Showing papers by "Mike Irwin published in 2002"

Evidence for Stellar Substructure in the Halo and Outer Disk of M31

Abstract: We report the discovery of significant stellar substructure in the halo and outer disk of our nearest large galactic neighbor, M31. Our deep panoramic survey with the Isaac Newton Telescope Wide Field Camera currently maps out an area of ≈25 deg2 around M31, extending along the semimajor axis to 55 kpc and is the first to allow an uninterrupted study of the density and color distribution of individual red giant branch stars across a large fraction of the halo of an external spiral galaxy. We find evidence for both spatial density and metallicity (as inferred from color information) variations, which are often, but not always, correlated. In addition to the previously reported giant stellar stream, the data reveal the presence of significant stellar overdensities at large radii close to the southwestern major axis, in the proximity of the very luminous globular cluster G1, and near the northeastern major axis, coinciding with and extending beyond the previously known northern spur. The most prominent metallicity variations are found in the southern half of the halo, where two large structures with above average metallicites are apparent; one of these coincides with the giant stellar stream while the other corresponds to a much lower level stellar enhancement. Our findings contrast with, but do not conflict with, past studies of the M31 halo and outer disk that have suggested a rather homogeneous stellar population at large radius: the bulk of our newly detected substructure lies in the previously uncharted far outer regions of the galaxy. We discuss the possible origin of the substructure observed and the implications it has for constraining the galaxy assembly process.

Uncovering cold dark matter halo substructure with tidal streams

Abstract: Models for the formation and growth of structure in a cold-dark-matter-dominated universe predict that galaxy haloes should contain significant substructure. Studies of the Milky Way, however, have yet to identify the expected few hundred subhaloes with masses greater than about 106 M⊙. Here we propose a test for the presence of subhaloes in the haloes of galaxies. We show that the structure of the tidal tails of ancient globular clusters is very sensitive to heating by repeated close encounters with the massive dark subhaloes. We discuss the detection of such an effect in the context of the next generation of astrometric missions, and conclude that it should be easily detectable with the GAIA data set. The finding of a single extended cold stellar stream from a globular cluster would support alternative theories, such as self-interacting dark matter, that give rise to smoother haloes.

The Kinematics and Metallicity of the M31 Globular Cluster System

Abstract: With the ultimate aim of distinguishing between various models describing the formation of galaxy halos (e.g., radial or multiphase collapse and random mergers), we have completed a spectroscopic study of the globular cluster system of M31. We present the results of deep intermediate-resolution fiber-optic spectroscopy of several hundred of the M31 globular clusters using the Wide Field Fibre Optic Spectrograph at the William Herschel Telescope in La Palma, Canary Islands. These observations have yielded precise radial velocities (±12 km s-1) and metallicities (±0.26 dex) for over 200 members of the M31 globular cluster population out to a radius of 15 from the galaxy center. Many of these clusters have no previous published radial velocity or [Fe/H] estimates, and the remainder typically represent significant improvements over earlier determinations. We present analyses of the spatial, kinematic, and metal abundance properties of the M31 globular clusters. We find that the abundance distribution of the cluster system is consistent with a bimodal distribution with peaks at [Fe/H] ~ -1.4 and -0.5. The metal-rich clusters demonstrate a centrally concentrated spatial distribution with a high rotation amplitude, although this population does not appear significantly flattened and is consistent with a bulge population. The metal-poor clusters tend to be less spatially concentrated and are also found to have a strong rotation signature.

The distant type Ia supernova rate

Abstract: We present a measurement of the rate of distant Type Ia supernovae derived using 4 large subsets of data from the Supernova Cosmology Project. Within this fiducial sample,which surveyed about 12 square degrees, thirty-eight supernovae were detected at redshifts 0.25--0.85. In a spatially flat cosmological model consistent with the results obtained by the Supernova Cosmology Project, we derive a rest-frame Type Ia supernova rate at a mean red shift z {approx_equal} 0.55 of 1.53 {sub -0.25}{sub -0.31}{sup 0.28}{sup 0.32} x 10{sup -4} h{sup 3} Mpc{sup -3} yr{sup -1} or 0.58{sub -0.09}{sub -0.09}{sup +0.10}{sup +0.10} h{sup 2} SNu(1 SNu = 1 supernova per century per 10{sup 10} L{sub B}sun), where the first uncertainty is statistical and the second includes systematic effects. The dependence of the rate on the assumed cosmological parameters is studied and the redshift dependence of the rate per unit comoving volume is contrasted with local estimates in the context of possible cosmic star formation histories and progenitor models.

