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|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.
|The Survey for Ionization in Neutral Gas Galaxies. I. Description and Initial Results|
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG),a census of star formation in H I-selected galaxies. The survey consistsof Hα and R-band imaging of a sample of 468 galaxies selected fromthe H I Parkes All Sky Survey (HIPASS). The sample spans three decadesin H I mass and is free of many of the biases that affect otherstar-forming galaxy samples. We present the criteria for sampleselection, list the entire sample, discuss our observational techniques,and describe the data reduction and calibration methods. This paperfocuses on 93 SINGG targets whose observations have been fully reducedand analyzed to date. The majority of these show a single emission linegalaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs ina single field. All of the targets in this sample are detected inHα, indicating that dormant (non-star-forming) galaxies withMHI>~3×107 Msolar are veryrare. A database of the measured global properties of the ELGs ispresented. The ELG sample spans 4 orders of magnitude in luminosity(Hα and R band), and Hα surface brightness, nearly 3 ordersof magnitude in R surface brightness and nearly 2 orders of magnitude inHα equivalent width (EW). The surface brightness distribution ofour sample is broader than that of the Sloan Digital Sky Survey (SDSS)spectroscopic sample, the EW distribution is broader than prism-selectedsamples, and the morphologies found include all common types ofstar-forming galaxies (e.g., irregular, spiral, blue compact dwarf,starbursts, merging and colliding systems, and even residual starformation in S0 and Sa spirals). Thus, SINGG presents a superior censusof star formation in the local universe suitable for further studiesranging from the analysis of H II regions to determination of the localcosmic star formation rate density.
|A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds|
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
|A FUSE Survey of High-Latitude Galactic Molecular Hydrogen|
Measurements of molecular hydrogen (H2) column densities arepresented for the first six rotational levels (J=0-5) for 73extragalactic targets observed with the Far Ultraviolet SpectroscopicExplorer (FUSE). All of these have a final signal-to-noise ratio largerthan 10 and are located at Galactic latitude |b|>20deg.The individual observations were calibrated with the FUSE calibrationpipeline CalFUSE version 2.1 or higher and then carefully aligned invelocity. The final velocity shifts for all the FUSE segments arelisted. H2 column densities or limits are determined for thesix lowest rotational (J) levels for each H I component in the line ofsight, using a curve-of-growth approach at low column densities(<16.5) and Voigt-profile fitting at higher column densities.Detections include 65 measurements of low-velocity H2 in theGalactic disk and lower halo. Eight sight lines yield nondetections forGalactic H2. The measured column densities range fromlogN(H2)=14 to 20. Strong correlations are found betweenlogN(H2) and T01, the excitation temperature ofthe H2, as well as between logN(H2) and the levelpopulation ratios (log[N(J')/N(J)]). The average fraction ofnuclei in molecular hydrogen [f(H2)] in each sight line iscalculated; however, because there are many H I clouds in each sightline, the physics of the transition from H I to H2 cannot bestudied. Detections also include H2 in 16intermediate-velocity clouds in the Galactic halo (out of 35 IVCs).Molecular hydrogen is seen in one high-velocity cloud (the Leading Armof the Magellanic Stream), although 19 high-velocity clouds areintersected; this strongly suggests that dust is rare or absent in theseobjects. Finally, there are five detections of H2 in externalgalaxies.
|Broadband Imaging of a Large Sample of Irregular Galaxies|
We present the results of UBV imaging of a large sample of irregulargalaxies: 94 Im systems, 24 blue compact dwarfs (BCDs), and 18 Smgalaxies. We also include JHK imaging of 42 of these galaxies. Thesample spans a large range in galactic parameters. Ellipse fit axialratios, inclinations, and position angles are derived, integratedphotometry and azimuthally averaged surface photometry profiles aredetermined, and exponential fits give the central surface brightnesses,scale lengths, and isophotal and half-power radii. These data are usedto address the shapes of Im galaxies, look for clues to pastinteractions in large-scale peculiarities, examine the nature andconsequences of bars, study color gradients and large-scale colorvariations, and compare the exponential disk profiles of the young andold stellar components. For example, color gradients exhibit a greatvariety and not all passbands are correlated. Bars are associated withhigher star formation rates. Many irregulars show a double-exponentialradial light profile that is steeper in the outer parts, and these arereproduced by a new model of star formation that is discussed in acompanion paper. Some galaxies, primarily BCDs, have double exponentialsthat are steeper (and bluer) in the inner parts, presumably fromcentralized star formation. Im-type galaxies have thicker, lessprominent dust layers than spiral galaxies because of their loweraverage surface densities and midplane extinctions.
