Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
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.
| Star Formation Properties of a Large Sample of Irregular Galaxies
We present the results of Hα imaging of a large sample ofirregular galaxies. Our sample includes 94 galaxies with morphologicalclassifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems.The sample spans a large range in galactic parameters, includingintegrated absolute magnitude (MV of -9 to -19), averagesurface brightness (20-27 mag arcsec-2), current starformation activity (0-1.3 Msolar yr-1kpc-2), and relative gas content(0.02-5Msolar/LB). The Hα images were usedto measure the integrated star formation rates, determine the extents ofstar formation in the disks, and compare azimuthally averaged radialprofiles of current star formation to older starlight. The integratedstar formation rates of Im galaxies normalized to the physical size ofthe galaxy span a range of a factor of 104 with 10% Imgalaxies and one Sm system having no measurable star formation at thepresent time. The BCDs fall, on average, at the high star formation rateend of the range. We find no correlation between star formation activityand proximity to other cataloged galaxies. Two galaxies located in voidsare similar in properties to the Sm group in our sample. The H IIregions in these galaxies are most often found within the Holmbergradius RH, although in a few systems H II regions are tracedas far as 1.7RH. Similarly, most of the star formation isfound within three disk scale lengths RD, but in somegalaxies H II regions are traced as far as 6RD. A comparisonof Hα surface photometry with V-band surface photometry shows thatthe two approximately follow each other with radius in Sm galaxies, butin most BCDs there is an excess of Hα emission in the centers thatdrops with radius. In approximately half of the Im galaxies Hα andV correspond well, and in the rest there are small to large differencesin the relative rate of falloff with radius. The cases with stronggradients in the LHα/LV ratios and with highcentral star formation rate densities, which include most of the BCDs,require a significant fraction of their gas to migrate to the center inthe last gigayear. We discuss possible torques that could have causedthis without leaving an obvious signature, including dark matter barsand past interactions or mergers with small galaxies or H I clouds.There is now a substantial amount of evidence for these processes amongmany surveys of BCDs. We note that such gas migration will also increasethe local pressure and possibly enhance the formation of massive denseclusters but conclude that the star formation process itself does notappear to differ much among BCD, Im, and Sm types. In particular, thereis evidence in the distribution function for Hα surface brightnessthat the turbulent Mach numbers are all about the same in these systems.This follows from the Hα distribution functions corrected forexponential disk gradients, which are log-normal with a nearly constantdispersion. Thus, the influence of shock-triggered star formation isapparently no greater in BCDs than in Im and Sm types.
| Galaxy interactions - poor starburst triggers. III. A study of a complete sample of interacting galaxies
We report on a study of tidally triggered star formation in galaxiesbased on spectroscopic/photometric observations in the optical/near-IRof a magnitude limited sample of 59 systems of interacting and merginggalaxies and a comparison sample of 38 normal isolated galaxies. From astatistical point of view the sample gives us a unique opportunity totrace the effects of tidally induced star formation. In contrast toresults from previous investigations, our global UBV colours do notsupport a significant enhancement of starforming activity in theinteracting/merging galaxies. We also show that, contrary to previousclaims, there is no significantly increased scatter in the colours ofArp galaxies as compared to normal galaxies. We do find support formoderate (a factor of ~ 2-3) increase in star formation in the verycentres of the interacting galaxies of our sample, contributingmarginally to the total luminosity. The interacting and in particularthe merging galaxies are characterized by increased far infrared(hereafter FIR) luminosities and temperatures that weakly correlate withthe central activity. The LFIR/LB ratio however,is remarkably similar in the two samples, indicating that truestarbursts normally are not hiding in the central regions of the FIRluminous cases. The gas mass-to-luminosity ratio in optical-IR ispractically independent of luminosity, lending further support to thepaucity of true massive starburst galaxies triggered byinteractions/mergers. We estimate the frequency of such cases to be ofthe order of ~ 0.1% of the galaxies in an apparent magnitude limitedsample. Our conclusion is that interacting and merging galaxies, fromthe global star formation aspect, generally do not differ dramaticallyfrom scaled up versions of normal, isolated galaxies. No drastic changewith redshift is expected. One consequence is that galaxy formationprobably continued over a long period of time and did not peak at aspecific redshift. The effects of massive starbursts, like blowoutscaused by superwinds and cosmic reionization caused by starburstpopulations would also be less important than what is normally assumed.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| The Taxonomy of Blue Amorphous Galaxies. II. Structure and Evolution
