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

# NGC 2976

Contents

### Images

DSS Images   Other Images

### Related articles

 Hαkinematics of the SINGS nearby galaxies survey - I*This is the first part of an Hαkinematics follow-up survey of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for28galaxies are presented. The observations were done on three differenttelescopes with Fabry-Perot of New Technology for the Observatoire dumont Megantic (FaNTOmM), an integral field photon-counting spectrometer,installed in the respective focal reducer of each telescope. The datareduction was done through a newly built pipeline with the aim ofproducing the most homogenous data set possible. Adaptive spatialbinning was applied to the data cubes in order to get a constantsignal-to-noise ratio across the field of view. Radial velocity andmonochromatic maps were generated using a new algorithm, and thekinematical parameters were derived using tilted-ring models. Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby GalaxiesMid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions. Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio CorrelationWe estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion. Masses of the local group and of the M81 group estimated from distortions in the local velocity fieldBased on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies. First Results from THINGS: The HI Nearby Galaxy SurveyWe describe The HI Nearby Galaxy Survey (THINGS), the largestprogramever undertaken at the Very Large Array to perform 21-cm HIobservations of thehighest quality ( 7'', ≤ 5 km s^{-1}resolution) ofnearby galaxies. The goal of THINGS is to investigatekeycharacteristics related to galaxy morphology, star formation andmassdistribution across the Hubble sequence. A sample of 34 objectswithdistances between 3 and 10 Mpc will be observed, covering a widerangeof evolutionary stages and properties. Data from THINGSwillcomplement SINGS, the Spitzer Infrared Nearby Galaxy Survey. Forthe THINGS sample, high-quality observations at comparable resolutionwillthus be available from the X-ray regime through to the radio partofthe spectrum. THINGS data can be used to investigate issues such asthesmall-scale structure of the ISM, its three-dimensional structure,the(dark) matter distribution and processes leading to starformation. Todemonstrate the quality of the THINGS data products, wepresent someprelimary HI maps here of four galaxies from the THINGSsample. Infrared Spectral Energy Distributions of Nearby GalaxiesThe Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities. Secular Evolution via Bar-driven Gas Inflow: Results from BIMA SONGWe present an analysis of the molecular gas distributions in the 29barred and 15 unbarred spirals in the BIMA CO (J=1-0) Survey of NearbyGalaxies (SONG). For galaxies that are bright in CO, we confirm theconclusion by Sakamoto et al. that barred spirals have higher moleculargas concentrations in the central kiloparsec. The SONG sample alsoincludes 27 galaxies below the CO brightness limit used by Sakamoto etal. Even in these less CO-bright galaxies we show that high central gasconcentrations are more common in barred galaxies, consistent withradial inflow driven by the bar. However, there is a significantpopulation of early-type (Sa-Sbc) barred spirals (6 of 19) that have nomolecular gas detected in the nuclear region and have very little out tothe bar corotation radius. This suggests that in barred galaxies withgas-deficient nuclear regions, the bar has already driven most of thegas within the bar corotation radius to the nuclear region, where it hasbeen consumed by star formation. The median mass of nuclear moleculargas is over 4 times higher in early-type bars than in late-type (Sc-Sdm)bars. Since previous work has shown that the gas consumption rate is anorder of magnitude higher in early-type bars, this implies that theearly types have significantly higher bar-driven inflows. The loweraccretion rates in late-type bars can probably be attributed to theknown differences in bar structure between early and late types. Despitethe evidence for bar-driven inflows in both early and late Hubble-typespirals, the data indicate that it is highly unlikely for a late-typegalaxy to evolve into an early type via bar-induced gas inflow.Nonetheless, secular evolutionary processes are undoubtedly present, andpseudobulges are inevitable; evidence for pseudobulges is likely to beclearest in early-type galaxies because of their high gas inflow ratesand higher star formation activity. High-Resolution Measurements of the Halos of Four Dark Matter-Dominated Galaxies: Deviations from a Universal Density ProfileWe derive rotation curves for four nearby, low-mass spiral galaxies anduse them to constrain the shapes of their dark matter density profiles.This analysis is based on high-resolution two-dimensional Hαvelocity fields of NGC 4605, NGC 5949, NGC 