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Comment regarding the functional form of the Schmidt law
Star formation rates on the galactic scale are describedphenomenologically by two distinct relationships, as emphasized recentlyby Elmegreen [Elmegreen, B.G., 2002. ApJ, 577, 206. astro-ph/0207114.].The first of these is the Schmidt law, which is a power-law relationbetween the star formation rate SFR and the column density Σ. Theother relationship is that there is a cutoff in the gas density belowwhich star formation shuts off. The purpose of this paper is to arguethat: (1) these two relationships can be accommodated by a singlefunctional form of the Schmidt law, (2) this functional form ismotivated by the hypothesis that star formation is a criticalphenomenon, and that as a corollary, (3) the existence of a sharp cutoffmay thus be an emergent property of galaxies, as was argued by Seiden[Seiden, P.E.,1983. ApJ, 266, 555.], as opposed to the classical viewthat this cutoff is due to an instability criterion.
| On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
| The Extended H I Rotation Curve and Mass Distribution of M31
New H I observations of Messier 31 (M31) obtained with the Effelsbergand Green Bank 100 m telescopes make it possible to measure the rotationcurve of that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotationcurve is nearly flat at a velocity of ~226 km s-1. A model ofthe mass distribution shows that at the last observed velocity point,the minimum dark-to-luminous mass ratio is ~0.5 for a total mass of3.4×1011 Msolar at R<35 kpc. This can becompared to the estimated Milky Way mass of 4.9×1011Msolar for R<50 kpc.
| 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.
| Oxygen and Nitrogen in Isolated Dwarf Irregular Galaxies
We present long-slit optical spectroscopy of 67 H II regions in 21 dwarfirregular galaxies to investigate the enrichment of oxygen, nitrogen,neon, sulfur, and argon in low-mass galaxies. Oxygen abundances areobtained via direct detection of the temperature-sensitive emissionlines for 25 H II regions; for the remainder of the sample, oxygenabundances are estimated from strong-line calibrations. The directabundance determinations are compared to the strong-line abundancecalibrations of both McGaugh and Pilyugin. While the McGaugh calibrationyields a statistical offset of 0.07 dex, the photoionization model gridtraces the appropriate isometallicity contour shape in theR23-O32 diagnostic diagram. In contrast, while thePilyugin calibration yields a negligible statistical offset, theresiduals in this strong-line calibration method are correlated withionization parameter. Thus, these observations indicate that oxygenabundances will be overestimated by the p-method for H II regions withlow-ionization parameters. Global oxygen and nitrogen abundances forthis sample of dwarf irregular galaxies are examined in the context ofopen- and closed-box chemical evolution models. While several galaxiesare consistent with closed-box chemical evolution, the majority of thissample has an effective yield ~1/4 of the expected yield for a constantstar formation rate and Salpeter IMF, indicating that either outflow ofenriched gas or inflow of pristine gas has occurred. The effective yieldstrongly correlates with MH/LB in the sense thatgas-rich galaxies are more likely to be closed systems. However, theeffective yield does not appear to correlate with other globalparameters such as dynamical mass, absolute magnitude, star formationrate, or surface brightness. In addition, open and closed systems arenot identified easily in other global abundance measures; for example,the observed correlation between luminosity and metallicity isconsistent with other recent results in the literature. A correlation isfound between the observed nitrogen-to-oxygen ratio and the color of theunderlying stellar population; redder dwarf irregular galaxies havehigher N/O ratios than blue dwarf irregular galaxies. The relativeabundance ratios are interpreted in the context of delayed release ofnitrogen and varied star formation histories.
