999by. This similarity also holds for the spectroscopicappearance, the only remarkable difference being the likely presence ofcarbon in pre-maximum spectra of SN 2005bl. A comparison study amongunderluminous SNe Ia is performed, based on a number ofspectrophotometric parameters. Previously reported correlations of thelight-curve decline rate with peak luminosity and (Si) are confirmed,and a large range of post-maximum SiII λ6355 velocity gradientsis encountered. 1D synthetic spectra for SN 2005bl are presented, whichconfirm the presence of carbon and suggest an overall low burningefficiency with a significant amount of leftover unburned material.Also, the Fe content in pre-maximum spectra is very low, which may pointto a low metallicity of the precursor. Implications for possibleprogenitor scenarios of underluminous SNe Ia are briefly discussed.Based on observations at ESO-Paranal, Programme 075.D-0662(B).E-mail: tauben@mpa-garching.mpg.de
| Spectroscopy of two PN candidates in IC10
We present the results of the first spectroscopic observations of twoplanetary nebula (PN) candidates in the Local Group dwarf irregulargalaxy IC10. Using several spectral classification diagrams, we showthat the brightest PN candidate (PN7) is not a PN, but rather a compactHII region consisting of two components with low electron numberdensities. After the rejection of this PN candidate, the IC10 PNluminosity function cut-off becomes very close to the standard value.With the compiled spectroscopic data for a large number of extragalacticPNe, we analyse a series of diagnostic diagrams to generate quantitativecriteria for separating PNe from unresolved HII regions. We show that,with the help of the diagnostic diagrams and the derived set ofcriteria, PNe can be distinguished from HII regions with an efficiencyof ~99.6 per cent. With the obtained spectroscopic data, we confirm thatanother, 1.7 mag fainter PN candidate (PN9) is a genuine PN. We arguethat, based on all currently available PNe data, IC10 is located at adistance of 725+63-33 kpc [distance modulus (m -M) = 24.30+0.18-0.10].Based on observations obtained at the 6-m SAO RAS (Special AstrophysicalObservatory of Russian Academy of Science) telescope.E-mail: akniazev@saao.ac.za (AYK); sap@sao.ru (SAP);zucker@ast.cam.ac.uk (DBZ)
| The Third Image of the Large-Separation Lensed Quasar SDSS J1029+2623
We identify a third image in the unique quasar lens SDSS J1029+2623, thesecond known quasar lens produced by a massive cluster of galaxies. Thespectrum of the third image shows similar emission and absorptionfeatures but has a redder continuum than the other two images, which canbe explained by differential extinction or microlensing. We alsoidentify several lensed arcs. Our observations suggest a complicatedstructure of the lens cluster at z~0.6. We argue that the three lensedimages are produced by a naked cusp on the basis of successful massmodels, the distribution of cluster member galaxies, and the shapes andlocations of the lensed arcs. Lensing by a naked cusp is quite rareamong galaxy-scale lenses but is predicted to be common amonglarge-separation lensed quasars. Thus the discovery can be viewed assupport for an important theoretical prediction of the standard colddark matter model.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. This work is based in part on data collected at SubaruTelescope, which is operated by the National Astronomical Observatory ofJapan. Use of the UH 2.2 m telescope for the observations is supportedby NAOJ.