The distant Type Ia supernova rate

Abstract: We present a measurement of the rate of distant Type Ia supernovae derived using 4 large subsets of data from the Supernova Cosmology Project. Within this fiducial sample, which surveyed about 12 square degrees, thirty-eight supernovae were detected at redshifts 0.25--0.85. In a spatially-flat cosmological model consistent with the results obtained by the Supernova Cosmology Project, we derive a rest-frame Type Ia supernova rate at a mean redshift $z\simeq0.55$ of $1.53 {^{+0.28}_{-0.25}} {^{+0.32}_{-0.31}} 10^{-4} h^3 {\rm Mpc}^{-3} {\rm yr}^{-1}$ or $0.58 {^{+0.10}_{-0.09}} {^{+0.10}_{-0.09}} h^2 {\rm SNu}$ (1 SNu = 1 supernova per century per $10^{10}$\Lbsun), where the first uncertainty is statistical and the second includes systematic effects. The dependence of the rate on the assumed cosmological parameters is studied and the redshift dependence of the rate per unit comoving volume is contrasted with local estimates in the context of possible cosmic star formation histories and progenitor models.

The Carlsberg Meridian Telescope CCD drift scan survey

Abstract: This paper contains the general data reduction methods used in processing the data from the Carlsberg Meridian Telescope CCD drift scan survey. An efficient method to calibrate the fluctuations in the positions of the images caused by atmospheric turbulence is described. The external accuracy achieved is 36 mas in right ascension and declination. A description of the recently released catalogue is given.

Substructure of the outer Galactic halo from the 2-Micron All-Sky Survey

Abstract: A pole-count analysis of the infrared 2-Micron All-Sky (2MASS) survey is presented, in order to identify faint stream-like great-circle structures within the halo of the Milky Way. Selecting stars with colours consistent with M-giant stars, we find a strong overdensity of sources on a stream with a pole (l=95°, b=13°), which corresponds to the pole of the orbit of the Sagittarius dwarf galaxy. This great-circle feature of width ∼12° contains ∼5 per cent of the late M-giants in the halo. No other stream-like structures are detected in M-giants in the 2MASS Second Incremental Data Release (2IDR), and in particular, we find no evidence for a stellar component to the Magellanic Stream. This suggests that the present accretion rate of low-mass satellites with a luminous component is very low, and the formation of the luminous component of the halo must have been essentially complete before the accretion of the Sagittarius dwarf galaxy, more than 3 Gyr ago. We also search for great-circle streams using almost all high-latitude (∣b∣>30°) sources in the 2IDR data set. No narrow great-circle streams of width 0.5°–2° were found, though we were only sensitive to relatively nearby (<17-kpc) remnants of massive (10 6 -M ⊙ ) globular clusters. If the Galactic potential is close to being spherical, as some recent observations suggest, the lack of observed great-circle streams is consistent with the presence of dark matter substructures in the halo. Although alternative explanations cannot be ruled out from our analysis of the 2IDR data set, future experiments with better statistics have the potential to reveal the heating effect of dark matter substructure on stellar streams.

A New Wide Pair of Cool White Dwarfs in the Solar Neighborhood

Abstract: We report the discovery of a wide pair (93'' angular separation) of extremely cool (Teff < 4000 K) white dwarfs with a very large common proper motion (~19 yr-1). The objects were discovered in a high proper motion survey in the poorly investigated southern sky region with δ < -60° using SuperCOSMOS Sky Survey (SSS) data. Both objects, SSSPM J2231-7514 and SSSPM J2231-7515, show featureless optical spectra. Fits of blackbody models to the spectra yield effective temperatures of 3810 and 3600 K, respectively, for the bright (V = 16.60) and faint (V = 16.87) components. Both degenerates are much brighter than other recent discoveries of cool white dwarfs with comparable effective temperatures and/or BJ-R colors. Therefore, they should be relatively nearby objects. The comparison with other cool white dwarfs and a photometric distance determination yield distance estimates between 9 and 14 pc. The latter seems to be more realistic since the good agreement of the proper motion of both components within the errors of about 8 mas yr-1 and the angular separation between the two stars support a distance of about 15 pc, with relatively small masses of the components. With a smaller distance we should be able to measure a differential proper motion due to orbital motion if the orbital plane is not strongly inclined and if the present orbital velocity vector is not close to the line of sight. The space velocity based on that distance and assumptions on radial velocity make the new pair of extremely cool white dwarfs probably some of the oldest members of the Galactic disk population, although the possibility that these objects are part of a Galactic halo dark matter component also cannot yet be ruled out.

