arXiv subjects

Rosemary F. G. Wyse

Publications and source records attributed to Rosemary F. G. Wyse.

At least 109 records · Page 6Linked to original sources

arXiv2004

Galactic Structure

Our Milky Way Galaxy is a typical large spiral galaxy, representative of the most common morphological type in the local Universe. We can determine the properties of individual stars in unusual detail, and use the characteristics of the stellar populations of the Galaxy as templates in understanding more distant galaxies. The star formation history and merging history of the Galaxy is written in its stellar populations; these reveal that the Galaxy has evolved rather quietly over the last ~10 Gyr. More detailed simulations of galaxy formation are needed, but this result apparently makes our Galaxy unusual if Lambda CDM is indeed the correct cosmological paradigm for structure formation. While our Milky Way is only one galaxy, a theory in which its properties are very anomalous most probably needs to be revised. Happily, observational capabilities of next-generation facilities should, in the the forseeable future, allow the aquisition of detailed observations for all galaxies in the Local Group.

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arXiv2003

The Millennium Galaxy Catalogue: Star counts and the Structure of the Galactic Stellar Halo

We derive a star catalogue generated from the images taken as part of the 37.5 sq. deg Millennium Galaxy Catalogue. These data, alone and together with colours gained from the Sloan Digital Sky Survey Early Data Release, allow the analysis of faint star counts (B(MGC) < 20) at high Galactic latitude (41 < b < 63), as a function of Galactic longitude (239 < l < 353). We focus here on the inner stellar halo, providing robust limits on the amplitude of substructure and on the large-scale flattening. In line with previous results, the thick disk, an old, intermediate-metallicity population, is clearly seen in the colour-magnitude diagram. We find that the Galactic stellar halo within ~10 kpc (the bulk of the stellar mass) is significantly flattened, with an axial ratio of (c/a) =0.56 +/- 0.01, again consistent with previous results. Our analysis using counts-in-cells, angular correlation functions and the Lee 2D statistic, confirms tidal debris from the Sagittarius dwarf but finds little evidence for other substructure in the inner halo, at heliocentric distances of < 5 kpc. This new quantification of the smoothness in coordinate space limits the contribution of recent accretion/disruption to the build-up of the bulk of the stellar halo.

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arXiv2003

Galaxy Populations and Evolution in Clusters IV: Deep HI Observations of Dwarf Ellipticals in the Virgo Cluster

We present in this paper the deepest Arecibo HI observations of Virgo cluster dwarf ellipticals (dEs) taken to date. Based on this data we argue that a significant fraction of Virgo cluster dEs recently underwent evolution. Our new observations consist of HI 21-cm line observations for 22 classified dE galaxies with optical radial velocities consistent with membership in the Virgo cluster. Cluster members VCC 390 and VCC 1713 are detected with HI masses M(HI) = 6*10^7 M_solar and 8*10^7 M_solar, respectively, while M(HI) in the remaining 20 dE galaxies have upper limits as low as 5*10^5 M_solar. We combine our results with those for 27 other Virgo cluster dEs with HI observations in the literature, 7 of which have HI detection claims. New optical images from the WIYN telescope of 5 of these HI-detected dEs, along with archival data, suggest that seven of the claimed detections are real, yielding a ~ 15% detection rate. These HI-detected classified dEs are preferentially located near the periphery of the Virgo cluster. Three Virgo dEs have observed HI velocity widths > 200 km/s, possibly indicating the presence of a large dark matter content, or transient extended HI. We discuss the possible origins of these objects and argue that they originate from field galaxies accreted onto high angular momentum orbits by Virgo in the last few Gyr. As a result these galaxies are slowly transformed within the cluster by gradual gas stripping processes, associated truncation of star formation, and passive fading of stellar populations. Low-mass early-type cluster galaxies are therefore currently being produced as the product of cluster environmental effects. We utilize our results to estimate the recent (past 1-3 Gyr) average mass accretion rate into the Virgo cluster, finding dM/dt ~ 50 M_solar/year.

