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|FISICA observations of the starburst galaxy, NGC 1569|
Using the Florida Image Slicer for Infrared Cosmology and Astrophysics(FISICA) we obtained observations of the dwarf starburst galaxy NGC1569. We present our JH band spectra, particularly noting the existenceof extended emission in Paschen β and He I.
|Programs for laser-AO assisted integral-field spectrometers on ionized flows|
An AO-assisted integral-field spectrograph is becoming the mostefficient tool with which to explore ionized gas outflows. It maps faintspectral lines that diagnose cloud dust content, gas pressure,excitation mechanism, and chemical abundances. Coupled with recentimprovements in photoionization models, the total mass hence flowenergetics can be estimated. Establishing a consistent dynamicalframework requires linking multi-frequency datasets to track the energyflow through its optimal-contrast emission in the various ISM phases. Ishow HST results on AGN, starburst nuclei, and Galactic Herbig-HaroObjects that need complementary 3D spectra at comparable spatialresolution to come soon from laser-guided AO + integral-fieldspectrographs at the William-Herschel and SOAR telescopes.
|Formation and evolution of late-type dwarf galaxies - I. NGC1705 and NGC1569|
We present one-zone chemical evolution models for two dwarf starburstgalaxies, NGC1705 and NGC1569. Though especially designed for the inner~1 kpc region, where numerous HII regions and most of the stars areobserved, the models also account for the presence of extended gaseousand dark matter haloes, and properly compute the binding energy of thegas heated by supernova explosions. Using information about the paststar formation history and initial mass function of the systemspreviously obtained from Hubble Space Telescope optical andnear-infrared colour-magnitude diagrams, we identify possible scenariosof chemical enrichment and development of galactic winds. We assume thatthe galactic winds are proportional to the Type II and Type Ia supernovarates. As a consequence, they do not necessarily go to zero when thestar formation stops. In order not to overestimate the currentmetallicity of the interstellar gas inferred from HII regionspectroscopy, we suggest that the winds efficiently remove from thegalaxies the metal-rich ejecta of dying stars. Conversely, requiring thefinal mass of neutral gas to match the value inferred from 21-cmobservations implies a relatively low efficiency of interstellar mediumentrainment in the outflow, thus confirming previous findings that thewinds driving the evolution of typical starbursts are differential.These conclusions could be different only if the galaxies accrete hugefractions of unprocessed gas at late times. By assuming standard stellaryields we obtain a good fit to the observed nitrogen-to-oxygen (N/O)ratio of NGC1569, while the mean N/O ratio in NGC1705 is overestimatedby the models. Reducing the extent of hot bottom burning inlow-metallicity intermediate-mass stars does not suffice to solve theproblem. Localized self-pollution from stars more massive than 60Msolar in NGC1705 and/or funnelling of larger fractions ofnitrogen through its winds are then left to explain the discrepancybetween model predictions and observations. Inspection of the log(N/O)versus log(O/H)+12 diagram for a large sample of dwarf irregular andblue compact dwarf galaxies in the literature favours the latterhypothesis, but the physical mechanisms responsible for such a selectiveloss of metals remain unclear.
|Near-Infrared [Fe II] Emission in Starburst Galaxies. I. Measured Properties|
We used the near-infrared [Fe II] emission line signature to detectsupernova remnants (SNRs) in the nearby starburst galaxies NGC 1569, NGC3738, and NGC 5253. The near-infrared narrowband imaging program has ledto the detection of 10 SNR candidates in NGC 1569, 7 in NGC 5253, andnone in NGC 3738. The luminosity of the SNRs candidates varies from 72to 780 Lsolar and from 69 to 331 Lsolar for NGC1569 and NGC 5253, respectively. Also, a spatially extended component tothe [Fe II] line emission is observed in NGC 1569 and NGC 5253. Thiscomponent dominates the integrated [Fe II] luminosity in both galaxies,the compact sources accounting for 14% and 7% of the total [Fe II]luminosity of NGC 1569 and NGC 5253, respectively.
