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Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Ultraviolet-to-Far-Infrared Properties of Local Star-forming Galaxies
We present the results of a multiwavelength study of nearby galaxiesaimed at understanding the relation between the ultraviolet andfar-infrared emission in star-forming galaxies. The data set comprisesnew ultraviolet (from HST STIS), ground-based Hα, and radiocontinuum observations, together with archival infrared data (from IRASand ISO). The local galaxies are used as benchmarks for comparison ofthe infrared-to-ultraviolet properties with two populations ofhigh-redshift galaxies: the submillimeter star-forming galaxies detectedby SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). Inaddition, the long wavelength baseline covered by the present dataenables us to compare the star formation rates (SFRs) derived from theobserved ultraviolet, Hα, infrared, and radio luminosities and togauge the impact of dust opacity in the local galaxies. We also derive anew calibration for the nonthermal part of the radio SFR estimator,based on the comparison of 1.4 GHz measurements with a new estimator ofthe bolometric luminosity of the star-forming regions. We find that moreactively star-forming galaxies show higher dust opacities, which is inline with previous results. We find that the local star-forming galaxieshave a lower Fλ(205 μm)/Fλ(UV)ratio by 2-3 orders of magnitude than the submillimeter-selectedgalaxies and may have a similar or somewhat higherFλ(205 μm)/Fλ(UV) ratio thanLBGs. The Fλ(205 μm)/Fλ(UV) ratioof the local galaxy population may be influenced by the cool dustemission in the far-infrared heated by nonionizing stellar populations,which may be reduced or absent in the LBGs.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Luminous Compact Blue Galaxies up to z~1 in the Hubble Space Telescope Ultra Deep Field. I. Small Galaxies or Blue Centers of Massive Disks?
We analyze 26 luminous compact blue galaxies (LCBGs) in the Hubble SpaceTelescope ACS Ultra Deep Field (UDF) at z~0.2-1.3, to determine whetherthese truly are small galaxies or, rather, bright central starburstswithin existing or forming large disk galaxies. Surface brightnessprofiles from UDF images reach fainter than rest-frame 26.5 B magarcsec-2 even for compact objects at z~1. Most LCBGs show asmaller, brighter component that is likely star-forming, and anextended, roughly exponential component with colors suggesting stellarages from >~100 Myr to a few gigayears. Scale lengths of the extendedcomponents are mostly <~2 kpc, more than 1.5-2 times smaller thanthose of nearby large disk galaxies like the Milky Way. Larger, very lowsurface brightness disks can be excluded down to faint rest-framesurface brightnesses (>~26 B mag arcsec-2). However, oneor two of the LCBGs are large, disklike galaxies that meet LCBGselection criteria because of a bright central nucleus, possibly aforming bulge. These results indicate that >~90% of high-z LCBGs aresmall galaxies that will evolve into small disk galaxies, or low-massspheroidal or irregular galaxies in the local universe, assuming passiveevolution and no significant disk growth. The data do not reveal signsof disk formation around small, H II galaxy-like LCBGs, nor do theysuggest a simple inside-out growth scenario for larger LCBGs with adisklike morphology. Irregular blue emission in distant LCBGs isrelatively extended, suggesting that nebular emission lines fromstar-forming regions sample a major fraction of an LCBG's velocityfield.
| NGC 7679: an anomalous, composite Seyfert 1 galaxy whose X-ray luminous AGN vanishes at optical wavelengths
Morphological disturbances and gas kinematics of the SB0 galaxy NGC 7679= Arp 216 are investigated to understand the history of this highlycomposite object, where AGN and starburst signatures dominate in theX-ray and optical/IR regime, respectively. Perturbations of the ionizedgas velocity field appear quite mild within 15'' (~5 kpc) of the center,so it can be straightforwardly modeled as a circularly rotating disk.Outside that radius, significant disturbances are seen. In particular,the eastern distorted arm as well as the huge neutral hydrogen bridgeconnecting NGC 7679 to the nearby Seyfert spiral NGC 7682 unambiguouslyrepresent the vestige of a close encounter of the two objects ~500 Myrago. The relationship of such a past event with the much more recent,centrally located starburst (not older than 20 Myr) cannot be easilyestablished. Together, the classification of NGC 7679 is less extremethan that proposed in the past, being simply a (disturbed) galaxy wherestarburst and AGN activity coexist with a starburst dominating thebolometric luminosity.
