Galactic Dark Halo Research Articles

Does the Milky Way Have a Maximal Disk?

The Milky Way is often considered to be the best example of a spiral for which the dark matter not only dominates the outer kinematics but also plays a major dynamical role in the inner galaxy: the Galactic disk is therefore said to be "submaximal." This conclusion is important to the understanding of the evolution of galaxies and the viability of particular dark matter models. The Galactic evidence rests on a number of structural and kinematic measurements, many of which have recently been revised. The new constraints not only indicate that the Galaxy is a more typical member of its class (Sb to Sc spirals) than previously thought but also require a reexamination of the question of whether the Milky Way disk is maximal. By applying to the Milky Way the same definition of "maximal disk" that is applied to external galaxies, it is shown that the new observational constraints are consistent with a Galactic maximal disk of reasonable M/L. In particular, the local disk column can be substantially less than the oft-quoted required Σ☉ ≈ 100 M☉ pc-2—as low as 40 M☉ pc-2 in the extreme case—and still be maximal, in the sense that the dark halo provides negligible rotation support in the inner Galaxy. This result has possible implications for any conclusion that rests on assumptions about the potential of the Galactic disk or dark halo and, in particular, for the interpretation of microlensing results along both LMC and bulge lines of sight.

Dwarf spheroidal satellite galaxies without dark matter

Self-consistent simulations of the interaction of initially spherical low-mass satellite galaxies with a massive Galactic dark halo suggest that observed apparent high mass-to-light ratios, (ML)obs, in dwarf-spheroidal (dSph) satellites may be obtained through successive and destructive perigalactic passages.The model satellites are disrupted after several perigalactic passages, but orbiting condensations in phase space can clearly be identified as remnants well after this event. An intrinsic (ML)true < 3 leads to long-lived model remnants that show, as seen from Earth, a tantalizingly close resemblance to observed dSph satellites with (ML)obs > 10. The reason for large (ML)obs is that the remnants have non-isotropic velocity dispersions, are non-spherical and are not in dynamical equilibrium. The remnants fade with time (i.e. orbital phase) and (ML)obs increases indicating a correlation between brightness and (ML)obs as in the observed dSph sample. The observed correlation between central surface brightness and integrated absolute magnitude is also approximately reproduced by model remnants of different dynamical age. A possibly useful method for probing the line-of-sight extension of a dSph galaxy is suggested.If it is true that most if not all dSph satellites are tidally modified remnants without dark matter then it is plausible that their progenitors may have formed as satellite galaxies in tidal tails during possible early merging events.

Comment on “Dispersion Velocity of Galactic Dark Matter Particles”

In a recent Letter (astro-ph/9605001), Cowsik et al. claim that a self-consistent treatment of the dark Galactic halo requires that the local velocity dispersion of dark-matter particles be $600$ km~s$^{-1}$ or greater, more than a factor of two larger than the canonical value of 270 km~s$^{-1}$. If true, this would significantly affect rates and signatures for detection of baryonic and nonbaryonic dark matter. We believe that this work is incorrect, probably because not all the observational constraints were taken into account and because the models were forced to satisfy arbitrary constraints on the halo density.

Microlensing by a Prolate All‐MACHO Halo

It is widely believed that dark matter halos are flattened, that is, closer to oblate than prolate. The evidence cited is based largely on observations of galaxies that do not look anything like our own and on numerical simulations that use ad hoc initial conditions. Given what we believe to be a doubt concerning the shape of the dark Galactic halo, we calculate the optical depth and event rate for microlensing of stars in the Large Magellanic Cloud assuming a wide range of models that include both prolate and oblate halos. We find, in agreement with previous analyses, that the optical depth for a spherical (E0) halo and for an oblate (E6) halo are roughly the same, essentially because two competing effects cancel approximately. However, the optical depth for an E6 prolate halo is reduced by ~35%. This means that an all-MACHO prolate halo with reasonable parameters for the Galaxy is consistent with the published microlensing event rate.

