arXiv subjects

M. J. Disney

Publications and source records attributed to M. J. Disney.

At least 19 recordsLinked to original sources

arXiv2013

Stellar Populations of Lyman Break Galaxies at z=1-3 in the HST/WFC3 Early Release Science Observations

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z=1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 sq. arcmin in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z=1-3 are selected using dropout selection criteria similar to high redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout selected LBGs is accurate to within few percent; (2) the UV spectral slope (beta) is redder than at high redshift (z>3), where LBGs are less dusty; (3) on average, LBGs at z=1-3 are massive, dustier and more highly star-forming, compared to LBGs at higher redshifts with similar luminosities (0.1L*<~L<~2.5L*), though their median values are similar within 1-sigma uncertainties. This could imply that identical dropout selection technique, at all redshifts, find physically similar galaxies; and (4) stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of ~0.46, and star-formation rates are proportional to their stellar masses with a logarithmic slope of ~0.90. These relations hold true --- within luminosities probed in this study --- for LBGs from z~1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z~2, but to avoid any selection biases, and for direct comparison with LBGs at z>3, a true Lyman break selection at z~2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties, and their evolution.

astro-ph.CO↗

arXiv2012

A blind HI survey in the Ursa Major region

We have conducted the first blind HI survey covering 480 deg^2 and a heliocentric velocity range from 300-1900 km/s to investigate the HI content of the nearby spiral-rich Ursa Major region and to look for previously uncatalogued gas-rich objects. Here we present the catalog of HI sources. The HI data were obtained with the 4-beam receiver mounted on the 76.2-m Lovell telescope (FWHM 12 arcmin) at the Jodrell Bank Observatory (UK) as part of the HI Jodrell All Sky Survey (HIJASS). We use the automated source finder DUCHAMP and identify 166 HI sources in the data cubes with HI masses in the range of 10^7 - 10^{10.5} M_sun. Our Ursa Major HI catalogue includes 10 first time detections in the 21-cm emission line. We identify optical counterparts for 165 HI sources (99 per cent). For 54 HI sources (33 per cent) we find numerous optical counterparts in the HIJASS beam, indicating a high density of galaxies and likely tidal interactions. Four of these HI systems are discussed in detail. We find only one HI source (1 per cent) without a visible optical counterpart out of the 166 HI detections. Green Bank Telescope (FWHM 9 arcmin) follow-up observations confirmed this HI source and its HI properties. The nature of this detection is discussed and compared to similar sources in other HI surveys.

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arXiv2012

A Panchromatic Catalog of Early-Type Galaxies at Intermediate Redshift in the Hubble Space Telescope Wide Field Camera 3 Early Release Science Field

In the first of a series of forthcoming publications, we present a panchromatic catalog of 102 visually-selected early-type galaxies (ETGs) from observations in the Early Release Science (ERS) program with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) of the Great Observatories Origins Deep Survey-South (GOODS-S) field. Our ETGs span a large redshift range, 0.35 < z < 1.5, with each redshift spectroscopically-confirmed by previous published surveys of the ERS field. We combine our measured WFC3 ERS and ACS GOODS-S photometry to gain continuous sensitivity from the rest-frame far-UV to near-IR emission for each ETG. The superior spatial resolution of the HST over this panchromatic baseline allows us to classify the ETGs by their small-scale internal structures, as well as their local environment. By fitting stellar population spectral templates to the broad-band photometry of the ETGs, we determine that the average masses of the ETGs are comparable to the characteristic stellar mass of massive galaxies, 11< log(M [Solar]) < 12. By transforming the observed photometry into the GALEX FUV and NUV, Johnson V, and SDSS g' and r' bandpasses we identify a noteworthy diversity in the rest-frame UV-optical colors and find the mean rest-frame (FUV-V)=3.5 and (NUV-V)=3.3, with 1$σ$ standard deviations approximately equal to 1.0. The blue rest-frame UV-optical colors observed for most of the ETGs are evidence for star-formation during the preceding gigayear, but no systems exhibit UV-optical photometry consistent with major recent (<~50 Myr) starbursts. Future publications which address the diversity of stellar populations likely to be present in these ETGs, and the potential mechanisms by which recent star-formation episodes are activated, are discussed.

