Contents
Images
Upload your image
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 Galaxies
Mid-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.
| Where Is the Molecular Hydrogen in Damped Lyα Absorbers?
We show in this paper why molecular millimeter absorption line searchesin DLAs have been unsuccessful. We use CO emission-line maps of localgalaxies to derive the H2 column density distributionfunction f(NH2) at z=0. We show that it forms anatural extension to f(NHI): the H2 distributionexceeds f(NHI) atNH~1022cm-2 and exhibits a power-lawdropoff with slope ~-2.5. Approximately 97% of the H2 massdensity ρH2 is in systems aboveNH2=1021 cm-2. We derive avalue ρH2=1.1×107h70 Msolar Mpc-3, which is ~25% themass density of atomic hydrogen. Yet the redshift number density ofH2 above this NH2 limit is only~3×10-4, a factor 150 lower than that for H I in DLAsat z=0. Furthermore, we show that the median impact parameter between aNH2>1021 cm-2 absorberand the center of the galaxy hosting the H2 gas is only 2.5kpc. On the basis of arguments related to the Schmidt law, we argue thatH2 gas above this column density limit is associated with alarge fraction of the integral star formation rate density. Evenallowing for an increased molecular mass density at higher redshifts,the derived cross sections indicate that it is very unlikely to identifythe bulk of the molecular gas in present quasar absorption linessamples. We discuss the prospects for identifying this molecular mass infuture surveys.
| The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium
We investigate the relationship between the velocity dispersion of thegas and the supernova (SN) rate and feedback efficiency withthree-dimensional numerical simulations of SN-driven turbulence in theinterstellar medium (ISM). Our simulations aim to explore the constancyof the velocity dispersion profiles in the outer parts of galactic disksat ~6-8 km s-1 and the transition to the starburst regime,i.e., high star formation rates (SFRs) associated with high velocitydispersions. With our fiducial value of the SN feedback efficiency(i.e., ε=0.25, corresponding to an injected energy per SN of0.25×1051 ergs), our results show that (1) SN drivingleads to constant velocity dispersions of σ~6 km s-1for the total gas and σHI~3 km s-1 for the HI gas, independent of the SN rate, for values of the rate between 0.01and 0.5 the Galactic value (ηG) (2) the position of thetransition to the starburst regime (i.e., location of sharp increase inthe velocity dispersion) at around SFR/area~=5×10-3 to10-2 Msolar yr-1 kpc-2observed in the simulations is in good agreement with the transition tothe starburst regime in the observations (e.g., NGC 628 and NGC 6949);(3) for the high SN rates, no H I gas is present in the simulations box;however, for the total gas velocity dispersion, there is good agreementbetween the models and the observations; (4) at the intermediate SNrates (η/ηG~0.5-1), taking into account the thermalbroadening of the H I line helps reach a good agreement in that regimebetween the models and the observations; and (5) forη/ηG<0.5, σ and σHI fallbelow the observed values by a factor of ~2. However, a set ofsimulations with different values of ɛ indicates that, forlarger values of the SN feedback efficiencies, velocity dispersions ofthe H I gas of the order of 5-6 km s-1 can be obtained, incloser agreement with the observations. The fact that forη/ηG<0.5, the H I gas velocity dispersions are afactor of ~2 smaller than the observed values could result from the factthat we might have underestimated the SN feedback efficiency. On theother hand, it might also be an indication that other physical processescouple to the stellar feedback in order to produce the observed level ofturbulence in galactic disks.
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| Palomar/Las Campanas Imaging Atlas of Blue Compact Dwarf Galaxies. II. Surface Photometry and the Properties of the Underlying Stellar Population
We present the results from an analysis of surface photometry of B, R,and Hα images of a total of 114 nearby galaxies(vhelio<4000 km s-1) drawn from the Palomar/LasCampanas Imaging Atlas of blue compact dwarf (BCD) galaxies. Surfacebrightness and color profiles for the complete sample have beenobtained. We determine the exponential and Sérsic profiles thatbest fit the surface brightness distribution of the underlying stellarpopulation detected in these galaxies. We also compute the (B-R) colorand total absolute magnitude of the underlying stellar population andcompared them to the integrated properties of the galaxies in thesample. Our analysis shows that the (B-R) color of the underlyingpopulation is systematically redder than the integrated color, except inthose galaxies where the integrated colors are strongly contaminated byline and nebular-continuum emission. We also find that galaxies withrelatively red underlying stellar populations [typically (B-R)>=1mag] show structural properties compatible with those of dwarfelliptical galaxies (i.e., a smooth light distribution, fainterextrapolated central surface brightness, and larger scale lengths thanBCD galaxies with blue underlying stellar populations). At least ~15% ofthe galaxies in the sample are compatible with being dwarf elliptical(dE) galaxies experiencing a burst of star formation. For the remainingBCD galaxies in the sample we do not find any correlation between therecent star formation activity and their structural differences withrespect to other types of dwarf galaxies.