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arXiv2002

Galaxy Populations and Evolution in Clusters III. The Origin of Low-Mass Galaxies in Clusters: Constraints from Stellar Populations

Low-mass galaxies in nearby clusters are the most numerous galaxy type in the universe, yet their origin and properties remain largely unknown. To study basic questions concerning these galaxies we present the results of a survey designed to constrain the characteristics and properties of the stellar populations in a magnitude complete sample of low-mass cluster galaxies (LMCGs) in the center of the Perseus cluster. Using deep, high-quality WIYN UBR images, we demonstrate that the 53 LMCGs in our sample have a significant scatter about the color-magnitude relationship at M_B > -15. By comparing single-burst stellar population models to our photometry, we argue that stellar populations in LMCGs all have ages > 1 Gyrs, with redder LMCGs having stellar metallicities [Fe/H] > -0.5. By assuming that the colors of LMCGs reflect metallicity, and have co-evolved with the giant ellipticals, we find a wide range of values, from solar to [Fe/H] ~ -3. We argue from this that LMCGs have multiple origins, and fundamentally differ from Local Group dwarf spheroidals/ellipticals. The inferred lower metallicities of the bluer LMCGs implies that these are possibly primordial galaxies formed through self enrichment and stellar feedback provided by winds from supernova. We also investigate several other formation scenarios for these LMCGS, including: self enrichment induced by the confinement of metals in halos by the intracluster medium, in situ formation out of intracluster gas, systems with extreme dark halos, and as remnants of higher mass systems. We conclude that roughly half of all low-mass cluster galaxies have stellar populations and kinematic properties, discussed in previous papers in this series, consistent with being the remnants of stripped galaxies accreted into clusters a few Gyrs ago.

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arXiv2002

Deciphering The Last Major Invasion of the Milky Way Galaxy

We present first results from a spectroscopic survey of 2000 F/G stars 0.5--5kpc from the Galactic plane, obtained with the 2dF facility on the AAT. These data show the mean rotation velocity of the thick disk about the Galactic center a few kpc from the plane is very different than expectation, being about 100km/s, rather than the predicted ~180km/s. We propose that our sample is dominated by stars from a disrupted satellite which merged with the disk of the Milky Way Galaxy some 10-12Gyr ago. We do not find evidence for the many substantial mergers expected in hierarchical clustering theories. We find yet more evidence that the stellar halo retains kinematic substructure, indicative of minor mergers

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arXiv2002

Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy: Implications for the Low-Mass Stellar Initial Mass Function at High Redshift

The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population - all of the stars being old and metal-poor - similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi Sph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to 0.3 solar masses. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way.

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arXiv2002

M31, M33 and the Milky Way: Similarities and Differences

The large galaxies in the Local Group, while all disk galaxies, have diverse stellar populations. A better understanding of these differences, and a physical understanding of the causes, requires more detailed study of the older populations. This presents a significant challenge to GAIA but the scientific returns are also significant.

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arXiv2002

The Dynamical Evolution of Substructure

The evolution of substructure embedded in non-dissipative dark halos is studied through N-body simulations of isolated systems, both in and out of initial equilibrium, complementing cosmological simulations of the growth of structure. We determine by both analytic calculations and direct analysis of the N-body simulations the relative importance of various dynamical processes acting on the clumps, such as the removal of material by global tides, clump-clump heating, clump-clump merging and dynamical friction. Our comparison between merging and disruption processes implies that spiral galaxies cannot be formed in a proto-system that contains a few large clumps, but can be formed through the accretion of many small clumps; elliptical galaxies form in a more clumpy environment than do spiral galaxies. Our results support the idea that the central cusp in the density profiles of dark halos is the consequence of self-limiting merging of small, dense halos. This implies that the collapse of a system of clumps/substructure is not sufficient to form a cD galaxy, with an extended envelope; plausibly subsequent accretion of large galaxies is required. Persistent streams of material from disrupted clumps can be found in the outer regions of the final system, and at an overdensity of around 0.75, can cover 10% to 30% of the sky.

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arXiv2002

Galaxy Populations and Evolution in Clusters II: Defining Cluster Populations

This paper presents quantitative techniques for studying, in an unbiased manner, the photometric and structural properties of galaxies in clusters, including a means to identify likely background objects in the absence of redshift information. We develop self-consistent and reproducible measurements of fundamental properties of galaxies such as radius, surface brightness, concentration of light and structural asymmetry. We illustrate our techniques through an application to deep UBR images, taken with the WIYN 3.5m telescope, of the central ~173 arcmin^2 (or 0.3 Mpc * 0.3 Mpc) of the cluster Abell 0146 (Perseus). Our techniques allow us to study the properties of the galaxy population in the center of Perseus down to M_B = -11. Using these methods, we describe and characterize a well-defined relation between absolute magnitude and surface brightness for galaxy cluster members across the entire range of galaxy luminosity from M_B = -20 to M_B = -11. The galaxies that are assigned by our techniques to the background show no such tight relationship between apparent magnitude and surface brightness, with the exception of those we identify as being members of a background cluster of galaxies at z ~ 0.55. We, however, find that at the fainter magnitudes, M_B > -16, there is a large scatter about the underlying color--magnitude relation defined by the brighter galaxies. Our analysis also indicates that the vast majority of Perseus galaxies are `normal', with little evidence for features associated with evolution; we however discuss the detailed properties of a handful of unusual galaxies. Finally, the galaxy luminosity function of the Perseus cluster center is computed, with a derived faint end slope of alpha = -1.44+/-0.04, similar to values found in other nearby clusters.