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arXiv2010

Hubble Space Telescope WFC3 Early Release Science: Emission-Line Galaxies from Infrared Grism Observations

We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6 microns from the Wide Field Camera 3 on the Hubble Space Telescope. These new infrared grism data augment previous optical Advanced Camera for Surveys G800L 0.6-0.95 micron grism data in GOODS-South from the PEARS program, extending the wavelength covereage well past the G800L red cutoff. The ERS grism field was observed at a depth of 2 orbits per grism, yielding spectra of hundreds of faint objects, a subset of which are presented here. ELGs are studied via the Ha, [OIII], and [OII] emission lines detected in the redshift ranges 0.2 2.

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arXiv2010

UV-dropout Galaxies in the GOODS-South Field from WFC3 Early Release Science Observations

We combine new high sensitivity ultraviolet (UV) imaging from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with existing deep HST/Advanced Camera for Surveys (ACS) optical images from the Great Observatories Origins Deep Survey (GOODS) program to identify UV-dropouts, which are Lyman break galaxy (LBG) candidates at z~1-3. These new HST/WFC3 observations were taken over 50 sq.arcmin in the GOODS-South field as a part of the Early Release Science program. The uniqueness of these new UV data is that they are observed in 3 UV/optical (WFC3 UVIS) channel filters (F225W, F275W and F336W), which allows us to identify three different sets of UV-dropout samples. We apply Lyman break dropout selection criteria to identify F225W-, F275W- and F336W-dropouts, which are z~1.7, 2.1 and 2.7 LBG candidates, respectively. Our results are as follows: (1) these WFC3 UVIS filters are very reliable in selecting LBGs with z~2.0, which helps to reduce the gap between the well studied z~>3 and z~0 regimes, (2) the combined number counts agrees very well with the observed change in the surface densities as a function of redshift when compared with the higher redshift LBG samples; and (3) the best-fit Schechter function parameters from the rest-frame UV luminosity functions at three different redshifts fit very well with the evolutionary trend of the characteristic absolute magnitude, and the faint-end slope, as a function of redshift. This is the first study to illustrate the usefulness of the WFC3 UVIS channel observations to select z<3 LBGs. The addition of the new WFC3 on the HST has made it possible to uniformly select LBGs from z~1 to z~9, and significantly enhance our understanding of these galaxies using HST sensitivity and resolution.

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arXiv2008

Galaxies appear simpler than expected

Galaxies are complex systems the evolution of which apparently results from the interplay of dynamics, star formation, chemical enrichment, and feedback from supernova explosions and supermassive black holes. The hierarchical theory of galaxy formation holds that galaxies are assembled from smaller pieces, through numerous mergers of cold dark matter. The properties of an individual galaxy should be controlled by six independent parameters including mass, angular-momentum, baryon-fraction, age and size, as well as by the accidents of its recent haphazard merger history. Here we report that a sample of galaxies that were first detected through their neutral hydrogen radio-frequency emission, and are thus free of optical selection effects, shows five independent correlations among six independent observables, despite having a wide range of properties. This implies that the structure of these galaxies must be controlled by a single parameter, although we cannot identify this parameter from our dataset. Such a degree of organisation appears to be at odds with hierarchical galaxy formation, a central tenet of the cold dark matter paradigm in cosmology.

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arXiv2006

Star formation history and evolution of gas-rich dwarf galaxies in the Centaurus A group

We analyse the properties of three unusual dwarf galaxies in the Centaurus A group discovered with the HIPASS survey. From their optical morphology they appear to be low surface brightness dwarf spheroidals, yet they are gas-rich (M_{HI}/L_{B} > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. Therefore these systems appear different from any dwarfs of the Local Group. They should be favoured hosts for starburst, whereas we find a faint star formation region in only one object. We have obtained 21-cm data and Hubble Space Telescope photometry in V and I bands, and have constructed Colour Magnitude Diagrams (CMDs) to investigate their stellar populations and to set a constraint on their age. From the comparison of the observed and model CMDs we infer that all three galaxies are at least older than 2 Gyr (possibly even as old as 10 Gyr) and remain gas-rich because their star formation rates (SFRs) have been very low (< 10^{-3} M_{sun}/yr) throughout. In such systems, star formation (SF) appears to have been sporadic and local, though one object (HIPASS J1321--31) has a peculiar red plume in its CMD suggesting that many of its stars were formed in a "miniburst" 300 - 500 Myr ago. The question of why there are no similar dwarf galaxies in the Local Group remains open.