| Infrared Spectral Energy Distributions of Nearby Galaxies
The 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.
| The Baryonic Tully-Fisher Relation of Galaxies with Extended Rotation Curves and the Stellar Mass of Rotating Galaxies
I investigate the baryonic Tully-Fisher relation for a sample ofgalaxies with extended 21 cm rotation curves spanning the range 20 kms-1<~Vf<=300 km s-1. A variety ofscalings of the stellar mass-to-light ratio Υ* areconsidered. For each prescription for Υ*, I give fitsof the form Md=AVxf.Presumably, the prescription that comes closest to the correct valuewill minimize the scatter in the relation. The fit with minimum scatterhas A=50 Msolar km-4 s4 andx=4. This relation holds over five decades in mass. Galaxy color,stellar fraction, and Υ* are correlated with eachother and with Md, in the sense that more massivegalaxies tend to be more evolved. There is a systematic dependence ofthe degree of maximality of disks on surface brightness. High surfacebrightness galaxies typically have Υ*~3/4 of themaximum disk value, while low surface brightness galaxies typicallyattain ~1/4 of this amount.
| On the Relevance of the Tremaine-Weinberg Method Applied to an Hα Velocity Field: Pattern Speed Determination in M100 (NGC 4321)
The relevance of the Tremaine-Weinberg (TW) method is tested formeasuring bar, spiral, and inner structure pattern speeds using agaseous velocity field. The TW method is applied to various simulatedbarred galaxies in order to demonstrate its validity in seven differentconfigurations, including star formation and/or dark matter halo. Thereliability of the different physical processes involved and of thevarious observational parameters is also tested. The simulations showthat the TW method could be applied to gaseous velocity fields to get agood estimate of the bar pattern speed, under the condition that regionsof shocks are avoided and measurements are confined to regions where thegaseous bar is well formed. We successfully apply the TW method to theHα velocity field of the Virgo Cluster galaxy M100 (NGC 4321) andderive pattern speeds of 55+/-5 km s-1 kpc-1 forthe nuclear structure, 30+/-2 km s-1 kpc-1 for thebar, and 20+/-1 km s-1 kpc-1 for the spiralpattern, in full agreement with published determinations using the samemethod or alternative ones.
| Ursa Major: A Missing Low-Mass CDM Halo?
The recently discovered Ursa Major dwarf spheroidal (dSph) galaxycandidate is about 5-8 times less luminous than the faintest previouslyknown dSphs, And IX, Draco, and Ursa Minor. In this Letter, we presentvelocity measurements of seven color-magnitude-selected Ursa Majorcandidate stars. Two of them are apparent nonmembers based onmetallicity and velocity, and the remaining five stars yield a systemicheliocentric velocity of v¯=-52.45+/-4.27 km s-1 and acentral line-of-sight velocity dispersion of1/2=9.3+11.7-1.2km s-1, with 95% confidence that1/2>6.5 km s-1. Assumingthat UMa is in dynamical equilibrium, it is clearly darkmatter-dominated and cannot be a purely stellar system like a globularcluster. It has an inferred central mass-to-light ratio of M/L~500Msolar/Lsolar and, based on our studies of otherdSphs, may possess a much larger total mass-to-light ratio. UMa isunexpectedly massive for its low luminosity-indeed, UMa appears to bethe most dark matter-dominated galaxy yet discovered. The presence of somuch dark matter in UMa immediately suggests that it may be a member ofthe missing population of low-mass galaxies predicted by the cold darkmatter (CDM) paradigm. Given the weak correlation between dSph mass andluminosity, it is entirely likely that a population of dark dwarfssurrounds our Galaxy.
