Contents
Images
Upload your image
DSS Images Other Images
Related articles
The Cool ISM in S0 Galaxies. II. A Survey of Atomic Gas
The place of lenticular galaxies within the range of types of galaxiesremains unclear. We previously reported the mass of molecular hydrogenfor a volume-limited sample of lenticular galaxies, where we saw thatthe amount of gas was less than that predicted by the return of stellarmass to the interstellar medium. Here we report observations of atomichydrogen (H I) for the same sample. Detections in several galaxies makemore compelling the case presented in our earlier paper that the mass ofcool gas in S0 galaxies cuts off at ~10% of what is expected fromcurrent models of gas return from stellar evolution. The molecular andatomic phases of the gas in our sample galaxies appear to be separateand distinct, both spatially and in velocity space. We propose that themolecular gas arises mostly from the stellar mass returned to thegalaxy, while the atomic hydrogen is mainly accumulated from externalsources (infall, captured dwarfs, etc.). While this proposal fits mostof the observations, it makes the presence of the upper mass cutoff evenmore mysterious.
| Oxygen and Nitrogen in Isolated Dwarf Irregular Galaxies
We present long-slit optical spectroscopy of 67 H II regions in 21 dwarfirregular galaxies to investigate the enrichment of oxygen, nitrogen,neon, sulfur, and argon in low-mass galaxies. Oxygen abundances areobtained via direct detection of the temperature-sensitive emissionlines for 25 H II regions; for the remainder of the sample, oxygenabundances are estimated from strong-line calibrations. The directabundance determinations are compared to the strong-line abundancecalibrations of both McGaugh and Pilyugin. While the McGaugh calibrationyields a statistical offset of 0.07 dex, the photoionization model gridtraces the appropriate isometallicity contour shape in theR23-O32 diagnostic diagram. In contrast, while thePilyugin calibration yields a negligible statistical offset, theresiduals in this strong-line calibration method are correlated withionization parameter. Thus, these observations indicate that oxygenabundances will be overestimated by the p-method for H II regions withlow-ionization parameters. Global oxygen and nitrogen abundances forthis sample of dwarf irregular galaxies are examined in the context ofopen- and closed-box chemical evolution models. While several galaxiesare consistent with closed-box chemical evolution, the majority of thissample has an effective yield ~1/4 of the expected yield for a constantstar formation rate and Salpeter IMF, indicating that either outflow ofenriched gas or inflow of pristine gas has occurred. The effective yieldstrongly correlates with MH/LB in the sense thatgas-rich galaxies are more likely to be closed systems. However, theeffective yield does not appear to correlate with other globalparameters such as dynamical mass, absolute magnitude, star formationrate, or surface brightness. In addition, open and closed systems arenot identified easily in other global abundance measures; for example,the observed correlation between luminosity and metallicity isconsistent with other recent results in the literature. A correlation isfound between the observed nitrogen-to-oxygen ratio and the color of theunderlying stellar population; redder dwarf irregular galaxies havehigher N/O ratios than blue dwarf irregular galaxies. The relativeabundance ratios are interpreted in the context of delayed release ofnitrogen and varied star formation histories.
| 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.
| Effects of episodic gas infall on the chemical abundances in galaxies
The chemical evolution of galaxies that undergo an episode of massiveand rapid accretion of metal-poor gas is investigated with models usingboth simplified and detailed nucleosynthesis recipes. The rapid decreaseof the oxygen abundance during infall is followed by a slower evolutionwhich leads back to the closed-box relation, thus forming a loop in theN/O-O/H diagram. For large excursions from the closed-box relation, themass of the infalling material needs to be substantially larger than thegas remaining in the galaxy, and the accretion rate should be largerthan the star formation rate. We apply this concept to the encounter ofhigh velocity clouds with galaxies of various masses, finding that theobserved properties of these clouds are indeed able to cause substantialeffects not only in low mass galaxies, but also in the partial volumesin large massive galaxies that would be affected by the collision.Numerical models with detailed nucleosynthesis prescriptions areconstructed. We assume star formation timescales and scaled yields thatdepend on the galactic mass, and which are adjusted to reproduce theaverage relations of gas fraction, oxygen abundance, and effectiveoxygen yield observed in irregular and spiral galaxies. The resultingexcursions in the N/O-O/H diagram due to a single accretion eventinvolving a high velocity cloud are found to be appreciable, which couldthus provide a contribution to the large scatter in the N/O ratio foundamong irregular galaxies. Nonetheless, the N/O-O/H diagram remains animportant indicator for stellar nucleosynthesis.
| Are interactions the primary triggers of star formation in dwarf galaxies?