|The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy|
We present the catalog of 477 spectra from the SerendipitousExtragalactic X-ray Source Identification (SEXSI) program, a surveydesigned to probe the dominant contributors to the 2-10 keV cosmic X-raybackground. Our survey covers 1 deg2 of sky to 2-10 keVfluxes of 1×10-14 ergs cm-2 s-1,and 2 deg2 for fluxes of 3×10-14 ergscm-2 s-1. Our spectra reach to R-band magnitudesof <~24 and have produced identifications and redshifts for 438 hardX-ray sources. Typical completeness levels in the 27 Chandra fieldsstudied are 40%-70%. The vast majority of the 2-10 keV selected sampleare active galactic nuclei (AGNs) with redshifts between 0.1 and 3; ourhighest redshift source lies at z=4.33. We find that few sources atz<1 have high X-ray luminosities, reflecting a dearth of high-mass,high-accretion-rate sources at low redshift, a result consistent withother recent wide-area surveys. We find that half of our sources showsignificant obscuration, with NH>1022cm-2, independent of unobscured luminosity. We classify 168sources as emission-line galaxies; all are X-ray-luminous(LX>1041 ergs s-1) objects withoptical spectra lacking both high-ionization lines and evidence of anonstellar continuum. The redshift distribution of these emission-linegalaxies peaks at a significantly lower redshift than does that of thesources we spectroscopically identify as AGNs. We conclude that few ofthese sources, even at the low-luminosity end, can be powered bystarburst activity. Stacking spectra for a subset of these sources in asimilar redshift range, we detect [Ne V] λ3426 emission, a clearsignature of AGN activity, confirming that the majority of these objectsare Seyfert 2 galaxies in which the high-ionization lines are diluted bystellar emission. We find a total of 33 objects lacking broad lines intheir optical spectra that have quasar X-ray luminosities(LX>1044 ergs s-1), the largestsample of such objects identified to date. In addition, we explore 17AGNs associated with galaxy clusters and find that the cluster-memberAGN sample has a lower fraction of broad-line AGNs than does thebackground sample.The majority of 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, andNASA. The Observatory was made possible by the generous financialsupport of the W. M. Keck Foundation.
|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.
|Three Years of ChaMPlane Northern Field WIYN Spectroscopy|
We present initial results of WIYN spectroscopic observations ofselected objects detected in the Chandra Multiwavelength Plane(ChaMPlane) Survey in fields toward the Galactic anticenter. ChaMPlaneis designed to identify low-luminosity X-ray sources, bothaccretion-powered and stellar coronal, in the Galaxy. It also includes awide-field optical imaging Survey conducted with the NOAO Mosaic camerasto identify optical counterparts, as well as Hα-selected objectsin the ~5 times larger field. We report spectroscopic classificationsfor 1069 objects in Galactic anticenter (i.e., northern) fields,resulting in 612 type determinations. These include 5 new cataclysmicvariables, 4 Be stars, 14 lithium-absorption stars, 182 stellar coronalsources (primarily dMe stars), and 30 new quasars. Bright opticalcounterparts of Chandra sources in this sample are most frequently dMestars, whereas a majority of the faintest (R>20.5 mag)spectroscopically classified Chandra source counterparts are quasars.The bulk of Hα-selected sources appears to be roughly evenlydivided between dMe stars and M stars at all magnitudes.
