Helmut Jerjen

The HIPASS catalogue - I. Data presentation

The H I Parkes All-Sky Survey (HIPASS) catalogue forms the largest uniform catalogue of H I sources compiled to date, with 4315 sources identified purely by their H I content. The catalogue data comprise the southern region δ< + 2 ◦ of HIPASS, the first blind H I survey to cover the entire southern sky. The rms noise for this survey is 13 mJy beam −1 and the velocity range is −1280 to 12 700 km s −1 . Data search, verification and parametrization methods are discussed along with a description of measured quantities. Full catalogue data are made available to the astronomical community including positions, velocities, velocity widths, integrated fluxes and peak flux densities. Also available are on-sky moment maps, position‐velocity moment maps and spectra of catalogue sources. A number of local large-scale features are observed in the space distribution of sources, including the super-Galactic plane and the Local Void. Notably, large-scale structure is seen at low Galactic latitudes, a region normally obscured at optical wavelengths.

Local-Group tests of dark-matter concordance cosmology - Towards a new paradigm for structure formation

Predictions of the concordance cosmological model (CCM) of the structures in the environment of large spiral galaxies are compared with observed properties of Local Group galaxies. Five new, most probably irreconcilable problems are uncovered: 1) A wide variety of published CCM models consistently predict some form of relation between dark-matter-mass and luminosity for the Milky Way (MW) satellite galaxies, but none is observed. 2) The mass function of luminous sub-haloes predicted by the CCM contains too few satellites with dark matter (DM) mass ≈10 7 Mwithin their innermost 300 pc than in the case of the MW satellites. 3) The Local Group galaxies and data from extragalactic surveys indicate there is a correlation between bulge-mass and the number of luminous satellites that is not predicted by the CCM. 4) The 13 new ultra-faint MW satellites define a disc-of-satellites (DoS) that is virtually identical to the DoS previously found for the 11 classical MW satellites, implying that most of the 24 MW satellites are correlated in phase-space. 5) The occurrence of two MW-type DM halo masses hosting MW-like galaxies is unlikely in the CCM. However, the properties of the Local Group galaxies provide information leading to a solution of the above problems. The DoS and bulge-satellite correlation suggest that dissipational events forming bulges are related to the processes forming phase-space correlated satellite populations. These events are well known to occur since in galaxy encounters energy and angular momentum are expelled in the form of tidal tails, which can fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star clusters. If Local Group satellite galaxies are to be interpreted as TDGs then the substructure predictions of the CCM are internally in conflict. All findings thus suggest that the CCM does not account for the Local Group observations and that therefore existing as well as new viable alternatives have to be further explored. These are discussed and natural solutions for the above problems emerge.

Tidal disruption of the Magellanic Clouds by the Milky Way

Interactions between galaxies are common and are an important factor in determining their physical properties such as position along the Hubble sequence and star-formation rate. There are many possible galaxy interaction mechanisms, including merging, ram-pressure stripping, gas compression, gravitational interaction and cluster tides. The relative importance of these mechanisms is often not clear, as their strength depends on poorly known parameters such as the density, extent and nature of the massive dark halos that surround galaxies. A nearby example of a galaxy interaction where the mechanism is controversial is that between our own Galaxy and two of its neighbours -- the Large and Small Magellanic Clouds. Here we present the first results of a new HI survey which provides a spectacular view of this interaction. In addition to the previously known Magellanic Stream, which trails 100 degrees behind the Clouds, the new data reveal a counter-stream which lies in the opposite direction and leads the motion of the Clouds. This result supports the gravitational model in which leading and trailing streams are tidally torn from the body of the Magellanic Clouds.Interactions between galaxies are common, and influence physical properties such as the global morphology and star-formation rate (Hubble type). Galaxies can interact in many different ways: they can merge together; they can pass through each other, with gas being stripped from the smaller of the two and compressed in the larger; and they can interact gravitationally (including, for example, tides in clusters). The relative importance of these mechanisms is often not clear, as the strength of each depends on poorly known parameters such as the density, extent and nature of the dark-matter haloes that surround galaxies. A nearby example of a galaxy interaction where the mechanism is controversial is that between our Galaxy and two of its neighbours, the Magellanic Clouds. Here we present the results of an atomic-hydrogen survey that help to elucidate this mechanism. Our data reveal a new stream of gas that lies in the opposite direction to the trailing Magellanic Stream and leads the motion of the Clouds. The existence of both leading and trailing streams supports a gravitational interaction whereby the streams are torn from the bodies of the Magellanic Clouds by tidal forces.