An optical velocity for the Phoenix dwarf galaxy

Abstract: We present the results of a Very Large Telescope observing programme carried out in service mode using FORS1 on ANTU in long-slit mode to determine the optical velocities of nearby low surface brightness galaxies. As part of our programme of service observations we obtained long-slit spectra of several members of the Phoenix dwarf galaxy from which we derive an optical heliocentric radial velocity of-13 +/- 9 km s(-1) . This agrees very well with the velocity of the most promising of the H i clouds seen around Phoenix, which has a heliocentric velocity of -23 km s(-1) , but is significantly different from the recently published optical heliocentric velocity of Phoenix of -52 +/- 6km s(-1) by Gallart et al.

The southern dwarf hunt: local group dwarf candidates in the southern sky

Abstract: We present observations of 82 Local Group dwarf galaxy candidates, of which 62 were chosen visually from ESO-SRC survey plates of the southern sky (32 of which were not previously cataloged) and the rest suggested by various sources in the literature. Two are the Local Group galaxies Antlia and Cetus; nine are more distant galaxies, though still within a few megaparsecs; 45 are background galaxies; seven are planetary (or other emission) nebulae; 15 are reflection or other Galactic nebulae; two are galaxy clusters; one is a Galactic star cluster; and one is a misidentified star. We conclude that there is no large population of faint Local Group dwarf galaxies of any familiar type awaiting discovery. We point out the danger of relying on a single type of data to reach conclusions about an object.

New nearby stars selected in a high proper motion survey by DENIS photometry

Abstract: We present new nearby stars extracted from a proper motion catalogue and having a DENIS counterpart. Their distances and spectral type are estimated using the DENIS colours. 107 stars are within 50 pc. 31 stars among them have previously measured distances. In addition, 40 stars may enter within the 50 pc limit depending on which population they belong to. 6 stars among them have already measured distances. 5 objects, LHS5045, L225-57, LP831-45, LHS1767, and WT792, are probably closer than 15 pc, with L225-57 at 9.5 pc. Most of these stars are M-type while 4 stars are white dwarfs. 88 M-dwarfs are disc stars, 14 belong to the thick disc and 1 to the spheroid.

New nearby stars selected in a high proper motion survey by DENIS photometry

Abstract: We present new nearby stars extracted from a proper motion catalogue and having a DENIS counterpart. Their distances and spectral type are estimated using the DENIS colours. 107 stars are within 50 pc. 31 stars among them have previously measured distances. In addition, 40 stars may enter within the 50 pc limit depending on which population they belong to. 6 stars among them have already measured distances. 5 objects, LHS5045, LP225-57, LP831-45, LHS1767, and WT792, are probably closer than 15 pc, with LP225-57 at 9.5 pc. Most of these stars are M-type while 4 stars are white dwarfs. 88 M-dwarfs are disc stars, 14 belong to the thick disc and 1 to the spheroid.

New nearby stars among bright APM high proper motion stars

Abstract: As part of a new southern sky survey for faint high proper motion stars based on Automatic Plate Measuring (APM) measurements of UK Schmidt Telescope plates, we have found a large number of previously unknown brighter objects. Spectroscopic follow-up observations with the European Southern Observatory New Technology Telescope of 15 of these new, relatively bright (12 0.45 arcsec yr - 1 ) show one-third of them to be nearby (d < 25 pc). Among the nearby stars is an M6 dwarf with strong emission lines at a spectroscopic distance of about 11 pc and an M4 dwarf at about 13 pc. Coupled with earlier South African Astronomical Observatory spectroscopic observations of three similar bright high proper motion stars, the success rate of finding nearby stars (d < 25 pc) is about 45 per cent. All newly discovered nearby stars have disc kinematics confirmed by radial velocity measurements from our spectra. In addition there are several high-velocity stars with halo kinematics in the sample, mainly subdwarfs, at about 60 to 110 pc distance. These high-velocity stars are interesting targets for further study of the Galactic escape velocity. One of the detected nearby high proper motion stars was formerly thought to be an M giant in the Small Magellanic Cloud. The spectrum of one M3 star shows a strong blue continuum, which is likely to signify the presence of a hot companion. Spectroscopic follow-up observations of high proper motion stars are shown to be an effective tool in the search for the missing stars in the Solar neighbourhood. Candidates for more extensive trigonometric parallax determination can be selected on the basis of the spectroscopic distance estimates.

