WIKISKY.ORG
 Home Getting Started To Survive in the Universe News@Sky Astro Photo The Collection Forum Blog New! FAQ Press Login

# NGC 7046

Contents

### Images

DSS Images   Other Images

### Related articles

 New Reference Galaxy Standards for H I Emission ObservationsWe have taken advantage of the improved baselines and higher sensitivityavailable with the upgraded Arecibo 305 m telescope to create a new H Ispectral line catalog of disk galaxies that can be used as a referencecatalog for anyone interested in 21 cm spectral line work. In all 108galaxies were observed, covering 24 hr of the sky at declinationsbetween 0° and 36° and velocities between 0 and 25,000 kms-1. The majority of the galaxies were observed at least twotimes on different nights to avoid problems with radio frequencyinterference, baseline fluctuations, etc. Comparing our measured valueswith all those available in the literature shows that although largeindividual variations may exist, the average difference between themeasurements is zero. In all we have considerable confidence in ourmeasurements, and the resulting catalog should be extremely useful as awell-defined reference catalog for anyone interested in 21 cm spectralline work. Mass-to-light ratios from the fundamental plane of spiral galaxy discsThe best-fitting two-dimensional plane within the three-dimensionalspace of spiral galaxy disc observables (rotational velocityvrot, central disc surface brightnessμ0=-2.5logI0 and disc scalelength h) has beenconstructed. Applying the three-dimensional bisector method ofregression analysis to a sample of ~100 spiral galaxy discs that spanmore than 4magarcsec-2 in central disc surface brightnessyields vrot\proptoI0.50\pm0.050\,h0.77\pm 0.07 (B band)and vrot\proptoI0.43\pm0.040\,h0.69\pm 0.07 (R band).Contrary to popular belief, these results suggest that in the B band,the dynamical mass-to-light ratio (within four disc scalelengths) islargely independent of the surface brightness, varying as I0.00\pm0.100\,h0.54\pm 0.14. Consistentresults were obtained when the range of the analysis was truncated byexcluding the low-surface-brightness galaxies. Previous claims thatM/LBvaries withI-1/20,Bareshown to be misleading and/or caused by galaxy selection effects - notall low-surface-brightness disc galaxies are dark matter dominated. Thesituation is, however, different in the near-infrared whereLK'~v4 and M/LK' is shown to vary asI-1/20,K\prime. Theoretical studies ofspiral galaxy discs should therefore not assume a constant M/L ratiowithin any given passband. The B-band dynamical mass-to-light ratio(within four disc scalelengths) has no obvious correlation with (B-R)disc colour, while in the K' band it varies as -1.25+/-0.28(B-R).Combining the present observational data with recent galaxy modelpredictions implies that the logarithm of the stellar-to-dynamical massratio is not a constant value, but increases as discs become redder,varying as 1.70+/-0.28(B-R). The UZC-SSRS2 Group CatalogWe apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers. An Investigation into the Prominence of Spiral Galaxy BulgesFrom a diameter-limited sample of 86 low-inclination (face-on) spiralgalaxies, the bulge-to-disk size and luminosity ratios and otherquantitative measurements for the prominence of the bulge are derived.The bulge and disk parameters have been estimated using aseeing-convolved Sérsic r1/n bulge and aseeing-convolved exponential disk that were fitted to the optical (B, R,and I) and near-infrared (K) galaxy light profiles. In general,early-type spiral galaxy bulges have Sérsic values of n>1, andlate-type spiral galaxy bulges have values of n<1. In the B band,only eight galaxies have a bulge shape parameter n consistent with theexponential value 1, and only five galaxies do in the K band. Use of theexponential bulge model is shown to restrict the range ofre/h and B/D values by more than a factor of 2. Applicationof the r1/n bulge models, unlike exponential bulge models,results in a larger mean re/h ratio for the early-type spiralgalaxies than for the late-type spiral galaxies, although this result isshown not to be statistically significant. The mean B/D luminosity ratiois, however, significantly larger (>3 σ) for the early-typespirals than for the late-type spirals. Two new parameters areintroduced to measure the prominence of the bulge. The first is thedifference between the central surface brightness of the galaxy and thesurface brightness level at which the bulge and disk contribute equally.The other test uses the radius at which the contribution from the diskand bulge light are equal, normalized for the effect of intrinsicallydifferent galaxy sizes. Both of these parameters reveal that theearly-type spiral galaxies appear'' to have significantly (more than 2σ in all passbands) bigger and brighter bulges than late-typespiral galaxies. This apparent contradiction with the re/hvalues can be explained with an iceberg-like