| Using Quantitative Spectroscopic Analysis to Determine the Properties and Distances of Type II Plateau Supernovae: SN 2005cs and SN 2006bp
We analyze the Type II plateau supernovae (SNe IIP) SN 2005cs and SN2006bp with the non-LTE model atmosphere code CMFGEN. We fit 13 spectrain the first month for SN 2005cs and 18 for SN 2006bp. Swift ultravioletphotometry and ground-based optical photometry calibrate each spectrum.Our analysis shows that both objects were discovered less than 3 daysafter they exploded, making these the earliest SN IIP spectra everstudied. They reveal broad and very weak lines from highly ionized fastejecta with an extremely steep density profile. We identify He IIλ4686 emission in the SN 2006bp ejecta. Days later, the spectraresemble the prototypical Type IIP SN 1999em, which had asupergiant-like photospheric composition. Despite the association of SN2005cs with possible X-ray emission, the emergent UV and optical lightcomes from the photosphere, not from circumstellar emission. We surmisethat the very steep density falloff we infer at early times may be afossil of the combined actions of the shock wave passage and radiationdriving at shock breakout. Based on tailored CMFGEN models, the directfitting technique and the expanding photosphere method both yielddistances and explosion times that agree within a few percent. We derivea distance to NGC 5194, the host of SN 2005cs, of 8.9+/-0.5 Mpc and17.5+/-0.8 Mpc for SN 2006bp in NGC 3953. The luminosity of SN 2006bp is1.5 times that of SN 1999em and 6 times that of SN 2005cs. Reliabledistances to SNe IIP that do not depend on a small range in luminosityprovide an independent route to the Hubble constant and improvedconstraints on other cosmological parameters.
| Gemini Spectroscopy and HST Imaging of the Stellar Cluster Population in Region B of M82
We present new spectroscopic observations of the stellar clusterpopulation of region B in the prototype starburst galaxy M82 obtainedwith the Gemini North 8.1 m telescope. By coupling the spectroscopy withUBVI photometry acquired with the Advanced Camera for Surveys (ACS) onthe Hubble Space Telescope (HST), we derive ages, extinctions, andradial velocities for seven young massive clusters (YMCs) in region B.We find the clusters to have ages between 80 and 200 Myr and velocitiesin the range 230-350 km s-1, while the extinctionsAV vary between ~1 and 2.5 mag. We also find evidence ofdifferential extinction across the faces of some clusters, which hindersthe photometric determination of ages and extinctions in these cases.The cluster radial velocities indicate that the clusters are located atdifferent depths within the disk and are on regular disk orbits. Ourresults overall contradict the findings of previous studies, in whichregion B was thought to be a bound region populated by intermediate-ageclusters that formed in an independent, offset starburst episode thatcommenced 600 Myr-1 Gyr ago. Our findings instead suggest that region Bis optically bright because of low-extinction patches, and that thisallows us to view the cluster population of the inner M82 disk, whichprobably formed as a result of the last encounter with M81. This studyforms part of a series of papers whose aim is to study the clusterpopulation of M82 using deep optical spectroscopy and multibandphotometry.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theScience and Technology Facilities Council (United Kingdom), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), CNPq (Brazil), and CONICET (Argentina).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program 10853.
| Discovery of Four Gravitationally Lensed Quasars from the Sloan Digital Sky Survey
We present the discovery of four gravitationally lensed quasars selectedfrom the spectroscopic quasar catalog of the Sloan Digital Sky Survey(SDSS). We describe imaging and spectroscopic follow-up observationsthat support the lensing interpretation of the following four quasars:SDSS J0832+0404 (image separation θ = 198, source redshiftzs = 1.115, lens redshift zl = 0.659); SDSSJ1216+3529 (θ = 149, zs = 2.012); SDSS J1322+1052(θ = 200, zs = 1.716); and SDSS J1524+4409 (θ =167, zs = 1.210, zl = 0.320). Each system has twolensed images. We find that the fainter image component of SDSSJ0832+0404 is significantly redder than the brighter component, perhapsbecause of differential reddening by the lensing galaxy. The lenspotential of SDSS J1216+3529 might be complicated by the presence of asecondary galaxy near the main lensing galaxy.