Dwarf galaxies play an important role in our understanding of galaxyformation and evolution, and starbursts are believed to affect thestructure and evolution of dwarf galaxies strongly. We have thereforeembarked on a systematic study of 12 of the nearest dwarf galaxiesthought to be undergoing bursts of star formation. These were selectedprimarily by their morphological type (blue ``amorphous'' galaxies). Weshow that these blue amorphous galaxies are not physicallydistinguishable from dwarfs selected as starbursting by other methods,such as blue compact dwarfs (BCDs) and H II galaxies. All these classesexhibit surface brightness profiles that are exponential in the outerregions (r>~1.5r_e) but often have a predominantly central blueexcess, suggesting a young burst in an older, redder galaxy. Typically,the starbursting ``cores'' are young (~10^7-10^8 yr) events compared tothe older (~10^9-10^10 yr) underlying galaxy (the ``envelope''). Theratio of the core to envelope in blue light ranges from essentially zeroto about 2. These starbursts are therefore modest events involving onlya few percent of the stellar mass. The envelopes have surfacebrightnesses that are much higher than typical dwarf irregular (dI)galaxies, so it is unlikely that there is a straightforward evolutionaryrelation between typical dIs and dwarf starburst galaxies. Instead wesuggest that amorphous galaxies may repeatedly cycle through starburstand quiescent phases, corresponding to the galaxies with strong andweak/absent cores, respectively. Once amorphous galaxies use up theavailable gas (either through star formation or galactic winds) so thatstar formation is shut off, the faded remnants would strongly resembledwarf elliptical galaxies. However, in the current cosmological epoch,this is evidently a slow process that is the aftermath of a series ofmany weak, recurring bursts. Present-day dE's must have experienced morerapid and intense evolution than this in the distant past.
| Morphology of star formation regions in irregular galaxies
The location of HII regions, which indicates the locus of present starformation in galaxies, is analysed for a large collection of 110irregular galaxies (Irr) imaged in Hα and nearby continuum. Theanalysis is primarily by visual inspection, although a two-dimensionalquantitative measure is also employed. The two different analyses yieldessentially identical results. HII regions appear preferentially at theedges of the light distribution, predominantly on one side of thegalaxy, contrary to what is expected from stochastic self-propagatingstar formation scenarios. This peculiar distribution of star-formingregions cannot be explained by a scenario of star formation triggered byan interaction with extragalactic gas, or by a strong one-armed spiralpattern.
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
| The Taxonomy of Blue Amorphous Galaxies. I. H alpha and UBVI Data
Dwarf galaxies play an important role in our understanding of galaxyformation and evolution. We have embarked on a systematic study of 12nearby dwarf galaxies (most of which have been classified as amorphous)selected preferentially by their blue colors. The properties of thegalaxies in the sample suggest that they are in a burst or postburststate. It seems likely that these amorphous galaxies are closely relatedto other "starburst" dwarfs such as blue compact dwarfs (BCDs) and H IIgalaxies but are considerably closer and therefore easier to study. Ifso, these galaxies may offer important insights into dwarf galaxyevolution. In an effort to clarify the role of starbursts inevolutionary scenarios for dwarf galaxies, we present H alpha and UBVIdata for our sample. Blue amorphous galaxies, like BCDs and H IIgalaxies, have surface brightness profiles that are exponential in theouter regions (r >~ 1.5re) but have a predominantly blue centralexcess, which suggests a young burst in an older, redder galaxy. Sevenof the galaxies have the bubble or filamentary H alpha morphology anddouble-peaked emission lines that are the signature of superbubbles orsuperwind activity. These galaxies are typically the ones with thestrongest central excesses. The underlying exponential galaxies are verysimilar to those found in BCDs and H II galaxies. How amorphous galaxiesfit into the dwarf irregular--"starburst dwarf"--dwarf ellipticalevolutionary debate is less clear. In this paper, we present our dataand make some preliminary comparisons between amorphous galaxies andother classes of dwarf galaxies. In a future companion paper, we willcompare this sample more quantitatively with other dwarf galaxy samplesin an effort to determine if amorphous galaxies are a physicallydifferent class of object from other starburst dwarfs such as BCDs and HII galaxies and also investigate their place in dwarf galaxy evolutionscenarios.