| Star Formation in H I-selected Galaxies. II. H II Region Properties
A sample of 69 galaxies with radial velocities less than 2500 kms-1 was selected from the H I Parkes All Sky Survey (HIPASS)to deduce details about star formation in nearby disk galaxies selectedwith no bias to optical surface brightness selection effects. Broadband(B and R) and narrowband (Hα) images were obtained for all ofthese objects. More than half of the sample galaxies are late-type,dwarf disks (mostly Sc and Sm galaxies). We have measured the propertiesof the H II regions on Hα continuum-subtracted images, using theHIIphot package developed by Thilker et al. All but one of the galaxiescontained at least one detectable H II region. Examination of theproperties of the H II regions in each galaxy revealed that thebrightest regions in higher surface brightness galaxies tend to be moreluminous than those in lower surface brightness galaxies. A higherfraction (referred to as the diffuse fraction) of the Hα emissionfrom lower surface brightness galaxies comes from diffuse ionized gas. HII region luminosity functions (LFs) co-added according to surfacebrightness show that the shapes of the LFs for the lowest surfacebrightness galaxies are different from those for typical spiralgalaxies. This discrepancy could be caused by the lowest surfacebrightness galaxies having somewhat episodic star formation or by themforming a relatively larger fraction of their stars outside of dense,massive molecular clouds. In general, the results imply that theconditions under which star formation occurs in lower surface brightnessgalaxies are different than in more typical, higher surface brightnessspiral galaxies.
| NGC 300: An Extremely Faint, Outer Stellar Disk Observed to 10 Scale Lengths
We have used the Gemini Multi-Object Spectrograph (GMOS) on the GeminiSouth 8 m telescope in exceptional conditions (0.6" FWHM seeing) toobserve the outer stellar disk of the Sculptor Group galaxy NGC 300 attwo locations. At our point-source detection threshold ofr'=27.0 (3 σ) mag, we trace the stellar disk out to aradius of 24', or 2.2R25, where R25 is the 25 magarcsec-2 isophotal radius. This corresponds to about 10 scalelengths in this low-luminosity spiral galaxy (MB=-18.6), orabout 14.4 kpc at a Cepheid distance of 2.0+/-0.07 Mpc. The backgroundgalaxy counts are derived in the outermost field, and these are within10% of the mean survey counts from both Hubble Deep Fields. Theluminosity profile is well described by a nucleus plus a simpleexponential profile out to 10 optical scale lengths. We reach aneffective surface brightness of μr'=30.5 magarcsec-2 (2 σ) at 55% completeness, which doubles theknown radial extent of the optical disk. These levels are exceedinglyfaint in the sense that the equivalent surface brightness in B or V isabout 32 mag arcsec-2. We find no evidence for truncation ofthe stellar disk. Only star counts can be used to reliably trace thedisk to such faint levels, since surface photometry is ultimatelylimited by nonstellar sources of radiation. In the Appendix, we derivethe expected surface brightness of one such source: dust scattering ofstarlight in the outer disk.
| H I Distribution and Kinematics of UGCA 86
We present 21 cm H I line and 408 MHz and 1.4 GHz continuum observationsof the Magellanic dwarf galaxy UGCA 86, made with the Dominion RadioAstrophysical Observatory (DRAO) Synthesis Telescope. UGCA 86 isdetected in the continuum at both frequencies, with 408 MHz flux densityS408=120+/-30 mJy and 1.4 GHz flux densityS1400=79+/-3 mJy. The H I structure of UGCA 86 is complex,with two separate components: a rotating disk and a highly elongatedspur that is kinematically disjunct from the disk. The H I disk iscentered on the optical galaxy with similar axial ratio and orientationof the major axis. An area of the disk with a peculiar velocity of ~25km s-1 relative to the regular rotation of the disk is foundon the southern side, where most of the star formation is concentrated.The spur is seen along the minor axis of UGCA 86 and overlaps in partwith the disk. Toward the optical center of UGCA 86, the velocitydifference between the spur and the disk is 40 km s-1, aboutone-third of the rotation velocity of the H I disk at 6 kpc from thecenter. This implies a large radial component of the orbital velocity ofthe spur and therefore a significantly noncircular orbit. The median H Ivelocity dispersion of the disk is 8.8 km s-1, similar toother (dwarf) galaxies. The H I velocity dispersion of the spur variesfrom 10 to 30 km s-1. A possible tidal origin of the spur isconsidered in view of the proximity of the large Scd galaxy IC 342.However, the orientation of the spur along the minor axis and itsspatial overlap with the disk suggest that the spur is located faroutside the plane of the H I disk. No evidence is found that the outer HI disk is warped, which poses a problem for the interpretation of thespur as a tidal tail induced by IC 342. Detailed modeling of the IC342/UGCA 86 system will be required before a tidal origin of the spurcan be dismissed conclusively. The possibility that the spur is part ofthe nascent cloud of UGCA 86 or the remains of a small dwarf galaxy ispresented as an alternative interpretation.
