Parametrization of the Galactic structure by two exponentials

We parametrized the total structure of the Galaxy in cylindrical coordinates up to z ∼ 10 kpc by using radial and vertical exponentials, covering the thin and thick disc and the inner spheroid. However, we allowed the scale height and scale length to vary continuously with distance from the Galactic plane. The standard deviations for the differences between stellar space densities estimated by means of the newly defined scale heights and scale lengths and the observed space densities for three absolute magnitude intervals, 5 < M (g ′) ≤ 6, 6 < M (g ′) ≤ 7, and 7 < M (g ′) ≤ 8, of the fields SA 114, ELAIS, and # 0952+5245 are rather small. The uncertainties of the scale heights are also small, indicating that this parameter depends very sensitively on the distance from the Galactic plane, whereas those of the scale lengths are larger. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The asymmetric structure of the Galactic halo

Using the stellar photometry catalogue based on the latest data release (DR4) of the Sloan Digital Sky Survey (SDSS), a study of the Galactic structure using star counts is carried out for selected areas of the sky. The sample areas are selected along a circle at a Galactic latitude of +60°, and 10 strips of high Galactic latitude along different longitudes. Direct statistics of the data show that the surface densities of ℓ from 180° to 360° are systematically higher than those of ℓ from 0° to 180°, defining a region of overdensity (in the direction of Virgo) and another one of underdensity (in the direction of Ursa Major) with respect to an axisymmetric model. It is shown by comparing the results from star counts in the ( g − r ) colour that the density deviations are due to an asymmetry of the stellar density in the halo. Theoretical models for the surface density profile are built and star counts are performed using a triaxial halo of which the parameters are constrained by observational data. Two possible reasons for the asymmetric structure are discussed.     

The structure of the Galactic halo: SDSS versus SuperCOSMOS

The halo structure at high Galactic latitudes near both the north and south poles is studied using Sloan Digital Sky Survey (SDSS) and SuperCOSMOS data. For the south cap halo, the archive of the SuperCOSMOS photographic photometry sky survey is used. The coincident source rate between SuperCOSMOS data in B _J band from 16.5 to 20.5 mag and SDSS data is about 92 per cent, in a common sky area in the south. While that in the R _F band is about 85 per cent from 16.5 to 19.5 mag. Transformed to the SuperCOSMOS system and downgraded to the limiting magnitudes of SuperCOSMOS, the star counts in the North Galactic Cap from SDSS show up to an  16.9 ± 6.3  per cent  asymmetric ratio (defined as relative fluctuations over the rotational symmetry structure) in the B _J band, and up to  13.5 ± 6.7  per cent  asymmetric ratio in the R _F band. From SuperCOSMOS B _J and R _F bands, the structure of the Southern Galactic hemisphere does not show the same obvious asymmetric structures as the northern sky does in both the original and downgraded SDSS star counts. An axisymmetric halo model with n = 2.8 and q = 0.7 can fit the projected number density from SuperCOSMOS fairly well, with an average error of about 9.17 per cent. By careful analysis of the difference of star counts between the downgraded SDSS northern halo data and SuperCOSMOS southern halo data, it is shown that no asymmetry can be detected in the South Galactic Cap at the accuracy of SuperCOSMOS, and the Virgo overdensity is likely a foreign component in the Galactic halo.     

Galactic models and white dwarf populations

We make use of a previous well-tested Galactic model, but describing the observational behaviour of the various stellar components in terms of suitable assumptions on their evolutionary status. In this way we are able to predict the expected distribution of Galactic white dwarfs (WDs), with results which appear in rather good agreement with recent estimates of the local WD luminosity function. The predicted occurrence of WDs in deep observations of selected Galactic fields is presented, and we discuss the role played by WDs in star counts. The effects on the theoretical predictions of different white dwarf evolutionary models, ages, initial mass functions and relations between progenitor mass and WD mass are also discussed.     

The field of NGC 6397 as a test for Galactic models

Taking advantage of recent Hubble Space Telescope ( HST ) data for field stars in the region of the Galactic globular cluster NGC 6397, we tested the predictions of several Galactic models with star counts reaching a largely unexplored range of magnitudes, down to V ∼26.5. After updating the input stellar ( V − I ) colours, we found that the two-component Bahcall–Soneira (B&S) model can be put into satisfactory agreement with observations for suitable choices of disc/spheroid luminosity functions (LFs). However, if one assumes the disc LF of Gould, Bahcall and Flynn together with the spheroid LF of Gould, Flynn and Bahcall, there is no way to reconcile the predicted and observed V -magnitude distribution. We also analysed the agreement between observed and predicted magnitude and colour distributions for two selected models with a thick disc component. Even in this case there are suitable combinations of model parameters and faint-magnitude LFs that can give a reasonable agreement with observational star counts in both magnitude and colour. However, the above-quoted combination of Gould et al. LFs again gives predictions in clear disagreement with observations.     

The structure of the Galactic bar

We present a deep near-infrared wide-angle photometric analysis of the structure of the inner Galactic bar and central disc. The presence of a triaxial structure at the centre of the Galaxy is confirmed, consistent with a bar inclined at  22°± 55  from the Sun—Galactic Centre line, extending to approximately 2.5 kpc from the Galactic Centre and with a rather small axis ratio. A feature at  ℓ=−98  not aligned with this triaxiality suggests the existence of a second structure in the inner Galaxy, a double triaxiality or an inner ring. We argue that this is likely to be the signature of the end of the Galactic bar, at approximately 2.5–3 kpc, which is circumscribed by an inner pseudo-ring. No thick dust lane preceding the bar is detected and a hole in the dust distribution of the disc inside the bar radius is inferred.     