|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.
|Warm Dust and Spatially Variable Polycyclic Aromatic Hydrocarbon Emission in the Dwarf Starburst Galaxy NGC 1705|
We present Spitzer observations of the nearby dwarf starburst galaxy NGC1705 obtained as part of the Spitzer Infrared Nearby Galaxies Survey.The galaxy morphology is very different shortward and longward of ~5μm: optical and short-wavelength IRAC imaging shows an underlying redstellar population, with the central super star cluster (SSC) dominatingthe luminosity; longer wavelength IRAC and MIPS imaging reveals warmdust emission arising from two off-nuclear regions that are offset by~250 pc from the SSC and that dominate the far-IR flux of the system.These regions show little extinction at optical wavelengths. The galaxyhas a relatively low global dust mass (~2 × 105Msolar, implying a global dust-to-gas mass ratio ~2-4 timeslower than the Milky Way average, roughly consistent with themetallicity decrease). The off-nuclear dust emission appears to bepowered by photons from the same stellar population responsible for theexcitation of the observed Hα emission; these photons areunassociated with the SSC (although a contribution from embedded sourcesto the IR luminosity of the off-nuclear regions cannot be ruled out).Low-resolution IRS spectroscopy shows moderate-strength PAH emission inthe 11.3 μm band in the more luminous eastern peak; no PAH emissionis detected in the SSC or the western dust emission complex. There issignificant diffuse emission in the IRAC 8 μm band after starlighthas been removed by scaling shorter wavelength data; the fact that IRSspectroscopy shows spatially variable PAH emission strengths compared tothe local continuum within this diffuse gas suggests caution in theinterpretation of IRAC diffuse 8 μm emission as arising from PAHcarriers alone. The nebular metallicity of NGC 1705 falls at thetransition level of ~0.35 Zsolar found by Engelbracht andcollaborators, below which PAH emission is difficult to detect; the factthat a system at this metallicity shows spatially variable PAH emissiondemonstrates the complexity of interpreting diffuse 8 μm emission ingalaxies. NGC 1705 deviates significantly from the canonicalfar-infrared versus radio correlation, having significant far-infraredemission but no detected radio continuum.
|Kinematics of Interstellar Gas in Nearby UV-selected Galaxies Measured with HST STIS Spectroscopy|
We measure Doppler shifts of interstellar absorption lines in HST STISspectra of individual star clusters in nearby UV-selected galaxies.Values for systemic velocities, which are needed to quantify outflowspeeds, are taken from the literature and verified with stellar lines.We detect outflowing gas in 8 of 17 galaxies via low-ionization lines(e.g., C II, Si II, Al II), which trace cold and/or warm gas. Thestarbursts in our sample are intermediate in luminosity (and mass) todwarf galaxies and luminous infrared galaxies (LIRGs), and we confirmthat their outflow speeds (ranging from -100 to nearly -520 kms-1, with an accuracy of ~80 km s-1) areintermediate to those previously measured in dwarf starbursts and LIRGs.We do not detect the outflow in high-ionization lines (such as C IV orSi IV); higher quality data will be needed to empirically establish howvelocities vary with the ionization state of the outflow. We do verifythat the low-ionization UV lines and optical Na I doublet give roughlyconsistent outflow velocities, solidifying an important link betweenstudies of galactic winds at low and high redshift. To obtain a highersignal-to-noise ratio (S/N), we create a local average compositespectrum and compare it to the high-z Lyman break composite spectrum. Itis surprising that the low-ionization lines show similar outflowvelocities in the two samples. We attribute this to a combination ofweighting toward higher luminosities in the local composite, as well asboth samples being, on average, brighter than the ``turnover''luminosity in the v-SFR relation.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program GO-9036.