| A radio study of the superwind galaxy NGC 1482
We present multifrequency radio continuum as well as HI observations ofthe superwind galaxy NGC 1482, with both the Giant Metrewave RadioTelescope (GMRT) and the Very Large Array (VLA). This galaxy has aremarkable hourglass-shaped optical emission-line outflow as well asbipolar soft X-ray bubbles on opposite sides of the galactic disc. Thelow-frequency, lower-resolution radio observations show a smoothstructure. From the non-thermal emission, we estimate the availableenergy in supernovae, and examine whether this would be adequate todrive the observed superwind outflow. The high-frequency,high-resolution radio image of the central starburst region located atthe base of the superwind bi-cone shows one prominent peak and moreextended emission with substructure. This image has been compared withthe infrared, optical red continuum, Hα, and soft and hard X-rayimages from Chandra to understand the nature and relationship of thevarious features seen at different wavelengths. The peak of the infraredemission is the only feature that is coincident with the prominent radiopeak, and possibly defines the centre of the galaxy.The HI observations with the GMRT show two blobs of emission on oppositesides of the central region. These are rotating about the centre of thegalaxy and are located at ~2.4 kpc from it. In addition, theseobservations also reveal a multicomponent HI absorption profile againstthe central region of the radio source, with a total width of ~250 kms-1. The extreme blue- and redshifted absorption componentsare at 1688 and 1942 km s-1, respectively, while the peakabsorption is at 1836 km s-1. This is consistent with theheliocentric systemic velocity of 1850 +/- 20 km s-1,estimated from a variety of observations. We discuss possibleimplications of these results.
| The Nature of Nearby Counterparts to Intermediate-Redshift Luminous Compact Blue Galaxies. II. CO Observations
We present the results of a single-dish beam-matched survey of the threelowest rotational transitions of CO in a sample of 20 local (D<~70Mpc) luminous compact blue galaxies (LCBGs). These ~L*, blue, highsurface brightness, starbursting galaxies were selected on the samecriteria used to define LCBGs at higher redshifts. Our detection ratewas 70%, with those galaxies havingLB<7×109 Lsolar not detected.We find that the H2 masses of local LCBGs range from6.6×106 to 2.7×109 Msolar,assuming a Galactic CO-to-H2 conversion factor. Combiningthese results with our earlier H I survey of the same sample, we findthat the ratio of molecular to atomic gas mass is low, typically 5%-10%.Using a large velocity gradient model, we find that the average gasconditions of the entire interstellar medium in local LCBGs are similarto those found in the centers of star-forming regions in our Galaxy andin the nuclear regions of other galaxies. Star formation rates,determined from IRAS fluxes, are a few Msolaryr-1, much higher per unit dynamical mass than normal spiralgalaxies. If this rate remains constant, the molecular hydrogendepletion timescales are short, ~10-200 Myr.
| Gas and Stars in an H I-Selected Galaxy Sample
We present the results of a J-band study of the H I-selected AreciboDual-Beam Survey and Arecibo Slice Survey galaxy samples using TwoMicron All Sky Survey data. We find that these galaxies span a widerange of stellar and gas properties. However, despite the diversitywithin the samples, we find a very tight correlation between luminosityand size in the J band, similar to that found in a previous paper byRosenberg & Schneider between the H I mass and size. We also findthat the correlation between the baryonic mass and the J-band diameteris even tighter than that between the baryonic mass and the rotationalvelocity.