5963, and NGC 6689 and COvelocity fields of NGC 4605 and NGC 5963. In combination with ourprevious study of NGC 2976, the full sample of five galaxies containsdensity profiles that span the range from αDM=0 toαDM=1.20, where αDM is the power-lawindex describing the central density profile. The scatter inαDM from galaxy to galaxy is 0.44, 3 times as large asin cold dark matter (CDM) simulations, and the mean density profileslope is αDM=0.73, shallower than that predicted by thesimulations. These results call into question the hypothesis that allgalaxies share a universal dark matter density profile. We show that oneof the galaxies in our sample, NGC 5963, has a cuspy density profilethat closely resembles those seen in CDM simulations, demonstrating thatwhile galaxies with the steep central density cusps predicted by CDM doexist, they are in the minority. In spite of these differences betweenobservations and simulations, the relatively cuspy density profiles wefind do not suggest that this problem represents a crisis for CDM.Improving the resolution of the simulations and incorporating additionalphysics may resolve the remaining discrepancies. We also find that fourof the galaxies contain detectable radial motions in the plane of thegalaxy. We investigate the hypothesis that these motions are caused by atriaxial dark matter halo and place lower limits on the ellipticity ofthe orbits in the plane of the disk of 0.043-0.175.Based on observations carried out at the WIYN Observatory. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory. The Local Group and Other Neighboring Galaxy GroupsOver the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. Molecular gas in compact galaxiesNew observations of eleven compact galaxies in the 12CO J =2{-}1 and J = 3{-}2 transitions are presented. From these observationsand literature data accurate line ratios in matched beams have beenconstructed, allowing the modelling of physical parameters. Matching asingle gas component to observed line ratios tends to produce physicallyunrealistic results, and is often not possible at all. Much betterresults are obtained by modelling two distinct gas components. In mostobserved galaxies, the molecular gas is warm (Tk = 50{-}150K) and at least partially dense (n(H2) ≥ 3000cm-3). Most of the gas-phase carbon in these galaxies is inatomic form; only a small fraction ( 5%) is in carbon monoxide.Beam-averaged CO column densities are low (of the order of1016 cm-2). However, molecular hydrogen columndensities are high (of the order of 1022 cm-2)confirming large CO-to- H2 conversion factors (typically X =1021{-}1022 cm-2/ {K kms-1}) found for low-metallicity environments by othermethods. From CO spectroscopy, three different types of molecularenvironment may be distinguished in compact galaxies. Type I (highrotational and isotopic ratios) corresponds to hot and dense molecularclouds dominated by star-forming regions. Type II has lower ratios,similar to the mean found for infrared-luminous galaxies in general, andcorresponds to environments engaged in, but not dominated by,star-forming activity. Type III, characterized by low 12CO(2-1)/(1-0) ratios, corresponds to mostly inactive environments ofrelatively low density. Thick disks and halos of spiral galaxies M 81, NGC 55 and NGC 300By using images from the HST/WFPC2/ACS archive, we have analyzed thespatial distribution of the AGB and RGB stars along the galactocentricradius of nearby spiral galaxies M 81, NGC 300 and NGC 55. Examiningcolor-magnitude diagrams and stellar luminosity functions, we gauge thestellar contents of the surroundings of the three galaxies. The redgiant population (RGB) identified at large galactocentric radii yields adistance of 3.85±0.08 Mpc for M 81, 2.12±0.10 Mpc for NGC55, and 2.00±0.13 Mpc for NGC 300, and a mean stellar metallicityof -0.65, -1.25, and -0.87 respectively. We find that there are twonumber density gradients of RGB stars along the radius, which correspondto the thick disk and halo components of the galaxies. We confirm thepresence of a metallicity gradient of evolved stars in these galaxies,based on the systematic changes of the color distribution of red giantstars. These results imply that the thick disk might be a generalfeature of spiral galaxies, and endorse a further investigation of theouter stellar edges of nearby spirals, which is critical in constrainingthe origin and evolution of galaxies.