|Hot Dust and Polycyclic Aromatic Hydrocarbon Emission at Low Metallicity: A Spitzer Survey of Local Group and Other Nearby Dwarf Galaxies|
We present Spitzer 4.5 and 8.0 μm imaging of 15 Local Group andnearby dwarf galaxies. We find that the diffuse 8 μm emission isspatially correlated with regions of active star formation. Our samplespans a range of >1 dex in nebular metallicity and 3 orders ofmagnitude in current star formation rate, allowing us to examine thedependence of emission from hot dust and PAHs on these parameters. Wedetect prominent diffuse 8 μm emission from the four most luminousgalaxies in the sample (IC 1613, IC 5152, NGC 55, and NGC 3109) and onlyvery low surface brightness emission from four others (DDO 216, SextansA, Sextans B, and WLM). These are the first spatially resolved images ofdiffuse 8 μm emission from such low-metallicity objects[12+log(O/H)~7.5]. We observe correlations of this emission with thecurrent star formation rate and the nebular metallicity of thesegalaxies. However, we also see evidence suggesting that other processesmay also have a significant effect on the generation of this emission.These systems all have evidence for old and intermediate-age starformation; thus, the lack of diffuse 8 μm emission cannot beattributed to low galaxy ages. Also, winds cannot explain the paucity ofthis emission, since high-resolution imaging of the neutral gas in theseobjects shows no evidence of blowout. We propose that the lack ofdiffuse 8 μm emission in low-metallicity systems may be due to thedestruction of dust grains by supernova shocks, assuming a longtimescale to regrow dust. It is likely that the observed weak emissionis at least partly due to a general absence of dust (including PAHs), inagreement with their low metallicities.
|Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies|
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
|The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822|
To test the existence of a possible radial gradient in oxygen abundanceswithin the Local Group dwarf irregular galaxy NGC 6822, we have obtainedoptical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6 mtelescope at ESO La Silla. The extent of the measured nebulae spansgalactocentric radii in the range between 0.05 and 2 kpc (over 4exponential scale lengths). In five H II regions (Hubble I, Hubble V,Kα, Kβ, KD 28e), the temperature-sensitive [O III]λ4363 emission line was detected, and direct oxygen abundanceswere derived. Oxygen abundances for the remaining H II regions werederived using bright-line methods. The oxygen abundances for threeA-type supergiant stars are slightly higher than nebular values atcomparable radii. Linear least-square fits to various subsets ofabundance data were obtained. When all of the measured nebulae areincluded, no clear signature is found for an abundance gradient. A fitto only newly observed H II regions with [O III] λ4363 detectionsyields an oxygen abundance gradient of -0.14+/-0.07 dexkpc-1. The gradient becomes slightly more significant(-0.16+/-0.05 dex kpc-1) when three additional H II regionswith [O III] λ4363 measurements from the literature are added.Assuming no abundance gradient, we derive a mean nebular oxygenabundance 12+log(O/H)=8.11+/-0.10 from [O III] λ4363 detectionsin the five H II regions from our present data; this mean valuecorresponds to [O/H]=-0.55.Based on EFOSC2 observations collected at the European SouthernObservatory, Chile: proposal 71.B-0549(A).
|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.
|A FUSE Survey of Interstellar Molecular Hydrogen toward High-Latitude AGNs|
We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE)survey of interstellar molecular hydrogen (H2) along 45 sightlines to AGNs at high Galactic latitudes (b>20deg). Most(39 out of 45) of the sight lines show detectable Galactic H2absorption from Lyman and Werner bands between 1000 and 1126 Å,with column densities ranging fromNH2=1014.17 to 1019.82cm-2. In the northern Galactic hemisphere, we identify manyregions of low NH2 (<=1015cm-2) between l=60deg and 180° and atb>54deg. These ``H2 holes'' provide valuable,uncontaminated sight lines for extragalactic UV spectroscopy, and a fewmay be related to the ``Northern Chimney'' (low Na I absorption) and the``Lockman Hole'' (low NHI). A comparison of high-latitudeH2 with 139 OB star sight lines surveyed in the Galactic disksuggests that high-latitude and disk H2 clouds may havedifferent rates of heating, cooling, and UV excitation. For rotationalstates J=0 and 1, the mean excitation temperature at high latitude,=124+/-8 K, is somewhat higher thanthat in the Galactic disk, =86+/-20K. For J>=2, the mean =498+/-28 K, and thecolumn-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate acomparable degree of UV excitation in the disk and low halo for sightlines with NH2>=1018cm-2. The distribution of molecular fractions at highlatitude shows a transition at lower total hydrogen column density(logNhlH~20.38+/-0.13) than in the Galactic disk(logNdiskH~20.7). If the UV radiation fields aresimilar in disk and low halo, this suggests an enhanced H2(dust-catalyzed) formation rate in higher density, compressed clouds,which could be detectable as high-latitude, sheetlike infrared cirrus.
|Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton|
We investigated the dynamical structure of 53 elliptical galaxies usingthe Chandra archival X-ray data. In X-ray-luminous galaxies, temperatureincreases with radius and gas density is systematically higher at theoptical outskirts, indicating the presence of a significant amount ofthe group-scale hot gas. In contrast, X-ray-dim galaxies show a flat ordeclining temperature profile against radius and the gas density isrelatively lower at the optical outskirts. Thus, it is found thatX-ray-bright and faint elliptical galaxies are clearly distinguished bythe temperature and gas density profile. The mass profile is well scaledby a virial radius r200 rather than an optical half-radiusre, is quite similar at (0.001-0.03)r200 betweenX-ray-luminous and dim galaxies, and smoothly connects to those profilesof clusters of galaxies. At the inner region of(0.001-0.01)r200 or (0.1-1)re, the mass profilewell traces a stellar mass with a constant mass-to-light ratio ofM/LB=3-10 Msolar/Lsolar. TheM/LB ratio of X-ray-bright galaxies rises up steeply beyond0.01r200 and thus requires a presence of massive dark matterhalo. From the deprojection analysis combined with the XMM-Newton data,we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 alsohave a high M/LB ratio of 20-30 at 20 kpc, comparable to thatof X-ray-luminous galaxies. Therefore, dark matter is indicated to becommon in elliptical galaxies; their dark matter distribution, as wellas that of galaxy clusters, almost follows the NFW profile.
|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.
|Mid-Infrared Images of Stars and Dust in Irregular Galaxies|
We present mid-IR to optical properties of 22 representative irregulargalaxies: 18 irregular (Im) galaxies, 3 blue compact dwarfs, and 1Magellanic-type spiral galaxy. The mid-IR is based on images from theSpitzer Space Telescope archives. The 3.6 and 4.5 μm bands and theUBVJHK images are used to examine disk morphology and the integrated andazimuthally averaged magnitudes and colors of stars. The nonstellarcontribution to the 4.5 μm images is used to trace hot dust. The 5.8and 8.0 μm images reveal emission from hot dust and polycyclicaromatic hydrocarbons (PAHs), and both may contribute to thesepassbands, although we refer to the nonstellar emission as PAH emission.We compare the 8.0 μm images to Hα. Im galaxies have no hiddenbars, and those with double-exponential optical light profiles have thesame at mid-IR. Most galaxies have similar optical and mid-IR scalelengths. Four galaxies have super star clusters that are not visible atoptical bands. Galaxies with higher area-normalized star formation rateshave more dust and PAH emission relative to starlight. Hot dust and PAHemission is found mostly in high surface brightness H II regions,implying that massive stars are the primary source of heating. Galaxieswith intense, widespread star formation have more extended PAH emission.The ratio of PAH to Hα emission is not constant on small scales.PAHs are associated with shells and giant filaments, so they are notdestroyed during shell formation.This work is based in part on archival data obtained with the SpitzerSpace Telescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under a contract with NASA.