We investigate the assumption that the trigger of star formation indwarf galaxies is interactions with other galaxies, in the context of asearch for a `primary' trigger of a first generation of stars. This iscosmologically relevant because the galaxy formation process consistsnot only of the accumulation of gas in a gravitational potential wellbut also of the triggering of star formation in this gas mass, and alsobecause some high-z potentially primeval galaxy blocks look like nearbystar-forming dwarf galaxies. We review theoretical ideas proposed toaccount for the tidal interaction triggering mechanism and present aseries of observational tests of this assumption using published data.We also show results of a search in the vicinity of a composite sampleof 96 dwarf late-type galaxies for interaction candidates showing starformation. The small number of possible perturbing galaxies identifiedin the neighbourhood of our sample galaxies, along with similar findingsfrom other studies, supports the view that tidal interactions may not berelevant as primary triggers of star formation. We conclude thatinteractions between galaxies may explain some forms of star formationtriggering, perhaps in central regions of large galaxies, but they donot seem to be significant for dwarf galaxies and, by inference, forfirst-time galaxies forming at high redshifts. Intuitive reasoning,based on an analogy with stellar dynamics, shows that conditions forprimary star formation triggering may occur in gas masses oscillating ina dark-matter gravitational potential. We propose this mechanism as aplausible primary trigger scenario, which would be worth investigatingtheoretically.
| [O II] as a Star Formation Rate Indicator
We investigate the [O II] emission line as a star formation rate (SFR)indicator using integrated spectra of 97 galaxies from the Nearby FieldGalaxies Survey (NFGS). The sample includes all Hubble types andcontains SFRs ranging from 0.01 to 100 Msolaryr-1. We compare the Kennicutt [O II] and Hα SFRcalibrations and show that there are two significant effects thatproduce disagreement between SFR([O II]) and SFR(Hα): reddeningand metallicity. Differences in the ionization state of the interstellarmedium do not contribute significantly to the observed differencebetween SFR([O II]) and SFR(Hα) for the NFGS galaxies withmetallicities log(O/H)+12>~8.5. The Kennicutt [O II]-SFR relationassumes a typical reddening for nearby galaxies; in practice, thereddening differs significantly from sample to sample. We derive a newSFR([O II]) calibration that does not contain a reddening assumption.Our new SFR([O II]) calibration also provides an optional correction formetallicity. Our SFRs derived from [O II] agree with those derived fromHα to within 0.03-0.05 dex. We show that the reddening, E(B-V),increases with intrinsic (i.e., reddening-corrected) [O II] luminosityfor the NFGS sample. We apply our SFR([O II]) calibration withmetallicity correction to two samples: high-redshift 0.8
| Palomar/Las Campanas Imaging Atlas of Blue Compact Dwarf Galaxies. I. Images and Integrated Photometry
We present B, R, and Hα images for a total of 114 nearby galaxies(vhelio<4000 km s-1) that, with exception ofnine objects, are classified as blue compact dwarfs (BCDs). BRintegrated magnitudes, Hα fluxes and Hα equivalent widthsfor all the objects in the sample are presented. A new set ofquantitative, observational criteria for a galaxy to be classified as aBCD is proposed. These criteria include a limit on the K-band luminosity(i.e., stellar mass; MK>-21 mag), peak surface brightness(μB,peak<22 mag arcsec-2), and color at thepeak surface brightness(μB,peak-μR,peak<~1). Hα emissionis detected in all but three sample galaxies. Typical color, absolutemagnitude, and Hα luminosity are (B-R)=0.7+/-0.3 mag,MB=-16.1+/-1.4 mag, and log (LHα)=40.0+/-0.6(ergs s-1). Galaxies morphologically classified as nE and iEBCDs within our sample show lower Hα equivalent widths and reddercolors, on average, than the iI- and i0-type BCDs. For most of thegalaxies the presence of an evolved stellar population is required toexplain their observed properties; only the most metal-poor BCDs (e.g.,I Zw 18, Tol 65) are still compatible with a pure, young burst. Theflux-calibrated and WCS-compliant images in this Atlas are individuallyavailable through the NASA/IPAC Extragalactic Database (NED) imageserver and collectively through a dedicated Web page.