| Mid-Infrared Images of Stars and Dust in Irregular Galaxies
We present mid-IR to optical properties of 22 representative irregulargalaxies: 18 irregular (Im) galaxies, 3 blue compact dwarfs, and 1Magellanic-type spiral galaxy. The mid-IR is based on images from theSpitzer Space Telescope archives. The 3.6 and 4.5 μm bands and theUBVJHK images are used to examine disk morphology and the integrated andazimuthally averaged magnitudes and colors of stars. The nonstellarcontribution to the 4.5 μm images is used to trace hot dust. The 5.8and 8.0 μm images reveal emission from hot dust and polycyclicaromatic hydrocarbons (PAHs), and both may contribute to thesepassbands, although we refer to the nonstellar emission as PAH emission.We compare the 8.0 μm images to Hα. Im galaxies have no hiddenbars, and those with double-exponential optical light profiles have thesame at mid-IR. Most galaxies have similar optical and mid-IR scalelengths. Four galaxies have super star clusters that are not visible atoptical bands. Galaxies with higher area-normalized star formation rateshave more dust and PAH emission relative to starlight. Hot dust and PAHemission is found mostly in high surface brightness H II regions,implying that massive stars are the primary source of heating. Galaxieswith intense, widespread star formation have more extended PAH emission.The ratio of PAH to Hα emission is not constant on small scales.PAHs are associated with shells and giant filaments, so they are notdestroyed during shell formation.This work is based in part on archival data obtained with the SpitzerSpace Telescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under a contract with NASA.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
| Stellar and Gas Properties of High H I Mass-to-Light Ratio Galaxies in the Local Universe
We present a multiwavelength study (BVRI-band photometry and H I lineinterferometry) of nine late-type galaxies selected from the HIPASSBright Galaxy Catalog on the basis of apparently high H I mass-to-lightratios (3Msolar/Lsolar,B5Msolar/Lsolar,B) galaxies are rare in thelocal universe. Extreme high-MHI/LB galaxiessuch as ESO 215-G?009 appear to have formed only the minimum number ofstars necessary to maintain the stability of their H I disks and couldpossibly be used to constrain galaxy formation models. They may alsohave been forming stars at a low, constant rate over their lifetimes.The best examples all have highly extended H I disks, are spatiallyisolated, and have normal baryonic content for their total masses butare deficient in stars. This suggests thathigh-MHI/LB galaxies are not lacking thebaryons to create stars but are underluminous, as they lack either theinternal or external stimulation for more extensive star formation.
| Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field
We present Hubble Space Telescope Advanced Camera for Surveys images andcolor-magnitude diagrams for 25 nearby galaxies with radial velocitiesVLG<500 km s-1. Distances are determined basedon the luminosities of stars at the tip of the red giant branch thatrange from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, arefound to be the nearest known representatives of blue compact dwarfobjects. Using high-quality data on distances and radial velocities of110 nearby field galaxies, we derive their mean Hubble ratio to be 68 kms-1 Mpc-1 with a standard deviation of 15 kms-1 Mpc-1. Peculiar velocities of most of thegalaxies, Vpec=VLG-68D, follow a Gaussiandistribution with σv=63 km s-1 but with atail toward high negative values. Our data display the known correlationbetween peculiar velocity and galaxy elevation above the LocalSupercluster plane. The small observed fraction of galaxies with highpeculiar velocities, Vpec<-500 km s-1, may beunderstood as objects associated with nearby groups (Coma I, Eridanus)outside the local volume.
| First Results from THINGS: The HI Nearby Galaxy Survey
We 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.
| On Star Formation and the Nonexistence of Dark Galaxies
We investigate whether a baryonic dark galaxy or ``galaxy withoutstars'' could persist indefinitely in the local universe, whileremaining stable against star formation. To this end, a simple model hasbeen constructed to determine the equilibrium distribution andcomposition of a gaseous protogalactic disk. Specifically, we determinethe amount of gas that will transit to a Toomre unstable cold phase viathe H2 cooling channel in the presence of a UV-X-ray cosmicbackground radiation field. All but one of the models are predicted tobecome unstable to star formation: we find that, in the absence of aninternal radiation field, the majority of gas will become Toomreunstable in all putative dark galaxies with baryonic masses greater than109 Msolar, and in at least half of those greaterthan 106 Msolar. Moreover, we find that all ourmodel objects would be detectable via H I line emission, even in thecase that star formation is potentially avoided. These results areconsistent with the nondetection of isolated extragalactic H I cloudswith no optical counterpart (galaxies without stars) by the H I ParkesAll-Sky Survey. Additionally, where star formation is predicted tooccur, we determine the minimum interstellar radiation field required torestore gravothermal stability, which we then relate to a minimum globalstar formation rate. This leads to the prediction of a previouslyundocumented relation between H I mass and star formation rate that isobserved for a wide variety of dwarf galaxies in the H I mass range108-1010 Msolar. The existence of sucha relation strongly supports the notion that the well-observedpopulation of dwarf galaxies represents the minimum rates ofself-regulating star formation in the universe.
| 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.