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. Cores of dark matter haloes correlate with stellar scalelengthsWe investigate in detail the mass distribution obtained by means ofhigh-resolution rotation curves of 25 galaxies of differentmorphological types. The dark matter contribution to the circularrotation velocity is well-described by resorting to a dark component,the density of which shows an inner core, i.e. a central constantdensity region. We find a very strong correlation between the coreradius size RC and the stellar exponential scalelengthRD: RC~=13[RD/(5kpc)]1.05kpc, and between RCand the galaxy dynamical mass at this distance,Mdyn(RC). These relationships would not beexpected if the core radii were the product of an incorrectdecomposition procedure, or the biased result of wrong or misunderstoodobservational data. The very strong correlation between the dark andluminous scalelengths found here seems to hold also for different Hubbletypes and opens new scenarios for the nature of the dark matter ingalaxies. Dynamical friction for dark halo satellites: effects of tidal mass loss and growing host potentialMotivated by observations of inner halo satellite remnants like the Sgrstream and ω Centauri, we develop fully analytical models to studythe orbital decay and tidal mass loss of satellites on eccentric orbitsin an isothermal potential of a host galaxy halo. The orbital decay rateis often severely overestimated if applying Chandrasekhar's formulawithout correcting for (i) the evaporation and tidal loss of thesatellite, and (ii) the contraction of satellite orbits due to adiabaticgrowth of the host galaxy potential over the Hubble time. As a satellitemigrates inwards, the increasing halo density affects the dynamicalfriction in two opposite ways: (1) it boosts the number of haloparticles swept in the gravitational wake' of the satellite, henceincreasing the drag on the satellite, and (2) it boosts the tide whichpeels off' the satellite, and reduces the amplitude of the wake. Thesecompeting processes can be modelled analytically for a satellite withthe help of an empirical formula for the mass-loss history. Theanalytical model agrees with more traditional numerical simulations oftidal mass loss and dynamical friction. Rapid mass loss due toincreasing tides at smaller and smaller radius makes it less likely forstreams or remnants of infalling satellites to intrude into the innerhalo (like the Sgr stream and ω Centauri) than to stay in theouter halo (like the Magellanic stream), hence any intermediate-masscentral black holes of the satellites are also probably hung up' atlarge distances as well. It is difficult for the black holes of thesatellites to come close enough to merge into the supermassive blackhole in the centre of the host potential unless the satellites startedwith (i) pericentres much smaller than the typical distances topresent-day observed satellites, and (ii) central density much higherthan in the often seen finite-density cores of observed satellites. Dark Matter in Dwarf Galaxies: High Resolution ObservationsWe present observations and analysis of rotation curves and dark matterhalo density profiles in the central regions of four nearby dwarfgalaxies. This observing program has been designed to overcome some ofthe limitations of other rotation curve studies that rely mostly onlongslit spectra. We find that these objects exhibit the full range ofcentral density profiles between ρ∝ r^0 (constant density) andρ∝ r-1 (NFW halo). This result suggests that thereis a distribution of central density slopes rather than a unique halodensity profile. Dark Matter in Galaxies: Observational overviewI review the observational side of the present state of the debate aboutthe dark matter in galaxies, with emphasis on the core/cusp problem inlow surface brightness galaxies, and the question of maximum /sub-maximum disks in spiral galaxies. Some remarks are made about thedwarf spheroidals around the Milky Way, and about elliptical galaxies. Infrared Array Camera (IRAC) Observations of M81Infrared Array Camera (IRAC) images of M81 show three distinctmorphological constituents: a smooth distribution of evolved stars withbulge, disk, and spiral arm components; a clumpy distribution of dustemission tracing the spiral arms; and a pointlike nuclear source. Thebulge stellar colors are consistent with M-type giants, and the diskcolors are consistent with a slightly younger population. The dustemission generally follows the blue and ultraviolet emission, but thereare large areas that have dust emission without ultraviolet and smallerareas with ultraviolet but little dust emission. The former arepresumably caused by extinction, and the latter may be due to cavitiesin the gas and dust created by supernova explosions. The nucleus appearsfainter at 8 μm than expected from ground-based 10 μm observationsmade 4 years ago. Spatially Resolved Ultraviolet, Hα, Infrared, and Radio Star Formation in M81We present Multiband Imaging Photometer for Spitzer (MIPS) observationsof M81 at 24, 70, and 160 μm. The grand design nature of M81 isclearly seen, showing two well-resolved spiral arms containing numerousbright star-forming regions. The MIPS images reveal a significant amountof cold dust associated with the spiral arms. We investigate thevariation of the ultraviolet (UV), Hα, and infrared (IR)luminosities and star formation rate (SFR) indicators across the face ofM81 using the MIPS images and