|Mid-Infrared High Spatial Resolution Observations of NGC 1569: Detection of Embedded Embryos of Star Formation|
We present high spatial resolution mid-infrared (MIR) imaging andspectroscopic observations of the dwarf galaxy NGC 1569 with COMICS onthe Subaru telescope. The [S IV] 10.5 μm image clearly shows fourpointlike sources and one extended source together with the diffuseemission. The brightest MIR source (MIR1) coincides with the intense HII region, which does not have a corresponding optical source and shouldthus be a source embedded in dust clouds. The infrared luminositysuggests that there are about 40 O7 stars associated with MIR1. Thesecond brightest source located ~1.5" south of MIR1 (MIR2) is detectedfor the first time. This source is not seen in the radio continuum.These characteristics can be accounted for if ionizing regions in thesource are compact and optically thick in the radio. The large infraredluminosity (~2×108 Lsolar) suggests thatthere are about 950 O7 stars in MIR2 and significant contributions fromsources other than O-type stars, such as less massive stars and/orpre-main-sequence stars. If this interpretation is correct, MIR2 couldbe in a very early phase of massive star formation. The unidentifiedinfrared (UIR) band at 11.2 μm is also detected in the spectrum ofMIR2. The strength of the UIR band decreases with the [S IV] emission,suggesting that the UIR band weakens and the band carriers are destroyedin strong radiation fields. Super star cluster (SSC) A is also detectedin the N-band images. The mid-infrared characteristics support thedifference in the ages between two components of SSC A.Based on the data collected at Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
|On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass|
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
|Mapping Large-Scale Gaseous Outflows in Ultraluminous Infrared Galaxies with Keck II ESI Spectra: Spatial Extent of the Outflow|
The kinematics of neutral gas and warm ionized gas have been mappedacross ultraluminous starburst galaxies using the Na Iλλ5890, 5896 absorption-line and Hα emission-lineprofiles, respectively, in Keck II ESI spectra. Blueshifted,interstellar absorption is found over extended regions, exceeding 15 kpcin several systems. An outflow diverging from the nuclear starburstwould have to reach large heights to cover this area in projection. Thescale height of the absorbing material could be lower, however, if theoutflow emanates from a larger region of the galaxy. The large velocitygradient discovered across several outflows is inconsistent with a flowdiverging from the nuclear starburst. Widespread star formation,triggered by the merger, probably drives these extended outflows viamechanical feedback from supernovae, although shocks generated by thegalaxy-galaxy merger may also contribute to the formation of a hot wind.In a typical ULIG, the mass carried by the cool phase of the outflow is~108 Msolar i.e., a few percent of the dynamicalmass in the starburst region. Assuming the starburst activity haspersisted for 10 Myr, the kinetic energy of the cool outflows is a fewpercent of the supernova energy. The cool wind is expected to beaccelerated by momentum deposition, possibly from radiation pressure aswell as the ram pressure of the hot, supernova-induced wind. Theturnaround radii of the cool outflows are at least ~30-90 kpc, whichpresents a significant Na I absorption cross section. If mostL>0.1L* galaxies pass through a luminous starburst phase,then relics of cool outflows will create a significant redshift-pathdensity. Galaxy formation models should include this cool phase of theoutflow in addition to a hot wind in feedback models.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
|Cosmological Implications of Dwarf Spheroidal Chemical Evolution|
The chemical properties of dwarf spheroidals in the local group areshown to be inconsistent with star formation being truncated after thereionization epoch (z~8). Enhanced levels of [Ba/Y] in stars in dwarfspheroidals like Sculptor indicate strong s-process production fromlow-mass stars whose lifetimes are comparable with the duration of thepre-reionization epoch. The chemical evolution of Sculptor is followedusing a model with SN II and SN Ia feedback and mass- andmetallicity-dependent nucleosynthetic yields for elements from H to Pb.We are unable to reproduce the Ba/Y ratio unless stars formed over aninterval long enough for the low-mass stars to pollute the interstellarmedium with s-elements. This robust result challenges the suggestionthat most of the local group dwarf spheroidals are fossils ofreionization and supports the case for large initial dark matter halos.
|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.