| Probing Halos of Galaxies at Very Large Radii Using Background QSOs
Gaseous halos of nine nearby galaxies (with redshifts cz<6000 kms-1) were probed at large galactocentric radii usingbackground quasars observed with the Hubble Space Telescope Goddard HighResolution Spectrograph and the Space Telescope Imaging Spectrograph.The projected quasar-galaxy separations range from 55 to 387h-175 kpc. Lyα absorption lines weresuccessfully detected in the spectra of five quasars, at impactparameters of up to ~170 h-175 kpc from the centerof the nearby galaxy, and in each case at wavelengths consistent withthe galaxy's redshift. Our observations include the lowest redshiftLyα lines detected to date. H I velocity fields were obtained atthe Very Large Array for three of the galaxies in our sample (in onecase the velocity field was available from the literature) to derivetheir rotation curves. When comparing the inner rotation curves of thegalaxies with the velocity at large radius provided by the Lyαline, it is apparent that it is very difficult to explain the observedLyα velocity as due to gas in an extended rotating disk. In mostcases, one would need to invoke large warps in the outer gas disks andalso thick gas disks to reconcile the observed velocities with thepredicted ones. Indeed, in one case, the Lyα line velocityindicates, in fact, counterrotation with respect to the inner diskrotation. In light of these results, we conclude that in a typicalgalaxy there is no longer detectable atomic gas corotating in anextended disk at radii greater than 35α-1, whereα-1 is the stellar disk exponential scale length. Thecosmic web is the most likely origin for the detected Lyα lines.Our observations confirm the recent Bowen et al. correlation ofequivalent widths with the local volume density of galaxies around thesight line, and the observed equivalent widths of the lines areconsistent with expectations of the cosmic web.Based on observations with the NASA ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555.
| A dwarf galaxy with a giant HI disk
We present Giant Meterwave Radio Telescope (GMRT) HI 21 cm images of anearby dwarf irregular galaxy NGC 3741 (MB -13.13)which show it to have a gas disk that extends to ~8.3 times its Holmbergradius. This makes it probably the most extended gas disk known. Ourobservations allow us to derive the rotation curve (which is flat in theouter regions) out to ~38 optical scale lengths. NGC 3741 has adynamical mass to light ratio of ~107 and is one of the“darkest” irregular galaxies known. However, the bulk of thebaryonic mass in NGC 3741 is in the form of gas and the ratio of thedynamic mass to the baryonic mass (~8), falls within the range that istypical for galaxies. Thus the dark matter halo of NGC 3741 has acquiredits fair share of baryons, but for some reason, these baryons have beenunable to collapse to form stars. A comparison of NGC 3741's dark haloproperties with those of a sample of galaxies with well measuredrotation curves suggests that if one has to reconcile the observationswith the expectation that low mass galaxies suffer fractionally greaterbaryon loss then baryon loss from halos occurs in such a way that, inthe net, the remaining baryons occupy a fractionally smaller volume ofthe total halo.
| The Baryonic Tully-Fisher relation revisited
The Baryonic Tully-Fisher relation (BTF) can be substantially improvedwhen considering that the galactic baryonic mass is likely to consistnot only of the detected baryons, stars and gas, but also of a darkbaryonic component proportional to the HI gas. The BTF relation isoptimally improved when the HI mass is multiplied by a factor of about3, but larger factors up to 11-16 still improve the fit over theoriginal one using only the detected baryons. The strength of thisimproved relation is quantified with up-to-date statistical tests suchas the Akaike Information Criterion or the Bayesian InformationCriterion. In particular they allow us to show that supposing a variableMstar/L ratio instead is much less significant. This resultreinforces the suggestion made in several recent works that mass withingalactic disks must be a multiple of the HI mass, and that galacticdisks are substantially, but not necessarily fully, self-gravitating.
| Dark Matter and the Tully-Fisher Law
I discuss the origin of the Tully-Fisher law in the context of theobserved scaling laws for dark halos.