|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
|A Spectroscopic Study of the Star-Forming Properties of the Center of NGC 4194|
We have obtained Space Telescope Imaging Spectrograph long-slit spectraof the central region of the advanced merger NGC 4194. The spectra coverthe wavelength ranges 1150-1750 Å in the UV and 2900-10270 Åin the visible. Results from the study of the properties of 14star-forming regions (knots) are presented. If the [N II] contributionis 40% of the combined Hα + [N II] flux, then the averageE(B-V)=0.7 mag. The metal abundances are approximately solar, withindividual knot abundances ranging from log(O/H)+12=8.1+/-0.5 to8.9+/-0.4. The Hα luminosities of the 14 observed knots yield atotal star formation rate (SFR) of ~46 Msolaryr-1. The sizes of the H II regions associated with the knotswere determined from L(Hβ) and range from ~28 to ~119 pc when afilling factor of 0.1 is assumed. The sizes are a factor of ~2.15smaller for a filling factor of 1. Using Starburst99, the EW(Hα +[N II]), and EW(Hβ), we estimate the ages of the star-formingregions to be 5.5-10.5 Myr. From ground-based spectra the effectivetemperatures of the H II regions are found to be ~11,000 K, and theelectron densities are determined to be ~530 cm-3. We findthat eight of the knots probably formed with a Salpeter initial massfunction truncated at an upper mass of 30 Msolar, and one ofthe knots likely formed with a standard Salpeter initial mass function.We suggest that the knots in our sample are the precursors of globularclusters. Two of the knots are in a region of flowing gas and are amongthe most massive, are the largest in radius, have the highest SFR, andare among the youngest of the knots.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute (STScI), which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
|Remarkable Disk and Off-Nuclear Starburst Activity in the Tadpole Galaxy as revealed by the Spitzer Space Telescope|
We present ground-based optical and Spitzer Space Telescope infraredimaging observations of the interacting galaxy UGC 10214, the Tadpolegalaxy (z=0.0310), focusing on the star formation activity in thenuclear, disk, spiral arms, and tidal tail regions. The ground-basedoptical data set spans a wavelength range between 0.3 and 0.8 μm, thenear-IR data set spans 1-2.2 μm, and the Spitzer IR data set spans3-70 μm. The major findings of this study are that the Tadpole isactively forming stars in the main disk outside of the nucleus and inthe tidal plume, with an estimated mean star formation rate of ~2-4Msolar yr-1. The most prominent sites of mid-IRemission define a ``ring'' morphology that, combined with the overallmorphology of the system, suggests the interaction may belong to therare class of off-center collisional ring systems that form bothshock-induced rings of star formation and tidal plumes. In starkcontrast to the disk star formation, the nuclear emission is solelypowered by older stars, with little evidence for ongoing star formationat the center of the Tadpole. Extranuclear star formation accounts for>50% of the total star formation in the disk and spiral arms,featuring infrared-bright ``hot spots'' that exhibit strong polycyclicaromatic hydrocarbon (PAH) emission, the band strength of which iscomparable to that of late-type star-forming disk galaxies. The tidaltail, which extends 2' (~75 kpc) into the intergalactic medium, ispopulated by supermassive star clusters, M~106Msolar, likely triggered by the galaxy-galaxy interactionthat has distorted UGC 10214 into its current ``tadpole'' shape. TheTadpole is therefore an example of an off-nuclear or tidal-tailstarburst, with several large sites of massive star formation in thedisk and in the plume, including the most prominent Hubble SpaceTelescope-revealed cluster, J160616.85+552640.6. The clusters exhibitremarkable IR properties, including exceptionally strong 24 μmemission relative to the underlying starlight, hot dust continuum, andPAH emission, with an estimated current star formation rate of ~0.1-0.4Msolar yr-1, representing >10% of the totalstar formation in the system. We estimate the mass of the largestcluster to be ~(1.4-1.6)×106 Msolar based onthe g'-band (0.5 μm) and near-IR (2.2 μm) integrated fluxes incombination with an assumed mass-to-light ratio appropriate to youngclusters, or large enough to be classified as a nascent dwarf galaxy orglobular cluster.
|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.
|Stellar clusters in dwarf galaxies|
We present new observations in the Ks (2.2 μm) and L' (3.7μm) infrared bands of a sample of blue dwarf galaxies with the largeraim of studying the population of massive stellar clusters, theoccurrence of dust-embedded stellar clusters, and their properties. AllKs images show a rich population of clusters, but only asmall fraction of them is bright in L'. Most L' sources have radiocounterparts. We derived the luminosity function in Ks forthe galaxies IC 4661 and NGC 5408, finding both to be consistent withthose of similar galaxies. We also compared the number of clusters andtheir luminosities with the star-formation rate of the host galaxies andfound no compelling evidence of correlation. We conclude that youngclusters and embedded clusters are a common feature of blue dwarfgalaxies and possibly of galaxies in general, we suggest that theiroccurrence is due to purely statistical effects rather than a phenomenonrelated to specific physical conditions. In this sense we expect theseobjects to be abundant at high red-shift.