|Molecular Gas in the Low-Metallicity, Star-forming Dwarf IC 10|
We present a complete survey of CO (1-->0) emission in the LocalGroup dwarf irregular IC 10. The survey, conducted with the BIMAinterferometer, covers the stellar disk and a large fraction of theextended H I envelope with the sensitivity and resolution necessary todetect individual giant molecular clouds (GMCs) at the distance of IC 10(950 kpc). We find 16 clouds with a total CO luminosity of1×106 K km s-1 pc2, equivalent to4×106 Msolar of molecular gas using theGalactic CO-to-H2 conversion factor. Observations with theARO 12 m find that BIMA may resolve out as much as 50% of the COemission, and we estimate the total CO luminosity as~2.2×106 K km s-1 pc2. We measurethe properties of 14 GMCs from high-resolution OVRO data. These cloudsare very similar to Galactic GMCs in their sizes, line widths,luminosities, and CO-to-H2 conversion factors, despite thelow metallicity of IC 10 (Z~1/5 Zsolar). Comparing the BIMAsurvey to the atomic gas and stellar content of IC 10, we find that mostof the CO emission is coincident with high surface density H I. IC 10displays a much higher star formation rate per unit molecular(H2) or total (H I+H2) gas than most galaxies.This could be a real difference or may be an evolutionary effect-thestar formation rate may have been higher in the recent past.
|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.
|Two Populations of Young Massive Star Clusters in Arp 220|
We present new optical observations of young massive star clusters inArp 220, the nearest ultraluminous infrared galaxy, taken in UBVI withthe Hubble Space Telescope ACS HRC camera. We find a total of 206probable clusters whose spatial distribution is centrally concentratedtoward the nucleus of Arp 220. We use model star cluster tracks todetermine ages, luminosities, and masses for 14 clusters with completeUBVI indices or previously published near-infrared data. We estimaterough masses for 24 additional clusters with I<24 mag from BVIindices alone. The clusters with useful ages fall into two distinctgroups: a ``young'' population (<10 Myr) and an intermediate-agepopulation (~=300 Myr). There are many clusters with masses clearlyabove 106 Msolar and possibly even above107 Msolar in the most extreme instances. Thesemasses are high enough that the clusters being formed in the Arp 220starburst can be considered to be genuine young globular clusters. Inaddition, this study allows us to extend the observed correlationbetween global star formation rate and maximum cluster luminosity bymore than 1 order of magnitude in star formation rate.
|On the Incidence and Kinematics of Strong Mg II Absorbers|
We present the results of two complementary investigations into thenature of strong (rest equivalent width, Wr>1.0 Å)Mg II absorption systems at high redshift. The first line of questioningexamines the complete Sloan Digital Sky Survey Data Release 3 set ofquasar spectra to determine the evolution of the incidence of strong MgII absorption. This search resulted in 7421 confirmed Mg II systems ofWr>1.0 Å, yielding a >95% complete statisticalsample of 4835 absorbers (systems detected in S/N>7 spectral regions)spanning a redshift range 0.350.8, indicating thatthe product of the number density and gas cross section of halos hostingstrong Mg II is unevolving at these redshifts. In contrast, one observesa decline in lMg(X) at z<0.8, which we interpret as adecrease in the gas cross section to strong Mg II absorption andtherefore a decline in the physical processes relevant to strong Mg IIabsorption. Perhaps uncoincidentally, this evolution roughly tracks theglobal evolution of the star formation rate density. Dividing thesystems in Wr subsamples, the lMg(X) curves showsimilar shape with lower normalization at higher Wr valuesand a more pronounced decrease in lMg(X) at z<0.8 forlarger Wr systems. We also present the results of a searchfor strong Mg II absorption in a set of 91 high-resolution quasarspectra collected on the ESI and HIRES spectrographs. These data allowus to investigate the kinematics of such systems at 0.81.0 Å werediscovered. These systems are characterized by the presence of numerouscomponents spread over an average velocity width of Δv~200 kms-1. Also, absorption due to more highly ionized species(e.g., Al III, C IV, Si IV) tends to display kinematic profiles similarto the corresponding Mg II and Fe II absorption. We consider all ofthese results in light of two competing theories previously introducedto explain strong Mg II absorption: post-starburst, supernova-drivengalactic winds and accreting gas in the halos of massive galaxies. Thelatter model is especially disfavored by the absence of evolution inlMg(X) at z>1. We argue that the strong Mg II phenomenonprimarily arises from feedback processes in relatively low mass galactichalos related to star formation.
|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.
|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.