The spatial distribution of the Milky Way and Andromeda satellite galaxies

There are two fundamentally different physical origins of faint satellite galaxies: cosmological sub-structures that contain shining baryons and the fragmentation of gas-rich tidal arms thrown out from interacting galaxies during hierarchical structure formation. The latter tidal-dwarf galaxies (TDG) may form populations with correlated orbital angular momenta about their host galaxies. The existence of TDGs is a stringent necessity because they arise as a result of fundamental physical principals. We determine the significance of the apparent disc-like distribution of Milky Way (MW) satellite galaxies. The distribution of the MW satellites is found to be inconsistent with an isotropic or prolate DM sub-structure distribution at a 99.5 per cent level including the recently discovered UMa and CVn dwarf spheroidal galaxies. The distribution is extremely oblate and inclined by about 88 ◦ with respect to the the MW disc. We also apply the methods to Andromeda’s (M31) satellite galaxies using two recently published data-sets. It can not be excluded that the whole population of M31 companions is drawn randomly from an isotropic parent distribution. However, two subsamples of Andromeda satellites are identified which have disc-like features. A kinematically motivated subsample of eight Andromeda satellites forms a pronounced disc-like distribution in both data-sets. The existence of this disc would be inconsistent with a CDM parent distribution of subhaloes if the disc is rotationally supported. The M31 satellite distribution is inclined by about 59 ◦ with respect to the M31 disc, and has virtually the same orientation as the disc derived for the whole M31 satellite sample. We present a new geometric method to set restrictions on possible locations of angular momentum vectors for Andromeda satellites. Our conclusion is that both, the MW and M31, may indeed have satellite galaxies derived from TDGs. Further, both host-discs and both identified discs-of-satellites are highly inclined relative to the supergalactic plane. The discs-of-satellites therefore cannot be created from individual accretion events from the supergalactic plane further supporting the possibility that they are of TDG origin.

The 1000 brightest hipass galaxies: The HI mass function and Omega(HI)

We present a new, accurate measurement of the H I mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest H I peak flux densities in the southern (delta<0D) hemisphere. This sample spans nearly 4 orders of magnitude in H I mass [ log (M-H I/M-O) + 2 log h(75)=6.8-10.6] and is the largest sample of H I-selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies and show that this is a robust method, insensitive to the effects of large-scale structure. The resulting H I mass function can be fitted satisfactorily with a Schechter function with faint-end slope α=-1.30. This slope is found to be dependent on morphological type, with late-type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the H I mass function, including peculiar motions of galaxies, large-scale structure, selection bias, and inclination effects, and we quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: U(H) I=(3.8P0.6)x10(-4) h(75)(-1). Low surface brightness galaxies contribute only similar to15% to this value, consistent with previous findings.

The Northern HIPASS catalogue – data presentation, completeness and reliability measures

The Northern HIPASS catalogue (NHICAT) is the northern extension of the HIPASS catalogue, HICAT. This extension adds the sky area between the declination (Dec.) range of +2 degrees 300 km s(-1). Sources with -300 < nu(hel) < 300 km s(-1) were excluded to avoid contamination by Galactic emission. In total, the entire HIPASS survey has found 5317 galaxies identified purely by their HI content. The full galaxy catalogue is publicly available at http://hipass.aus-vo.org.

The Invisibles: A Detection Algorithm to Trace the Faintest Milky Way Satellites

A specialized data-mining algorithm has been developed using wide-field photometry catalogs, enabling systematic and efficient searches for resolved, extremely low surface brightness satellite galaxies in the halo of the Milky Way (MW). Tested and calibrated with the Sloan Digital Sky Survey Data Release 6 (SDSS-DR6) we recover all 15 MW satellites recently detected in SDSS, six known MW/Local Group dSphs in the SDSS footprint, and 19 previously known globular and open clusters. In addition, 30 point-source overdensities have been found that correspond to no cataloged objects. The detection efficiencies of the algorithm have been carefully quantified by simulating more than three million model satellites embedded in star fields typical of those observed in SDSS, covering a wide range of parameters including galaxy distance, scale length, luminosity, and Galactic latitude. We present several parameterizations of these detection limits to facilitate comparison between the observed MW satellite population and predictions. We find that all known satellites would be detected with >90% efficiency over all latitudes spanned by DR6 and that the MW satellite census within DR6 is complete to a magnitude limit of MV ?6.5 and a distance of 300 kpc. Assuming all existing MW satellites contain an appreciable old stellar population and have sizes and luminosities comparable with currently known companions, we predict lower and upper limit totals of 52 and 340 MW dwarf satellites within ~260 kpc if they are uniformly distributed across the sky. This result implies that many MW satellites still remain undetected. Identifying and studying these elusive satellites in future survey data will be fundamental to test the dark matter distribution on kpc scales.