|Extragalactic Science with Tunable Filters|
Tunable filters provide unique capabilities to carry out a wide array ofextragalactic projects. The emphasis of this review is on sciencerelating to starburst and active galaxies. Future avenues of researchwith 8-meter class telescopes equipped with tunable filters are alsodiscussed briefly.
|Nucleated dwarf elliptical galaxies in the Virgo cluster|
Using images from a charge-coupled device survey with the Wide FieldCamera on the Isaac Newton Telescope, we performed B- and I-bandphotometry on 156 Virgo cluster dwarf elliptical (dE) galaxies, 25candidate new cluster dwarfs, and nine candidate field dwarfs. Galaxieswere modelled with Sérsic profiles, using both 1Dχ2 and 2D cross-correlation methods, with nuclei modelledas point sources. The intensity profiles of 50 galaxies previouslyclassified as dE, dE?, or ? are more accurately fitted if a nucleus isincluded, and this results in the majority of dwarfs now beingclassified as nucleated dwarf ellipticals (dE,N). Some faint galaxieswith B magnitudes of 18-21 have particularly large relative nuclei,while a small number have apparent central dimmings. For cluster dE,Ngalaxies the nucleus magnitude is correlated with the magnitude of thehost galaxy. The profile parameters of dE and dE,N galaxies are notsignificantly different, and there is no evident discontinuity inrelative nucleus size between non-nucleated and nucleated dwarfs,suggesting that they may form a continuum. Nuclei are on average redderthan their underlying galaxies, though a spread of relative colours wasfound, and two-fifths of nuclei are bluer. Formation mechanisms ofnuclei are discussed: at least some appear to have formed in an alreadyexisting non-nucleated galaxy, though others may have formedsimultaneously with their galaxies and subsequently evolved within them.
|The dynamics and high-energy emission of conductive gas clouds in supernova-driven galactic superwinds|
Superwinds from starburst galaxies are multiphase outflows that sweep upand incorporate ambient galactic disc and halo gas. The interaction ofthis denser material with the more diffuse hot wind gas is thought togive rise to the OVI emission and absorption in the far ultraviolet(FUV) and the soft thermal X-ray emission observed in superwinds. Inthis paper, we present high-resolution hydrodynamical models of warmionized clouds embedded in a superwind, and compare the OVI and softX-ray properties to the existing observational data. These modelsinclude thermal conduction, which we show plays an important role inshaping both the dynamics and radiative properties of the resultingwind/cloud interaction. Heat conduction stabilizes the cloud byinhibiting the growth of Kelvin-Helmholtz and Rayleigh-Taylorinstabilities, and also generates a shock wave at the cloud's surfacethat compresses the cloud. This dynamical behaviour influences theobservable properties. We find that while OVI emission and absorptionalways arises in cloud material at the periphery of the cloud, most ofthe soft X-ray arises in the region between the wind bow shock and thecloud surface, and probes either wind or cloud material depending on thestrength of conduction and the relative abundances of the wind withrespect to the cloud. In general, only a small fraction (<~1 percent) of the wind mechanical energy intersecting a cloud is radiatedaway at ultraviolet (UV) and X-ray wavelengths, with more wind energygoing into accelerating the cloud. Clouds in relatively slow cool windsradiate a larger fraction of their energy, which are inconsistent withobservational constraints. Models with heat conduction at Spitzer-levelsare found to produce observational properties closer to those observedin superwinds than models with no thermal conduction, in particular, interms of the OVI to X-ray luminosity ratio, but cloud life times areuncomfortably short (<~1 Myr) compared to the dynamical ages of realwinds. We experimented with reducing the thermal conductivity for oneset of model parameters, and found that even when we reduced conductionby a factor of 25 that the simulations retained the beneficialhydrodynamical stability and low OVI to X-ray luminosity ratio found inthe Spitzer-level conductive models, while also having reducedevaporation rates. Although more work is required to simulate clouds forlonger times and to investigate cloud acceleration and thermalconduction at sub-Spitzer levels in a wider range of models, we concludethat thermal conduction can no longer be ignored in superwinds.