|Chemical Enrichment of the Complex Hot ISM of the Antennae Galaxies. II. Physical Properties of the Hot Gas and Supernova Feedback|
We investigate the physical properties of the interstellar medium (ISM)in the merging pair of galaxies known as the Antennae (NGC 4038/4039),using the deep co-added ~411 ks Chandra ACIS-S data set. The method ofanalysis and some of the main results from the spectral analysis, suchas metal abundances and their variations from ~0.2 to ~20-30 timessolar, are described in Paper I (Baldi et al.). In the present paper weinvestigate in detail the physics of the hot emitting gas, derivingmeasures for the hot gas mass (~107 Msolar),cooling times (107-108 yr), and pressure(3.5×10-11-2.8×10-10 dynecm-2). In at least one of the two nuclei (NGC 4038), the hotgas pressure is significantly higher than the CO pressure, implying thatshock waves may be driven into the CO clouds. Comparison of the metalabundances with the average stellar yields predicted by theoreticalmodels of SN explosions points to SNe of Type II as the maincontributors of metals to the hot ISM. There is no evidence of anycorrelation between radio-optical star formation indicators and themeasured metal abundances. Although due to uncertainties in the averagegas density we cannot exclude that mixing may have played an importantrole, the short time required to produce the observed metal masses(<~2 Myr) suggests that the correlations are unlikely to have beendestroyed by efficient mixing. More likely, a significant fraction ofType II SN ejecta may be in a cool phase, in grains, or escaping in hotwinds. In each case, any such fraction of the ejecta would remainundetectable with soft X-ray observations.
|Environment and luminosity of supernova remnants|
The explosion of supernovae and the evolution of their remnants (SNRs)accelerate cosmic rays over a vast range of timescales. Magnetic fieldscan be investigated indirectly through one of the observationalsignatures of this acceleration, namely radio synchrotron emission. Withthe aim of better understanding the role of the magnetic field insupernova evolution, we explore the variation of SNR radio luminositieswith physical conditions in the surrounding interstellar medium. With adata set that comprises more than 90 individual SNRs in 10 galaxies, anda range of 3000 in ISM density and 104 in radio synchrotronluminosity, we find a significant correlation between the twoquantities. The observed trends support the hypothesis that adiabaticcompression of magnetic fields by itself is insufficient to explain theradio emission of the brighter and more luminous in SNRs.
|Weak redshift discretisation in the Local Group of galaxies?|
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
|Massive Star Cluster Populations in Irregular Galaxies as Probable Younger Counterparts of Old Metal-rich Globular Cluster Populations in Spheroids|
Peak metallicities of metal-rich populations of globular clusters(MRGCs) belonging to early-type galaxies and spheroidal subsystems ofspiral galaxies (spheroids) of different mass fall within the somewhatconservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible ageeffects are taken into account, this metallicity range might becomesmaller. Irregular galaxies such as the Large Magellanic Cloud (LMC),with longer timescales of formation and lower star formation (SF)efficiency, do not contain old MRGCs with [Fe/H]>-1.0, but they areobserved to form populations of young/intermediate-age massive starclusters (MSCs) with masses exceeding 104 Msolar.Their formation is widely believed to be an accidental process fullydependent on external factors. From the analysis of available data onthe populations and their hosts, including intermediate-age populousstar clusters in the LMC, we find that their most probable meanmetallicities fall within -0.7<=[Fe/H]<=-0.3, as the peakmetallicities of MRGCs do, irrespective of signs of interaction.Moreover, both the disk giant metallicity distribution function (MDF) inthe LMC and the MDFs for old giants in the halos of massive spheroidsexhibit a significant increase toward [Fe/H]~-0.5. That is in agreementwith a correlation found between SF activity in galaxies and theirmetallicity. The formation of both the old MRGCs in spheroids and MSCpopulations in irregular galaxies probably occurs at approximately thesame stage of the host galaxies' chemical evolution and is related tothe essentially increased SF activity in the hosts around the samemetallicity that is achieved very early in massive spheroids, later inlower mass spheroids, and much later in irregular galaxies. Changes inthe interstellar dust, particularly in elemental abundances in dustgrains and in the mass distribution function of the grains, may be amongthe factors regulating star and MSC formation activity in galaxies.Strong interactions and mergers affecting the MSC formation presumablyplay an additional role, although they can substantially intensify theinternally regulated MSC formation process. Several implications of oursuggestions are briefly discussed.