| The Relationship between Stellar Light Distributions of Galaxies and Their Formation Histories
A major problem in extragalactic astronomy is the inability todistinguish in a robust, physical, and model-independent way how galaxypopulations are physically related to each other and to their formationhistories. A similar, but distinct, and also long-standing question iswhether the structural appearances of galaxies, as seen through theirstellar light distributions, contain enough physical information tooffer this classification. We argue through the use of 240 images ofnearby galaxies that three model-independent parameters measured on asingle galaxy image reveal its major ongoing and past formation modesand can be used as a robust classification system. These parametersquantitatively measure: the concentration (C), asymmetry (A), andclumpiness (S) of a galaxy's stellar light distribution. When combinedinto a three-dimensional ``CAS'' volume all major classes of galaxies invarious phases of evolution are cleanly distinguished. We argue thatthese three parameters correlate with important modes of galaxyevolution: star formation and major merging activity. This is arguedthrough the strong correlation of Hα equivalent width andbroadband colors with the clumpiness parameter S, the uniquely largeasymmetries of 66 galaxies undergoing mergers, and the correlation ofbulge to total light ratios, and stellar masses, with the concentrationindex. As an obvious goal is to use this system at high redshifts totrace evolution, we demonstrate that these parameters can be measured,within a reasonable and quantifiable uncertainty with available data outto z~3 using the Hubble Space Telescope GOODS ACS and Hubble Deep Fieldimages.
| The Cool Interstellar Medium in S0 Galaxies. I. A Survey of Molecular Gas
Lenticular galaxies remain remarkably mysterious as a class.Observations to date have not led to any broad consensus about theirorigins, properties, and evolution, although they are often thought tohave formed in one big burst of star formation early in the history ofthe universe and to have evolved relatively passively since then. Inthat picture, current theory predicts that stellar evolution returnssubstantial quantities of gas to the interstellar medium; most isejected from the galaxy, but significant amounts of cool gas might beretained. Past searches for that material, though, have provided unclearresults. We present results from a survey of molecular gas in avolume-limited sample of field S0 galaxies selected from the NearbyGalaxies Catalog. CO emission is detected from 78% of the samplegalaxies. We find that the molecular gas is almost always located insidethe central few kiloparsecs of a lenticular galaxy, meaning that ingeneral it is more centrally concentrated than in spirals. We combineour data with H I observations from the literature to determine thetotal masses of cool and cold gas. Curiously, we find that, across awide range of luminosity, the most gas-rich galaxies have ~10% of thetotal amount of gas ever returned by their stars. That result isdifficult to understand within the context of either monolithic orhierarchical models of evolution of the interstellar medium.
| Neutral Hydrogen Mapping of Virgo Cluster Blue Compact Dwarf Galaxies
A new installment of neutral hydrogen mappings of blue compact dwarf(BCD) galaxies, as defined by optical morphology, in and near the VirgoCluster is presented. The primary motivation was to search for outlyingclouds of H I as potential interactive triggers of enhanced starformation, and therefore the mapped galaxies were selected for large H Imass, large optical diameter, and large velocity profile width.Approximately half the sample proved to have one or more small, lowcolumn density, star-free companion clouds, either detached or appearingas an appendage in our maps, at a resolution on the order of 4 kpc.Comparison is made with a sample of similarly mapped field BCD galaxiesdrawn from the literature; however, the Virgo Cluster sample of mappedBCDs is still too small for conclusive comparisons to be made. We found,on the one hand, little or no evidence for ram-pressure stripping nor,on the other, for extremely extended low column density H I envelopes.The H I rotation curves in most cases rise approximately linearly andslowly, as far out as we can trace the gas.