| Evolution of Galaxies in Triaxial Halos with Figure Rotation
Firstly, we demonstrate that unusually large outer HI spiral armsobserved in NGC 2915 can form in an extended gas disk embedded in amassive triaxial dark matter halo with slow figure rotation, through thestrong gravitational torque of the rotating halo. Secondly, we showthat the figure rotation of a triaxial dark matter halo can influencedynamical evolution of disk galaxies by using fully self-consistentnumerical simulations. We particularly describe the formation processesof ``halo-triggered'' bars in thin galactic disks dominated by darkmatter halos with figure rotation and discuss the origin of stellar barsin low luminosity, low surface brightness (LSB) disk galaxies. Thirdly,we provide several implications of the present numerical results interms of triggering mechanism of starbursts in galaxies and stellar barformation in high redshifts.
| Rotating Halos and Heavy Disks: The Case of NGC 2915
NGC 2915 is a blue compact dwarf galaxy embedded in an extended, lowsurface brightness HI disk with a bar and two-armed spiral structure.Common mechanisms are unable to explain those patterns and disk darkmatter or a rotating triaxial dark halo were proposed as alternatives.Hydrodynamical simulations were run for each case and compared toobservations using customized column density and kinematic constraints.The spiral structure can be accounted for by an unseen bar or triaxialhalo, but the large bar mass or halo pattern frequency required make itunlikely that the spiral is driven by an external perturber. Inparticular, the spin parameter lambda is much higher than predicted bycurrent CDM structure formation scenarios. Massive disk models show thatwhen the gas surface density is scaled up by a factor of about 10, thedisk develops a spiral structure matching the observed one in perturbeddensity as well as velocity. This suggests that the disk of NGC 2915contains much more mass than is visible, tightly linked to the neutralhydrogen. A classic (quasi-)spherical halo is nevertheless stillrequired, as increasing the disk mass further to fit the circularvelocity curve would make the disk violently unstable.
| Rotating Halos and Heavy Disks: the Case of NGC 2915
NGC 2915 is a blue compact dwarf galaxy embedded in an extended, lowsurface brightness HI disk with a bar and two-armed spiral structure.Common mechanisms are unable to explain those patterns and disk darkmatter or a rotating triaxial dark halo were proposed as alternatives.Hydrodynamical simulations were run for each case and compared toobservations using customized column density and kinematic constraints.The spiral structure can be accounted for by an unseen bar or triaxialhalo, but the large bar mass or halo pattern frequency required make itunlikely that the spiral is driven by an external perturber. Inparticular, the spin parameter lambda is much higher than predicted bycurrent CDM structure formation scenarios. Massive disk models show thatwhen the gas surface density is scaled up by a factor of about 10, thedisk develops a spiral structure matching the observed one in perturbeddensity as well as velocity. This suggests that the disk of NGC 2915contains much more mass than is visible tightly linked to the neutralhydrogen. A classic (quasi-)spherical halo is nevertheless stillrequired, as increasing the disk mass further to fit the circularvelocity curve would make the disk violently unstable
| HI Tidal Tails, Bridges and Clouds
There is plenty of intergalactic H I gas without any obvious stellarcontent ranging from (1) extended gas envelopes around some normal andpeculiar galaxies, (2) tidal tails/bridges in interacting or merginggalaxy systems, (3) large-scale rings around early type galaxies, and(4) detached clouds at varying distances from associated galaxies, butthere are few or no isolated H I clouds.The HIPASS Bright Galaxy Catalog, which covers the whole southern sky,contains only one definite extragalactic H I cloud which is locatedclose to the galaxy NGC 2442 whereas it is sensitive to isolated H Iclouds with MHI > 106 × D2Mȯ. The space density of H I clouds is therefore about1/1000th that of galaxies with the same MHI.
| The Rotation Curves of Dwarf Galaxies: A Problem for Cold Dark Matter?