| Scaleheights of 486 southern spiral galaxies and some statistical correlation
Based on Peng's method (1988), we obtain scaleheights of 486 southernspiral galaxies, the images of which are taken from the Digitized SkySurvey at Xinglong Station of Beijing Astronomical Observatory. Thefitted spiral arms of 70 galaxies are compared with their images to gettheir optimum inclinations. The scaleheights of other 416 ones arelisted in Table A1 in Appendix. After compiling and analyzing the data,we find some statistical correlations. The most interesting results arethat a flatter galaxy is bluer and looks brighter, and galaxies becomeflatter along the Hubble sequence Sab -- Scd. Based on photographic dataof the National Geographic Society -- Palomar Observatory Sky Survey(NGS-POSS) obtained using the Oschin Telescope Palomar Mountain. TheNGS-POSS was funded by a grant from the National Geographic Society tothe California Institute of Technology. The plates were processed intothe present compressed digital form with their permission. The DigitizedSky Survey was produced at the Space Telescope Science Institute underUS Government grant NAG W-2166. Table A1 is available in electronic fromonly, via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| CO and HI in a southern sample of interacting galaxies. I. The data
Using SEST, the Parkes antenna and the Australia Telescope CompactArray, we have made a survey of the ^12CO(1-0) and HI emission of anoptically-selected sample of ~60 southern interacting and merginggalaxies. In this paper we present the data and determine global massesof neutral gas (in molecular and atomic form) for the observed galaxies.We have detected HI in 26 systems and found that these galaxies haveless than 15% of their gas in molecular form. Figure 3 in its entirelyis only available in electronic form at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A study of a complete sample of interacting galaxies. II. Images, colour distributions and spectra.
We present broadband images, two-colour maps and spectra of a magnitudelimited sample of interacting and merging galaxies.
| Spectroscopic Observations of ARP / Madore Interacting Galaxies - Part Two - Galaxies with Tails Loops of Material or Debris
We present spectroscopic observations of 103 galaxies from a sample ofinteracting galaxies with tails, loops of material or debris. Radialvelocities, relative emission-line intensity measurements and opticalclassifications are presented. Three new Seyfert candidates areidentified. The frequency of Seyfert-type nuclei in our sample of`strongly' interacting galaxies (3.9 per cent; 4/103) is notsignificantly different from that of the interacting doubles ofcomparably sized galaxies (3.1 per cent; 4/129), which are presumably atan earlier stage of interaction than the galaxies observed in thepresent-study. However, the lack of a suitable control sample for ourinteracting samples prevents us from confirming whether interactions andmergers enhance Seyfert nuclear activity compared to that of non-interacting galaxies. A large fraction of the galaxies in our sampleshow strong H II region type emission lines, which indicate ongoingenhanced star formation activity. Since the systems are at a relativelyearly stage of the merger process and are near the peak of theirstarburst activities, the interstellar gas in the disc must be collectedin the nuclear region on time-scales less than about a few X 10^8^-10^9^yr. Strong H II region type emission lines superimposed on a strongstellar Balmer absorption spectrum are seen in many of the systems inour sample, suggesting a possible recurrent starburst or propagation ofstar-forming regions within the galaxy. High-resolution imaging studiesare required for further analysis of the sample.