| V1647 Orionis: One Year Into Quiescence
We present new optical, near-IR, and mid-IR observations of the youngeruptive variable star V1647 Orionis that went into outburst in late2004 for approximately two years. Our observations, taken one year afterthe star had faded to its pre-outburst optical brightness, show thatV1647 Ori is still actively accreting circumstellar material. We compareand contrast these data with existing observations of the source fromboth pre-outburst and outburst phases. From near-IR spectroscopy weidentify photospheric absorption features for the first time that allowus to constrain the classification of the young star itself. Ourbest-fit spectral type is M0 ± 2 sub-classes with a visualextinction of 19 ± 2 magnitudes and a K-band veiling ofrK ~ 1.5 ± 0.2. We estimate that V1647 Ori has aquiescent bolometric luminosity of ~ 9.5 Lsun and a massaccretion rate of ~ 1 × 10-6 Msunyr-1. Our derived mass and age, from comparison withevolutionary models, are 0.8 ± 0.2 Msun and lsim0.5Myr, respectively. The presence toward the star of shock-excited optical[S II] and [Fe II] emission as well as near-IR H2 and [Fe II]emission perhaps suggests that a new Herbig-Haro flow is becomingvisible close to the star.
| The metal abundance of circumnuclear star-forming regions in early-type spirals. Spectrophotometric observations
We have obtained long-slit observations in the optical and near-infraredof 12 circumnuclear HII regions [circumnuclear star-forming regions(CNSFR)] in the early-type spiral galaxies NGC2903, 3351 and 3504 withthe aim of deriving their chemical abundances. Only for one of theregions, the [SIII] λ6312Å was detected providing, togetherwith the nebular [SIII] lines at λλ9069, 9532Å, avalue of the electron temperature of . A semi-empirical method for thederivation of abundances in the high metallicity regime is presented.We obtain abundances which are comparable to those found in highmetallicity disc HII regions from direct measurements of electrontemperatures and consistent with solar values within the errors. Theregion with the highest oxygen abundance is R3+R4 in NGC3504, 12 +log(O/H) = 8.85, about 1.5 solar if the solar oxygen abundance is set atthe value derived by Asplund, Grevesse & Sauval, 12 +log(O/H)solar = 8.66 +/- 0.05. Region R7 in NGC3351 has thelowest oxygen abundance of the sample, about 0.6 times solar. In all theobserved CNSFR the O/H abundance is dominated by theO+/H+ contribution, as is also the case for highmetallicity disc HII regions. For our observed regions, however, alsothe S+/S2+ ratio is larger than one, contrary towhat is found in high metallicity disc HII regions for which, ingeneral, the sulphur abundances are dominated byS2+/H+.The derived N/O ratios are in average larger than those found in highmetallicity disc HII regions and they do not seem to follow the trend ofN/O versus O/H which marks the secondary behaviour of nitrogen. On theother hand, the S/O ratios span a very narrow range between 0.6 and 0.8of the solar value.As compared to high metallicity disc HII regions, CNSFR show values ofthe O23 and the N2 parameters whose distributions are shiftedto lower and higher values, respectively, hence, even though theirderived oxygen and sulphur abundances are similar, higher values wouldin principle be obtained for the CNSFR if pure empirical methods wereused to estimate abundances. CNSFR also show lower ionization parametersthan their disc counterparts, as derived from the [SII]/[SIII]. Theirionization structure also seems to be different with CNSFR showingradiation field properties more similar to HII galaxies than to dischigh metallicity HII regions.
| An optical spectroscopic HR diagram for low-mass stars and brown dwarfs in Orion
The masses and temperatures of young low-mass stars and brown dwarfs instar-forming regions are not yet well established because ofuncertainties in the age of individual objects and the spectraltype-temperature scale appropriate for objects with ages of only a fewMyr. Using multi-object optical spectroscopy, 45 low-mass stars andbrown dwarfs in the Trapezium Cluster in Orion have been classified and44 of these confirmed as bona fide cluster members. The spectral typesobtained have been converted to effective temperatures using atemperature scale intermediate between those of dwarfs and giants, whichis suitable for young pre-main-sequence objects. The objects have beenplaced on a Hertzsprung-Russell (HR) diagram overlaid with theoreticalisochrones. The low-mass stars and the higher mass substellar objectsare found to be clustered around the 1 Myr isochrone, while many of thelower mass substellar objects are located well above this isochrone. Anaverage age of 1 Myr is found for the majority of the objects. Assumingcoevality of the sources and an average age of 1 Myr, the masses of theobjects have been estimated and range from 0.018 to0.44Msolar. The spectra also allow an investigation of thesurface gravity of the objects by measurement of the sodium doubletequivalent width. With one possible exception, all objects have lowgravities, in line with young ages, and the Na indices for the Trapeziumobjects lie systematically below those of young stars and brown dwarfsin Chamaeleon, suggesting that the 820 nm Na index may provide asensitive means of estimating ages in young clusters.
