Monthly Notices of the Royal Astronomical Society Letters

Zwaan, M. A.; Meyer, M. J.; Staveley-Smith, L.; Webster, R. L.

doi: 10.1111/j.1745-3933.2005.00029.xpmid: N/A

We use the catalogue of 4315 extragalactic H i 21-cm emission-line detections from the H i Parkes All Sky Survey (HIPASS) to calculate the most accurate measurement of the H i mass function (HIMF) of galaxies to date. The completeness of the HIPASS sample is well characterized, which enables an accurate calculation of space densities. The HIMF is fitted with a Schechter function with parameters α = −1.37 ± 0.03 ± 0.05, log (M*Hi/M⊙) = 9.80 ± 0.03 ± 0.03 h−275, and θ* = (6.0 ± 0.8 ± 0.6) × 10−3h375 Mpc−3 dex−1 (random and systematic uncertainties at 68 per cent confidence limit), in good agreement with calculations based on the HIPASS Bright Galaxy Catalogue, which is a complete, but smaller, sub-sample of galaxies. The cosmological mass density of H i in the local Universe is found to be ΩH i = (3.5 ± 0.4 ± 0.4) × 10−4h75−1. This large homogeneous sample allows us to test whether the shape of the HIMF depends on local galaxy density. We find tentative evidence for environmental effects in the sense that the HIMF becomes steeper toward higher density regions, ranging from α ≈ −1.2 in the lowest density environments to α ≈ −1.5 in the highest density environments probed by this blind H i survey. This effect appears stronger when densities are measured on larger scales.

Lah, P.; Kiss, L. L.; Bedding, T. R.

doi: 10.1111/j.1745-3933.2005.00033.xpmid: N/A

We present an analysis of the 3D structure of the Magellanic Clouds, using period-luminosity (P-L) relations of pulsating red giants in the OGLE-II sample. By interpreting deviations from the mean P-L relations as distance modulus variations, we examine the 3D distributions of the sample. The results for the Large Magellanic Cloud, based solely on stars below the tip of the Red Giant Branch (LMC), confirm previous results on the inclined and possibly warped bar of the LMC. The depth variation across the OGLE-II field is about 2.4 kpc, interpreted as the distance range of a thin but inclined structure. The inclination angle is about 29°. A comparison with OGLE-II red clump distances revealed intriguing differences that seem to be connected to the red clump reddening correction. A spatially variable red clump population in the LMC can explain the deviations, which may have a broader impact on our understanding of the LMC formation history. For the Small Magellanic Cloud, we find a complex structure showing patchy distribution scattered within 3.2 kpc of the mean. However, the larger range of the overall depth on every line-of-sight is likely to smooth out significantly the real variations.

Begum, Ayesha; Chengalur, Jayaram N.; Karachentsev, I. D.; Sharina, M. E.

doi: 10.1111/j.1745-3933.2005.00040.xpmid: N/A

The nearby galaxy HIZSS003 was recently discovered during a blind H i survey of the Zone of Avoidance. Follow-up Very Large Array (VLA) observations as well as optical and nearinfrared imaging and spectroscopy confirm that it is a low-metallicity dwarf irregular galaxy. However, there are two puzzling aspects of these observations: (i) current star formation, as traced by Hα emission, is confined to a small region at the edge of the VLA HI image; and (ii) the metallicity of the older red giant branch stars is higher than that of the gas in the H ii region. We present high spatial and velocity resolution Giant Metrewave Radio Telescope observations which resolve these puzzles by showing that HIZSS003 is actually a galaxy pair and that the H ii region lies at the centre of a much smaller companion galaxy (HIZSS003B) to the main galaxy (HIZSS003A). The HI emission from these two galaxies overlaps in projection, but can be separated in velocity space. HIZSS003B has an H i mass of 2.6 × 106M⊙, and a highly disturbed velocity field. Since the velocity field is disturbed, an accurate rotation curve cannot be derived; however, the dynamical mass indicated is ∼5 × 107M⊙. For the bigger galaxy HIZSS003A we derive an HI mass of 1.4 × 107M⊙. The velocity field of this galaxy is quite regular, and from its rotation curve we derive a total dynamical mass of ∼6.5 × 108M⊙.

Cooray, Asantha; Furlanetto, Steven R.