Limits on the Halo White Dwarf Component of Baryonic Dark Matter from the Hubble Deep Field

The MACHO collaboration lensing event statistics suggest that a significant fraction of the dark Galactic halo can be comprised of baryonic matter in the form of white dwarf stars with masses between 0.1 and 1.0 M☉. Such a halo white dwarf population, in order to have escaped detection by those who observe the white dwarf luminosity function of the disk, must have formed from an old population. The observations indicate that the number of halo white dwarfs per cubic parsec per unit bolometric magnitude is less than 10-5 at 10-4.5 L☉; the number must rise significantly at lower luminosities to provide the needed baryonic halo mass. Such white dwarfs may easily escape detection in most current and earlier surveys. Though it is limited in angular extent, the Hubble Deep Field (HDF) probes a sufficient volume of the Galactic halo to provide interesting limits on the number of halo white dwarf stars and on the fraction of the halo mass that they can make up. If the HDF field can be probed for stars down to V = 29.8 then the MACHO result suggests that there could be up to 12 faint halo white dwarfs visible in the HDF. Finding (or not finding) these stars in turn places interesting constraints on star formation immediately following the formation of the galaxy.

Hubble Deep Field Constraint on Baryonic Dark Matter

We use a new technique to search for faint red stars in the Hubble deep field. We distinguish unambiguously between stars and galaxies to I = 26.3. Our results place strong and general constraints on the I-band luminosity of the constituents of the Galactic dark halo.

The MACHO Project Large Magellanic Cloud Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo

The MACHO Project is a search for dark matter in the form of massive compact objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected (~1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with 2 < < 200 days to be τ2002=2.9+ 1.4−0.9×10−7 based upon our eight event sample. This exceeds the optical depth, τbackgnd = 0.5 × 10-7, expected from known stars, and the difference is to be compared with the optical depth predicted for a composed entirely of MACHOs: τhalo = 4.7 × 10-7. To compare with Galactic models, we perform likelihood analyses on the full eight-event sample and a six-event subsample (which allows for two events to be caused by a nonhalo background). This gives a fairly model-independent estimate of the mass in MACHOs within 50 kpc of 2.0+ 1.2−0.7×1011 M☉, which is about half of the standard halo value. We also find a most probable MACHO mass of 0.5+ 0.3−0.2 M☉, although this value is strongly model dependent. In addition, the absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs: objects from 10-4 M☉ to 0.03 M☉ contribute 20% of the dark halo.

The MACHO Project First-Year Large Magellanic Cloud Results: The Microlensing Rate and the Nature of the Galactic Dark Halo

Since July 1992, the MACHO project has been carrying out long-term photometric monitoring of over 20 million stars in the Magellanic Clouds and Galactic bulge. Our aim is to search for the very rare gravitational microlensing events predicted if the dark halo of our Galaxy is comprised of massive compact halo objects (hereafter MACHOs). We have now analyzed most of the first year{close_quote}s LMC data, comprising 9.5 million light curves of stars with an average of 235 observations each. Automated selection procedures applied to this sample show three events consistent with microlensing; the first detected is very striking (Alcock and coworkers), and two are of modest amplitude. We have evaluated our experimental detection efficiency using a range of detailed Monte Carlo simulations, including the addition of artificial stars to real data frames. Using a ``standard`` halo density profile, we find that a halo comprised entirely of MACHOs in the mass range 3{times}10{sup {minus}4} to 0.06 {ital M}{sub {circle_dot}} would predict more than 15 detected events in this data set, and objects around 3{times}10{sup {minus}3} {ital M}{sub {circle_dot}} would predict 25 events; thus a standard spherical halo cannot be dominated by objects in this mass range. Assuming all three events aremore » microlensing by halo objects, and fitting a naive spherical halo model to our data yields a MACHO halo fraction {ital f}=0.19{sup +0.16}{sub -0.10}, a total mass in MACHOs (inside 50 kpc) of 7.6{sup +6}{sub -4}{times}10{sup 10} {ital M}{sub {circle_dot}}, and a microlensing optical depth 8.8{sup +7}{sub -5}{times}10{sup -8} (68% confidence level). Should only one of these events be microlensing, this could be explained in terms of previously known populations. We have explored a wide range of halo models and find that, while our constraints on the MACHO fraction are quite model dependent, constraints on the total mass in MACHOs within 50 kpc are quite secure.« less