| Probing the Multiphase Interstellar Medium of the Dwarf Starburst Galaxy NGC 625 with Far Ultraviolet Spectroscopic Explorer Spectroscopy
We present new Far Ultraviolet Spectroscopic Explorer (FUSE)spectroscopy of the dwarf starburst galaxy NGC 625. These observationsprobe multiple phases of the interstellar medium (ISM), including thecoronal, ionized, neutral, and molecular gas. This nearby (D=3.9+/-0.2Mpc) system shows a clear detection of outflowing coronal gas as tracedby O VI λ1032 absorption. The centroid of the O VI profile isblueshifted with respect to the galaxy systemic velocity by ~30 kms-1, suggesting a low-velocity outflow. The implied O VIvelocity extent is found to be 100+/-20 km s-1, which isfully consistent with the detected H I outflow velocity found in radiosynthesis observations. We detect multiple lines of diffuseH2 absorption from the ISM of NGC 625; this is one of only afew extragalactic systems with FUSE detections of H2 lines inthe Lyman and Werner bands. We find a potential abundance offset betweenthe neutral and nebular gas that exceeds the errors on the derivedcolumn densities. Since such an offset has been found in multiple dwarfgalaxies, we discuss the implications of a lower-metallicity halosurrounding the central star-forming regions of dwarf galaxies. Theapparent offset may be due to saturation of the observed O I line, andhigher signal-to-noise ratio (S/N) observations are required to resolvethis issue.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985.
| The Arecibo Legacy Fast ALFA Survey. I. Science Goals, Survey Design, and Strategy
The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims tomap ~7000 deg2 of the high Galactic latitude sky visible fromArecibo, providing a H I line spectral database covering the redshiftrange between -1600 and 18,000 km s-1 with ~5 kms-1 resolution. Exploiting Arecibo's large collecting areaand small beam size, ALFALFA is specifically designed to probe the faintend of the H I mass function in the local universe and will provide acensus of H I in the surveyed sky area to faint flux limits, making itespecially useful in synergy with wide-area surveys conducted at otherwavelengths. ALFALFA will also provide the basis for studies of thedynamics of galaxies within the Local Supercluster and nearbysuperclusters, allow measurement of the H I diameter function, andenable a first wide-area blind search for local H I tidal features, H Iabsorbers at z<0.06, and OH megamasers in the redshift range0.16
| Infrared Properties of Star-forming Dwarf Galaxies. I. Dwarf Irregular Galaxies in the Local Volume
A sample of 34 dwarf irregular galaxies (dIs) in the Local Volume, mostnearer than 5 Mpc, has been imaged in the near-infrared (NIR) in J andKs at the Canada-France-Hawaii Telescope (CFHT) in Hawaii andthe Observatorio Astronómico Nacional in the Sierra San PedroMártir, in Mexico. Absolute magnitudes in Ks rangefrom -14 to -18. In the CFHT images, stars brighter thanMKs~-7.5 were resolved. We show that the resolvedcomponent comprises more than 50% of the light from star formationbursts within the last 3 Gyr. In most cases, the resolved populationdown to MKs=-7.5 represents less than 5% of thetotal NIR flux in Ks, with fractions in J being 1.5-2 timeslarger. Thus, the NIR light of dIs can be considered to be predominantlycontributed by stars older than about 4 Gyr. Although exponential atlarge radii, surface brightness profiles for the unresolved componentflatten in the centers. They can be fitted across the whole range ofradii with a hyperbolic secant (sech) defined as a function of twoparameters: the central surface brightness and the scale length of theexponential. With respect to this model, only two galaxies (NGC 1569 andNGC 3738) show an excess of flux in the center, both of which arehosting starbursts. Isophotal, total, and fitted sech magnitudes havebeen calculated for all galaxies for which the unresolved component wasdetected, along with semimajor axes at μJ=23 magarcsec-2 and μKs=22 magarcsec-2. The scale length and the semimajor axes correlatelinearly with absolute isophotal magnitude. The same is true for colorsand the central brightness. More luminous dIs tend to be larger, redder,and brighter in the center. The fraction of light contributed by youngstars is independent of both luminosity and central surface brightness.The Tully-Fisher relation shows considerable scatter, but residuals aretied to surface brightness. The galaxies appear to lie in a``fundamental plane'' defined by the sech absolute magnitude, the sechcentral surface brightness, and the H I line width. The rms of residualsin MK is only 0.4 mag, which implies that the plane can beused to evaluate the distances of star-forming dwarfs. Corrections fortilt do not reduce the residuals, so line widths must be governedpredominantly by random motions. Color-magnitude diagrams (CMDs) arepresented for 29 galaxies in which stars were resolved. Most show afinger centered around J-Ks=1 mag. In some cases, there is ared tail extending to J-Ks=2.5 mag. Most color profilesconstructed for the unresolved component show a remarkably constantJ-Ks=0.8-1.0 mag, matching the color of the finger in theCMDs.Based on observations obtained at the Canada-France-Hawaii Telescope,which is operated by the National Research Council of Canada, the CentreNational de la Recherche Scientifique de France, and the University ofHawaii; also based on data acquired at OAN-SPM in Mexico.