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arXiv2001

Populations of Dwarfs in Clusters of Galaxies: Environmental Connections

Despite their apparent fragile appearance, dwarf spheroidals are the most common galaxy type in clusters. In this paper we consider some of the issues associated with two major models for the origin of these dwarfs: primeval galaxies which formed with the cluster and the modification of accreted systems. We argue that the present observational evidence, derived from the Virgo and Perseus clusters, points to infall as the origin of many of these objects.

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arXiv2001

On the Nature of the NGC 1275 System

Sub-arcsecond images, taken in B, R, and H-Alpha filters, and area spectroscopy obtained with the WIYN 3.5-m telescope provide the basis for an investigation of the unusual structures in the stellar body and ionized gas in and around the Perseus cluster central galaxy, NGC 1275. Our H-Alpha filter is tuned to gas at the velocity of NGC 1275, revealing complex, probably unresolved, small-scale features in the extended ionized gas, located up to 50/h kpc from NGC 1275. The mean H-Alpha surface brightness varies little along the outer filaments; this, together with the complex excitation state demonstrated by spectra, imply that the filaments are likely to be tubes, or ribbons, of gas. The morphology, location and inferred physical parameters of the gas in the filaments are consistent with a model whereby the filaments form through compression of the intracluster gas by relativistic plasma emitted from the active nucleus of NGC 1275. Imaging spectroscopy with the Densepak fiber array on WIYN suggests partial rotational support of the inner component of low velocity ionized gas. We confirm and extend evidence for features in the stellar body of NGC 1275, and identify outer stellar regions containing very blue, probably very young, star clusters. We interpret these as evidence for recent accretion of a gas-rich system, with subsequent star formation. We suggest that two main processes, which may be causally connected, are responsible for the rich phenomenology of the NGC 1275 system -- NGC 1275 experienced a recent merger/interaction with a group of gas-rich galaxies, and recent outflows from its AGN have compressed the intracluster gas, and perhaps the gas in the infalling galaxies, to produce a complex web of filaments. (Abridged)

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arXiv2001

Dynamics and the Origin of Low-Mass Galaxies in the Virgo Cluster

Early-type dwarfs are the most common galaxy in the local universe, yet their origin and evolution remain a mystery. Various cosmological scenarios predict that dwarf-like galaxies in dense areas are the first to form and hence should be the oldest stellar systems in clusters. By using radial velocities of early-type dwarfs in the Virgo cluster we demonstrate that these galaxies are not an old cluster population but have signatures of production from the infall of field galaxies. Evidence of this includes the combined large dispersions and sub-structure in spatial and kinematic distributions for Virgo early-type dwarfs, and a velocity dispersion ratio with giant ellipticals expected for virialized and accreted populations. We also argue that these galaxies cannot originate from accreted field dwarfs, but must have physically evolved from a precursor population, of different morphology, that fell into Virgo some time in the past.

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arXiv2001

The Thick Disk-Halo Interface

The star formation history of a galaxy, explicitely here our Milky Way Galaxy, where the most detailed information is attainable, is the convolution of two functions. One function describes the rate of formation of the stars which are today in the Galaxy. The second describes the assembly of those stars into the present Galactic potential well. There is direct evidence that this assembly continues today, with both stars and gas being assembled into, or at least rearranged in, the Galactic potential. But is this recent accretion significant? Was the last significant accretion the formation of the thick disk, some 10Gyr ago?. Recent spectroscopic studies support this unexpected result, while dynamical studies find increasing numbers of specific examples of smaller scale more recent accretion. We present early results for one specific such survey, the Anglo Australian Old Stellar populations Survey, to illustrate current studies.

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arXiv2000

The Merging History of the Milky Way Disk

The stellar populations of the stellar halo and of the thick disk of the Milky Way reveal much about the merging history that our Galaxy, a typical large disk galaxy, has experienced. Our current understanding, described here, implies a rather quiet evolution for the disk, back to redshifts of order 2.