QSOs from the variability and proper motion survey in the M 3 field

Abstract: We present results of the spectroscopical follow-up observations of QSO candidates from a combined variability and proper motion (VPM) survey in a � 10 square degrees region centered on the globular cluster M3. The search is based on a large number of digitised Schmidt plates with a time-baseline of three decades. This paper reviews the candidate selection, the follow-up spectroscopy, and general properties of the resulting QSO sample. In total, 175 QSOs and Sey1s were identified among the objects from the VPM survey, with 114 QSOs and 10 Sey1s up to the pre-estimated 90% completeness limit of the survey at Blim � 19.7. The redshift range of the QSOs is 0.4 < z < 3. Among the 80 QSO candidates of highest priority we confirm 75 QSOs/Sey1s and 2 NELGs. We present magnitudes, colours, redshifts, and variability indices for all 181 identified QSOs/Sey1s/NELGs and spectra for the 77 QSOs/Sey1s/NELGs from our spectroscopic follow-up observations. The VPM survey uses selection criteria that are not directly relying on the spectral energy distribution of QSOs. It is therefore remarkable that the properties of the VPM QSOs do not significantly differ from those of samples from colour selection or slitless spectroscopy. In particular, we do not detect a substantial number of unusually red QSOs. The total surface density of the brighter QSOs (17 � B � 18.5) in our search field is found to be by a factor of � 1.8 larger than that derived from previous optical surveys.

A search for star formation around the Galactic halo B-type star PHL 346 with the 2dF spectrograph

Abstract: A preliminary search for stars that may have formed coevally with the apparently young halo B-type star PHL 346 has been performed with the 2dF multifibre spectrograph on the Anglo-Australian Telescope (AAT). Candidates were selected for spectroscopy from APM scans of B and R Schmidt plates centred on PHL 346. A total of 476 stars of spectral type A or F were found; radial velocity estimates and more accurate spectral type assignments narrowed the number of possible coeval candidates to 6 A-type and 14 F-type stars. A statistical analysis of these results using a comparison with a control field suggests that the number of A-type or F-type candidate stars around PHL 346 is not unexpected, and that they need not be associated with PHL 346. A number of ways to improve the project are suggested.

QSOs from a Variability-and-Proper Motion Survey

Abstract: We report on a search for QSOs via variability and zero-proper motion. Candidates were selected by means of indices for overall variability, long-term variability, proper motion and image structure measured on a large number of Schmidt plates with a time-baseline of three decades. Spectroscopic follow-up observations of the candidates brighter than the pre-estimated completeness limit, B lim ≈ 19.7, of the survey revealed about 200 QSOs and Seyfert 1s with redshifts z ≈ 0 — 3. We describe the survey strategy and discuss the properties of the resulting QSO sample.

The Kinematics and Metallicity of the M31 Globular Cluster System

Abstract: With the ultimate aim of distinguishing between various models describing the formation of galaxy halos (e.g. radial or multi-phase collapse, random mergers), we have completed a spectroscopic study of the globular cluster system of M31. We present the results of deep, intermediate-resolution, fibre-optic spectroscopy of several hundred of the M31 globular clusters using the Wide Field Fibre Optic Spectrograph (WYFFOS) at the William Herschel Telescope in La Palma, Canary Islands. These observations have yielded precise radial velocities (+/-12 km/s) and metallicities (+/-0.26 dex) for over 200 members of the M31 globular cluster population out to a radius of 1.5 degrees from the galaxy center. Many of these clusters have no previous published radial velocity or [Fe/H] estimates, and the remainder typically represent significant improvements over earlier determinations. We present analyses of the spatial, kinematic and metal abundance properties of the M31 globular clusters. We find that the abundance distribution of the cluster system is consistent with a bimodal distribution with peaks at [Fe/H] = -1.4 and -0.5. The metal-rich clusters demonstrate a centrally concentrated spatial distribution with a high rotation amplitude, although this population does not appear significantly flattened and is consistent with a bulge population. The metal-poor clusters tend to be less spatially concentrated and are also found to have a strong rotation signature.

AAT/WFI Observations of the Extragalactic HI Cloud HIPASS J1712-64

Abstract: AAT/WFI optical images of a candidate extragalactic HI cloud, HIPASS J1712–64, are presented. The g and r band CCD mosaic camera frames were processed using a new data pipeline recently installed at the AAO. The resultant stacked images reach significantly deeper levels than those of previous published optical imaging of this candidate, providing a detection limit M g ~ −7 at a distance of 3 Mpc, the inferred distance to HIPASS J1712–64. However, detailed analysis of the images fails to uncover any stellar population associated with the HI emission. If this system is a member of the Local Group then it is pathologically different to other members. Hence, our observations reinforce earlier suggestions that this HI cloud is most likely Galactic in origin and not a Local Volume dwarf galaxy.