| Gas and Stars in an H I-Selected Galaxy Sample
We present the results of a J-band study of the H I-selected AreciboDual-Beam Survey and Arecibo Slice Survey galaxy samples using TwoMicron All Sky Survey data. We find that these galaxies span a widerange of stellar and gas properties. However, despite the diversitywithin the samples, we find a very tight correlation between luminosityand size in the J band, similar to that found in a previous paper byRosenberg & Schneider between the H I mass and size. We also findthat the correlation between the baryonic mass and the J-band diameteris even tighter than that between the baryonic mass and the rotationalvelocity.
| DDO 88: A Galaxy-sized Hole in the Interstellar Medium
We present an H I and optical study of the gas-rich dwarf irregulargalaxy DDO 88. Although the global optical and H I parameters of DDO 88are normal for its morphological type, it hosts a large (3 kpc diameter)and unusually complete ring of enhanced H I emission. The normalappearance of this galaxy in the optical and the outer regions of the HI give no hint of the presence of the striking H I ring in the innerregions. The gas ring is located at approximately one-third of the totalH I radius and one-half the optically defined Holmberg radius, andcontains 30% of the total H I of the galaxy. The ring surrounds acentral depression in the H I distribution. If the H I ring and centraldepression in the gas were formed by the energy input from winds andsupernova explosions of massive stars formed in a starburst, as isthought often to be the case, the star-forming event would have formed0.1%-1% of the total stellar mass of the galaxy. However, the UBV colorsin the H I hole are not bluer than the rest of the galaxy, as would beexpected if an unusual star-forming event had taken place thererecently, although there is an old (~1-3 Gyr), red cluster near thecenter of the hole that is massive enough to have produced the hole inthe H I. An age estimate for the ring is uncertain, however, because itis not observed to be expanding. An expansion model produces a lowerestimate of 0.5 Gyr, but the presence of faint star formation regionsassociated with the ring indicates a much younger age. We also estimatethat the ring could have dispersed by now if it is older than 0.5 Gyr.This implies that the ring is younger than 0.5 Gyr. A younger age wouldindicate that the red cluster did not produce the hole and ring.Therefore, uncertainties prevent us from concluding that the cluster andthe H I hole are definitely related. If this ring and the depression inthe gas that it surrounds were not formed by stellar winds andsupernovae, this would indicate that some other, currently unidentified,mechanism is operating.
| Fast growth of supermassive black holes in galaxies
We report on a calculation of the growth of the mass of supermassiveblack holes at galactic centers from dark matter and Eddington - limitedbaryonic accretion. Assuming that dark matter halos are made of fermionsand harbor compact degenerate Fermi balls of masses from 103Mȯ to 106 Mȯ, we find thatdark matter accretion can boost the mass of seed black holes from about 5 Mȯ to 103-4 Mȯ blackholes, which then grow by Eddington-limited baryonic accretion tosupermassive black holes of 106{-9} Mȯ. Wethen show that the formation of the recently detected supermassive blackhole of 3× 109 Mȯ at a redshift of z =6.41 in the quasar SDSS J114816.64+525150.3 could be understood if theblack hole completely consumes the degenerate Fermi ball and then growsby Eddington-limited baryonic accretion. In the context of this model weconstrain the dark matter particle masses to be within the range from 12keV/c2 to about 450 keV/c2. Finally we investigatethe black hole growth dependence on the formation time and on the massof the seed black hole. We find that in order to fit the observed datapoint of M_BH 3 × 109 Mȯ and z 6.41, dark matter accretion cannot start later than about 2× 108 years and the seed BH cannot be greater thanabout 104 Mȯ. Our results are in fullagreement with the WMAP observations that indicate that the first onsetof star formation might have occurred at a redshift of z 15 - 20.For other models of dark matter particle masses, correspondingconstraints may be derived from the growth of black holes in the centerof galaxies.