doi: 10.1111/j.1745-3933.2005.00035.xpmid: N/A

While emission and absorption lines of the 21-cm spin-flip transition of neutral hydrogen are intrinsically unpolarized, a magnetic field creates left- and right-handed polarized components through the Zeeman effect. Here we consider the resulting polarization of the redshifted 21-cm background from the intergalactic medium before reionization. The polarization is most readily detectable in regions with a strong gradient in the mean brightness temperature. In principle, this can open a new window on the evolution of intergalactic magnetic fields. One possible approach is an extended integration of an individual target during this era, such as the Mpc-scale H ii regions inferred to surround quasars at z ∼ 6.5. The differential intensity between the two polarization states can be used as a probe of the magnetic field at the edge of the H ii region. We estimate that the Square Kilometer Array (SKA) could (ignoring systematics) detect B ∼ 200 (10) µG coherent over several kpc with an observational bandwidth of 100 (2) kHz. Beyond individual sources, the statistical properties of wide-field 21-cm polarization maps, such as the angular power spectrum, can be used to constrain the large-scale magnetic field. In this case, the SKA can detect B ∼ 100 µG fields coherent over many Mpc. The magnetic field can be measured in any epoch over which the 21-cm background changes rapidly (e.g. because the ionized fraction or spin temperature change). With either of these methods, a secure detection will require high-precision removal of any polarized foregrounds, which is likely to be a substantial challenge. Our estimates show the level to which the foregrounds must be removed to extract useful cosmological information. Even ignoring foregrounds, the resulting constraints are relatively weak compared to theoretical expectations, but they nevertheless offer a unique direct probe of magnetic fields in the high-redshift universe.

Khan, Sophia A.; Benford, Dominic J.; Clements, David L.; Moseley, S. Harvey; Shafer, Richard A.; Sumner, Timothy J.

doi: 10.1111/j.1745-3933.2005.00024.xpmid: N/A

We have resolved the ultraluminous infrared galaxy (ULIRG) IRAS 09111–1007 with the new 350-µm-optimized Second Generation Submillimeter High Angular Resolution Camera (SHARC II), and present the first submillimetre fluxes and images for the system. IRAS 09111–1007 comprises two interacting luminous infrared galaxies (LIRGs) with a projected nuclear separation of 39 h71−1 kpc. The western galaxy is roughly four times more luminous in the submillimetre than its eastern counterpart. It is an extremely bright LIRG with an active galactic nucleus (AGN). The classification of the eastern source is uncertain: it could be a Seyfert 2 galaxy or a LINER. We highlight IRAS 09111–1007 as a system that necessitates further study: a double AGN ULIRG whose molecular gas content differs from that of other widely separated pairs, and whose ULIRG phase might not be explained by current multiple-merger and/or final-stage ULIRG scenarios.

Baumgardt, H.; Grebel, E. K.; Kroupa, P.

doi: 10.1111/j.1745-3933.2005.00021.xpmid: N/A

We propose to determine the stellar velocity dispersions of globular clusters in the outer halo of the Milky Way in order to decide whether the dynamics of the Universe on large scales is governed by dark matter or modified Newtonian dynamics (MOND). We show that, for a number of Galactic globular clusters, both the internal and the external accelerations are significantly below the critical acceleration parameter a0 of MOND. This leads to velocity dispersions in the case of MOND which exceed their Newtonian counterparts by up to a factor of 3, providing a stringent test for MOND. Alternatively, in cases where high velocity dispersions are found, these would provide the first evidence that globular clusters are dark matter dominated.

Fuhrmann, Klaus

doi: 10.1111/j.1745-3933.2005.00032.xpmid: N/A

The very recent suggestion for the identification of a faint G-type subgiant, dubbed ‘Tycho G’, as the binary progenitor of Tycho Brahe's 1572 Type Ia supernova by Ruiz-Lapuente et al. is essentially based on the peculiar space velocity of the star. Here we demonstrate that the kinematics of Tycho G are likewise in keeping with the stars of the Milky Way's thick-disc population. Hence we may well be seeing an object that is only coincidentally passing in the vicinity of the supernova remnant, or Tycho Brahe and his contemporaries may indeed have witnessed the late explosion of one of the first stars in the Galaxy.

Sereno, M.

doi: 10.1111/j.1745-3933.2005.00026.xpmid: N/A

Gravitational lensing by a spinning deflector in translational motion relative to the observer is discussed in the weak field, slow motion approximation. The effect of rotation, which generates an intrinsic gravito-magnetic field, separates from that due to radial motion. Corrections to the lens equation, deflection angle and time delay are derived.

Roux, Brice Le; Ragazzoni, Roberto

doi: 10.1111/j.1745-3933.2005.00027.xpmid: N/A

We show that by coupling an occulting mask to a wavefront sensor one can achieve sensitivities that exceed by a factor of 2 in variance the Poissonian limit in wavefront sensing. Examples of applications are the extension to fainter objects of conventional adaptive optics, and the increase in the sky coverage of natural guide star multiconjugated adaptive optics.

Kanbur, S. M.; Fernando, I.

doi: 10.1111/j.1745-3933.2005.00025.xpmid: N/A

In this Letter, we analyse period—colour and amplitude-colour relations at minimum, mean and maximum V-band light for 6391 RRab stars in the Large Magellanic Cloud obtained by the MACHO project. Specifically, we find that colour and amplitude are nearly independent of period at minimum light, but that there exists a definite relation between period and colour and amplitude and colour at maximum light. These two properties are consistent with earlier work suggesting that these observed properties are, at least to some extent, explained by the application of the Stefan-Boltzmann law and the interaction of the photosphere and hydrogen ionization front at minimum light. When we examine the slope of the period—colour relation as a function of phase, we find that the slope varies significantly with phase and is small for a wide range of phases around minimum light. This suggests that another factor that needs to be considered when trying to understand RR Lyrae observed properties is their behaviour at different phases during a pulsation cycle.