Galactic winds are the primary mechanism by which energy and metals arerecycled in galaxies and are deposited into the intergalactic medium.New observations are revealing the ubiquity of this process,particularly at high redshift. We describe the physics behind thesewinds, discuss the observational evidence for them in nearbystar-forming and active galaxies and in the high-redshift universe, andconsider the implications of energetic winds for the formation andevolution of galaxies and the intergalactic medium. To inspire futureresearch, we conclude with a set of observational and theoreticalchallenges.
|The Classification of Galaxies: Early History and Ongoing Developments|
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
|Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis|
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
|Palomar/Las Campanas Imaging Atlas of Blue Compact Dwarf Galaxies. II. Surface Photometry and the Properties of the Underlying Stellar Population|
We present the results from an analysis of surface photometry of B, R,and Hα images of a total of 114 nearby galaxies(vhelio<4000 km s-1) drawn from the Palomar/LasCampanas Imaging Atlas of blue compact dwarf (BCD) galaxies. Surfacebrightness and color profiles for the complete sample have beenobtained. We determine the exponential and Sérsic profiles thatbest fit the surface brightness distribution of the underlying stellarpopulation detected in these galaxies. We also compute the (B-R) colorand total absolute magnitude of the underlying stellar population andcompared them to the integrated properties of the galaxies in thesample. Our analysis shows that the (B-R) color of the underlyingpopulation is systematically redder than the integrated color, except inthose galaxies where the integrated colors are strongly contaminated byline and nebular-continuum emission. We also find that galaxies withrelatively red underlying stellar populations [typically (B-R)>=1mag] show structural properties compatible with those of dwarfelliptical galaxies (i.e., a smooth light distribution, fainterextrapolated central surface brightness, and larger scale lengths thanBCD galaxies with blue underlying stellar populations). At least ~15% ofthe galaxies in the sample are compatible with being dwarf elliptical(dE) galaxies experiencing a burst of star formation. For the remainingBCD galaxies in the sample we do not find any correlation between therecent star formation activity and their structural differences withrespect to other types of dwarf galaxies.
|Warm-Hot Gas in and around the Milky Way: Detection and Implications of O VII Absorption toward LMC X-3|
X-ray absorption lines of highly ionized species such as O VII at aboutzero redshift have been firmly detected in the spectra of several activegalactic nuclei. However, the location of the absorbing gas remains asubject of debate. To separate the Galactic and extragalacticcontributions to the absorption, we have obtained Chandra LETG-HRC andFar Ultraviolet Spectroscopic Explorer observations of the black holeX-ray binary LMC X-3. We clearly detect the O VII Kα absorptionline with an equivalent width of 20(14, 26) mÅ (90% confidencerange). The Ne IX Kα absorption line is also detected, albeitmarginally. A joint analysis of these lines, together with thenondetection of the O VII Kβ and O VIII Kα lines, gives thetemperature, velocity dispersion, and hot oxygen column density as1.3(0.7,1.8)×106 K, 79(62,132) km s-1, and1.9(1.2, 3.2) ×1016 cm-2, assuming acollisional ionization equilibrium of the X-ray-absorbing gas and aGalactic interstellar Ne/O number ratio of 0.18. The X-ray data allow usto place a 95% confidence lower limit to the Ne/O ratio as 0.14, but theupper limit is not meaningfully constrained. The O VII line centroid andits relative shift from the Galactic O I Kα absorption line,detected in the same observations, are inconsistent with the systemicvelocity of LMC X-3 (+310 km s-1). The far-UV spectrum showsO VI absorption at Galactic velocities, but no O VI absorption isdetected at the LMC velocity at greater than 3 σ significance. Themeasured Galactic O VI column density is higher than the value predictedfrom the O VII-bearing gas, indicating multiphase absorption. Both thenonthermal broadening and the decreasing scale height with theincreasing ionization state further suggest an origin of the highlyionized gas in a supernova-driven galactic fountain. In addition, weestimate the warm and hot electron column densities from our detected OII Kα line in the LMC X-3 X-ray spectra and from the dispersionmeasure of a pulsar in the LMC vicinity. We then infer the O/H ratio ofthe gas to be >~8×10-5, consistent with thechemically enriched galactic fountain scenario. We conclude that theGalactic hot interstellar medium should in general substantiallycontribute to zero-redshift X-ray absorption lines in extragalacticsources.
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