| Effects of episodic gas infall on the chemical abundances in galaxies
The chemical evolution of galaxies that undergo an episode of massiveand rapid accretion of metal-poor gas is investigated with models usingboth simplified and detailed nucleosynthesis recipes. The rapid decreaseof the oxygen abundance during infall is followed by a slower evolutionwhich leads back to the closed-box relation, thus forming a loop in theN/O-O/H diagram. For large excursions from the closed-box relation, themass of the infalling material needs to be substantially larger than thegas remaining in the galaxy, and the accretion rate should be largerthan the star formation rate. We apply this concept to the encounter ofhigh velocity clouds with galaxies of various masses, finding that theobserved properties of these clouds are indeed able to cause substantialeffects not only in low mass galaxies, but also in the partial volumesin large massive galaxies that would be affected by the collision.Numerical models with detailed nucleosynthesis prescriptions areconstructed. We assume star formation timescales and scaled yields thatdepend on the galactic mass, and which are adjusted to reproduce theaverage relations of gas fraction, oxygen abundance, and effectiveoxygen yield observed in irregular and spiral galaxies. The resultingexcursions in the N/O-O/H diagram due to a single accretion eventinvolving a high velocity cloud are found to be appreciable, which couldthus provide a contribution to the large scatter in the N/O ratio foundamong irregular galaxies. Nonetheless, the N/O-O/H diagram remains animportant indicator for stellar nucleosynthesis.
| 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.
| Dwarf and Normal Spiral Galaxies: are they Self-Similar?
The investigation presented here was focused on clarifying the existenceof dwarf spiral galaxies as a separate group from classical spirals.First, a list of spiral galaxies with small sizes was obtained.Information on colors, luminosities, morphologies and chemical contentwas searched for in the literature for these galaxies. Using thisinformation, it can be concluded that dwarf spirals are not likely to bethe tail of the distribution of classical galaxies. On the contrary,significant differences in some of the most important properties ofspiral galaxies, such as the metallicity gradient and the bar frecuency,were found. In any case, further and more accurate observations areneeded for a definitive answer.
| 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.
| Dynamical friction for dark halo satellites: effects of tidal mass loss and growing host potential
Motivated 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 which`peels 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.
| Are interactions the primary triggers of star formation in dwarf galaxies?
We investigate the assumption that the trigger of star formation indwarf galaxies is interactions with other galaxies, in the context of asearch for a `primary' trigger of a first generation of stars. This iscosmologically relevant because the galaxy formation process consistsnot only of the accumulation of gas in a gravitational potential wellbut also of the triggering of star formation in this gas mass, and alsobecause some high-z potentially primeval galaxy blocks look like nearbystar-forming dwarf galaxies. We review theoretical ideas proposed toaccount for the tidal interaction triggering mechanism and present aseries of observational tests of this assumption using published data.We also show results of a search in the vicinity of a composite sampleof 96 dwarf late-type galaxies for interaction candidates showing starformation. The small number of possible perturbing galaxies identifiedin the neighbourhood of our sample galaxies, along with similar findingsfrom other studies, supports the view that tidal interactions may not berelevant as primary triggers of star formation. We conclude thatinteractions between galaxies may explain some forms of star formationtriggering, perhaps in central regions of large galaxies, but they donot seem to be significant for dwarf galaxies and, by inference, forfirst-time galaxies forming at high redshifts. Intuitive reasoning,based on an analogy with stellar dynamics, shows that conditions forprimary star formation triggering may occur in gas masses oscillating ina dark-matter gravitational potential. We propose this mechanism as aplausible primary trigger scenario, which would be worth investigatingtheoretically.
| Observational Constraints on the Physical Parameters of Dark Matter Halos
After looking at the difference in the mass distribution between massivespiral and dwarf irregular (dIrr) and low surface brightness (LSB)galaxies, the central Dark Matter (DM) concentration (flat vs cuspy) indwarf and LSB galaxies, derived from observations, will be examined. Wewill then present what kind of observational constraints can be put onthe total mass and total extent of DM halos from the studies ofindividual galaxies, small groups, satellites' dynamics and tidal tailsof interacting systems. Finally, we will discuss how limits on thephysical parameters of DM halos could be set by deriving extendedrotation curves beyond the HI radius (r > rHI), usingeither Lyα absorption or Hα emission observations.