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arXiv2006

The existence and detection of optically dark galaxies by 21cm surveys

One explanation for the disparity between Cold Dark Matter (CDM) predictions of galaxy numbers and observations could be that there are numerous dark galaxies in the Universe. These galaxies may still contain baryons, but no stars, and may be detectable in the 21cm line of atomic hydrogen. The results of surveys for such objects, and simulations that do/do not predict their existence, are controversial. In this paper we use an analytical model of galaxy formation, consistent with CDM, to firstly show that dark galaxies are certainly a prediction of the model. Secondly, we show that objects like VIRGOHI21, a dark galaxy candidate recently discovered by us, while rare are predicted by the model. Thirdly, we show that previous 'blind' HI surveys have placed few constraints on the existence of dark galaxies. This is because they have either lacked the sensitivity and/or velocity resolution or have not had the required detailed optical follow up. We look forward to new 21cm blind surveys (ALFALFA and AGES) using the Arecibo multi-beam instrument which should find large numbers of dark galaxies if they exist.

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arXiv2006

The Northern HIPASS catalogue - Data presentation, completeness and reliability measures

The Northern HIPASS catalogue (NHICAT) is the northern extension of the HIPASS catalogue, HICAT (Meyer et al. 2004). This extension adds the sky area between the declination range of +2 deg < dec. < +25.5 deg to HICAT's declination range of -90 deg < dec. < +2 deg. HIPASS is a blind HI survey using the Parkes Radio Telescope covering 71% of the sky (including this northern extension) and a heliocentric velocity range of -1,280 km/s to 12,700 km/s . The entire Virgo Cluster region has been observed in the Northern HIPASS. The galaxy catalogue, NHICAT, contains 1002 sources with v_hel > 300 km/s . Sources with -300 km/s < v_hel < 300 km/s 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 .

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arXiv2005

A Dark Galaxy in the Virgo Cluster Imaged at 21-cm

Dark Matter supposedly dominates the extragalactic, yet no totally dark structure of galactic proportions has ever been convincingly identified. Earlier (Minchin et al. 2005) we suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster at Jodrell Bank using single-dish observations (Davies et al. 2004), was probably such a dark galaxy because of its broad line-width (~ 200 km/s) unaccompanied by any visible gravitational source to account for it. Now we have managed to image VIRGOHI 21 in the neutral-hydrogen line, and indeed we find what appears to be a dark, edge-on, spinning disc with the mass and diameter of a typical spiral galaxy. Moreover the disc has unquestionably interacted with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor invariably responsible for such a feature. Published numerical models (Vollmer, Huchtmeier & van Driel 2005) of NGC 4254 call for a close interaction ~ 10^8 years ago with a perturber of 10^11 solar masses. This we take as completely independent evidence for the massive nature of VIRGOHI 21.

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arXiv2005

The HIPASS Catalogue: III - Optical Counterparts & Isolated Dark Galaxies

We present the largest catalogue to date of optical counterparts for HI radio-selected galaxies, Hopcat. Of the 4315 HI radio-detected sources from the HI Parkes All Sky Survey (Hipass) catalogue, we find optical counterparts for 3618 (84%) galaxies. Of these, 1798 (42%) have confirmed optical velocities and 848 (20%) are single matches without confirmed velocities. Some galaxy matches are members of galaxy groups. From these multiple galaxy matches, 714 (16%) have confirmed optical velocities and a further 258 (6%) galaxies are without confirmed velocities. For 481 (11%), multiple galaxies are present but no single optical counterpart can be chosen and 216 (5%) have no obvious optical galaxy present. Most of these 'blank fields' are in crowded fields along the Galactic plane or have high extinctions. Isolated 'Dark galaxy' candidates are investigated using an extinction cut of ABj < 1 mag and the blank fields category. Of the 3692 galaxies with an ABj extinction < 1 mag, only 13 are also blank fields. Of these, 12 are eliminated either with follow-up Parkes observations or are in crowded fields. The remaining one has a low surface brightness optical counterpart. Hence, no isolated optically dark galaxies have been found within the limits of the Hipass survey.