| Redshift-Distance Survey of Early-Type Galaxies: Spectroscopic Data
We present central velocity dispersions and Mg2 line indicesfor an all-sky sample of ~1178 elliptical and S0 galaxies, of which 984had no previous measures. This sample contains the largest set ofhomogeneous spectroscopic data for a uniform sample of ellipticalgalaxies in the nearby universe. These galaxies were observed as part ofthe ENEAR project, designed to study the peculiar motions and internalproperties of the local early-type galaxies. Using 523 repeatedobservations of 317 galaxies obtained during different runs, the dataare brought to a common zero point. These multiple observations, takenduring the many runs and different instrumental setups employed for thisproject, are used to derive statistical corrections to the data and arefound to be relatively small, typically <~5% of the velocitydispersion and 0.01 mag in the Mg2 line strength. Typicalerrors are about 8% in velocity dispersion and 0.01 mag inMg2, in good agreement with values published elsewhere.
| Redshift-Distance Survey of Early-Type Galaxies: Circular-Aperture Photometry
We present R-band CCD photometry for 1332 early-type galaxies, observedas part of the ENEAR survey of peculiar motions using early-typegalaxies in the nearby universe. Circular apertures are used to tracethe surface brightness profiles, which are then fitted by atwo-component bulge-disk model. From the fits, we obtain the structuralparameters required to estimate galaxy distances using theDn-σ and fundamental plane relations. We find thatabout 12% of the galaxies are well represented by a pure r1/4law, while 87% are best fitted by a two-component model. There are 356repeated observations of 257 galaxies obtained during different runsthat are used to derive statistical corrections and bring the data to acommon system. We also use these repeated observations to estimate ourinternal errors. The accuracy of our measurements are tested by thecomparison of 354 galaxies in common with other authors. Typical errorsin our measurements are 0.011 dex for logDn, 0.064 dex forlogre, 0.086 mag arcsec-2 for<μe>, and 0.09 for mRC,comparable to those estimated by other authors. The photometric datareported here represent one of the largest high-quality and uniformall-sky samples currently available for early-type galaxies in thenearby universe, especially suitable for peculiar motion studies.Based on observations at Cerro Tololo Inter-American Observatory (CTIO),National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation (NSF);European Southern Observatory (ESO); Fred Lawrence Whipple Observatory(FLWO); and the MDM Observatory on Kitt Peak.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| An H I Survey of Actively Star-forming Dwarf Galaxies
We present the results of H I 21 cm observations of 139 activelystar-forming dwarf galaxies obtained with the 305 m radio telescope atArecibo Observatory. Our sample consists of all objects cataloged inobjective-prism surveys for UV-excess or emission-line galaxiespublished prior to the start of the survey that have luminosities belowMB=-17.0 and that are located within the declination limitsof the Arecibo telescope. Galaxies from the Markarian, Michigan, Case,Wasilewski, Haro, and Zwicky lists are included. The sample spans a widerange of both H I gas content and star formation levels. A total of 122objects (88%) were detected; 82 galaxies have been observed for thefirst time in H I. The median velocity width for our sample is 88 kms-1, and the median H I gas mass is 3.0×108Msolar. In general, the sample galaxies are gas-rich, with anaverage MHI/LB=1.3 after correcting for theluminosity enhancement due to the starburst. The progenitors of many ofthe star-forming dwarfs have higher MHI/LB thantypically seen in samples of nearby ``normal'' galaxies, emphasizingtheir distinct nature.
| Multi-wavelength study of a new sample of blue compact dwarf galaxies. IV. Optical multi-pupil spectroscopy of 18 objects
We present the multi-pupil integral field spectroscopy of 18 BlueCompact Dwarf Galaxies performed at the 6 m telescope of the SAO(Russia). We produce an atlas of the Hα , [OIII], [SII]emission-line intensity distributions, the excitation parameter([OIII]/Hβ ratio), the continuum maps in blue and red and thevelocity fields as derived from the 2D spectra. We provide detailedindividual descriptions of the objects with a table of theirspectroscopic properties. From the atlas and the velocity maps, we haveclassified the sample according to the following scheme: - Nine objectsshow spatially coincident distributions with respect to their emissionlines, excitation parameter and the continuum. In this case, thesegalaxies are repeating star-formation sequences at the same spatiallocation. Mrk 1416 also belongs to this class of BCDG but has asecondary site of star formation. - Eight objects in which thedistributions in emission lines brightness, excitation parameter and incontinuum strength differ from each other at various degrees. Theseobjects are galaxies with several sites of star formation of variousages and spatial locations. They can be ideal templates for testing themodels of star formation. - From the 2-D [SII] emission-linedistribution we find a superbubble structure in one object and possiblyin 2 others. - Ten objects show an overall quite regular radial velocitygradient. On the other hand, in 8 objects, the kinematical major axis ofthis regular velocity field does not align with the photometric majoraxis, suggesting dynamical disturbances, gas/star decoupling and/orinstabilities. - Six objects have irregular velocity fields, implyingthat the rotation of a gaseous disk is not their dominant dynamicalsignature. - SBS 1723+565B is not a dwarf galaxy but a giant irregulargalaxy or an interacting system experiencing a high star formation rate.Based on observations performed with the Special AstrophysicalObservatory (Russia) 6 m. Telescope.