archival UV and Hα images. Forregions in M81, we find that UV and Hα SFRs (uncorrected for dustattenuation) are always lower than the IR SFR. The cause of thisbehavior is dust attenuation and/or using SFR calibrations appropriatefor entire galaxies, not regions in galaxies. The characteristics of thedust attenuation for the regions indicate the dust grains and/orgeometry are different from those in starburst galaxies. The behavior ofthe infrared-radio correlation in M81 is seen to vary from the globalaverage, with variations correlated with the morphology of M81. Star Formation Properties of a Large Sample of Irregular GalaxiesWe 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. A New Nonparametric Approach to Galaxy Morphological ClassificationWe present two new nonparametric methods for quantifying galaxymorphology: the relative distribution of the galaxy pixel flux values(the Gini coefficient or G) and the second-order moment of the brightest20% of the galaxy's flux (M20). We test the robustness of Gand M20 to decreasing signal-to-noise ratio (S/N) and spatialresolution and find that both measures are reliable to within 10% forimages with average S/N per pixel greater than 2 and resolutions betterthan 1000 and 500 pc, respectively. We have measured G andM20, as well as concentration (C), asymmetry (A), andclumpiness (S) in the rest-frame near-ultraviolet/optical wavelengthsfor 148 bright local normal'' Hubble-type galaxies (E-Sd) galaxies, 22dwarf irregulars, and 73 0.05 A Catalog of Neighboring GalaxiesWe present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra. Is There a Universal Dark Matter Density Profile?Over the last several years, three crucial shortcomings of the Cold DarkMatter (CDM) model have been discovered on galaxy-size scales. Wepresent new observations addressing one of these problems: the centraldensity problem. We derive rotation curves for four nearby, low-massspiral galaxies and use them to constrain the shapes of their darkmatter density profiles. This analysis is based on high-resolutiontwo-dimensional Hα and CO velocity fields, which we use in orderto overcome some of the limitations of other rotation curve studies thatrely mostly on long-slit spectra or low-resolution HI observations. Incombination with our previous study of NGC 2976, the full sample of fivegalaxies contains dark matter density profiles that span the range fromconstant-density to steeper than predicted by CDM simulations. Thescatter in density profile slopes from galaxy to galaxy is substantiallylarger than in the simulations, and the mean slope we derive isshallower than predicted. These results call into question thehypothesis that all galaxies share a universal dark matter densityprofile. In spite of these differences between observations andsimulations, the relatively cuspy density profiles we find do notsuggest that this problem represents a crisis for CDM. Improving theresolution of the simulations and incorporating additional physics mayresolve the remaining discrepancies. We also find that four of thegalaxies contain detectable radial motions in the plane of the galaxy.We investigate the hypothesis that these motions are caused by atriaxial dark matter halo, and place lower limits on the ellipticity ofthe orbits in the plane of the disk of a few percent.This research was partially supported by the National ScienceFoundation. Deprojecting spiral galaxies using Fourier analysis. Application to the Frei sampleWe present two methods that can be used to deproject spirals, based onFourier analysis of their images, and discuss their potential andrestrictions. Our methods perform particularly well for galaxies moreinclined than 50° or for non-barred galaxies moreinclined than 35°. They are fast and straightforward touse, and thus ideal for large samples of galaxies. Moreover, they arevery robust for low resolutions and thus are appropriate for samples ofcosmological interest. The relevant software is available from us uponrequest. We use these methods to determine the values of the positionand inclination angles for a sample of 79 spiral galaxies contained inthe Frei et al. (\cite{frei96}) sample. We compare our results with thevalues found in the literature, based on other methods. We findstatistically very good agreementTable 7 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/415/849 Cold dust and molecular gas towards the centers of Magellanic type galaxies and irregulars. I. The dataWe present 1300 μm continuum emission measurements and observationsof the 12CO (1-0) and (2-1) transition towards the centers of64 Magellanic type galaxies (Sdm/Sm) and irregulars (Im/I0/Irr). Thesources are selected to have IRAS flux densities S100 μm≥1000 mJy and optical diameters mainly below 180 arcsec. We wereable to detect 12CO towards 41 and the continuum emissiontowards 28 galaxies. In addition, we obtained the corresponding data fora set of 6 complementary galaxies of