AAT/WFI observations of the Extragalactic H I Cloud HIPASS J1712-64

Abstract: AAT/WFI optical images of a candidate extragalactic HI cloud, HIPASS J1712-64, are presented. The g and r-band CCD mosaic camera frames were processed using a new data pipeline recently installed at the AAO. The resultant stacked images reach significantly deeper levels than those of previous published optical imaging of this candidate, providing a detection limit M_g -7 at a distance of 3Mpc, the inferred distance to HIPASS J1712-64. However, detailed analysis of the images fails to uncover any stellar population associated with the HI emission. If this system is a member of the Local Group then it is pathologically different to other members. Hence, our observations reinforce earlier suggestions that this HI cloud is most likely Galactic in origin and not a Local Volume dwarf galaxy.

The Hubble Diagram of Type Ia Supernovae as a Function of Host Galaxy Morphology

Abstract: (Abridged) We present new results on the Hubble diagram of distant type Ia supernovae (SNe Ia) segregated according to the type of host galaxy. This makes it possible to check earlier evidence for a cosmological constant by explicitly comparing SNe residing in galaxies likely to contain negligible dust with the larger sample. The cosmological parameters derived from these SNe Ia hosted by presumed dust-free early-type galaxies supports earlier claims for a cosmological constant, which we demonstrate at 5 sigma significance, and the internal extinction implied is small even for late-type systems (A_B<0.2). Thus, our data demonstrate that host galaxy extinction is unlikely to systematically dim distant SNe Ia in a manner that would produce a spurious cosmological constant. We classify the host galaxies of 39 distant SNe discovered by the Supernova Cosmology Project (SCP) using the combination of HST STIS imaging, Keck ESI spectroscopy and ground-based broad-band photometry. We compare with a low-redshift sample of 25 SNe Ia. The scatter observed in the SNe Ia Hubble diagrams correlates closely with host galaxy morphology. We find the scatter in the SNe Ia Hubble diagram is smallest for SNe occurring in early-type hosts and largest for those occurring in late-type galaxies. Moreover, SNe residing in early-type hosts appear only ~0.14+/-0.09 mag brighter in their light-curve-width-corrected luminosity than those in late-type hosts, implying only a modest amount of dust extinction even in the late-type systems.

The WFCAM/UKIDSS data archive: problems and opportunities

Abstract: The UKIRT Wide Field Camera (WFCAM) is an IR mosaic camera that represents an enormous leap in deep IR survey capability. It will be used as both an open time facility, and to perform a public IR Deep Sky Survey (the UKIDSS project), starting in early 2004. Here we present current plans for the data archive system, which will be provided as a standard service for all UK WFCAM data whether private or public survey data. The data rate is an order of magnitude larger than any previous survey experiment. WFCAM is therefore a crucial stepping stone between current day surveys such as SuperCOSMOS, APM and SDSS, and future facilities such as VISTA and the LSST. Pipeline processing presents a technical challenge, but the strongest challenges come in operation and curation of such a pipeline and of the rapidly accumulating database. For the public archive, there is little technical challenge in simply storing the data, and the real challenge comes in the rapidly increasing expectations of the user community for the kind of on-line services available with the archive. We describe three levels of archive service and the challenges they present, and discuss the hardware and software solutions we are likely to deploy.

An Automated Search for Extrasolar Planet Transits

Abstract: We are setting up a new search for transiting extrasolar planets using the 0.5m Automated Patrol Telescope at Siding Spring Observatory, Australia. We will begin regular observations in September 2002. We expect to find ~7 new planets per year.

Massive Stars in the SMC: A Spectroscopic Investigation of the IMF

Abstract: The relative proximity and low metallicity of the Magellanic Clouds permit observations of resolved stellar populations in environments different to those found in the Galaxy. Using the multi-fibre 2dF spectrograph at the Anglo-Australian Telescope we have obtained over 4000 intermediate-resolution blue spectra of the massive-star population of the SMC. The spectra were visually classified in the MK system and used to construct an observed H-R diagram (Figure 1). This represents nearly a tenfold increase in the number of stars with spectral types in the SMC (cf. Massey, 2002).