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arXiv2004

The cosmological significance of Low Surface Brightness galaxies found in a deep blind neutral-hydrogen survey

We have placed limits on the cosmological significance of gas-rich low surface-brightness (LSB) galaxies as a proportion of the total population of gas-rich galaxies by carrying out a very deep survey (HIDEEP) for neutral hydrogen (HI) with the Parkes multibeam system. Such a survey avoids the surface-brightness selection effects that limit the usefulness of optical surveys for finding LSB galaxies. To complement the HIDEEP survey we have digitally stacked eight 1-hour R-band Tech Pan films from the UK Schmidt Telescope covering 36 square degrees of the survey area to reach a very deep isophotal limit of 26.5 R mag/sq. arcsec. At this level, we find that all of the 129 HI sources within this area have optical counterparts and that 107 of them can be identified with individual galaxies. We have used the properties of the galaxies identified as the optical counterparts of the HI sources to estimate the significance of LSB galaxies (defined to be those at least 1.5 magnitudes dimmer in effective surface-brightness than the peak in the observed distribution seen in optical surveys). We calculate the contribution of LSB galaxies to the total number, neutral hydrogen density, luminosity density, baryonic mass density, dynamical mass density and cross-sectional area of gas-rich galaxies. We do not find any `Crouching Giant' LSB galaxies such as Malin 1, nor do we find a population of extremely low surface-brightness galaxies not previously found by optical surveys. Such objects must either be rare, gas-poor or outside the survey detection limits.

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arXiv2003

Properties of three gas-rich dwarfs in the Centaurus A group

We present HST/WFPC2 observations (F555W, F814W) and ATCA high resolution HI maps of three gas-rich dwarf galaxies in the Centaurus A group discovered in two blind 21-cm surveys (HIPASS, HIDEEP). We compare their individual properties and discuss their star formation history. Although we can not constrain the age of the oldest population from the diagrams very well, the presence of an extended population of red giant stars suggests that these systems were not formed recently. The presence of asymptotic giant branch (AGB) stars in two out of three dwarfs, sets a lower limit on the age of about 6 Gyr.

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arXiv2003

HIDEEP - An extragalactic blind survey for very low column-density neutral hydrogen

We have carried out an extremely long integration-time (9000 s/beam) 21-cm blind survey of 60 square degrees in Centaurus using the Parkes multibeam system. We find that the noise continues to fall as sqrt(t) throughout, enabling us to reach an HI column-density limit of 4.2 x 10^18 cm^-2 for galaxies with a velocity width of 200 km/s in the central 32 square degree region, making this the deepest survey to date in terms of column density sensitivity. The HI data are complemented by very deep optical observations from digital stacking of multi-exposure UK Schmidt Telescope R-band films, which reach an isophotal level of 26.5 R mag arcsec^-2 (~27.5 B mag arcsec^-2). 173 HI sources have been found, 96 of which have been uniquely identified with optical counterparts in the overlap area. There is not a single source without an optical counterpart. Although we have not measured the column-densities directly, we have inferred them from the optical sizes of their counterparts. All appear to have a column-density of N_HI = 10^(20.65 +/- 0.38). This is at least an order of magnitude above our sensitivity limit, with a scatter only marginally larger than the errors on N_HI. This needs explaining. If confirmed it means that HI surveys will only find low surface brightness (LSB) galaxies with high M_HI/L_B. Gas-rich LSB galaxies with lower HI mass to light ratios do not exist. The paucity of low column-density galaxies also implies that no significant population will be missed by the all-sky HI surveys being carried out at Parkes and Jodrell Bank.

