Sebastian Gurovich

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 dark matter haloes of dwarf galaxies: a challenge for the Λ cold dark matter paradigm?

The cold dark matter halo mass function is much steeper than the galaxy stellar mass function on galactic and subgalactic scales. This difference is usually reconciled by assuming that the galaxy formation efficiency drops sharply with decreasing halo mass, so that virtually no dwarf galaxies form in halos less massive than ~ 10^10 M_sun. In turn, this implies that, at any given radius, the dark mass enclosed by a galaxy must exceed a certain minimum. We use rotation curves of dwarf galaxies compiled from the literature to explore whether their enclosed mass is consistent with these constraints. We find that almost one half of the dwarfs in our sample with stellar mass between 10^6-10^7 M_sun are at odds with this restriction: either they live in halos with masses substantially below 10^10 M_sun or there is a mechanism capable of reducing the dark mass enclosed by some of the faintest dwarfs. Neither possibility is easily accommodated within the standard LCDM scenario. Extending galaxy formation to halos well below 10^10 M_sun would lead to large numbers of dwarf galaxies in excess of current estimates; at the same time, the extremely low stellar mass of the systems involved makes it unlikely that baryonic effects can reduce their dark matter content. Resolving this challenge seems to require new insights into dwarf galaxy formation, or perhaps a radical revision of the prevailing paradigm.

The Baryonic Tully–Fisher Relation

We validate the baryonic Tully–Fisher (TF) relation by exploring the Tully–Fisher (TF) and BTF properties of optically and HI-selected disk galaxies. The data includes galaxies from Sakai et al. (2000) calibrator sample, McGaugh et al. (2000: M2000) I-band sample, and 18 newly acquired HI-selected field dwarf galaxies observed with the ANU 2.3-m telescope and the ATNF Parkes telescope (Gurovich 2005a). As in M2000, we re-cast the TF and BTF relations as relationships between baryon mass and W20. First we report some numerical errors in M2000. Then, we calculate weighted bi-variate linear fits to the data, and finally we compare the fits of the intrinsically fainter dwarfs with the brighter galaxies of Sakai et al. (2000). With regards to the local calibrator disk galaxies of Sakai et al. (2000), our results suggest that the BTF relation is indeed tighter than the TF relation and that the slopes of the BTF relations are statistically flatter than the equivalent TF relations. Further, for the fainter galaxies which include the I-band M2000 and HI-selected galaxies of Gurovichs sample, we calculate a break from a simple power law model because of what appears to be real cosmic scatter. Not withstanding this point, the BTF models are marginally better models than the equivalent TF ones with slightly smaller Χred2 values.

The dark matter halos of dwarf galaxies: a challenge for the LCDM paradigm?

The cold dark matter halo mass function is much steeper than the galaxy stellar mass function on galactic and subgalactic scales. This difference is usually reconciled by assuming that the galaxy formation efficiency drops sharply with decreasing halo mass, so that virtually no dwarf galaxies form in halos less massive than ~ 10^10 M_sun. In turn, this implies that, at any given radius, the dark mass enclosed by a galaxy must exceed a certain minimum. We use rotation curves of dwarf galaxies compiled from the literature to explore whether their enclosed mass is consistent with these constraints. We find that almost one half of the dwarfs in our sample with stellar mass between 10^6-10^7 M_sun are at odds with this restriction: either they live in halos with masses substantially below 10^10 M_sun or there is a mechanism capable of reducing the dark mass enclosed by some of the faintest dwarfs. Neither possibility is easily accommodated within the standard LCDM scenario. Extending galaxy formation to halos well below 10^10 M_sun would lead to large numbers of dwarf galaxies in excess of current estimates; at the same time, the extremely low stellar mass of the systems involved makes it unlikely that baryonic effects can reduce their dark matter content. Resolving this challenge seems to require new insights into dwarf galaxy formation, or perhaps a radical revision of the prevailing paradigm.

Radio galaxies in the Sloan Digital Sky Survey: spectral index–environment correlations

ABSTRACT We analyze optical and radio properties of radiogalaxies detected in the Sloan DigitalSky Survey (SDSS). The sample of radio sources are selected from the catalogue ofKimball & Ivezi´c (2008) with flux densities at 325, 1400 and 4850 MHz, using WENSS,NVSS and GB6 radio surveys and from flux measurements at 74 MHz taken from VLALow-frequency Sky Survey (Cohen et al. 2006). We study radiogalaxy spectral prop-erties using radio colour-colour diagrams and find that our sample follows a singlepower law from 74 to 4850 MHz. The spectral index vs. spectroscopic redshift relation(α − z) is not significant for our sample of radio sources. We analyze a subsample ofradio sources associated with clusters of galaxies identified from the maxBCG cata-logue and find that about 40% of radio sources with ultra steep spectra (USS, α < −1,where S ν ∝ ν α ) are associated with galaxy clusters or groups of galaxies. We constructa Hubble diagram of USS radio sources in the optical r band up to z ∼ 0.8 and com-pare our results with those for normal galaxies selected from different optical surveysand find that USS radio sources are around as luminous as the central galaxies in themaxBCG cluster sample and typically more than 4 magnitudes brighter than normalgalaxies at z ∼ 0.3.We study correlations between spectral index, richness and luminosity of clustersassociated with radio sources. We find that USS at low redshift are rare, most of themreside in regions of unusually high ambient density, such of those found in rich clusterof galaxies. Our results also suggest that clusters of galaxies associated with steeperthan the average spectra have higher richness counts and are populated by luminousgalaxiesin comparison with those environments associated to radiosources with flatterthan the average spectra. A plausible explanation for our results is that radio emissionis more pressure confined in higher gas density environments such as those found inrich clusters of galaxies and as a consequence radio lobes in rich galaxy clusters willexpand adiabatically and lose energy via synchrotron and inverse Compton losses,resulting in a steeper radio spectra.Keywords: surveys – radio continuum: general – radio continuum: galaxies – galax-ies: high-redshift

The dark matter haloes of dwarf galaxies: a challenge for the Λ cold dark matter paradigm?: The dark matter haloes of dwarf galaxies

The cold dark matter halo mass function is much steeper than the galaxy stellar mass function on galactic and subgalactic scales. This difference is usually reconciled by assuming that the galaxy formation efficiency drops sharply with decreasing halo mass, so that virtually no dwarf galaxies form in halos less massive than ~ 10^10 M_sun. In turn, this implies that, at any given radius, the dark mass enclosed by a galaxy must exceed a certain minimum. We use rotation curves of dwarf galaxies compiled from the literature to explore whether their enclosed mass is consistent with these constraints. We find that almost one half of the dwarfs in our sample with stellar mass between 10^6-10^7 M_sun are at odds with this restriction: either they live in halos with masses substantially below 10^10 M_sun or there is a mechanism capable of reducing the dark mass enclosed by some of the faintest dwarfs. Neither possibility is easily accommodated within the standard LCDM scenario. Extending galaxy formation to halos well below 10^10 M_sun would lead to large numbers of dwarf galaxies in excess of current estimates; at the same time, the extremely low stellar mass of the systems involved makes it unlikely that baryonic effects can reduce their dark matter content. Resolving this challenge seems to require new insights into dwarf galaxy formation, or perhaps a radical revision of the prevailing paradigm.