A 20 cm2 CCD mosaic camera for a dark matter search part I : Mechanics, optics and cryogeny

A 20cm 2 CCD mosaic camera has been especially built to search for dark galactic halo objects by the gravitational microlensing effect. The sensitive area is made of 16 edge-buttable CCDs developped by Thomson-CTS, with 23×23 µm 2 pixels. The 35 kg camera housing and mechanical equipment is presented. The associated electronics and data acquisition system are described in a separate paper. The camera resides at the focal plane of a 40 cm, f/10, Ferson reflector. The instrument has been in operation since December 1991 at the La Silla Observatory (ESO).

INTEGRAL, SNOBs, and baryonic dark matter

view Abstract Citations (5) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS INTEGRAL, SNOBs, and Baryonic Dark Matter Gilmore, Gerard Abstract The gamma-ray surface brightness distribution is proportional to the surface mass distribution and to the flux density of cosmic rays. Given adequate gamma-ray data one can determine both these distributions. Local enhancements in the cosmic-ray intensity, visible as high surface brightness fluctuations which do not correspond to mass density enhancements, signal regions of enhanced cosmic-ray acceleration. Such regions are expected to occur in those places where multiple supernova remnants and many hot OB stars are found-- SNOBs. SNOBs probably dominate the energetics of the interstellar medium (ISM) as well as the cosmic-ray flux. They sometimes also blow through the thin disk ISM, creating 'chimneys' which feed high-excitation material into the Galactic halo. High spatial resolution gamma-ray imaging provides a unique means to study such phenomena. In places where the cosmic-ray flux is more uniformly extended, particularly at high Galactic latidudes, the gamma-ray surface brightness is a tracer of the surface mass density. One may use this to determine what fraction of the 'dark' massive Galactic halo is made of baryonic matter in a form which is capable of interacting with cosmic rays. Current data allow an upper limit of only 15% of the total mass of the Galactic dark halo to be baryonic matter which is not in compact objects. Publication: The Astrophysical Journal Supplement Series Pub Date: June 1994 DOI: 10.1086/192011 Bibcode: 1994ApJS...92..539G Keywords: Dark Matter; Galactic Cosmic Rays; Galactic Halos; Gamma Ray Astronomy; Gamma Rays; Hot Stars; Supernova Remnants; Baryons; Mass Distribution; Massive Stars; Milky Way Galaxy; Space Radiation; ISM: COSMIC RAYS; COSMOLOGY: DARK MATTER; GAMMA RAYS: THEORY full text sources ADS |

A model of the galactic dark halo

view Abstract Citations (9) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Model of the Galactic Dark Halo Wollman, E. R. Abstract Results are presented of an analysis of the galactic dark halo which shows that several properties of galaxies can be understood on the basis of the assumption that the dark halo is composed of large grains of solid matter. Direct collisions among these large grains vaporize the grains. The vapor, which appears as hot halo gas, flows inward and forms a luminous core, the visible galaxy. The expression for the mass infall rate, which governs the star formation rate, is consistent with the Tully-Fisher relation. Gray extinction by the grains may account for the paucity of detectable quasars at large redshifts. Because the grains are large, they are an effective cosmic thermalizing agent. Their thermalizing action smooths inhomogeneities in the primordial microwave background radiation while providing a source of nonprimordial background. The expected thermal microwave emission from galactic grain halos is near the current limits of sensitivity of relevant measurements and provides a possible test of the model. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171407 Bibcode: 1992ApJ...392...80W Keywords: Dark Matter; Galactic Halos; Interstellar Matter; Milky Way Galaxy; Astronomical Models; Interstellar Extinction; Quasars; Red Shift; Relic Radiation; Astrophysics; COSMOLOGY: COSMIC MICROWAVE BACKGROUND; COSMOLOGY: DARK MATTER; GALAXIES: GENERAL; GALAXY: HALO full text sources ADS |