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arXiv2003

The 1000 Brightest HIPASS Galaxies: HI Mass Function and Omega_HI

We present a new accurate measurement of the HI mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest HI peak flux densities in the southern hemisphere (Koribalski et al. 2003). This sample spans nearly four orders of magnitude in HI mass (from log M_HI/M_sun=6.8 to 10.6, H0=75) and is the largest sample of HI 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 HI mass function can be fitted satisfactorily with a Schechter function with faint-end slope alpha=-1.30. This slope is found to be dependent on morphological type, with later type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the HI mass function, including peculiar motions of galaxies, large scale structure, selection bias, and inclination effects, and quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: Omega_HI=(3.8 +/- 0.6) x 10^{-4}. Low surface brightness galaxies contribute only 15% to this value, consistent with previous findings.

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arXiv2003

First Results from the HI Jodrell All Sky Survey: Inclination-Dependent Selection Effects in a 21-cm Blind Survey

Details are presented of the HI Jodrell All Sky Survey (HIJASS). HIJASS is a blind neutral hydrogen (HI) survey of the northern sky, being conducted using the multibeam receiver on the Lovell Telescope at Jodrell Bank. HIJASS covers the velocity range -3500 km/s to 10000 km/s with a velocity resolution of 18.1 km/s and a spatial positional accuracy of ~2.5 arcmin. Thus far about 1115 sq deg have been surveyed. We describe the methods of detecting galaxies within the HIJASS data and of measuring their HI parameters. The properties of the resulting HI-selected sample of galaxies are described. Of the 222 sources so far confirmed, 170 (77 per cent) are clearly associated with a previously catalogued galaxy. A further 23 sources (10 percent) lie close (within 6 arcmin) to a previously catalogued galaxy for which no previous redshift exists. A further 29 sources (13 per cent) do not appear to be associated with any previously catalogued galaxy. The distributions of peak flux, integrated flux, HI mass and cz are discussed. We show, using the HIJASS data, that HI self-absorption is a significant, but often overlooked, effect in galaxies with large inclination angles to the line of sight. Properly accounting for it could increase the derived HI mass density of the local Universe by at least 25 per cent. The effect this will have on the shape of the HI Mass Function will depend on how self-absorption affects galaxies of different morphological types and HI masses. We also show that galaxies with small inclinations to the line of sight may also be excluded from HI-selected samples, since many such galaxies will have observed linewidths which are too narrow for them to be distinguished from narrow-band radio frequency interference.

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arXiv2002

Where has all the neutral hydrogen gone?

Our extremely deep survey for extragalactic HI (HIDEEP) finds no intergalactic gas clouds, and no galaxies with HI at inferred global column-densities below 10^20 cm^-2 when we could have detected such objects down to a completeness limit of 4 x 10^18 cm^-2. We speculate that low surface-density hydrogen is either ionised or locked up in ``frozen discs'', i.e. structures where the local Ly-alpha is insufficient to excite the 21-cm transition to a spin-temperature above the cosmic background. Such ``frozen discs'' might be responsible for many QSOALSs.

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arXiv2002

The Large Scale Distribution of Neutral Hydrogen in the Fornax Region

Using HIPASS data, we have searched for HI in a ~25x25 sq.deg. region centred on the Fornax cluster. Within a velocity search range of 300 - 3700 km/s and a lower flux limit of ~40 mJy, 110 galaxies with HI emission were detected, one of which is previously uncatalogued. None of the detections has early-type morphology. Previously unknown velocities for 14 galaxies have been determined, with a further 4 velocity measurements being significantly dissimilar to published values. Identification of an optical counterpart is relatively unambiguous for more than ~90% of our HI galaxies. The galaxies appear to be embedded in a sheet at the cluster velocity which extends for more than 30 deg across the search area. At the nominal cluster distance of ~20 Mpc, this corresponds to an elongated structure more than 10 Mpc in extent. A velocity gradient across the structure is detected, with radial velocities increasing by \~500 km/s from SE to NW. The clustering of galaxies evident in optical surveys is only weakly suggested in the spatial distribution of our HI detections. Our results suggest a considerable deficit of HI-rich galaxies in the centre of the cluster. However, relative to the field, there is a 3(+/-1)-fold excess of HI-rich galaxies in the outer parts of the cluster where galaxies may be infalling towards the cluster for the first time.

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