| Dust properties of UV bright galaxies at z ~ 2
We investigate the properties of the extinction curve in the rest-frameUV for a sample of 34 UV-luminous galaxies at 2 < z < 2.5,selected from the FORS Deep Field (FDF) spectroscopic survey. A newparametric description of the rest-frame UV spectral energy distributionis adopted; its sensitivity to properties of the stellar populations orof dust attenuation is established with the use of models. The latterare computed by combining composite stellar population models andcalculations of radiative transfer of the stellar and scatteredradiation through the dusty interstellar medium (ISM) for a dust/starsconfiguration describing dust attenuation in local starbursts. In thefavoured configuration the stars are enveloped by a shell with atwo-phase, clumpy, dusty ISM. The distribution of the z 2UV-luminous FDF galaxies in several diagnostic diagrams shows that theirextinction curves range between those typical of the Small and LargeMagellanic Clouds (SMC and LMC, respectively). For the majority ofstrongly reddened objects having a UV continuum slope β > -0.4 asignificant 2175 Å absorption feature (or "UV bump") is inferred,indicating an LMC-like extinction curve. On the other hand, the UVcontinua of the least reddened objects are mostly consistent withSMC-like extinction curves, lacking a significant UV bump, as for thesample of local starbursts investigated by Calzetti and collaborators.Furthermore, the most opaque (β 0) and, thus (for ourmodels), dustiest UV-luminous FDF galaxies tend to be among the mostmetal-rich, most massive, and largest systems at z 2, indicating< Z > 0.5 {-} 1 Zȯ, < Mstars> 6 × 1010 Mȯ, and 4 kpc, respectively. The presence of the UVbump does not seem to depend on the total metallicity, as given by theequivalent width (EW) of the C IV doublet. Conversely, it seems to beassociated with a large average EW of the six most prominentinterstellar low-ionisation absorption lines falling in the FORSspectra. The average EW of these saturated lines offers a proxy for theISM topology. We interpret these results as the evidence for adifference in the properties of the dusty ISM among the most evolvedUV-luminous, massive galaxies at z 2.
| Spectroscopic study of blue compact galaxies. V. Oxygen abundance and the metallicity-luminosity relation
This is the fifth paper in a series studying the stellar components,star formation histories, star formation rates and metallicities of ablue compact galaxy (BCG) sample. Based on our high-quality ground-basedspectroscopic observations, we have determined the electrontemperatures, electron densities, nitrogen abundances and oxygenabundances for 72 star-forming BCGs in our sample, using differentoxygen abundance indicators. The oxygen abundance covers the range 7.15< 12 + log (O/H)< 9.0, and nitrogen is found to be mostly aproduct of secondary nucleosynthesis for 12 + log (O/H)>8.2 andapparently a product of primary nucleosynthesis for 12 + log (O/H)<8.2. To assess the possible systematic differences among differentoxygen abundance indicators, we have compared oxygen abundances of BCGsobtained with the Te method, R23 method, P method,N2 method and O3N2 method. The oxygen abundances derived from theTe method are systematically lower by 0.1-0.25 dex than thosederived from the strong line empirical abundance indicators, consistentwith previous studies based on region samples. We confirm the existenceof the metallicity-luminosity relation in BCGs over a large range ofabundances and luminosities. Our sample of galaxies shows that the slopeof the metallicity-luminosity relation for the luminous galaxies(~-0.05) is slightly shallower than that for the dwarf galaxies(~-0.17). An offset was found in the metallicity-luminosity relation ofthe local galaxies and that of the intermediate redshift galaxies. Itshows that the metallicity-luminosity relation for the emission linegalaxies at high redshift is displaced to lower abundances, higherluminosities, or both.
| Principal component analysis of International Ultraviolet Explorer galaxy spectra
We analyse the UV spectral energy distribution of a sample of normalgalaxies listed in the International Ultraviolet Explorer (IUE) NewlyExtracted Spectra (INES) Guide No. 2 - Normal Galaxies using a principalcomponent analysis. The sample consists of the IUE short-wavelength (SW)spectra of the central regions of 118 galaxies, where the IUE apertureincluded more than 1 per cent of the galaxy size. The principalcomponents are associated with the main components observed in theultraviolet (UV) spectra of galaxies. The first component, accountingfor the largest source of diversity, may be associated with the UVcontinuum emission. The second component represents the UV contributionof an underlying evolved stellar population. The third component issensitive to the amount of activity in the central regions of galaxiesand measures the strength of star-formation events.In all the samples analysed here, the principal component representativeof star-forming activity accounts for a significant percentage of thevariance. The fractional contribution to the spectral energydistribution (SED) by the evolved stars and by the young population aresimilar.Projecting the SEDs on to their eigenspectra, we find that none of thecoefficients of the principal components can outline an internalcorrelation or can correlate with the optical morphological types. In asubsample of 43 galaxies, consisting of almost only compact and BCDgalaxies, the third principal component defines a sequence related tothe degree of starburst activity of the galaxy.