| High-Resolution Spectroscopy of [Ne II] Emission from TW Hydrae
We present high-resolution echelle spectra of [Ne II] 12.81 μmemission from the classical T Tauri star (CTTS) TW Hya, obtained withMICHELLE on Gemini North. The line is centered at the stellar radialvelocity and has an intrinsic FWHM of 21+/-4 km s-1. The linewidth is broader than other narrow emission lines typically associatedwith the disk around TW Hya. If formed in a disk, the line broadeningcould result from turbulence in a warm disk atmosphere, Keplerianrotation at an average distance of 0.1 AU from the star, or aphotoevaporative flow from the optically thin region of the disk. Weplace upper limits on the [Ne II] emission flux from the CTTSs DP Tauand BP Tau.
| Spectrophotometric Investigations of Blue Compact Dwarf Galaxies: Markarian 35
We present results from a detailed spectrophotometric analysis of theblue compact dwarf galaxy Mrk 35 (Haro 3), based on deep optical (BVRI)and near-IR (JHK) imaging, Hα narrowband observations, andlong-slit spectroscopy. The optical emission of the galaxy is dominatedby a central young starburst, with a barlike shape, while an underlyingcomponent of stars, with elliptical isophotes and red colors, extendsmore than 4 kpc from the galaxy center. High-resolution Hα andcolor maps allow us to identify the star-forming regions, to spatiallydiscriminate them from the older stars, and to recognize several dustpatches. We derive colors and Hα parameters for all the identifiedstar-forming knots. Observables derived for each knot are corrected forthe contribution of the underlying older stellar population, thecontribution by emission lines, and from interstellar extinction, andcompared with evolutionary synthesis models. We find that thecontributions of these three factors are by no means negligible and thatthey significantly vary across the galaxy. Therefore, carefulquantification and subtraction of emission lines, galaxy hostcontribution, and interstellar reddening at every galaxy position areessential to derive the properties of the young stars in blue compactdwarfs. We find that we can reproduce the colors of all the knots withan instantaneous burst of star formation and the Salpeter initial massfunction with an upper mass limit of 100 Msolar. In all casesthe knots are just a few Myr old. The underlying population of stars hascolors consistent with being several Gyr old.
| Stellar Populations of Luminous Evolved Galaxies at z ~ 1.5
Observational evidence has been mounting over the past decade that atleast some luminous (~2L*) galaxies have formed nearly all of theirstars within a short period of time, only (1-2)×109 yrafter the big bang. These are examples of the first major episodes ofstar formation in the universe and provide insights into the formationof the earliest massive galaxies. We have examined in detail the stellarpopulations of six z~1.5 galaxies that appear to be passively evolving,using both ground- and space-based photometry covering rest-frame UV tovisible wavelengths. In addition, we have obtained medium-resolutionspectroscopy for five of the six galaxies, covering the rest-frame UVportion of the spectrum. Spectral synthesis modeling for four of thesegalaxies favors a single burst of star formation more than 1 Gyr beforethe observed epoch. The other two exhibit slightly younger ages with ahigher dust content and evidence for a small contribution from eitherrecent star formation or active nuclei. The implied formation redshiftsfor the oldest of these sources are consistent with previous studies ofpassive galaxies at high redshift, and improved stellar modeling hasshown these results to be quite robust. It now seems clear that anyvalid galaxy formation scenario must be able to account for thesemassive (~2×1011 Msolar) galaxies at veryearly times in the universe.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. Results are also based in part on data collected at theSubaru Telescope, which is operated by the National AstronomicalObservatory of Japan, and on observations made with the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program GO-10418.
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