scenario, in which thebulges in late-type spiral galaxies are relatively submerged in theirdisk. This can be achieved by varying the relative stellar density whilemaintaining the same effective bulge-to-disk ratio. The B/D luminosityratio and the concentration index C31, in agreement with paststudies, are positively correlated and decrease as one moves along thespiral Hubble sequence toward later spiral galaxy types, although forgalaxies with large extended bulges the concentration index no longertraces the B/D luminosity ratio in a one-to-one fashion. A strong(Spearman's rank-order correlation coefficient, rs=0.80) andhighly significant positive correlation exists between the shape, n, ofthe bulge light profile and the bulge-to-disk luminosity ratio. Theabsolute bulge magnitude-logn diagram is used as a diagnostic tool forcomparative studies with dwarf elliptical and ordinary ellipticalgalaxies. At least in the B band these objects occupy distinctlydifferent regions of this parameter space. While the dwarf ellipticalgalaxies appear to be the faint extension to the brighter ellipticalgalaxies, the bulges of spiral galaxies do not; for a given luminositythey have a noticeably smaller shape parameter and hence a more dramaticdecline of stellar density at large radii. Nearby Optical Galaxies: Selection of the Sample and Identification of GroupsIn 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. Optical Long-Slit Spectroscopy of a Sample of Spiral GalaxiesWe have analyzed long-slit spectra of a sample of 11 spiral galaxies. Weidentify several disk H II regions in each galaxy and provideclassifications for the nuclear spectra. We find seven nuclear H IIregion galaxies, including four starburst nucleus galaxies, and fourLINERs. Arcsecond Positions of UGC GalaxiesWe 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. A strong correlation between bar strength and global star forming activity in isolated barred galaxiesI have studied the relation between the global star formation activityand the bar structure in a sample of isolated barred galaxies. The starformation activity was quantified via the ratio between the IRAS fluxesat 25 mu m and 100 mu m. Two parameters were chosen to define the barstructure: the strength of the bar and the relative projected barlength. The strength of the bar was defined by epsilon_ {b}=10(1-b/a),where a and b are the projected semi-major and semi-minor bar axis. Therelative bar length was defined as: 2Lb/D25, whereL_ {b} is one half of the projected total bar length and D25is the diameter of the 25 mag arcsec-2 magnitude isophote inthe B band. We found a strong correlation between the star formationactivity and epsilon_ {b}. The regression line is given bylog(I25/I100)=-1.81+0.093 epsilon_ {b}, with acorrelation coefficient of 0.9. The link is not so evident between therelative projected bar length and the star formation activity. But, itis noted that there is enhanced star formation activity in galaxies withstrong bars and small relative bar lengths,0.1<2Lb/D25<0.22. Total magnitude, radius, colour indices, colour gradients and photometric type of galaxiesWe 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. Gas Mass Fractions and the Evolution of Spiral GalaxiesWe show that the gas mass fraction of spiral galaxies is stronglycorrelated with luminosity and surface brightness. It is not correlatedwith linear size. Gas fraction varies with luminosity and surfacebrightness at the same rate, indicating evolution at fixed size. Dimgalaxies are clearly less evolved than bright ones, having consumed only~ \frac {1}{2} of their gas. This resolves the gas consumption paradox,since there exist many galaxies with large gas reservoirs. Thesegas-rich galaxies must have formed the bulk of their stellar populationsin the last half of a Hubble time. The existence of such immaturegalaxies at z = 0 indicates that either galaxy formation is a lengthy oreven ongoing process, or the onset of significant star formation can bedelayed for arbitrary periods in tenuous gas disks. Near-infrared and optical broadband surface photometry of 86 face-on disk dominated galaxies. II. A two-dimensional method to determine bulge and disk parameters.In this Paper I present a new two-dimensional decomposition technique,which models the surface photometry of a galaxy with an exponentiallight profile for both bulge and disk and, when necessary, with aFreeman bar. The new technique was tested for systematic errors on bothartificial and real data and compared with widely used one-dimensionaldecomposition techniques, where the luminosity profile of the galaxy isused. The comparisons indicate that a decomposition of thetwo-dimensional image of the galaxy with an exponential light profilefor both bulge and disk yields the most reproducible and representativebulge and disk parameters. An extensive error analysis was made todetermine the reliability of the model parameters. If the