| The Nature of Nearby Counterparts to Intermediate-Redshift Luminous Compact Blue Galaxies. I. Optical/H I Properties and Dynamical Masses
We present single-dish H I spectra obtained with the Green BankTelescope, along with optical photometric properties from the SloanDigital Sky Survey, of 20 nearby (D<~70 Mpc) luminous compact bluegalaxies (LCBGs). These ~L*, blue, high surface brightness, starburstinggalaxies were selected using the same criteria as were used to defineLCBGs at higher redshifts. We find that these galaxies are gas-rich,with MHI ranging from 5×108 to8×109 Msolar andMHIL-1B ranging from 0.2 to 2Msolar L-1solar, consistent with avariety of morphological types of galaxies. We find that the dynamicalmasses (measured within R25) span a wide range, from 1 to1×1011 Msolar. However, at least half havedynamical mass-to-light ratios smaller than those of nearby galaxies ofall Hubble types, as found for LCBGs at intermediate redshifts. Bycomparing line widths and effective radii with local galaxy populations,we find that LCBGs are consistent with the dynamical mass properties ofMagellanic (low luminosity) spiral galaxies and the more massiveirregular and dwarf elliptical galaxies, such as NGC 205.
| Subgalactic Clumps at High Redshift: A Fragmentation Origin?
We investigate the origin of the clumpy structures observed at highredshift, such as chain galaxies. We use a three-dimensionalchemodynamical simulation describing the dynamics of stars and atwo-phase interstellar medium, as well as feedback processes from thestars. For high efficiency of energy dissipation in the cold cloudmedium, the initially gaseous disk fragments and develops severalmassive clumps of gas and stars. We follow the evolution of theindividual clumps and determine their masses, metallicities, andvelocities. A few dynamical times after fragmentation of the disk, theclumps merge to build a massive bulge. Calculating Hubble SpaceTelescope (HST) and UBVRIJHKLM colors, including absorption byinterstellar dust, we determine the morphologies and colors of thismodel in HST images. Several peculiar morphological structures seen inthe Hubble Deep Field can be well explained by a fragmented galacticdisk model, including chain galaxies and objects consisting of severalnearby knots.
| Superassociations and Stellar Complexes in Galaxies
The basic characteristics of stellar complexes and superassociations, aswell as the differences between these kinds of gigantic groups of youngstars, are discussed. The main difference is that superassociations arethe result of induced (triggered) star formation, while the stars andclusters in stellar complexes develop as a result of the spontaneousprocesses typical of galactic gaseous disks.
| Spectroscopic study of blue compact galaxies. IV. Star formation rates and gas depletion timescales
This is the fourth paper in a series studying star formation rates,stellar components, metallicities, and star formation histories of ablue compact galaxy (BCG) sample. Using Hα, [O II]λ3727,infrared (IR), radio (1.4 GHz) luminosities and neutral hydrogen (H I)gas masses, we estimated star formation rates and gas depletiontimescales of 72 star-forming BCGs. The star formation rates of the BCGsin our sample span nearly four orders of magnitude, from approximately10-2 to 102 Mȯ yr-1,with a median star formation rate of about 3 Mȯyr-1. The typical gas depletion timescale of BCGs is aboutone billion years. Star formation could be sustained at the currentlevel only on a timescale significantly lower than the age of theuniverse before their neutral gas reservoir is completely depleted. Toassess the possible systematic differences among different starformation rate indicators, we compared the star formation rates derivedfrom Hα, [O II]λ3727, IR, and radio luminosities, andinvestigated the effects from underlying stellar absorption and dustextinction. We found that subtracting underlying stellar absorption isvery important to calculate both dust extinction and star formation rateof galaxies. Otherwise, the intrinsic extinction will be overestimated,the star formation rates derived from [O II]λ3727 and Hαwill be underestimated (if the underlying stellar absorption and theinternal extinction were not corrected from the observed luminosity) oroverestimated (if an overestimated internal extinction were used forextinction correction). After both the underlying stellar absorption andthe dust extinction were corrected, a remarkably good correlationemerges among Hα, [O II]λ3727, IR and radio star formationrate indicators. Finally, we find a good correlation between themeasured star formation rate and the absolute blue magnitude,metallicity, interstellar extinction of BCGs. Our results indicate thatfaint, low-mass BCGs have lower star formation rates.Star formation rates and gas depletion timescales of BCGs are availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/425/417
| Starbursts in barred spiral galaxies. VI. HI observations and the K-band Tully-Fisher relation