| Dynamical condition of neutral hydrogen envelopes of dwarf galaxies and their possible morphological evolution
We investigate the star-formation history of gas-rich dwarf galaxies,taking account of the dynamical evolution of their neutral hydrogen (HI)envelope. Gas-rich dwarfs are classified into blue compact dwarfs (BCDs)and dwarf irregulars (dIrrs). In this paper, their HI envelope isclearly shown not to be blown away by their stellar feedback. This isconcluded since the observed star-formation rate (SFR) of gas-richdwarfs is generally smaller than a critical SFR, psicrit, atwhich stellar feedback accelerates the HI envelope to the escapevelocity. From this standpoint and the chemical property of sample BCDs,we suggest two possibilities; (1) the HI gas in the envelope of BCDs isconsumed to fuel their star-formation; and (2) BCDs have a similarstar-formation history. We also discuss morphological evolution amongdwarf galaxies. As long as gas-rich dwarfs are isolated, it is difficultfor them to evolve into dwarf ellipticals (dEs). When the HI envelope ingas-rich dwarfs is consumed in subsequent star-formation, amorphological exchange between BCDs and dIrrs is still expected,consistent with previous studies. If the SFR of gas-rich dwarfs was muchhigher than psicrit in the past, interestingly, anevolutionary scenario from dEs to gas-rich dwarfs is possible.
| Kinematical data on early-type galaxies. VI.
We present the result of spectroscopic observations of a sample of 73galaxies, completing the database published in this series of articles.The sample contains mostly low-luminosity early-type objects, includingfour dwarfs of the Local Group (in particular, deep spectra of NGC 205),15 dEs or dS0s in the Virgo cluster, and UGC 05442, a spheroidal dwarfof the M 81 group. We have measured the central velocity dispersion forall but one object, and determined the major-axis rotation andvelocity-dispersion profiles for 59 objects. For the current sample ofdiffuse (or dwarf) elliptical galaxies, we have compared stellarrotation to velocity dispersion; the analysis suggests that theseobjects may be nearly rotationally flattened, and therefore thatanisotropy may be less important than previously thought. Based onobservations collected at the Observatoire de Haute-Provence. Table 1 isalso, and Tables 2 and 4 only, available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/371
| The Morphologies of Dwarf Markarian Galaxies
The morphologies of the 96 dwarf (M(B) -17m) galaxies in the Markariancatalog are determined from the digitized Schmidt plates obtained forthe construction of the Hubble Space Telescope Guide Star Catalog. Thefraction of double nucleus galaxies within the dwarf Markarian galaxiesis determined to be twice that found for all galaxies in the Markariancatalog. In addition to the 12 previously known cases, four definite andtwo probable galaxies with double nuclei are identified. The fraction ofdwarf Markarian galaxies with bright star forming regions is found to betwice that of Virgo cluster dwarf galaxies. No Elliptical galaxies arefound in the sample. Galaxies with blue compact dwarf and S0morphologies are more often found to contain unresolved regions of UVexcess emission. Dwarf Markarian galaxies with different morphologicalstructures and spectral classes are found to have similar FIRproperties.