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arXiv2000

HST Color-Magnitude Data for Globular Clusters: I. Transformations Between STIS LP Magnitudes and WFPC2 F606W and F814W Magnitudes

We present deep Hubble Space Telescope optical imaging observations of two Galactic globular clusters, 47 Tuc and M15, using the Space Telescope Imaging Spectrograph (STIS) longpass (LP) filter and the Wide Field and Planetary Camera 2 (WFPC2) F606W and F814W filters. These globular clusters have very different metallicities ([Fe/H] = -0.7 dex for 47 Tuc, and [Fe/H] = -2.2 dex for M15), essentially spanning the metallicity distribution of the entire Milky Way globular cluster system, and were chosen to investigate the relationship between magnitudes in the non-standard STIS LP system and the better characterized WFPC2 magnitudes. We determine this relationship by combining our new STIS data with archival WFPC2 data, taking care to provide a robust and reliable transformation. Examining the 47 Tuc and M15 data separately, there is no evidence for a metallicity dependence in the transformation relations. This allows us to combine the data for both clusters, and also include the stars in the Small Magellanic Cloud that lie within the field of view of the 47 Tuc images, to determine final transformations that are valid for main sequence stars with colors in the range 0.3 < V606-I814 < 1.8. These final relations predict STIS magnitudes within 0.05 mag (1 sigma) of those measured for stars in both clusters. In an appendix, we discuss the differences between our empirical transformation relations and those predicted by synthetic photometry.

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arXiv2000

Where Do We Stand?

I review the understanding of bulges that emerged from the lively discussions and presentations during the meeting, and emphasize areas for future work. The evidence is for a diversity of `bulges', and of formation mechanisms.

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arXiv2000

The Formation of the Hubble Sequence of Disk Galaxies: The Effects of Early Viscous Evolution

We investigate a model of disk galaxies whereby viscous evolution of the gaseous disk drives material inwards to form a proto-bulge. We start from the standard picture of disk formation through the settling of gas into a dark halo potential well, with the disk initially coming into centrifugal equilibrium with detailed conservation of angular momentum. We derive generic analytic solutions for the disk-halo system after adiabatic compression of the dark halo, with free choice of the input virialized dark halo density profile and of the specific angular momentum distribution. We derive limits on the final density profile of the halo in the central regions. Subsequent viscous evolution of the disk is modelled by a variation of the specific angular momentum distribution of the disk, providing analytic solutions to the final disk structure. The assumption that the viscous evolution timescale and star formation timescale are similar leads to predictions of the properties of the stellar components. Focusing on small `exponential' bulges, ones that may be formed through a disk instability, we investigate the relationship between the assumed initial conditions, such as halo `formation', or assembly, redshift $z_f$, spin parameter $λ$, baryonic fraction $F$, and final disk properties such as global star formation timescale, gas fraction, and bulge-to-disk ratio. We find that the present properties of disks, such as the scale length, are compatible with a higher initial formation redshift if the re-distribution by viscous evolution is included than if it is ignored. We also quantify the dependence of final disk properties on the ratio $F/λ$, thus including the possibility that the baryonic fraction varies from galaxy to galaxy, as perhaps may be inferred from the observations.

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arXiv1999

The Low-Mass Stellar IMF at High Redshift: Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy

Low-mass stars, those with main-sequence lifetimes that are of order the age of the Universe, provide unique constraints on the Initial Mass Function (IMF) when they formed. Star counts in systems with simple star-formation histories are particularly straightforward to interpret, and those in old systems allow one to determine the low-mass stellar IMF at large look-back times and thus at high redshift. We present the faint stellar luminosity function (based on optical HST data) in an external galaxy, the Ursa Minor dwarf Spheroidal (dSph). This relatively-nearby (distance 70kpc) companion galaxy to the Milky Way has a stellar population with narrow distributions of age and of metallicity, remarkably similar to that of a classical halo globular cluster such as M92 or M15, i.e. old and metal-poor. Contrasting with globular clusters, the internal velocity dispersion of the Ursa Minor dSph indicates the presence of significant amounts of dark matter. We find that the main sequence stellar luminosity function of the Ursa Minor dSph, and implied IMF, down to 0.4 of a solar mass is indistinguishable from that of the halo globular clusters M92 and M15. Thus the low mass stellar IMF for stars that formed at high redshift is invariant in going from a low-surface-brightness, dark-matter-dominated external galaxy, to a globular cluster within the Milky Way.

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