This paper reports a study of the effect of a bar on the neutralhydrogen (HI) content of starburst and Seyfert galaxies. We also makecomparisons with a sample of ``normal'' galaxies and investigate howwell starburst and Seyfert galaxies follow the fundamental scalingTully-Fisher (TF) relation defined for normal galaxies. 111 Markarian(Mrk) IRAS galaxies were observed with the Nançay radiotelescope,and HI data were obtained for 80 galaxies, of which 64 are newdetections. We determined the (20 and 50%) linewidths, the maximumvelocity of rotation and total HI flux for each galaxy. Thesemeasurements are complemented by data from the literature to form asample of Mrk IRAS (74% starburst, 23% Seyfert and 3% unknown) galaxiescontaining 105 unbarred and 113 barred ones. Barred galaxies have lowertotal and bias-corrected HI masses than unbarred galaxies, and this istrue for both Mrk IRAS and normal galaxies. This robust result suggeststhat bars funnel the HI gas toward the center of the galaxy where itbecomes molecular before forming new stars. The Mrk IRAS galaxies havehigher bias-corrected HI masses than normal galaxies. They also showsignificant departures from the TF relation, both in the B and K bands.The most deviant points from the TF relation tend to have a strongfar-infrared luminosity and a low oxygen abundance. These resultssuggest that a fraction of our Mrk IRAS galaxies are still in theprocess of formation, and that their neutral HI gas, partly of externalorigin, has not yet reached a stationary state.Based on observations obtained at the large radiotelescope ofObservatoire de Nançay, operated by Observatoire de Paris.Tables 5 and 6 are only (and Table 4 also) available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/515
| Gas physics, disk fragmentation, and bulge formation in young galaxies
We investigate the evolution of star-forming gas-rich disks, using a 3Dchemodynamical model including a dark halo, stars, and a two-phaseinterstellar medium with feedback processes from the stars. We show thatgalaxy evolution proceeds along very different routes depending onwhether it is the gas disk or the stellar disk which first becomesunstable, as measured by the respective Q-parameters. This in turndepends on the uncertain efficiency of energy dissipation of the coldcloud component from which stars form.When the cold gas cools efficiently and drives the instability, thegalactic disk fragments and forms a number of massive clumps of starsand gas. The clumps spiral to the center of the galaxy in a fewdynamical times and merge there to form a central bulge component in astrong starburst. When the kinetic energy of the cold clouds isdissipated at a lower rate, stars form from the gas in a more quiescentmode, and an instability only sets in at later times, when the surfacedensity of the stellar disk has grown sufficiently high. The system thenforms a stellar bar, which channels gas into the center, evolves, andforms a bulge whose stars are the result of a more extended starformation history.We investigate the stability of the gas-stellar disks in both regimes,as well as the star formation rates and element enrichment. We study themorphology of the evolving disks, calculating spatially resolved coloursfrom the distribution of stars in age and metallicity, including dustabsorption. We then discuss morphological observations such as clumpystructures and chain galaxies at high redshift as possible signatures offragmenting, gas-rich disks. Finally, we investigate abundance ratiodistributions as a means to distinguish the different scenarios of bulgeformation.
| The evolution of stars and gas in starburst galaxies
In systems undergoing starbursts the evolution of the young stellarpopulation is expected to drive changes in the emission-line properties.This evolution is usually studied theoretically, with a combination ofevolutionary synthesis models for the spectral energy distribution ofstarbursts and photoionization calculations. In this paper we present amore empirical approach to this issue. We apply empirical populationsynthesis techniques to samples of starburst and HII galaxies in orderto measure their evolutionary state and correlate the results with theiremission-line properties. A couple of useful tools are introduced thatgreatly facilitate the interpretation of the synthesis: (1) anevolutionary diagram, the axes of which are the strengths of the young,intermediate age and old components of the stellar population mix; and(2) the mean age of stars associated with the starburst, . These toolsare tested with grids of theoretical galaxy spectra and found to workvery well even when only a small number of observed properties(absorption-line equivalent widths and continuum colours) is used in thesynthesis.Starburst nuclei and HII galaxies are found to lie on a well-definedsequence in the evolutionary diagram. Using the empirically defined meanstarburst age in conjunction with emission-line data, we have verifiedthat the equivalent widths of Hβ and [OIII] decrease for increasing. The same evolutionary trend was identified for line ratios indicativeof the gas excitation, although no clear trend was identified formetal-rich systems. All these results are in excellent agreement withlong-known, but little tested, theoretical expectations.