| Kinematic Constraints on Evolutionary Scenarios for Blue Compact Dwarf Galaxies. I. Neutral Gas Dynamics
We present the results of high spatial resolution, H I synthesisobservations of six blue compact dwarf (BCD) galaxies. Optically, theselected galaxies have smooth, symmetric isophotes and thus are the mostlikely of the BCD class to fade into an object morphologically similarto a dwarf elliptical when the current starburst ends. The neutral gasin all six galaxies appears to be rotationally supported, however,indicating that true morphological transformation from a BCD to a dEwill require significant loss of angular momentum. Based on the observedneutral gas dynamics of these and other BCDs, it is unlikely thatpresent-day BCDs will evolve directly into dwarf ellipticals after astarburst phase. We discuss alternative evolutionary scenarios for BCDsand place them within the larger context of galaxy formation andevolution models. In general, BCDs appear to have steeper rotationcurves than similar luminosity, low surface brightness dwarf galaxies.BCDs have centrally concentrated mass distributions (stars, gas, anddark matter) and have lower specific angular momenta. Based on diskinstability analyses, steeply rising rotation curves result in higherthreshold densities for the onset of star formation. These resultssuggest that angular momentum may play a crucial role in themorphological evolution of low-mass galaxies: galaxies with low angularmomenta will be able to collapse into small, compact galaxies, whilegalaxies with high angular momenta will be more diffuse systems.Furthermore, because the star formation threshold density is higher inlow angular momenta systems, star formation will be delayed until anextremely high surface density is reached. Thus, angular momentum may bethe fundamental parameter that determines whether a low-mass galaxy willhave centrally concentrated stellar and gaseous distributions and bemore susceptible to a burst mode of star formation.
| The Evolutionary Status of Isolated Dwarf Irregular Galaxies. II. Star Formation Histories and Gas Depletion
The results of UBV and Hα imaging of a large sample of isolateddwarf irregular galaxies are interpreted in the context of compositestellar population models. The observed optical colors are best fittedby composite stellar populations that have had approximately constantstar formation rates for at least 10 Gyr. The galaxies span a range ofcentral surface brightness, from 20.5 to 25.0 mag arcsec-2there is no correlation between surface brightness and star formationhistory. Although the current star formation rates are low, it ispossible to reproduce the observed luminosities without a majorstarburst episode. The derived gas depletion timescales are long,typically ~20 Gyr. These results indicate that dwarf irregular galaxies(dI's) will be able to continue with their slow, but constant, starformation activity for at least another Hubble time. The sample ofisolated dI's is compared with a sample of starbursting dwarf galaxiestaken from the literature. The starbursting dwarf galaxies have manysimilar properties; the main difference between these two types ofgas-rich dwarf galaxies is that the current star formation isconcentrated in the center of the starbursting systems, while it is muchmore distributed in the quiescent dI's. This results in pronounced colorgradients for the starbursting dwarf galaxies, while the majority of thequiescent dwarf irregular galaxies have minor or nonexistent colorgradients. Thus, the combination of low current star formation rates,blue colors, and the lack of significant color gradients indicates thatstar formation percolates slowly across the disks of normal dwarfgalaxies in a quasi-continuous manner.
| Lopsidedness in dwarf irregular galaxies
We quantify the amplitude of the lopsidedness, the azimuthal angularasymmetry index and the concentration of star-forming regions, asrepresented by the distribution of the Hα emission, in a sample of78 late-type irregular galaxies. We bin the observed galaxies into twogroups representing blue compact galaxies (BCDs) andlow-surface-brightness dwarf galaxies (LSBs). The light distribution isanalysed with a novel algorithm, which allows detection of details inthe light distribution pattern. We find that while the asymmetry of theunderlying continuum light, representing the older stellar generations,is relatively small, the Hα emission is very asymmetric and iscorrelated in position angle with the continuum light. We show that theconcentration of continuum light is correlated with the Hαconcentration; this implies that the young star formation has the samespatial properties as the older stellar populations, but that theseproperties are more strongly expressed by the young stars. We test amodel of random star formation over the extent of a galaxy by simulatingHii regions in artificial dwarf galaxies. A galaxy is traced by assumingred star clusters distributed on an underlying exponential disc ofradius twice the scalelength. The disc is allowed to change in apparentmagnitude, scaleradius, position angle and ellipticity. We compare theasymmetry-concentration distribution predicted by the simulations withthe real observed distribution; we find that only LSBs match thedistribution predicted by the model. The reason is that, independentlyof the number of Hii regions, LSBs show no particular location of Hiiregions, whereas BCDs show current star formation activity restrictedvery much to the central parts of the galaxies. A consideration of theproperties of the continuum light leads to the conclusion that most ofLSBs can be approximated by exponential discs of radius twice theirscalelength; BCDs call, however, for much more concentrated underlyingsystems, with smaller scalelengths than assumed in the simulations. Theimplication is that random star formation over the full extent of agalaxy may be generated in LSB dwarf irregular galaxies but not in BCDgalaxies.