different morphological type.Based on observations collected at ESO, La Silla, Chile and IRAM, PicoVeleta, Spain.The full version of Figs. \ref{spec1.fig} and \ref{spec2.fig} is onlyavailable in electronic form at http://www.edpsciences.org The Small Magellanic Cloud in the far infrared. II. Global propertiesWe present global IR properties of the Small Magellanic Cloud using anISOPHOT 170 μm map and re-visited IRAS data. The integrated 170 μmflux is 14 950 ± 2300 Jy, and the integrated IR flux isF1-1000 = 7.48 × 10-10 W m-2.Discrete sources contribute 28%, 29%, and 36% to the integrated flux at60 μm, 100 μm, and 170 μm, respectively. Most of the totalemission arises from diffuse emission regions surrounding the HIIregions. The SED can be modelled by the composition of a 45 K, a 20.5 Kand a 10 K blackbody component with emissivity index n = 2. A colortemperature map is constructed from the ISOPHOT 170 μm and the IRAS100 μm HiRes maps. The average dust temperature is TD =20.3 K. The total dust mass is found to be MD = 7.8×105 Mȯ, yielding a gas-to-dust mass ratioMgas/Mdust ≈ 540, a value 25 times lower thanfound by former studies. The global star formation rate is estimated toSFRtotal ≈ 0.05 Mȯ yr-1. Thecomparison with other irregular galaxies reveals the SMC as quiet withno peculiar FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA. The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxiesWe discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe 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/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/ SINGS: The SIRTF Nearby Galaxies SurveyThe SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging andspectroscopic survey of 75 nearby galaxies. Its primary goal is tocharacterize the infrared emission of galaxies and their principalinfrared-emitting components, across a broad range of galaxy propertiesand star formation environments. SINGS will provide new insights intothe physical processes connecting star formation to the interstellarmedium properties of galaxies and provide a vital foundation forunderstanding infrared observations of the distant universe andultraluminous and active galaxies. The galaxy sample and observingstrategy have been designed to maximize the scientific and archivalvalue of the data set for the SIRTF user community at large. The SIRTFimages and spectra will be supplemented by a comprehensivemultiwavelength library of ancillary and complementary observations,including radio continuum, H I, CO, submillimeter, BVRIJHK, Hα,Paα, ultraviolet, and X-ray data. This paper describes the mainastrophysical issues to be addressed by SINGS, the galaxy sample and theobserving strategy, and the SIRTF and other ancillary data products. First results from the HI Jodrell All Sky Survey: inclination-dependent selection effects in a 21-cm blind surveyDetails are presented of the HI Jodrell All Sky Survey (HIJASS). HIJASSis a blind neutral hydrogen (HI) survey of the northern sky (δ> 22°), being conducted using the multibeam receiver on theLovell Telescope (full width at half-maximum beamwidth 12 arcmin) atJodrell Bank. HIJASS covers the velocity range -3500 to 10 000 kms-1, with a velocity resolution of 18.1 km s-1 andspatial positional accuracy of ~2.5 arcmin. Thus far about 1115deg2 of sky have been surveyed. The average rms noise duringthe early part of the survey was around 16 mJy beam-1.Following the first phase of the Lovell Telescope upgrade (in 2001), therms noise is now around 13 mJy beam-1. We describe themethods of detecting galaxies within the HIJASS data and of measuringtheir HI parameters. The properties of the resulting HI-selected sampleof galaxies are described. Of the 222 sources so far confirmed, 170 (77per cent) are clearly associated with a previously catalogued galaxy. Afurther 23 sources (10 per cent) lie close (within 6 arcmin) to apreviously catalogued galaxy for which no previous redshift exists. Afurther 29 sources (13 per cent) do not appear to be associated with anypreviously catalogued galaxy. The distributions of peak flux, integratedflux, HI mass and cz are discussed. We show, using the HIJASS data, thatHI self-absorption is a significant, but often overlooked, effect ingalaxies with large inclination angles to the line of sight. Properlyaccounting for it could increase the derived HI mass density of thelocal Universe by at least 25 per cent. The effect that this will haveon the shape of the HI mass function will depend on how self-absorptionaffects galaxies of different morphological types and HI masses. We alsoshow that galaxies with small inclinations to the line of sight may alsobe excluded from HI-selected samples, since many such galaxies will haveobserved velocity widths that are too narrow for them to bedistinguished from narrow-band radio-frequency interference. This effectwill become progressively more serious for galaxies with smallerintrinsic velocity