| Quantifying high z galaxy selection and visibility with the COSMOPACK tool
We have developed COSMOPACK, a set of tools that transform images ofreal galaxies to depict their appearance at a given redshift as observedwith a given telescope and camera. The transformation includesK-corrections, change of observing band, repixelation to the scale ofthe observing system, convolution by the seeing, and noise from sky,detector, and dark current. We show two applications of COSMOPACK: 1)the visibility of a number of galaxy types, and 2) the recovery ofeffective radii and effective surface brightness as a function ofredshift in the COSMOS survey.
| On the formation of star clusters in the merger NGC 6240
We identified star clusters in archived Hubble Space Telescope/WideField Planetary Camera 2 (HST/WFPC2) images of the merger andultraluminous infrared galaxy NGC 6240, with the aim of investigatingwhether star cluster properties (luminosity, age and mass) in such anextreme environment differ from those of clusters in less luminousstarburst galaxies. We found 54 star clusters in all of the F450W, F547Mand F814W exposures, of which 41 are located in the main body of NGC6240 and 13 are located in the galactic tails. Given that only twocolours are available to derive two independent variables (clusterreddening and age), we adopted an ad hoc procedure to derive clusterparameters statistically under the assumption that the clustermetallicity is like that in the Large Magellanic Cloud. The colours ofeach cluster are fitted to STARBURST99 models of fixed mass and variableages and reddenings. All cluster reddening and age solutions withχ2 < 1 are considered to be consistent with the data.Masses are derived by scaling the luminosity of the models withbest-fitting χ2 < 1 by the observed V luminosity,after correction for reddening and distance. Therefore, each cluster isdescribed by a range of reddening values, ages and masses; for each ofthese parameters, we derive probability functions. Thus we infer thatthe most probable age of the observed clusters is between 5 and 13 Myrand their most probable mass is about (1-2) × 105Msolar. A low probability exists for clusters as massive as108 Msolar, as well as for the trend that the meancluster mass increases towards the double nuclei of NGC 6240. Comparisonwith star clusters in starburst galaxies seems to point to more massiveclusters being formed in more massive galaxies and gas-rich mergers,while the overall cluster mass distribution might be relativelyindependent of the details of the associated starburst where dense,massive clusters preferentially form.
| The PDS versus Markarian starburst galaxies: comparing strong and weak IRAS emitter at 12 and 25 μm in the nearby Universe
The characteristics of the starburst galaxies from the Pico dos Diassurvey (PDS) are compared with those of the nearby ultraviolet (UV)bright Markarian starburst galaxies, having the same limit in redshift(vh < 7500 km s-1) and absolute B magnitude(MB < -18). An important difference is found: theMarkarian galaxies are generally undetected at 12 and 25 μm in IRAS.This is consistent with the UV excess shown by these galaxies andsuggests that the youngest star-forming regions dominating thesegalaxies are relatively free of dust.The far-infrared selection criteria for the PDS are shown to introduce astrong bias towards massive (luminous) and large size late-type spiralgalaxies. This is contrary to the Markarian galaxies, which are found tobe remarkably rich in smaller size early-type galaxies. These resultssuggest that only late-type spirals with a large and massive disc arestrong emitters at 12 and 25 μm in IRAS in the nearby Universe.The Markarian and PDS starburst galaxies are shown to share the sameenvironment. This rules out an explanation of the differences observedin terms of external parameters. These differences may be explained byassuming two different levels of evolution, the Markarian being lessevolved than the PDS galaxies. This interpretation is fully consistentwith the disc formation hypothesis proposed by Coziol et al. to explainthe special properties of the Markarian SBNG.
| Evolutionary spectral energy distribution diagnostics of starburst galaxies: signature of bimodality
We construct an evolutionary spectral energy distribution (SED) model ofa starburst region, from the ultraviolet to submillimetre wavelengths.This model allows us to derive the star formation rate, optical depth bydust and apparent effective radius of starburst regions at variouswavelengths; as a result, the intrinsic surface brightness of starburstregions can be derived. Using this SED model, we analyse 16ultraviolet-selected starburst galaxies and 10 ultraluminous infraredgalaxies. The derived star formation rates and optical depths arecompared with emission-line measurements and are found to be consistent.The derived apparent effective radii are also consistent withobservations. From the SED analysis, we find a bimodal property of thestar formation rate with the optical depth and the compactness ofstellar distributions. While mild starbursts have a limiting intrinsicsurface brightnessLbolr-2e~= 1012Lsolar kpc-2, intense starbursts tend to be moreheavily obscured and concentrated within a characteristic scale ofre~= 0.3 kpc. We suggest that the mild starbursts can betriggered by a self-gravitating disc instability in which feedback iseffective in the shallow gravitational potential. On the other hand, theintense starbursts can be induced via an external dynamical perturbationsuch as galaxy merging, in which feedback is less effective owing to thedeep gravitational potential attained by the large gas concentrationwithin the central starburst region.