Dynamics of cooling gas in galactic dark halos

view Abstract Citations (234) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamics of Cooling Gas in Galactic Dark Halos Navarro, Julio F. ; Benz, Willy Abstract The dynamical evolution of both collisionless 'dark' particles and a dissipative gaseous component in a flat universe is simulated in order to investigate the formation process of the luminous components of galaxies at the center of galactic dark halos. The present models assume that the gas amounts to 10 percent of the mass of the universe, and that both the gas and the dark matter are identically distributed in phase space at high redshifts. The results confirm previous suggestions that the merging history of the surrounding halo is a key factor in the determination of a galaxy's morphological type. The baryonic component is found to lose more angular momentum than could be predicted by dissipationless simulations. The formation of dense, slowly rotating baryonic cores at the center of galactic dark halos is observed, which explains the origin of the slow rotation and large densities observed in spheroids and elliptical galaxies. Publication: The Astrophysical Journal Pub Date: October 1991 DOI: 10.1086/170590 Bibcode: 1991ApJ...380..320N Keywords: Cooling Flows (Astrophysics); Dark Matter; Galactic Evolution; Galactic Structure; Gas Dynamics; Halos; Astronomical Models; Collisionless Plasmas; Computational Astrophysics; Elliptical Galaxies; Galactic Rotation; Astrophysics; COSMOLOGY; GALAXIES: FORMATION; GALAXIES: INTERNAL MOTIONS full text sources ADS |

Mutual sensitivity of halo and disk parameters in a two‐component model

AbstractThe mutual interaction between galactic dark halos and visible matter are explored by modelling a galaxy as two homogeneous, concentric and coaxial spheroidal subsystems, one completely lying within the other. The tensor virial theorem in its extended appropriate form is used and a general method outlined, then an application to the Galaxy is done. The values of equatorial semiaxis and axis ratio of the bright subsystem, and of the velocity components of both subsystems, deduced from a number of known observational data, are used in the computations as input parameters. Discussing the results, special effort has been devoted to the mutual sensitivity of halo and disk parameters which we understand in a twofold manner: 1) numerical interdependence between the present parameters of the visible Galaxy and the ones of dark matter, and 2) dependence of the bright component evolution on the characteristics of the dark halo, in which the former was embedded since the beginning. At present, due to the smallness of dark matter inside the bright disk, the halo‐disk interaction is so weak that few informations can be obtained; consequently, the solutions for dark masses and their axis ratios vary in a large range. Nevertheless, if one demands a spherical and isotropic halo, then from ad = 90 kpc a dark mass Md = 1.5 · 1012 M⊙ results in agreement with other independent estimations. Probably this was not the situation in the past when the disk was bigger and contained more dark mass. The dependence on Md/Mb and εd/εb was likely affecting the star formation rate and in this way contributing to the present morphology of the Galaxy and other galaxies.

Brown dwarfs as dark galactic halos

The possibility that the dark matter in galactic halos can consist of brown dwarf stars is considered. The radiative signature for such halos consisting solely of brown dwarfs is calculated, and the allowed range of brown dwarf masses, the initial mass function (IMF), the stellar properties, and the density distribution of the galactic halo are discussed. The prediction emission from the halo is compared with existing observations. It is found that, for any IMF of brown dwarfs below the deuterium burning limit, brown dwarf halos are consistent with observations. Brown dwarf halos cannot, however, explain the recently observed near-IR background. It is shown that future satellite missions will either detect brown dwarf halos or place tight constraints on the allowed range of the IMF.

The warp of the Galactic stellar disk detected in IRAS source counts

view Abstract Citations (63) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Warp of the Galactic Stellar Disk Detected in IRAS Source Counts Djorgovski, S. ; Sosin, Craig Abstract About 90,000 IRAS point sources have been used as disk tracers in order to explore the possibility of warp in the Galactic stellar disk. The results imply that the Galactic stellar disk is warped at large radii in a way similar to the H-I layer, and that the warp is an important characteristic of the Galaxy as a whole. It is suggested that the warp may be a long-lasting phenomenon, possibly caused by asymmetries of the mass distribution in the outer regions of the Galactic dark halo. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/185446 Bibcode: 1989ApJ...341L..13D Keywords: Infrared Sources (Astronomy); Milky Way Galaxy; Galactic Structure; Hydrogen; Infrared Astronomy Satellite; Mass Distribution; Astrophysics; GALAXIES: THE GALAXY; INFRARED: GENERAL full text sources ADS | data products SIMBAD (3)