widths. If, as we might expect, galaxies with smallerintrinsic velocity widths have smaller HI masses, then compensating forthis effect could significantly steepen the faint-end slope of thederived HI mass function. The Relationship between Stellar Light Distributions of Galaxies and Their Formation HistoriesA 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 BIMA Survey of Nearby Galaxies (BIMA SONG). II. The CO DataThe BIMA Survey of Nearby Galaxies is a systematic imaging study of the3 mm CO J=1-0 molecular emission within the centers and disks of 44nearby spiral galaxies. The typical spatial resolution of the survey is6" or 360 pc at the average distance (12 Mpc) of the sample. Thevelocity resolution of the CO observations is 4 km s-1,though most maps are smoothed to 10 km s-1 resolution. For 33galaxies, multifield observations ensured that a region >~190"(=10 kpc) in diameter was imaged. For the remaining 11galaxies, which had smaller optical diameters and were on averagefarther away, single-pointing observations imaged a 100" diameter(=11 kpc) region. The sample was not chosen based on CO orinfrared brightness; instead, all spirals were included that met theselection criteria of vsolar<=2000 km s-1,δ>=-20deg, i<=70deg,D25<70', and BT<11.0. Thedetection rate was 41/44 sources or 93%; of the three nondetections, one(M81) is known to have CO emission at locations outside the survey fieldof view. Fully sampled single-dish CO data were incorporated into themaps for 24 galaxies; these single-dish data comprise the most extensivecollection of fully sampled, two-dimensional single-dish CO maps ofexternal galaxies to date. We also tabulate direct measurements of theglobal CO flux densities for these 24 sources. For the remaining 20sources, we collected sensitive single-dish spectra in order to evaluatethe large-scale flux recovery. We demonstrate that the measured ratiosof flux density recovered are a function of the signal-to-noise of theinterferometric data. We examine the degree of central peakedness of themolecular surface density distributions and show that the distributionsexhibit their brightest CO emission within the central 6" in only 20/44or 45% of the sample. We show that all three Local Group spiral galaxieshave CO morphologies that are represented in SONG, though the Milky WayCO luminosity is somewhat below the SONG average, and M31 and M33 arewell below average. This survey provides a unique public database ofintegrated intensity maps, channel maps, spectra, and velocity fields ofmolecular emission in nearby galaxies. It also lays the groundwork forextragalactic surveys by more powerful future millimeter-wavelengthinterferometers like CARMA and ALMA. High-Resolution Measurements of the Dark Matter Halo of NGC 2976: Evidence for a Shallow Density ProfileWe have obtained two-dimensional velocity fields of the dwarf spiralgalaxy NGC 2976 in Hα and CO. The high spatial (~75 pc) andspectral (13 and 2 km s-1, respectively) resolution of theseobservations, along with our multicolor optical and near-infraredimaging, allows us to measure the shape of the density profile of thedark matter halo with good precision. We find that the total (baryonicplus dark matter) mass distribution of NGC 2976 follows aρtot~r-0.27+/-0.09 power law out to a radiusof 1.8 kpc, assuming that the observed radial motions provide nosupport. The density profile attributed to the dark halo is evenshallower, consistent with a nearly constant density of dark matter overthe entire observed region. A maximal disk fit yields an upper limit tothe K-band stellar mass-to-light ratio (M*/LK) of0.09+0.15-0.08Msolar/LsolarK(including systematic uncertainties), with the caveat that forM*/LK>0.19Msolar/LsolarKthe dark matter density increases with radius, which is unphysical.Assuming0.10Msolar/LsolarK<~M*/LK<=0.19Msolar/LsolarK,the dark matter density profile lies betweenρDM~r-0.17 andρDM~r-0.01. Therefore, independent of anyassumptions about the stellar disk or the functional form of the densityprofile, NGC 2976 does not contain a cuspy dark matter halo. We alsoinvestigate some of the systematic effects that can hamper rotationcurve studies and show that (1) long-slit rotation curves are far morevulnerable to systematic errors than two-dimensional velocity fields,(2) NGC 2976 contains radial motions that are as large as 90% of therotational velocities at small radii, and (3) the Hα and COvelocity fields of NGC 2976 agree within their uncertainties, with atypical scatter between the two velocities of 5.3 km s-1 atany position in the galaxy.Based on observations carried out at the WIYN Observatory. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory.
Submit a new article

• - No Links Found -

### Member of following groups:

#### Observation and Astrometry data

 Constellation: Ursa Major Right ascension: 09h47m15.50s Declination: +67Â°55'03.0" Aparent dimensions: 5.888′ × 3.02′

Catalogs and designations: