Decrease In Rotation Velocity Research Articles

Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago.

In the cold dark matter cosmology, the baryonic components of galaxies-stars and gas-are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark-matter halo. In the local (low-redshift) Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius-a hallmark of the dark-matter model. Comparisons between the dynamical mass, inferred from these velocities in rotational equilibrium, and the sum of the stellar and cold-gas mass at the peak epoch of galaxy formation ten billion years ago, inferred from ancillary data, suggest high baryon fractions in the inner, star-forming regions of the disks. Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (owing to the chosen stellar initial-mass function and the calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves (showing rotation velocity as a function of disk radius) for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of a combination of two main factors: first, a large fraction of the massive high-redshift galaxy population was strongly baryon-dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early (high-redshift) Universe efficiently condensed at the centres of dark-matter haloes when gas fractions were high and dark matter was less concentrated.

Factors Effect on Metal Detecting System using Mobile Robot

metal detection mobile robot has got to the summit of the utilized methods in landmines and unexploded ordnance detection techniques, while it is used in other applications like archeology, treasure and mines prospection and underground metallic services search in different scales. This paper uses a path planning, without supervisor robot at three environments. The path plan supposes that metal is safe so it cover the entire area in rectangle approach with detection of metal. The program which has been written in c++.net saves path of the robot and the detected metals locations in Microsoft Excel file. The program does not need a supervisor since it deals with all the possibilities of path planning. The three outdoor environments which were experimented are paved land, sand land and grassland. The metal detection autonomous mobile robot has succeeded to move in desired navigation strategy. The experimental work shows that the increase of translation velocity leads to increase in error, the paved land has the least error and the sand land has the biggest error, the decrease in rotational velocity decreases the elapsed time in turnover and stroke and at low velocities the effects of the land types have diminished. KeywordsDetection, Mobile Robot, Autonomous, path planning.

RADIAL VELOCITIES OF THREE POORLY STUDIED CLUSTERS AND THE KINEMATICS OF OPEN CLUSTERS

We present radial velocities for stars in the field of the open star clusters Berkeley 44, Berkeley 81, and NGC 6802 from spectra obtained using the Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5 m telescope. These clusters are of intermediate age (1-3 Gyr), located within the solar Galactocentric radius, and have no previous radial velocity measurements. We find mean radial velocities of -9.6 +/- 3.0 km s^-1, 48.1 +/- 2.0 km s^-1, and 12.4 +/- 2.8 km s^-1 for Be 44, Be 81, and NGC 6802, respectively. We present an analysis of radial velocities of 134 open clusters of a wide range of ages using data obtained in this study and the literature. Assuming the system of clusters rotates about the Galactic center with a constant velocity, we find older clusters exhibit a slower rotation and larger line-of-sight (LOS) velocity dispersion than younger clusters. The gradual decrease in rotational velocity of the cluster system with age is accompanied by a smooth increase in LOS velocity dispersion, which we interpret as the effect of heating on the open cluster system over time.

HALOGAS: Extraplanar gas in NGC 3198

We present the analysis of new, deep HI observations of the spiral galaxy NGC 3198, as part of the HALOGAS (Westerbork Hydrogen Accretion in LOcal GAlaxieS) survey, with the main aim of investigating the presence, amount, morphology and kinematics of extraplanar gas. We present models of the HI observations of NGC 3198: the model that matches best the observed data cube features a thick disk with a scale height of ~3 kpc and an HI mass of about 15% of the total HI mass; this thick disk also has a decrease in rotation velocity as a function of height (lag) of 7-15 km/s/kpc (though with large uncertainties). This extraplanar gas is detected for the first time in NGC 3198. Radially, this gas appears to extend slightly beyond the actively star-forming body of the galaxy (as traced by the Halpha emission), but it is not more radially extended than the outer, fainter parts of the stellar disk. Compared to previous studies, thanks to the improved sensitivity we trace the rotation curve out to larger radii. We model the rotation curve in the framework of MOND (Modified Newtonian Dynamics) and we confirm that, with the allowed distance range we assumed, fit quality is modest in this galaxy, but the new outer parts are explained in a satisfactory way.

HALOGAS: H I OBSERVATIONS AND MODELING OF THE NEARBY EDGE-ON SPIRAL GALAXY NGC 4565

We present 21-cm observations and models of the neutral hydrogen in NGC 4565, a nearby, edge-on spiral galaxy, as part of the Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. These models provide insight concerning both the morphology and kinematics of HI above, as well as within, the disk. NGC 4565 exhibits a distinctly warped and asymmetric disk with a flaring layer. Our modeling provides no evidence for a massive, extended HI halo. We see evidence for a bar and associated radial motions. Additionally, there are indications of radial motions within the disk, possibly associated with a ring of higher density. We see a substantial decrease in rotational velocity with height above the plane of the disk (a lag) of -40 +5/-20 km/s/kpc and -30 +5/-30 km s/kpc in the approaching and receding halves, respectively. This lag is only seen within the inner ~4.75' (14.9 kpc) on the approaching half and ~4.25' (13.4 kpc) on the receding, making this a radially shallowing lag, which is now seen in the HI layers of several galaxies. When comparing results for NGC 4565 and those for other galaxies, there are tentative indications of high star formation rate per unit area being associated with the presence of a halo. Finally, HI is found in two companion galaxies, one of which is clearly interacting with NGC 4565.

