arXiv subjects

Mike Irwin

Publications and source records attributed to Mike Irwin.

At least 73 records · Page 4Linked to original sources

arXiv2006

A near zero velocity dispersion stellar component in the Canes Venatici dwarf spheroidal galaxy

We present a spectroscopic survey of the newly-discovered Canes Venatici dwarf galaxy using the Keck/DEIMOS spectrograph. Two stellar populations of distinct kinematics are found to be present in this galaxy: an extended, metal-poor component, of half-light radius 7'.8(+2.4/-2.1), which has a velocity dispersion of 13.9(+3.2/-2.5) km/s, and a more concentrated (half-light radius 3'.6(+1.1/-0.8) metal-rich component of extremely low velocity dispersion. At 99% confidence, the upper limit to the central velocity dispersion of the metal-rich population is 1.9 km/s. This is the lowest velocity dispersion ever measured in a galaxy. We perform a Jeans analysis on the two components, and find that the dynamics of the structures can only be consistent if we adopt extreme (and unlikely) values for the scale length and velocity dispersion of the metal-poor population. With a larger radial velocity sample and improved measurements of the density profile of the two populations, we anticipate that it will be possible to place strong constraints on the central distribution of the dark matter in this galaxy.

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arXiv2006

The Monitor project: Rotation of low-mass stars in the open cluster M34

We report on the results of a V and i-band time-series photometric survey of M34 (NGC 1039) using the Wide Field Camera (WFC) on the Isaac Newton Telescope (INT), achieving better than 1% precision per data point for 13 <~ i <~ 17. Candidate cluster members were selected from a V vs V-I colour-magnitude diagram over 14 < V < 24 (0.12 <~ M/Msun <~ 1.0), finding 714 candidates, of which we expect ~ 400 to be real cluster members (taking into account contamination from the field). The mass function was computed, and found to be consistent with a log-normal distribution in dN/dlogM. Searching for periodic variable objects in the candidate members gave 105 detections over the mass range 0.25 < M/Msun < 1.0. The distribution of rotation periods for 0.4 < M/Msun < 1.0 was found to peak at ~ 7 days, with a tail of fast rotators down to periods of ~ 0.8 days. For 0.25 < M/Msun < 0.4 we found a peak at short periods, with a lack of slow rotators (eg. P >~ 5 days, consistent with the work of other authors (eg. Scholz & Eisloffel 2004) at very low masses. Our results are interpreted in the context of previous work, finding that we reproduce the same general features in the rotational period distributions. A number of rapid rotators were found with velocities ~ a factor of two lower than in the Pleiades, consistent with models of angular momentum evolution assuming solid body rotation without needing to invoke core-envelope decoupling.

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arXiv2006

Resolving the Stellar Outskirts of M31 and M33

Many clues about the galaxy assembly process lurk in the faint outer regions of galaxies. The low surface brightnesses of these parts pose a significant challenge for studies of diffuse light, and few robust constraints on galaxy formation models have been derived to date from this technique. Our group has pioneered the use of extremely wide-area star counts to quantitatively address the large-scale structure and stellar content of external galaxies at very faint light levels. We highlight here some results from our imaging and spectroscopic surveys of M31 and M33.

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arXiv2005

Structural parameters for the M31 dwarf spheroidals

(Abridged) The projected structures and integrated properties of the Andromeda I, II, III, V, VI, VII and Cetus dwarf spheroidal galaxies are analysed based upon resolved counts of red giant branch stars. For each object, we have derived isopleth maps, surface brightness profiles, absolute magnitudes, central surface brightnesses, and a large number of other morphological parameters. Our analysis probes to larger radius and fainter surface brightnesses than most previous studies and as a result we find that the galaxies are generally larger and brighter than has previously been recognised. In particular, the luminosity of Andromeda V is found to be consistent with the higher metallicity value which has been derived for it. Andromeda I shows strong evidence of tidal disruption and S-shaped tidal tails are clearly visible. On the other hand, Cetus does not show any evidence of tidal truncation, let alone disruption. Andromeda II shows compelling evidence of a large excess of stars at small radius and suggests that this galaxy consists of a secondary core component. Comparing the M31 dwarf spheroidal population with the Galactic population, we find that the scale radii of the M31 population are larger than those for the Galactic population by at least a factor of two, for all absolute magnitudes. We also find that the two populations are offset from one another in the central surface brightness - luminosity relation. Finally, we find that the M31 dwarf spheroidals show the same correlation with distance-from-host as shown by the Galactic population, such that dwarf spheroidals with a higher central surface brightness are found further from their host. This again suggests that environment plays a significant role in dwarf galaxy evolution.

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arXiv2005

The satellite distribution of M31

(Abridged) The spatial distribution of the Galactic satellite system plays an important role in Galactic dynamics and cosmology, where its successful reproduction is a key test of simulations of galaxy halo formation. Here, we examine its representative nature by conducting an analysis of the 3-dimensional spatial distribution of the M31 subgroup of galaxies. We begin by a discussion of distance estimates and incompleteness concerns, before revisiting the question of membership of the M31 subgroup. Comparison of the distribution of M31 and Galactic satellites relative to the galactic disks suggests that the Galactic system is probably modestly incomplete at low latitudes by ~20%. We find that the radial distribution of satellites around M31 is more extended than the Galactic subgroup; 50% of the Galactic satellites are found within ~100kpc of the Galaxy, compared to ~200kpc for M31. We search for ``ghostly streams'' of satellites around M31, in the same way others have done for the Galaxy, and find several. The lack of M31-centric kinematic data, however, means we are unable to probe whether these streams represent real physical associations. Finally, we find that the M31 satellites are asymmetrically distributed with respect to our line-of-sight to this object, so that the majority of its satellites are on its near side with respect to our line-of-sight. We quantify this result and find it to be significant at the ~3 sigma level. Until such time as a satisfactory explanation for this finding is presented, our results warn against treating the M31 subgroup as complete, unbiased and relaxed.

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arXiv2005

A Minor Axis Surface Brightness Profile for M31

We use data from the Isaac Newton Telescope Wide Field Camera survey of M31 to determine the surface brightness profile of M31 along the south-east minor axis. We combine surface photometry and faint red giant branch star counts to trace the profile from the innermost regions out to a projected radius of 4 degrees (~55 kpc) where the V-band surface brightness is 32 mag per square arcsec; this is the first time the M31 minor axis profile has been mapped over such a large radial distance using a single dataset. We confirm the finding by Pritchet & van den Bergh (1994) that the minor axis profile can be described by a single de Vaucouleurs law out to a projected radius of 1.4 degrees or ~20 kpc. Beyond this, the surface brightness profile flattens considerably and is consistent with either a power-law of index -2.3 or an exponential of scalelength 14 kpc. The fraction of the total M31 luminosity contained in this component is ~2.5%. While it is tempting to associate this outer component with a true Population II halo in M31, we find that the mean colour of the stellar population remains approximately constant at V-i~1.6 from 0.5-3.5 degrees along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components.

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arXiv2005

A Keck/DEIMOS Kinematic Study of Andromeda IX: dark matter on the smallest galactic scales

We present the results of a kinematic survey of the dwarf spheroidal satellite of M31, And-IX, which appears to be the lowest surface brightness and also the faintest galaxy (M_V = -8.3) found to date. Using Keck/DEIMOS spectroscopic data, we have measured its velocity relative to M31, its velocity dispersion, and its metallicity. It exhibits a significant velocity dispersion sigma_v = 6.8+3.0-2.0km/s, which coupled with the low luminosity implies a very high mass to V-band light ratio, M/L ~ 93+120-50msun/lsun (M/L > 17msun/lsun at 99% confidence). Unless strong tidal forces have perturbed this system, this smallest of galaxies is a highly dark matter dominated system.

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arXiv2005

The Stellar Populations of M31 Halo Substructure

We present the first results from our survey of stellar substructure in the outskirts of M31 using the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. We discuss the stellar populations associated with five prominent stellar overdensities discovered during the course of our panoramic ground-based imaging survey with the Isaac Newton Telescope Wide-Field Camera (INT WFC); a sixth pointing targets a region of `clean' halo. The colour-magnitude diagrams (CMDs), which contain between ~10,000--90,000 stars and reach four magnitudes below the horizontal branch, reveal clear variations in morphology between most fields, indicating that the age and/or metallicity mix of stars is not constant at large radius. This directly confirms the existence of large-scale population inhomogeneities within the halo of M31 and lends further support to the notion that M31 has formed, at least in part, through satellite accretions. We find a striking similarity between the populations of the giant stellar stream and those of another overdensity, the NE shelf, which lies north-east of the galaxy center. If these overdensities are associated with the same population, then the difference in their red clump magnitudes implies the NE shelf lies in front of the stream by several tens of kpc, in good agreement with recent orbit calculations for the stream progenitor.

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arXiv2004

A Keck/Deimos Survey of Red Giant Branch Stars in the Outskirts of M31

We are using the DEIMOS multi-object spectrograph on the Keck II 10m telescope to conduct a spectroscopic survey of red giant branch stars in the outskirts of M31. To date, velocities have been obtained for most of the major substructures in the halo as well as at several positions in the far outer disk and inner halo. First results concerning the giant stellar stream and major axis substructures are presented here.

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arXiv2003

CIV Absorbers in z>4 Quasars: Tracing Early Galactic Halos Evolution

We use 29 z>4 quasar spectra to build a homogeneous sample of high-redshift CIV absorbers. We use these data to calculate the number density, n(z), of W_rest(CIV)>0.30 A CIV doublets. We find that n(z) increases with time from z ~ 4.5. In addition, W_rest(CIV)>0.15 A CIV systems are more numerous than the former at all redshifts. On the contrary, n(z) of W_rest(MgII)>0.30 A MgII doublets decreases with time, in agreement with results from studies of LLS number densities. Below z<3, none of all the classes of absorbers show signs of evolution. We interpret this as the formation of galactic envelopes from smaller halos. Furthermore, the doublet ratio DR=W(CIV 1548)/W(CIV 1550) is found to decrease with time, a signature of the increase of the mean CIV column density. Finally, the W(SiIV 1393)/W(CIV 1548) ratios in strong absorbers (W_rest(CIV 1548) > 0.50 A) is found to be approximately constant from z=3.5 to z=2.5 and then to decrease with time. This result suggests that the highest column density absorbers are not sensitive to changes in the ionization continuum.

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arXiv2002

Kinematic outliers in the LMC: constraints on star-star microlensing

Although a decade of microlensing searches towards the Large Magellanic Cloud (LMC) has detected 13-25 possible microlensing events, the nature and the location of the lenses, being either halo machos or LMC stars, remains a subject of debate. The star-star lensing models generically predict the existence of a small population (more than about 5 percent of stars with a spatial and kinematic distribution different from the thin, young disc of the LMC. Here we present the results of a large spectroscopic survey of the LMC, consisting of more than 1300 radial velocities measured accurately with the 2dF instrument. In this large sample, no evidence is found for any extraneous population over the expected LMC and Galactic components. Any additional, kinematically distint, population can only be present at less than the 1 percent level. We discuss the significance of this finding for the LMC self-lensing models.

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arXiv2002

An Optical Velocity for the Phoenix Dwarf Galaxy

We present the results of a VLT observing program 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 program of service observations we obtained long-slit spectra of several members of the Phoenix dwarf galaxy from which we derive an optical helio-centric radial velocity of -13 +/- 9km/s. This agrees very well with the velocity of the most promising of the HI clouds seen around Phoenix, which has a helio-centric velocity of -23 km/s, but is significantly different to the recently published optical heliocentric velocity of Phoenix of -52 +/- 6 km/s of Gallart et al. (2001).

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arXiv2002

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

We report the discovery of significant stellar substructure in the halo and outer disk of our nearest large galactic neighbour, M31. Our deep panoramic survey with the Isaac Newton Telescope Wide Field Camera currently maps out an area of ~25 square degrees around M31, extending along the semi-major 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 colour information) variations, which are often, but not always, correlated. In addition to the previously reported giant stellar stream (Ibata 2001b), the data reveal the presence of significant stellar overdensities at large radii close to the south-western major axis, in the proximity of the very luminous globular cluster G1, and near the north-eastern 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 which 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.

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arXiv2002

The Southern Dwarf Hunt: Local Group Dwarf Candidates in the Southern Sky

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 catalogued) 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.

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arXiv2001

The SuperCOSMOS Sky Survey. Paper II: Image detection, parameterisation, classification and photometry

In this, the second in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the methods for image detection, parameterisation, classification and photometry. We demonstrate the internal and external accuracy of our object parameters. Using examples from the first release of data, the South Galactic Cap survey, we show that our image detection completeness is close to 100% to within 1.5 mag of the nominal plate limits. We show that for the Bj survey data, the image classification is externally > 99% reliable to Bj = 19.5. Internally, the image classification is reliable at a level of > 90% to Bj=21, R=19. The photometric accuracy of our data is typically 0.3 mag with respect to external data for m > 15. Internally, the relative photometric accuracy in restricted position and magnitude ranges can be as accurate as 5% for well exposed stellar images. Colours (B-R or R-I) are externally accurate to 0.07 mag at Bj = 16.5 rising to 0.16 mag at Bj = 20.

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arXiv2001

The SuperCOSMOS Sky Survey. Paper III: Astrometry

In this, the third in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the astrometric properties of the database. We describe the algorithms employed in the derivation of the astrometric parameters of the data, and demonstrate their accuracies by comparison with external datasets using the first release of data, the South Galactic Cap survey. We show that the celestial coordinates, which are tied to the International Celestial Reference Frame via the Tycho-2 reference catalogue, are accurate to better than +/- 0.2 arcsec at J,R=19,18 rising to +/- 0.3 arcsec at J,R=22,21 with positional dependent systematic effects from bright to faint magnitudes at the +/- 0.1 arcsec level. The proper motion measurements are shown to be accurate to typically +/- 10 mas/yr at J,R=19,18 rising to +/- 50 mas/yr at J,R=22,21 and are tied to zero using the extragalactic reference frame. We show that the zeropoint errors in the proper motions are < 1 mas/yr for R > 17 and are no larger than 10 mas/yr for R < 17 mas/yr.

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arXiv2001

Cosmological Evolution of the Universe Neutral Gas Mass Measured by Quasar Absorption Systems

The cosmological evolution of neutral hydrogen is an efficient way of tracing structure formation with redshift. It indicates the rate of evolution of gas into stars and hence the gas consumption and rate star formation history of the Universe. In measuring HI, quasar absorbers have proven to be an ideal tool and we use observations from a recent survey for high-redshift quasar absorption systems together with data gathered from the literature to measure the cosmological comoving mass density of neutral gas. This paper assumes Omega_M=0.3, Omega_lambda=0.7 and h=0.65.

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arXiv2001

Absorption Systems in the Spectra of 66 z > 4 Quasars

We present high signal-to-noise, 5 A resolution (FWHM) spectra of 66 z \ga 4 bright quasars obtained with the 4 m Cerro Tololo Inter-American Observatory and 4.2 m William Hershel telescopes. The primary goal of these observations was to undertake a new survey for intervening absorption systems detected in the spectra of background quasars. We look for both Lyman-limit systems (column densities N(HI) > 1.6 * 10^{17} atoms cm-2) and damped Ly-alpha systems (column densities N(HI) > 2 * 10^{20} atoms cm-2). This work resulted in the discovery of 49 Lyman-limit systems, 15 of which are within 3000 km s-1 of the quasar emission and thus might be associated with the quasar itself, 26 new damped Ly-alpha absorption candidates, 15 of which have z>3.5 and numerous metal absorption systems. In addition ten of the quasars presented here exhibit intrinsic broad absorption lines.

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