|The Starburst-Interstellar Medium Interaction in NGC 1569. II. Small-Scale Examination of Nebular Emission, H II Region Size Distribution, and H II Region Luminosity Function|
As the nearest dramatic example of a poststarburst galaxy driving agalactic wind, NGC 1569 is an ideal test environment to understand theimpact of ``feedback'' from massive star lives and deaths on thesurrounding interstellar medium. We present Hubble Space Telescope WideField Planetary Camera 2 narrowband imagery of NGC 1569 in an attempt tounderstand the underlying ionizing emission mechanisms on a 3 pc scaleand to generate a H II region size distribution and luminosity function.We use [O III]/Hβ and [S II]/Hα ratio maps to find thatnonphotoionizing mechanisms (e.g., shocks) are responsible for 10%+/-3%of the Hα emission, 2.5-3 times larger than results from similargalaxies. Note that our method of determining this result is differentfrom these past results, a point that we discuss further in this paper.The area of the nonphotoionized region is 10%-23% of the total. Ourresults for NGC 1569 indicate that these nonphotoionized areas do notlie in low surface brightness regions exclusively. A comparison withmultiwavelength point-source catalogs of NGC 1569 indicates that thedominant nonphotoionizing mechanisms are shocks from supernovae or windsfrom massive stars. To explain this large percentage of nonphotoionizedemission, we suggest that NGC 1569 is, indeed, in a poststarburst phase,as previous authors have claimed. We also derive slopes for the H IIregion luminosity function (-1.00+/-0.08) and size distribution(-3.02+/-0.27). The luminosity slope, although shallow, is similar toprevious work on this galaxy and other irregular galaxies. The sizedistribution slope is shallower than previous slopes found for irregulargalaxies, but our slope value fits into their confidence intervals, andvice versa. Within 4 pc of the 10-20 Myr old super star clusters A1, A2,and B, no bright H II regions exist to a luminosity limit of2.95×1036 ergs s-1, suggesting that thewinds and shocks have effectively terminated star formation in thissmall cavity. In the three annular regions around the super starclusters, both the H II region luminosity function and H II region sizedistribution are consistent with respect to one another and the galaxyas a whole. The H II region surface densities within the annuli remainthe same as the annuli are moved away from the super star clusters.These results indicate that feedback effects in NGC 1569 are confined tothe immediate vicinity of the most recent massive star formation eventon scales of ~1 pc.
|The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31|
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
|Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field|
We present Hubble Space Telescope Advanced Camera for Surveys images andcolor-magnitude diagrams for 25 nearby galaxies with radial velocitiesVLG<500 km s-1. Distances are determined basedon the luminosities of stars at the tip of the red giant branch thatrange from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, arefound to be the nearest known representatives of blue compact dwarfobjects. Using high-quality data on distances and radial velocities of110 nearby field galaxies, we derive their mean Hubble ratio to be 68 kms-1 Mpc-1 with a standard deviation of 15 kms-1 Mpc-1. Peculiar velocities of most of thegalaxies, Vpec=VLG-68D, follow a Gaussiandistribution with σv=63 km s-1 but with atail toward high negative values. Our data display the known correlationbetween peculiar velocity and galaxy elevation above the LocalSupercluster plane. The small observed fraction of galaxies with highpeculiar velocities, Vpec<-500 km s-1, may beunderstood as objects associated with nearby groups (Coma I, Eridanus)outside the local volume.