| A survey for extra-H II region ionized gas structures in irregular galaxies
We have conducted a deep H-alpha imaging survey of 51 irregular andamorphous galaxies with the objective of searching for large ionized gasstructures outside of normal H II regions. In this sample 12% of thegalaxies contain at least one ionized supershell (radius greater than orequal 300 pc), 24% contain supergiant ionized filaments which are notobviously connected with a particular star forming region, and 27%contain one or more of these types of structures. For the most part,large ionized gas structures are found in galaxies that are engaging inintense star formation or that contain at least one unusually largeconcentration of massive stars. Thus, in most galaxies these structuresare likely to have been produced by massive stars. However, there is alarge range in the properties of the host galaxies, and not all galaxiesthat are actively forming stars or that contain luminous H II regionsalso contain supershells or giant filaments of ionized gas. As giant HII regions appear to be more common than large scale ionized features,we statistically find either that not all giant H II regions willproduce supershells or giant filaments or that the extra-H II regionstructures have a shorter lifetime than the giant H II regionsthemselves. Two galaxies are particularly noteworthy. DDO 75 is a smallgalaxy with a low level of star formation activity but whichnevertheless hosts several remarkably large ionized supershells. Wecannot explain the origin and ionization of these structures. Bycontrast, DDO 50 has a neutral interstellar medium which containsnumerous holes but which has no extraordinary ionized gas structures.The most likely explanations are either that the H I structures in DDO50 are older than the lives of the massive stars that produced them orthat the H I holes were formed by a process not directly associated withconcentrations of massive stars. Only 2 out of 15 normal irregularsviewed at inclination angles greater than 60 deg, plus two galaxies thatare clearly in an abnormal state, show evidence for structures that areor could be extending out of the disk of the galaxy. In three of thesesystems the filaments which are farthest from the disks are roughlyparallel to the galactic planes, and therefore have the morphologiesexpected of stalled supershells. Thus, galactic disk breakouts orchimneys containing optically visible ionized gas cannot be a frequentphenomenon in irregular galaxies and, even when they do occur, we cannotassume that a galactic wind is also present.
| Southern Sky Redshift Survey - The catalog
The catalog of radial velocities for galaxies which comprise thediameter-limited sample of the Southern Sky Redshift Survey ispresented. It consolidates the data of observations carried out at theLas Campanas Observatory, Observatorio Nacional, and South AfricanAstronomical Observatory. The criteria used for the sample selection aredescribed, as well as the observational procedures and the techniqueutilized to obtain the final radial velocities. The intercomparisonbetween radial velocity measurements from different telescopes indicatesthat the final data base is fairly homogeneous with a typical error ofabout 40 km/s. The sample is at present 90 percent complete, and themissing galaxies are predominantly objects with very low surfacebrightness for which it is very difficult to obtain optical redshifts.
| A study of a complete sample of interacting galaxies. I - Presentation of the sample and the UBVRIJHK photometry
An investigation is presented on what effects galaxy-galaxy interactionhas on the properties of the involved galaxies. A magnitude limitedsample of interacting galaxies is presented, together with a controlsample of isolated galaxies. The Cousins UBVRI and Johnson JHKphotometry of all galaxies included in the samples is also presented.
| Mass-to-Light Ratios of Binary Galaxies. III. Analysis
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987ApJS...64..427S&db_key=AST
| Colors and the evolution of amorphous galaxies
UBVRI and H-alpha photometric observations are presented for 16amorphous galaxies and a comparison sample of Magellanic irregular (Im)and Sc spiral galaxies. These data are analyzed in terms ofstar-formation rates and histories in amorphous galaxies. Amorphousgalaxies have mean global colors and star-formation rates per unit areathat are similar to those in giant Im systems, despite differences inspatial distributions of star-forming centers in these two galacticstructural classes. Amorphous galaxies differ from giant Im systems inhaving somewhat wider scatter in relationships between B - V and U - Bcolors, and between U - B and L(H-alpha)/L(B). This scatter isinterpreted as resulting from rapid variations in star-formation ratesduring the recent past, which could be a natural consequence of theconcentration of star-forming activity into centrally located,supergiant young stellar complexes in many amorphous galaxies. While theunusual spatial distribution and intensity of star formation in someamorphous galaxies is due to interactions with other galaxies, severalamorphous galaxies are relatively isolated and thus the processes mustbe internal. The ultimate evolutionary fate of rapidly evolvingamorphous galaxies remains unknown.
| Southern Galaxy Catalogue.
Not Available
| An optical and H I study of late-type low surface brightness galaxies
Neutral hydrogen and optical parameters are presented for 151 galaxiesof low surface brightness selected from UK Schmidt plates. The 21-cm H Iline was detected in 100 of these systems. It is found that the galaxiesshow the same trends of global properties with type as samples of brightgalaxies, while the data are consistent with the low surface brightness(LSB) galaxies being of systematically lower mass than bright galaxiesof the same type and linear dimensions. A constant value of hydrogenmass/(linear dimension)/squared is strongly suggested for LSB and brightgalaxies.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
INHABITED SKY