The 1000 brightest HIPASS galaxies: The H I mass function and ω

We present a new, accurate measurement of the H I mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest H I peak flux densities in the southern (delta<0D) hemisphere. This sample spans nearly 4 orders of magnitude in H I mass [ log (M-H I/M-O) + 2 log h(75)=6.8-10.6] and is the largest sample of H I-selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies and show that this is a robust method, insensitive to the effects of large-scale structure. The resulting H I mass function can be fitted satisfactorily with a Schechter function with faint-end slope α=-1.30. This slope is found to be dependent on morphological type, with late-type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the H I mass function, including peculiar motions of galaxies, large-scale structure, selection bias, and inclination effects, and we quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: U(H) I=(3.8P0.6)x10(-4) h(75)(-1). Low surface brightness galaxies contribute only similar to15% to this value, consistent with previous findings.

Surface BR Photometry of Newly Discovered Dwarf Elliptical Galaxies in the Nearby Sculptor and Centaurus A Groups

Prompted by the recent finding of a large number of gas-rich dwarf irregular galaxies in the nearby southern galaxy groups Sculptor (Scl) and Centaurus A (Cen A) we carried out a complementary survey to search for members of the gas-poor dwarf elliptical (dE) galaxy family. In total 18 dE candidates were identified on SRC Sky Survey films covering the group regions. For five dEs in the Scl group and five in the Cen A group, membership information is available from surface brightness fluctuations distance measurements; two dEs in each group also have independent redshift measurements. The remaining eight galaxies have been associated to the groups based on their morphology. We acquired deep B- and R-band CCD images for all galaxies and determined their photometric and structural parameters. According to the magnitude range covered, -14 < MB < -9 (not counting the bright dS0 NGC 59), these objects are local dwarf spheroidal (dSph) analogues. The surface brightness profiles of most dwarfs are strongly nonexponential but are perfectly explained by the three-parametric Sersic law. The best-fitting Sersic parameters are compared with corresponding data for Virgo dEs and dSphs. The present dEs follow the relations between the Sersic parameters and absolute B-band magnitude for Virgo dEs and local dwarfs, thus confirming the membership in the Scl and Cen A groups, respectively. Most of our dwarfs exhibit color gradients in (B-R)0, in the sense of a reddening outward. We also show and discuss the sky distributions and B-band luminosity functions of the Scl and Cen A groups. The completeness magnitude for both populations (from the present survey) is estimated to be M ≈ -13. We find that the composite luminosity function of four nearby galaxy groups is well fitted by a Schechter function down to M = -14 with a faint end slope α = -1.29(±0.10). In the appendices we present best-fitting Sersic profile parameters for the dSph members of the Local Group and introduce two new dwarf irregulars found in the Cen A group.

Intracluster light and the extended stellar envelopes of cD galaxies: an analytical description

We have analysed deep R-band images, down to a limiting surface brightness of 26.5 R-mag arcsec −2 (equivalent to ∼28 B-mag arcsec −2 ), of five cD galaxies to determine the shape of the surface brightness profiles of their extended stellar envelopes. Both de Vaucouleurs R 1/4 and Sersic R 1/n models, on their own, provide a poor description of the surface brightness profiles of cD galaxies. This is due to the presence of outer stellar envelopes, thought to have accumulated over the merger history of the central cluster galaxy and also from the tidal stripping of galaxies at larger cluster radii. We therefore simultaneously fit two Sersic functions to measure the shape of the inner and outer components of the cD galaxies. We show that, for three out of our five galaxies, the surface brightness profiles are best fitted by an inner Sersic model, with indices n ∼ 1-6, and an outer exponential component. For these systems, the galaxy-to-envelope size ratio is 0.1-0.4 and the contribution of the stellar envelope to the total R-band light (i.e. galaxy + envelope) is around 60-80 per cent (based on extrapolation to a 300 kpc radius). The exceptions are NGC 6173, for which our surface brightness profile modelling is consistent with just a single component (i.e. no envelope) and NGC 4874 which appears to have an envelope with a de Vaucouleurs, rather than exponential, profile.