| Stellar populations in local star-forming galaxies - I. Data and modelling procedure
We present an analysis of the integrated properties of the stellarpopulations in the Universidad Complutense de Madrid (UCM) Survey ofHα-selected galaxies. In this paper, the first of a series, wedescribe in detail the techniques developed to model star-forminggalaxies using a mixture of stellar populations, and taking into accountthe observational uncertainties. We assume a recent burst of starformation superimposed on a more evolved population. The effects of thenebular continuum, line emission and dust attenuation are taken intoaccount. We also test different model assumptions, including the choiceof specific evolutionary synthesis model, initial mass function, starformation scenario and the treatment of dust extinction. Quantitativetests are applied to determine how well these models fit ourmultiwavelength observations for the UCM sample. Our observations spanthe optical and near-infrared, including both photometric andspectroscopic data. Our results indicate that extinction plays a keyrole in this kind of study, revealing that low- and high-obscuredobjects may require very different extinction laws and must be treateddifferently. We also demonstrate that the UCM Survey galaxies are bestdescribed by a short burst of star formation occurring within aquiescent galaxy, rather than by continuous star formation. A detaileddiscussion on the inferred parameters, such as the age, burst strength,metallicity, star formation rate, extinction and total stellar mass forindividual objects, is presented in Paper II of this series.
| 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.
| Spatial Analysis of the Hα Emission in the Local Star-forming UCM Galaxies
We present a photometric study of the Hα emission in theUniversidad Complutense de Madrid (UCM) Survey galaxies. This workcomplements our previously published spectroscopic data. We study thelocation of the star-forming knots, their intensity and concentration,and the relationship of these properties with those of the host galaxy.We also estimate that the amount of Hα emission that arises fromthe diffuse ionized gas is about 15%-30% of the total Hα flux fora typical UCM galaxy. This percentage seems to be independent of theHubble type. Conversely, we find that an ``average'' UCM galaxy harborsa star formation event with 30% of its Hα luminosity arising froma nuclear component. The implications of these results for higherredshift studies are discussed, including the effects of galaxy size andthe depth of the observations. A correlation between the star formationrate and the Balmer decrement is observed, but such correlation breaksdown for large values of the extinction. Finally, we recalculate theHα luminosity function and star formation rate density of thelocal universe using the new imaging data. Our results point out that,on average, spectroscopic observations detected about one-third of thetotal emission-line flux of a typical UCM galaxy. The new valuesobtained for the Hα luminosity density and the star formation ratedensity of the local universe are 1039.3+/-0.2 ergss-1 Mpc-3 andρSFR=0.016+0.007-0.004Msolar yr-1 Mpc-3 (H0=50 kms-1 Mpc-1, ΩM=1.0, Λ=0).The corresponding values for the ``concordance cosmology''(H0=70 km s-1 Mpc-1,ΩM=0.3, Λ=0.7) are 1039.5+/-0.2 ergss-1 Mpc-3 andρSFR=0.029+0.008-0.005Msolar yr-1 Mpc-3.