|Infrared Properties of Star-forming Dwarf Galaxies. II. Blue Compact Dwarf Galaxies in the Virgo Cluster|
A sample of 16 blue compact dwarf galaxies (BCDs) in the Virgo Clusterhas been imaged in the near-infrared (NIR) in J and Ks on the2.1 m telescope at the Observatorio Astronómico Nacional in theSierra San Pedro Mártir in Mexico. Isophotes as faint asμJ=24 mag arcsec-2 andμKs=23 mag arcsec-2 have beenreached in most of the targets. Surface brightness profiles can befitted across the whole range of radii by the sum of two components: ahyperbolic secant (sech) function, which is known to fit the lightprofiles of dwarf irregular galaxies (dIs), and a Gaussian component,which quantifies the starburst near the center. Isophotal and totalfitted NIR magnitudes have been calculated, along with semimajor axes atμJ=23 mag arcsec-2 andμKs=22 mag arcsec-2. The diffuseunderlying component and the young starburst have been quantified usingthe profile fitting. Most color profiles show a constant color, betweenJ-Ks=0.7 and 0.9 mag. The diffuse component represents theoverwhelming majority of the NIR light for most BCDs, with the starburstenhancing the flux by less than about 0.3 mag. Linear correlations werefound between the sech scale length and the sech magnitude and betweenthe sech semimajor axis and the sech magnitude. Overall, galaxies withmore luminous diffuse components are larger and brighter in the center.The central burst correlates with the diffuse component, with brighterBCDs having stronger starbursts, suggesting that more massive objectsare forming stars more efficiently. BCDs lie on the ``fundamentalplane'' defined by dIs in Paper I, following the same relation betweensech absolute magnitude, sech central surface brightness, and thehydrogen line width W20, although the scatter is larger thanfor the dIs. On the other hand, correlations between the sech absolutemagnitude and the sech central surface brightness in Ks forBCDs and dIs are equally good, indicating that BCD line widths may beenhanced by turbulence or winds.These data were acquired at the Observatorio Astronómico Nacionalin the Sierra San Pedro Mártir, Mexico.
|Distribution of extinction and star formation in NGC 1569|
We investigate the spatial distribution of the intrinsic extinction inthe starburst dwarf galaxy NGC 1569 using an extinction map of the wholegalaxy derived from the mbox H α/ mbox H β emission lineratio. We differentiate the extinction in the H ii regions from theextinction of the diffuse gas. The intrinsic extinction showsconsiderable variations over the plane of the galaxy, from negligibleextinction up to AV = 0.8 mag. The extinction map shows smallscale clumpy structures possibly due to a clumpy dust distribution. Wealso identify a shell structure in this map, for which we establish acausal relation with the expanding gas structure produced by the stellarwinds coming from the Super Star Clusters (SSC) in the center of thegalaxy. The comparison of the spatial profiles of the extinction, dustand gaseous emissions crossing the border of the shell shows a layeredstructure; the peak of this mbox H α distribution lies closest tothe SSC A, followed outwards by the peak of the extinction and at astill larger distance by the bulk of the atomic gas. We suggest that theextinction shell has been produced by the SSC and that it can beexplained by the accumulation of dust at the border of this ionized gasstructure.
|[CII] 158 μm emission and metallicity in photon dominated regions|
We study the effects of a metallicity variation on the thermal balanceand [CII] fine-structure line strengths in interstellar photon dominatedregions (PDRs). We find that a reduction in the dust-to-gas ratio andthe abundance of heavy elements in the gas phase changes the heatbalance of the gas in PDRs. The surface temperature of PDRs decreases asthe metallicity decreases except for high density (n>106cm-3) clouds exposed to weak (χ< 100) FUV fields wherevibrational H2-deexcitation heating dominates over photoelectric heatingof the gas. We incorporate the metallicity dependence in our KOSMA-τPDR model to study the metallicity dependence of [CII]/CO line ratios inlow metallicity galaxies. We find that the main trend in the variationof the observed CII/CO ratio with metallicity is well reproduced by asingle spherical clump, and does not necessarily require an ensemble ofclumps as in the semi-analytical model presented by Bolatto et al.(1999).