Lensing of extended sources by dark galactic halos

view Abstract Citations (22) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Lensing of Extended Sources by Dark Galactic Halos Narasimha, D. ; Chitre, S. M. Abstract Two aspects of gravitational lensing are mainly addressed in this paper: (i) the role of dark matter as a lensing agent and (ii) imaging of extended background sources. An attempt is made to reproduce the observed image configurations of lens systems 2016 + 112, 0023 + 171, 2345 + 007, and 1635 + 267 by invoking the assistance of dark halos around the lensing galaxies. The models that can reproduce the observed features of these systems reasonably well consist of normal galaxies accompanied by galactic halos of size 75-150 kpc and mass 1-2 X 10^13^ M_sun_. It is demonstrated that from the observational standpoint the image features resulting from the lensing of extended sources are not reproduced by the models based only on point sources. Speculative arguments are presented about the nature of an unobserved lensing agent in the case of 2345 + 007 and 1635 + 267. Should a substantial amount of matter exist in the universe in the form of nonluminous clumps of size ~50 kpc and mass ~10^12^ M_sun_ obeying the Schuster law of density distribution, then the lensing action of individual clumps can account for some of the observed image configurations. A promising method to detect such a lens in the case of 2345 + 007 would be to undertake radio observations of the 21 cm line emission at the ~10 mJy level in the frequency range of 500-900 MHz. Publication: The Astronomical Journal Pub Date: February 1989 DOI: 10.1086/114983 Bibcode: 1989AJ.....97..327N Keywords: Dark Matter; Gravitational Lenses; Radio Sources (Astronomy); Background Radiation; Brightness Distribution; Density Distribution; Image Analysis; Lyman Alpha Radiation; Very Long Base Interferometry; Astrophysics; GRAVITATION; GALAXIES: GENERAL full text sources ADS | data products NED (10) SIMBAD (4)

A cosmologically significant population of galaxies dominated by very young star formation

A population of galaxies whose light is dominated at all wavelengths from B to K by a very young stellar population has been found in an extremely deep optical and IR small-field survey. These galaxies must represent one of the main star-forming epochs in the history of the universe and approximately 10-100 percent of the metals must have formed in them. They must have a redshift less than 3.5 and may constitute either a genuine protogalaxy population or a rejuvenation phase of galaxies that formed earlier. The faint I band star counts in the field eliminate the possibility that the Galactic dark halo may be composed of hydrogen-burning main-sequence stars. 16 references.

N-body Simulations of Galaxy Formation

Modern N-body techniques allow the study of galaxy formation in the wider context of the formation of large-scale structure in the Universe. The results of such a study within the cold dark matter cosmogony are described. Dark galactic halos form at relatively recent epochs. Their properties and abundance are similar to those inferred for the halos of real galaxies. Massive halos tend to form preferentially in high density regions and as a result the galaxies that form within them are significantly more clustered than the underlying mass. This natural bias may be strong enough to reconcile the observed clustering of galaxies with the assumption that Ω = 1.

Gravitinos as the cold dark matter in an [formula omitted] universe

In attempts to simultaneously have galaxy formation with adiabatic fluctuations, dark galactic halos and a critical Ω = 1, recent models have been proposed with a heavy (cold) species which decays and a lighter (hot or warm) one which is stable. Surprisingly enough, these models are very constraining. Independent of the particle physics model, the decay width of the heavy particle must be O(10 −40) GeV. Such a scale is only natural in theories involving gravity where one expects Γ ∼ M H 3/ M P 2, where M H is the mass of the heavy particle and M P is the Planck mass. In this paper we suggest that the heavy particle might be the gravitino which is present in all supergravity theories. This model would then require that the gravitino decay products not include photons, indicating that the lightest supersymmetric particle (LSP) must be something other than the photino. An acceptable candidate for the LSP might be the axino, the supersymmetric partner of the axion.