model with anexponential bulge profile is a reasonable description of a galaxy, themaximum errors in the derived model parameters are of order 20%. Theuncertainties in the model parameters will increase, if the exponentialbulge function is replaced by other often used bulge functions as the deVaucouleurs law. All decomposition methods were applied to the opticaland near-infrared data set presented by de Jong & van der Kruit(1994), which comprises 86 galaxies in six passbands. An image database. II. Catalogue between δ=-30deg and δ=70deg.A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect. Near-infrared and optical broadband surface photometry of 86 face-on disk dominated galaxies. I. Selection, observations and data reduction.We present accurate surface photometry in the B, V, R, I, H and Kpassbands of 86 spiral galaxies. The galaxies in this statisticallycomplete sample of undisturbed spirals were selected from the UGC tohave minimum diameters of 2' and minor over major axis ratios largerthan 0.625. This sample has been selected in such a way that it can beused to represent a volume limited sample. The observation and reductiontechniques are described in detail, especially the not often useddriftscan technique for CCDs and the relatively new techniques usingnear-infrared (near-IR) arrays. For each galaxy we present radialprofiles of surface brightness. Using these profiles we calculated theintegrated magnitudes of the galaxies in the different passbands. Weperformed internal and external consistency checks for the magnitudes aswell as the luminosity profiles. The internal consistency is well withinthe estimated errors. Comparisons with other authors indicate thatmeasurements from photographic plates can show large deviations in thezero-point magnitude. Our surface brightness profiles agree within theerrors with other CCD measurements. The comparison of integratedmagnitudes shows a large scatter, but a consistent zero-point. Thesemeasurements will be used in a series of forthcoming papers to discusscentral surface brightnesses, scalelengths, colors and color gradientsof disks of spiral galaxies. High signal-to-noise ratio observations of H I in 243 galaxiesThe 21 cm spectral-line system of the Arecibo Observatory was used tomeasure neutral hydrogen emission from 243 faint galaxies. Most lie nearthe plane of the Local Supercluster. All observations reach anunsmoothed signal-to-noise ratio of at least 7.0; the average for theset is 23. The resulting data are used to estimate H I masses, systemicvelocities, and accurate profile widths at 20 percent, 25 percent, 50percent, and 80 percent of peak intensity levels. The widths are used tocalibrate directly the bias introduced by popular data-smoothingoperation. The data include observations of 65 objects with previouslyunknown redshifts. Morphology of spiral galaxies. I - General propertiesRed Palomar Sky Survey plates are scanned to characterize a completesample of 605 spiral galaxies north of declination -33 deg havinginclination angle less than 56 deg and blue diameter 2-15 arcmin. Theselection of the data and the reduction and parameter-extractionprocedures are explained, and the data and the results of statisticalanalysis are presented in tables and graphs. Findings reported include alow frequency of occurrence for small inclination angles (suggestingdistortion of outer structures), similar distributions of central diskbrightness for types Sa-Sc but not for types Sd-Sm (where mean valuesare smaller), fewer late-type galaxies with large exponential-disk scalelengths, no galaxies with both high central brightness and large scalelength (indicating a limit on angular momentum in galaxy formation), anda correlation between mean surface brightness and absolute magnitude forlater-type galaxies but not for types Sa-Scd. Velocity calibration of the Las Campanas redshift surveyThe Center for Astrophysics redshift survey is being extended bySchectman et al. (1983) to the south galactic cap. Their initial resultsprovide independent estimates of both emission Ve velocities and theabsorption line Vc velocities, such that (Ve-Vc) = 54 + or - 13 km/s. HI line observations of 40 of the sample are presented here, to provide asecure velocity scale calibration. The Vc velocities are found to have a32 + or - 9 km/s zero-point error. Redshifts for 115 galaxies near the equatorNew redshifts for 115 bright galaxies located near the celestial equatorare reported. The spectra were observed with a blue-sensitivephoton-counting Reticon on the 100-in. DuPont telescope, and theredshifts were derived using the data-analysis system developed for theCfA Redshift Survey. Comparisons with other measured redshifts suggestthat these data are similar in quality to the redshifts measured at Mt.Hopkins for the CfA Redshift Survey; the velocity zero point is good to10 or 15 km/s, with a typical error of 35 km/s for the individualmeasurements.
Submit a new article