| Spectrophotometry of Nearby Field Galaxies: The Data
We have obtained integrated and nuclear spectra as well as U, B, Rsurface photometry for a representative sample of 196 nearby galaxies.These galaxies span the entire Hubble sequence in morphological type, aswell as a wide range of luminosities (MB=-14 to -22). Here wepresent the spectrophotometry for these galaxies. The selection of thesample and the U, B, R surface photometry is described in a companionpaper. Our goals for the project include measuring the current starformation rates and metallicities of these galaxies, and elucidatingtheir star formation histories, as a function of luminosity andmorphology. We thereby extend the work of Kennicutt to lower luminositysystems. We anticipate that our study will be useful as a benchmark forstudies of galaxies at high redshift. We describe the observing, datareduction, and calibration techniques and demonstrate that ourspectrophotometry agrees well with that of Kennicutt. The spectra spanthe range 3550-7250 Å at a resolution (FWHM) of ~6 Å andhave an overall relative spectrophotometric accuracy of ~+/-6%. Wepresent a spectrophotometric atlas of integrated and nuclear rest-framespectra as well as tables of equivalent widths and synthetic colors. Theatlas and tables of measurements will be made available electronically.We study the correlations of galaxy properties determined from thespectra and images. Our findings include: (1) galaxies of a givenmorphological class display a wide range of continuum shapes andemission-line strengths if a broad range of luminosities are considered,(2) emission-line strengths tend to increase and continua tend to getbluer as the luminosity decreases, and (3) the scatter on the generalcorrelation between nuclear and integrated Hα emission-linestrengths is large.
| Surface Photometry of Nearby Field Galaxies: The Data
We have obtained integrated spectra and multifilter photometry for arepresentative sample of ~200 nearby galaxies. These galaxies span theentire Hubble sequence in morphological type, as well as a wide range ofluminosities (MB=-14 to -22) and colors (B-R=0.4-1.8). Herewe describe the sample selection criteria and the U, B, R surfacephotometry for these galaxies. The spectrophotometric results will bepresented in a companion paper. Our goals for the project includemeasuring the current star formation rates and metallicity of thesegalaxies, and elucidating their star formation histories, as a functionof luminosity and morphology. We thereby extend the work of Kennicutt tolower luminosity systems. We anticipate that our study will be useful asa benchmark for studies of galaxies at high redshift. We discuss theobserving, data reduction, and calibration techniques and show that ourphotometry agrees well with previous work in those cases in whichearlier data are available. We present an atlas of images, radialsurface brightness profiles, and color profiles as well as tables ofderived parameters. The atlas and tables of measurements will be madeavailable electronically. We study the correlations of galaxy propertiesdetermined from the galaxy images. Our findings include the following:(1) colors determined within the effective radius correlate better withmorphological type than with MB and (2) 50% of thelow-luminosity galaxies are bluest in their centers.
| The Evolutionary Status of Isolated Dwarf Irregular Galaxies. I. UBV and Hα Imaging Observations
The results of UBV and Hα imaging of a large sample of gas-rich,isolated, dwarf irregular galaxies are presented. The majority of thelow-luminosity systems in this sample have no known neighbors within200-400 kpc and thus are unlikely to have had significant interactionswithin the last several gigayears. The new observations confirm thatdwarf irregular galaxies are blue systems, with median values for thesample of 0.42+/-0.040.05 in B-V and-0.22+/-0.040.07 in U-B. Analysis of the derivedsurface photometry indicates that most of these systems can be wellfitted by single exponential disks and have only minor color gradients.Furthermore, the observed H II regions are distributed sparsely acrossthe optical disk. H II region luminosity functions were constructed forthose systems with a sufficient number of H II regions; the derivedpower-law indices are generally shallow, with a median value of-1.61+/-0.310.24.
| Stellar population synthesis for blue compact galaxies.