|The luminosity function of young star clusters: implications for the maximum mass and luminosity of clusters|
We introduce a method to relate a possible truncation of the starcluster mass function at the high mass end to the shape of the clusterluminosity function (LF). We compare the observed LFs of five galaxiescontaining young star clusters with synthetic cluster population modelswith varying initial conditions. The LF of the SMC, the LMC and NGC 5236are characterized by a power-law behavior N d L ∝L-α d L, with a mean exponent of < α> = 2.0± 0.2. This can be explained by a cluster population formed witha constant cluster formation rate, in which the maximum cluster mass perlogarithmic age bin is determined by the size-of-sample effect andtherefore increases with log (age/yr). The LFs of NGC 6946 and M 51 arebetter described by a double power-law distribution or a Schechterfunction. When a cluster population has a mass function that istruncated below the limit given by the size-of-sample effect, the totalLF shows a bend at the magnitude of the maximum mass, with the age ofthe oldest cluster in the population, typically a few Gyr due todisruption. For NGC 6946 and M 51 this suggests a maximum mass of M_max= 0.5-1×10^6 Mȯ, although the bend is only a 1-2σ detection. Faint-ward of the bend the LF has the same slope asthe underlying initial cluster mass function and bright-ward of the bendit is steeper. This behavior can be well explained by our populationmodel. We compare our results with the only other galaxy for which abend in the LF has been observed, the "Antennae" galaxies (NGC4038/4039). There the bend occurs brighter than in NGC 6946 and M 51,corresponding to a maximum cluster mass of M_max =1.3-2.5×106 Mȯ. Hence, if the maximumcluster mass has a physical limit, then it can vary between differentgalaxies. The fact that we only observe this bend in the LF in the"Antennae" galaxies, NGC 6946 and M 51 is because there are enoughclusters available to reach the limit. In other galaxies there might bea physical limit as well, but the number of clusters formed or observedis so low, that the LF is not sampled up to the luminosity of the bend.The LF can then be approximated with a single power-law distribution,with an index similar to the initial mass function index.
|Dynamical mass estimates for two luminous star clusters in galactic merger remnants|
We present high-dispersion spectra of two extremely massive starclusters in galactic merger remnants, obtained using the UVESspectrograph mounted on the ESO Very Large Telescope. One cluster, W30,is located in the ~500 Myr old merger remnant NGC 7252 and has avelocity dispersion and effective radius of σ=27.5±2.5 kms-1 and Reff=9.3±1.7 pc, respectively. Theother cluster, G114, located in the ~3 Gyr old merger remnant NGC 1316,is much more compact, Reff=4.08±0.55 pc, and has avelocity dispersion of σ=42.1±2.8 km s-1. Thesemeasurements allow an estimate of the virial mass of the two clusters,yielding Mdyn(W30)=1.59(±0.26)× 10^7Mȯ and Mdyn(G114)=1.64(±0.13)×10^7 Mȯ. Both clusters are extremely massive, being morethan three times heavier than the most massive globular clusters in theGalaxy. For both clusters we measure light-to-mass ratios, which whencompared to simple stellar population (SSP) models of the appropriateage, are consistent with a Kroupa-type stellar mass function. Usingmeasurements from the literature we find a strong age dependence on howwell SSP models (with underlying Kroupa or Salpeter-type stellar massfunctions) fit the light-to-mass ratio of clusters. Based on this resultwe suggest that the large scatter in the light-to-mass ratio of theyoungest clusters is not due to variations in the underlying stellarmass function, but instead to the rapidly changing internal dynamics ofyoung clusters. Based on sampling statistics we argue that while W30 andG114 are extremely massive, they are consistent with being the mostmassive clusters formed in a continuous power-law cluster massdistribution. Finally, based on the positions of old globular clusters,young massive clusters (YMCs), ultra-compact dwarf galaxies (UCDs) anddwarf-globular transition objects (DGTOs) in κ-space we concludethat 1) UCDs and DGTOs are consistent with the high mass end of starclusters and 2) YMCs occupy a much larger parameter space than oldglobular clusters, consistent with the idea of preferential disruptionof star clusters.
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