Ionization, Kinematics, and Extent of the Diffuse Ionized Gas Halo of NGC 5775

We present key results from deep spectra of the diffuse ionized gas halo of the edge-on galaxy NGC 5775. [N II] λ6583 has been detected up to z ≈ 13 kpc above the plane in one of two vertically oriented long slits—making this the spiral galaxy with the greatest spectroscopically detected halo extent in emission. Key diagnostic line ratios have been measured up to z ≈ 8 kpc, allowing the source of ionization and physical state to be probed. Ionization by a dilute radiation field from massive stars in the disk can explain some of the line ratio behavior, but departures from this picture are clearly indicated, most strongly by the rise of [O III]/Hα with z. Velocities of the gas in both slits approach the systemic velocity of the galaxy at several kiloparsecs above the plane. We interpret this trend as a decrease in rotation velocity with z, with essentially no rotation at heights of several kiloparsecs. Such a trend was observed in the edge-on galaxy NGC 891, but here much more dramatically. This falloff is presumably due to the gravitational potential changing with z, but will also depend on the hydrodynamic nature of the disk-halo cycling of gas and projection effects. More detailed modeling of the ionization and kinematics of this and other edge-ons will be presented in future papers.

Declining rotation curves - The end of a conspiracy?

view Abstract Citations (278) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Declining Rotation Curves: The End of a Conspiracy? Casertano, Stefano ; van Gorkom, J. H. Abstract New observations of H I rotation curves at the Very Large Array have uncovered two galaxies with rotation curves declining between l and 3 optical radii. The velocity decrease is large, more than 50 km s^-1^(~25% of the maximum rotation velocity ) , and is present on both sides of the galaxies; projection effects can be ruled out. We interpret the decrease in rotation velocity as an indication of a large ratio of luminous to dark mass in the luminous regions of these systems. An analysis of the current observations combined with rotation curves from the literature shows a clear correlation between peak circular velocity of a galaxy, its central surface brightness and the slope of the rotation curve in the outer parts. This correlation indicates a weakening of the well-known conspiracy between luminous and dark matter, and may provide evidence in favor of the idea that dark matter is baryonic. Although we do not sample the full morphological Hubble sequence, a strong correlation between slope of the rotation curve and morphological type is found. This result seems to support earlier suggestions that the ratio between the mass in dark and luminous matter might be the critical parameter that controls the Hubble sequence. Publication: The Astronomical Journal Pub Date: April 1991 DOI: 10.1086/115759 Bibcode: 1991AJ....101.1231C Keywords: Angular Velocity; Dark Matter; Galactic Rotation; Luminosity; Spiral Galaxies; Galactic Mass; Very Large Array (Vla); Astrophysics; GALAXIES: INTERNAL MOTIONS full text sources ADS | data products SIMBAD (31) NED (2)

Age-rotation relationship for late-type main-sequence stars

With advancing spectral type and increasing age, late main-sequence stars exhibit monotonic decrease in rotational velocity. It is of great interest to extend the rotation-age relationship to stars of later spectral type. In recent times it has become possible to measure directly the rotational periods from the photometric modulation by Ca II H and K line emission. There have also been successful attempts to relate the chromospheric activity as manifested through Ca II H and K lines to the rotation period, and it was shown that the fraction of total stellar luminosity in Ca II H and K lines, corrected for photospheric contribution, is a function of a single parameter related to P and B-V. In the present investigation, this rotation-activity relation is utilized to infer the rotation periods as a function of spectral type. The period versus B-V plot is employed as a basis to infer that the rotational period of main-sequence stars is a single-valued function of mass (B-V color) and age.

Angular velocity gradients in the solar convection zone

view Abstract Citations (81) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Angular velocity gradients in the solar convection zone. Gilman, P. A. ; Foukal, P. V. Abstract Numerical calculations of Boussinesq nonaxisymmetric convection in a rotating spherical shell are reported which were performed to study how convection in the supergranule layer redistributes angular momentum. It is found that supergranules are at best weakly influenced by rotation and can be largely responsible for the radial gradient of angular velocity observed in the thin supergranule layer below the photosphere. The results indicate that convection in a thin spherical shell weakly influenced by rotation can produce a substantial outward decrease in rotational velocity that approaches the limit predicted for radially moving particles that conserve their angular momentum. This phenomenon is shown to provide a plausible explanation for the observed difference in angular velocity between sunspots and the photospheric plasma. Publication: The Astrophysical Journal Pub Date: May 1979 DOI: 10.1086/157052 Bibcode: 1979ApJ...229.1179G Keywords: Angular Velocity; Convection; Solar Granulation; Solar Rotation; Velocity Distribution; Angular Momentum; Boussinesq Approximation; Compressibility Effects; Fluid Flow; Mathematical Models; Photosphere; Sunspots; Thin Walled Shells; Solar Physics; Convection:Sun; Solar Atmosphere:Motions; Solar Granulation:Supergranulation; Solar Rotation full text sources ADS |