We address the issue of accuracy in recovering density profiles fromobservations of rotation curves of galaxies. We ``observe'' and analyzeour models in much the same way as observers do the real galaxies. Ourmodels include stellar disks, disks with bars, and small bulges. We findthat the tilted-ring model analysis produces an underestimate of thecentral rotational velocity. In some cases the galaxy halo densityprofile seems to have a flat core, while in reality it does not. Weidentify three effects that explain the systematic biases: inclination,small bulge, and bar. Inclination effects are due to the finitethickness of the disk, bar, or bulge. Admixture of a nonrotating bulgecomponent reduces the rotational velocity. A small (200-500 pc) bulgemay be overlooked, leading to systematic bias even on relatively large(~1 kpc) distances. In the case of a disk with a bar, the underestimateof the circular velocity is larger because of a combination ofnoncircular motions and random velocities. The effect of the bar dependson the angle that the bar makes with the line of sight. Signatures ofbars can be difficult to detect in the surface brightness profiles ofthe model galaxies. The variations of inclination angle and isophoteposition angle with radius are more reliable indicators of bar presencethan the surface brightness profiles. The systematic biases in thecentral ~1 kpc of galaxies are not large. Each effect separately givestypically a few km s-1 error, but the effects add up. In somecases the error in circular velocity was a factor of 2, but typically weget about a 20% effect. The result is the false inference that thedensity profile of the halo flattens in the central parts. Ourobservations of real galaxies show that for a large fraction of galaxiesthe velocity of gas rotation (as measured by emission lines) is veryclose to the rotation of the stellar component (as measured byabsorption lines). This implies that the systematic effects discussed inthis paper are also applicable both for the stars and emission-line gas.
| Figure Rotation of Cosmological Dark Matter Halos
We have analyzed galaxy- and group-sized dark matter halos formed in ahigh-resolution ΛCDM numerical N-body simulation in order tostudy the rotation of the triaxial figure, a property in principleindependent of the angular momentum of the particles themselves. Suchfigure rotation may have observational consequences, such as triggeringspiral structure in extended gas disks. The orientation of the major andminor axes are compared at five late snapshots of the simulation. Haloswith significant substructure or that appear otherwise disturbed areexcluded from the sample. We detect smooth figure rotation in 288 of the317 halos in the sample. The pattern speeds follow a lognormaldistribution centered atΩp=0.148hkms-1kpc-1 with a widthof 0.83. These speeds are an order of magnitude smaller than required toexplain the spiral structure of galaxies such as NGC 2915. The axisabout which the figure rotates aligns very well with the halo minor axisin 85% of the halos and with the major axis in the remaining 15% of thehalos. The figure rotation axis is usually reasonably well aligned withthe angular momentum vector. The pattern speed is correlated with thehalo spin parameter λ but shows no correlation with the halomass. The halos with the highest pattern speeds show particularly strongalignment between their angular momentum vectors and their figurerotation axes. The figure rotation is coherent outside0.12rvir. The measured pattern speed and degree of internalalignment of the figure rotation axis drops in the innermost region ofthe halo, which may be an artifact of the numerical force softening. Theaxis ratios show a weak tendency to become more spherical with time.
| Pattern Speeds of BIMA SONG Galaxies with Molecule-dominated Interstellar Mediums Using the Tremaine-Weinberg Method
We apply the Tremaine-Weinberg method of pattern speed determination todata cubes of CO emission in six spiral galaxies from the BIMA Survey ofNearby Galaxies, each with an interstellar medium dominated by moleculargas. We compare derived pattern speeds with estimates based on othermethods, usually involving the identification of a predicted behavior atone or more resonances of the pattern(s). In two cases (NGC 1068 and NGC4736), we find evidence for a central bar pattern speed that is greaterthan that of the surrounding spiral and roughly consistent with previousestimates. However, the spiral pattern speed in both cases is muchlarger than previous determinations. For the barred spirals NGC 3627 andNGC 4321, the method is insensitive to the bar pattern speed (the bar ineach is nearly parallel to the major axis; in this case the method willnot work), but for the former galaxy the spiral pattern speed foundagrees with previous estimates of the bar pattern speed, suggesting thatthese two structures are part of a single pattern. For the latter, thespiral pattern speed found is in agreement with several previousdeterminations. For the flocculent spiral NGC 4414 and the ``Evil Eye''galaxy NGC 4826, the method does not support the presence of alarge-scale coherent pattern. We also apply the method to a simulatedbarred galaxy in order to demonstrate its validity and to understand itssensitivity to various observational parameters. In addition, we studythe results of applying the method to a simulated, clumpy axisymmetricdisk with no wave present. The Tremaine & Weinberg method in thiscase may falsely indicate a well-defined pattern.