| A Far Ultraviolet Spectroscopic Explorer Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo
Far Ultraviolet Spectroscopic Explorer (FUSE) data are used toinvestigate the molecular hydrogen (H2) content ofintermediate-velocity clouds (IVCs) in the lower halo of the Milky Way.We analyze interstellar absorption toward 56 (mostly extragalactic)background sources to study H2 absorption in the Lyman andWerner bands in 61 IVC components at H I column densities>=1019 cm-2. For data with good signal-to-noiseratio (S/N) (~9 per resolution element and higher), H2 in IVCgas is convincingly detected in 14 cases at column densities varyingbetween ~1014 and ~1017 cm-2. We findan additional 17 possible H2 detections in IVCs in FUSEspectra with lower S/N. The molecular hydrogen fractions, f, varybetween 10-6 and 10-3, implying a dense, mostlyneutral gas phase that is probably related to the cold neutral medium(CNM) in these clouds. If the H2 stays information-dissociation equilibrium, the CNM in these clouds can becharacterized by compact (D~0.1 pc) filaments with volume densities onthe order of nH~30 cm-3. The relatively highdetection rate of H2 in IVC gas implies that the CNM in theseclouds is ubiquitous. More dense regions with much higher molecularfractions may exist, but it would be difficult to detect them inabsorption because of their small size.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| Star formation rate in galaxies from UV, IR, and Hα estimators
Infrared (IR) luminosity of galaxies originating from dust thermalemission can be used as an indicator of the star formation rate (SFR).Inoue et al. (\cite{inoue00}, IHK) have derived a formula for theconversion from dust IR luminosity to SFR by using the following threequantities: the fraction of Lyman continuum luminosity absorbed by gas(f), the fraction of UV luminosity absorbed by dust (epsilon ), and thefraction of dust heating from old (ga 108 yr) stellarpopulations (eta ). We develop a method to estimate those threequantities based on the idea that the various way of SFR estimates fromultraviolet (UV) luminosity (2000 Å luminosity), Hαluminosity, and dust IR luminosity should return the same SFR. Afterapplying our method to samples of galaxies, the following results areobtained in our framework. First, our method is applied to a sample ofstar-forming galaxies, finding that f ~ 0.6, epsilon ~ 0.5, and eta ~0.4 as representative values. Next, we apply the method to a starburstsample, which shows larger extinction than the star-forming galaxysample. With the aid of f, epsilon , and eta , we are able to estimatereliable SFRs from UV and/or IR luminosities. Moreover, the Hαluminosity, if the Hα extinction is corrected by using the Balmerdecrement, is suitable for a statistical analysis of SFR, because thesame {correction factor for the Lyman continuum extinction (i.e. 1/f)}is applicable to both normal and starburst galaxies over all the rangeof SFR. The metallicity dependence of f and epsilon is also tested:Only the latter proves to have a correlation with metallicity. As anextension of our result, the local (z=0) comoving density of SFR can beestimated with our dust extinction corrections. We show that all UV,Hα , and IR comoving luminosity densities at z=0 give a consistentSFR per comoving volume ( ~ 3x 10-2h M_sun yr-1Mpc-3). Useful formulae for SFR estimate are listed.Tables 1 and 2, and Appendix A are only available in electronic form athttp://www.edpsciences.org
| The WSRT wide-field H I survey. I. The background galaxy sample
We have used the Westerbork array to carry out an unbiased wide-fieldsurvey for H I emission features, achieving an RMS sensitivity of about18 mJy/Beam at a velocity resolution of 17 km s-1 over 1800deg2 and between -1000 < VHel <+6500 kms-1. The primary data consists of auto-correlation spectrawith an effective angular resolution of 49' FWHM, althoughcross-correlation data were also acquired. The survey region is centeredapproximately on the position of Messier 31 and is Nyquist-sampled over60x 30o in RA x Dec. More than 100 distinct features aredetected at high significance in each of the two velocity regimes(negative and positive LGSR velocities). In this paper we present theresults for our H I detections of external galaxies at positive LGSRvelocity. We detect 155 external galaxies in excess of 8sigma inintegrated H I flux density. Plausible optical associations are foundwithin a 30' search radius for all but one of our H I detections in DSSimages, although several are not previously cataloged or do not havepublished red-shift determinations. Our detection without a DSSassociation is at low galactic latitude. Twenty-three of our objects aredetected in H I for the first time. We classify almost half of ourdetections as ``confused'', since one or more companions is catalogedwithin a radius of 30' and a velocity interval of 400 km s-1.We identify a handful of instances of significant positional offsetsexceeding 10 kpc of unconfused optical galaxies with the associated H Icentroid, possibly indicative of severe tidal distortions or uncatalogedgas-rich companions. A possible trend is found for an excess of detectedH I flux in unconfused galaxies within our large survey beam relative tothat detected previously in smaller telescope beams, both as function ofincreasing distance and increasing gas mass. This may be an indicationfor a diffuse gaseous component on 100 kpc scales in the environment ofmassive galaxies or a population of uncataloged low mass companions. Weuse our galaxy sample to estimate the H I mass function from our surveyvolume. Good agreement is found with the HIPASS BGC results, but onlyafter explicit correction for galaxy density variations with distance.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/406/829 and Fig. 3 is onlyavailable in electronic form at http://www.edpsciences.org
| 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
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