| Scaling Laws for Dark Matter Halos in Late-Type and Dwarf Spheroidal Galaxies
Published mass models fitted to galaxy rotation curves are used to studythe systematic properties of dark matter (DM) halos in late-type anddwarf spheroidal (dSph) galaxies. Halo parameters are derived by fittingnon-singular isothermals to (V^2 - Vvis^2)1/2,where V(r) is the observed rotation curve and Vvis is therotation curve of the visible matter. The latter is calculated from thesurface brightness assuming that the mass-to-light ratio M/L is constantwith radius. ``Maximum disk'' values of M/L are adjusted to fit as muchof the inner rotation curve as possible without making the halo have ahollow core. Rotation curve decomposition becomes impossible fainterthan absolute magnitude M_B ≃ -14, where V becomes comparable tothe velocity dispersion of the gas. To increase the luminosity rangefurther, we include dSph galaxies, which are physically related tospiral and irregular galaxies. Combining the data, we find that DM halossatisfy well defined scaling laws analogous to the ``fundamental plane''relations for elliptical galaxies. Halos in less luminous galaxies havesmaller core radii r_c, higher central densities ρ_0, and smallercentral velocity dispersions σ. Scaling laws provide new anddetailed constraints on the nature of DM and on galaxy formation andevolution. Some simple implications include:1 -- A single, continuous physical sequence of increasing mass extendsfrom dSph galaxies with M_B ≃ -7.6 to Sc I galaxies with M_B≃ -22.4.2 -- The high DM densities in dSph galaxies are normal for such tinygalaxies. Since virialised density depends on collapse redshiftzcoll, ρ_0 ∝ (1 + zcoll)^3, the smallestdwarfs formed at least Δ zcoll ≃ 7 earlier thanthe biggest spirals.3 -- The high DM densities of dSphs implies that they are real galaxiesformed from primordial density fluctuations. They are not tidalfragments. Tidal dwarfs cannot retain even the low DM densities of theirgiant-galaxy progenitors. In contrast, dSphs have higher DM densitiesthan do giant-galaxy progenitors.4 -- The fact that, as luminosity decreases, dwarf galaxies become muchmore numerous and also more nearly dominated by DM raises thepossibility that there exists a large population of objects that arecompletely dark. Such objects are a canonical prediction of cold DMtheory. If they exist, ``empty halos'' are likely to be small and dense-- that is, darker versions of Draco and UMi.5 -- The slopes of the DM parameter correlations provide a measure ongalactic mass scales of the slope n of the power spectrum|δk|2 ∝ k^n of primordial densityfluctuations. Our preliminary results, not yet corrected for baryoniccompression of DM, give n ≃ -1.9 ± 0.2. This is consistentwith cold DM theory.
| 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.
| Chemical Abundances of H II Regions in the Starburst Galaxy NGC 1705
We report optical spectroscopy of 16 H II regions in NGC 1705 and [OIII] λ4363 detections for the first time in five H II regions.The resulting mean oxygen abundance derived directly from measuredelectron temperatures is 12+log(O/H)=8.21+/-0.05, which corresponds to[O/H]=-0.45, or 35% of the solar value. There are no significant spatialinhomogeneities in [O III] λ4363 oxygen abundances from H IIregions at a radius approximately 10" from the super star cluster. In HII regions where [O III] λ4363 was not measured, oxygenabundances derived with bright-line methods (accurate only to 0.2 dex)are in agreement with direct values of the oxygen abundance. Faint,narrow He II λ4686 emission is found in two H II regions, but theimplied contribution from O+3 to the total oxygen abundanceis only 0.01 dex. The mean argon-, neon-, and nitrogen-to-oxygenabundance ratios are consistent with mean values for other dwarfirregular galaxies, blue compact dwarf galaxies, and H II galaxies atcomparable oxygen abundances. Interestingly, the nitrogen-to-oxygenabundance ratio in the ionized H II gas agrees with the value for theneutral H I, even though the metallicity of the neutral gas may be afactor of 6 lower than that of the ionized gas. This may be indicativeof low-metallicity gas in the halo of the galaxy. Extinction values,AV, derived from observed Balmer line ratios along lines ofsight to H II regions are in the range between 0 and 0.9 mag.Significant and variable extinction may have important effects on theinterpretation of resolved stellar populations and derived starformation histories. With respect to the metallicity-luminosity andmetallicity-gas fraction diagnostics, the measured oxygen abundance forNGC 1705 is comparable to those of Local Group dwarf irregular galaxiesat a given luminosity and gas fraction. Simple chemical evolution modelssuggest that the galaxy is quickly evolving into a gas-poor dwarfgalaxy.Based on EFOSC2 observations collected at the European SouthernObservatory, Chile.
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