| Star Formation in H I-Selected Galaxies. I. Sample Characteristics
A sample of 69 galaxies with radial velocities of less than 2500 kms-1 was selected from the H I Parkes All-Sky Survey (HIPASS)and imaged in broadband B and R and narrowband Hα, to deducedetails about star formation in nearby disk galaxies while avoidingsurface brightness selection effects. The sample is dominated bylate-type, dwarf disks (mostly Sc and Sm galaxies) with exponential diskscale lengths of ~1-5 kpc. The HIPASS galaxies, on average, have lowerstar formation rates (SFRs), are bluer, and have lower surfacebrightness than an optically selected sample. H II regions were detectedin all but one of the galaxies. Many galaxies had as few as two to fiveH II regions. The galaxies' Hα equivalent widths, colors, and SFRsper unit of H I mass are best explained by young mean ages (~3-5 Gyr,according to Schmidt-law models) with star formation histories in whichthe SFRs were higher in the past. Comparison of the surface brightnesscoverage of the HIPASS galaxies with that of an optically selectedsample shows that such a sample may miss ~10% of the local galaxy numberdensity and could possibly miss as much as 3%-4% of the SFR density. Theamount lower surface brightness galaxies contribute to the totalluminosity density may be insignificant, but this conclusion is somewhatdependent on how the fluxes of these objects are determined.
| The Pattern Speeds of M51, M83, and NGC 6946 Using CO and the Tremaine-Weinberg Method
In spiral galaxies in which the molecular phase dominates the ISM, themolecular gas as traced by CO emission will approximately obey thecontinuity equation on orbital timescales. The Tremaine-Weinberg methodcan then be used to determine the pattern speed of such galaxies. Wehave applied the method to single-dish CO maps of three nearby spirals,M51, M83, and NGC 6946, to obtain estimates of their pattern speeds:38+/-7, 45+/-8, and 39+/-8 km s-1 kpc-1,respectively, and we compare these results to previous measurements. Wealso analyze the major sources of systematic errors in applying theTremaine-Weinberg method to maps of CO emission.
| ESO 215-G?009: An Extreme H I-Rich Dwarf Irregular Galaxy
We present deep BVRI band images and H I line observations of thenearby, low surface brightness galaxy ESO 215-G?009, which were obtainedwith the Australian National University 2.3 m telescope and theAustralia Telescope Compact Array, respectively. ESO 215-G?009 wasselected from the HIPASS Bright Galaxy Catalog because it has the secondhighest H I mass-to-light ratio of the galaxies with measured B-bandapparent magnitudes. We find that it is an isolated dwarf irregulargalaxy with an old stellar population. We place an upper limit on thecurrent star formation rate of ~2.5×10-3Msolar yr-1. The extended H I disk showsregular rotation (vrot=51+/-8 km s-1) and, at acolumn density of ~5.0×1019 atoms cm-2, canbe traced out to over 6 times the Holmberg radius of the stellarcomponent (radius at μB=26.6 mag arcsec-2).After foreground star subtraction, we measure a B-band apparentmagnitude of 16.13+/-0.07 mag within a radius of 80". The H I fluxdensity is 122+/-4 Jy km s-1 within a radius of 370". Given aGalactic extinction of AB=0.95+/-0.15 mag, we derive an H Imass-to-light ratio of 22+/-4 Msolar/Lsolar,Bfor ESO 215-G?009. To our knowledge this is the highestMHI/LB ratio for a galaxy to be confirmed byaccurate measurement to date.
| A Catalog of Neighboring Galaxies
We 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.
| Discovery of an Extended Gas Disk Around UGC 5288
HI synthesis observations of UGC 5288 reveal that the majority of its HIis located beyond the stellar component in a well ordered, slightlywarped, disk. The HI flux density spatially coincident with the stellarcomponent corresponds to less than 1/8 of the total gas content of thiscompact dwarf irregular galaxy. However, despite the presence ofsignificant gas mass at large radii, the gas density of the outer gasdisk is more than a factor of 10 below the Toomre instability threshold.Similar to NGC 2915, the optical morphology of UGC 5288 is highlysuggestive of a bar-like object embedded in an extended, low density,gaseous disk. Possible origins of the diffuse gas disk around UGC 5288include (1) the disk is the remnant of a galactic fountain (perhapscreated after a past star burst episode); (2) the disk is tidal debriscreated during a recent encounter; and (3) the disk is a remnant of thenascent material from which the galaxy collapsed. We discussobservational constraints on the origin and evolution of the gas disk;in particular, given its local environment and past star formationhistory, we argue that the extended, quiescent, gas disk associated withUGC 5288 is primordial in nature.
| Cold dust and molecular gas towards the centers of Magellanic type galaxies and irregulars. I. The data
We 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
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