Not Available
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| HI observations of blue compact galaxies from the first and second Byurakan surveys
We present the results of a neutral hydrogen survey of 79 galaxies froma statistical sample of 88 Blue Compact Galaxies (BCGs) selected fromthe First and Second Byurakan objective prism surveys to have a HIIregion-like spectrum, an equivalent width of the [O III] lambda 5007line larger than ~ 50 Å, and a velocity <= 6000 km s(-1) . Thedetection rate for the statistical sample is 74%. HI masses rangebetween 4 10(7)M_sun) and 5 10(9) M_sun with the HI mass distributionpeaking at 3 10(8) M_sun. The full width at half-maximum of the HIprofile varies between ~ 30 km s(-1) and 160 km s(-1) , with a mean of ~92 km s(-1) . These small widths are characteristic of dwarf galaxies.For comparison, we have also observed an additional 92 BCGs with weakerstar formation and/or larger distances, and/or interesting astrophysicalproperties. These in general have larger widths and HI masses.Tables~1,~2,~3 also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Multi-spectral study of a new sample of Blue Compact Dwarf Galaxies. II. B and R surface photometry of 22 southern objects
We present the results of surface photometry on a new sample of BlueCompact Dwarf galaxies (BCDGs), in continuation to a previous paper(Doublier et al. 1997, hereafter Paper I). The 22 galaxies (plus twocompanions) discussed in the present paper have been selected in theSouthern Hemisphere, from several lists. An atlas containing isophotalmaps, surface brightnesses and B-R color profiles of the sample isgiven, together with the tables containing the photometric parameters.The results are consistent with those obtained in Paper I for objectsselected from the Byurakan Surveys in the Northern hemisphere.Similarly, we find about one fourth of the BCDGs showing a dominantr(1/4) brightness distribution component, one fourth of the BCDG showinga dominant exponential surface brightness profile, and about half ofthem show composite brightness distributions. Integrated properties,colors, mean surface brightnesses and luminosity-radius relations areinvestigated and discussed for the objects presented in this paper andPaper I. We found that r(1/4) BCDGs tend to show a different behaviourcompared to the exponential BCDGs, with respect to colors, compactnessand luminosity-radius relations. We also include a brief study of thesurroundings of the galaxies, where we find several candidatecompanions. Based on observations collected at the 1.54 m DanishTelscope at the European Southern observatory (La Silla, Chile).
| CO Emission in Low-Luminosity, H I-rich Galaxies
We present ^12CO 1 --> 0 observations of 11 low-luminosity (M_B >-18), H I-rich dwarf galaxies. Only the three most metal-rich galaxies,with 12 + log (O/H) ~ 8.2, are detected. Very deep CO spectra of sixextremely metal-poor systems [12 + log (O/H) <= 7.5] yield only lowupper limits on the CO surface brightness, I_CO < 0.1 K km s^-1.Three of these six have never before been observed in a CO line, whilethe others now have much more stringent upper limits. For the very lowmetallicity galaxy Leo A, we do not confirm a previously reporteddetection in CO, and the limits are consistent with another recentnondetection. We combine these new observations with data from theliterature to form a sample of dwarf galaxies that all have COobservations and measured oxygen abundances. No known galaxies with 12 +log (O/H) < 7.9 (Z < 0.1 Z_ȯ) have been detected in CO. Mostof the star-forming galaxies with higher [12 + log (O/H) > 8.1]metallicities are detected at similar or higher I_CO surfacebrightnesses. The data are consistent with a strong dependence of theI_CO/M_H_2=X_CO conversion factor on ambient metallicity. The strikinglylow upper limits on some metal-poor galaxies lead us to predict that theconversion factor is nonlinear, increasing sharply below ~1/10 of thesolar metallicity [12 + log (O/H) <= 7.9].
| CO in blue compact and star burst galaxies
(12) CO(J=1->0) observations of 34 blue compact and star burstgalaxies are presented. Although these galaxies are experiencingvigorous star formation at the current epoch, CO has been detected inonly five of them. The five detections reported in this paper are all ingalaxies with relatively red colours, (B-V)0>0.4. The newobservations, when combined with previously published data on CO inBCGs, indicate that CO luminosity decreases with absolute luminosity ofBCGs. Since the absolute luminosity of a galaxy is correlated with itsmetallicity, these results confirm that low metallicity BCGs have lowabundances of CO gas. We also show that the star formation ratedetermined from the H_β\ luminosity is lower than that determinedfrom the far infrared luminosity.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