Detection of the old stellar component of the major Galactic bar

We present near-infrared colour–magnitude diagrams and star counts for a number of regions along the Galactic plane. It is shown that along the l =27°, b =0° line of sight there is a feature at 5.7±0.7 kpc with a density of stars at least a factor of 2 and probably more than a factor of 5 times that of the disc at the same position. This feature forms a distinct clump on an H versus J − H diagram and is seen at all longitudes from the bulge to about l =28°, but at no longitude greater than this. The distance to the feature at l =20° is about 0.5 kpc further than at l =27°, and by l =10° it has merged with, or has become, the bulge. Given that at l =27° and l =21° there is also a clustering of very young stars, the only component that can reasonably explain what is seen is a bar with half-length of around 4 kpc and a position angle of about 43°±7°.     

On the Galactic disc age–metallicity relation

A comparison is made between the age–metallicity relations obtained from four different types of studies: F and G stars in the solar neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be −0.09 dex kpc⁻¹ and most likely constant in time. We do not correct for the vertical gradient, because its existence and value are not firmly established.
Stars and clusters trace a similar age–metallicity relation, showing an excess of rather metal-rich objects in the age range 5–9 Gyr. Galactic structure studies tend to give a more metal-poor relation than chemical evolution models. Neither relation explains the presence of old, relatively metal-rich stars and clusters. This might be caused by uncertainties in the ages of the local stars, or pre-enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.     

Dark haloes of spiral galaxies: ISO photometry

We exclude hydrogen-burning stars, of any mass above the hydrogen-burning limit and any metallicity, as significant contributors to the massive haloes deduced from rotation curves to dominate the outer parts of spiral galaxies. We present and analyse images of four nearly edge-on bulgeless spiral galaxies (UGC 711, NGC 2915, UGC 12426, UGC 1459) obtained with ISOCAM (The CAMera instrument on board the Infrared Space Observatory ) at 14.5 and 6.75 μm. Our sensitivity limit for detection of any diffuse infrared emission associated with the dark haloes in these galaxies is a few tens of μJy per 6 × 6 arcsec² pixel, with this limit currently set by remaining difficulties in modelling the non-linear behaviour of the detectors. All four galaxies show zero detected signal from extended non-disc emission, consistent with zero halo-like luminosity density distribution. The 95 per cent upper limit on any emission, for NGC 2915 in particular, allows us to exclude very low mass main-sequence stars ( M  > 0.08 M⊙) and young brown dwarfs (≲1 Gyr) as significant contributors to dark matter in galactic haloes. Combining our results with those of the Galactic microlensing surveys, which exclude objects with M  < 0.01 M⊙, excludes almost the entire possible mass range of compact baryonic objects from contributing to Galactic dark matter.     

Galactic structure from the Two Micron Galactic Survey

The Two Micron Galactic Survey (TMGS) is the most sensitive large scale point source near infrared survey of the Galactic Plane yet attempted. The Galactic plane has been drift-scanned in several regions over the areas -5° <l < 30°, |b | 15° and 30° <l < 180°, |b | 5°. The survey is complete in the areas covered to magnitudem _k = +9.8. So far, a total area of 255 square degrees has been mapped and 500000 objects have been detected, the majority of these in the Galactic plane and with no visible counterparts. In this contribution we use data from the TMGS to probe the star distribution within the Galactic disk.     

Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes

We present a method which separates field dwarfs and field giants by their 2MASS and V apparent magnitudes. This method is based on spectroscopically selected standards and is hence reliable.We applied it to stars in two fields, SA 54 and SA 82, and we estimated a full set of Galactic model parameters for giants including their total local space density. Our results are in agreement with the ones given in the recent literature. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Inversion of stellar statistics equation for the Galactic bulge

A method based on Lucy's iterative algorithm is developed to invert the equation of stellar statistics for the Galactic bulge and is then applied to the K -band star counts from the Two-Micron Galactic Survey in a number of off-plane regions (10°>| b |>2°, | l |<15°). The top end of the K -band luminosity function is derived and the morphology of the stellar density function is fitted to triaxial ellipsoids, assuming a non-variable luminosity function within the bulge. The results, which have already been outlined by López-Corredoira et al., are shown in this paper with a full explanation of the steps of the inversion: the luminosity function shows a sharp decrease brighter than M _K =−8.0  mag when compared with the disc population; the bulge fits triaxial ellipsoids with the major axis in the Galactic plane at an angle with the line of sight to the Galactic centre of 12° in the first quadrant; the axial ratios are 1:0.54:0.33, and the distance of the Sun from the centre of the triaxial ellipsoid is 7860 pc. The major–minor axial ratio of the ellipsoids is found not to be constant, the best fit to the gradient being K _z =(8.4±1.7)×exp(− t /(2000±920) pc), where t is the distance along the major axis of the ellipsoid in parsecs. However, the interpretation of this is controversial. An eccentricity of the true density-ellipsoid gradient and a population gradient are two possible explanations. The best fit for the stellar density, for 1300 pc< t <3000 pc, is calculated for both cases, assuming an ellipsoidal distribution with constant axial ratios, and when K _z is allowed to vary. From these, the total number of bulge stars is ∼3×10¹⁰ or ∼4×10¹⁰, respectively.     

Near-infrared star counts as a test of a Galactic bar structure

We present survey data in the narrow-band L filter (nb L ), taken at UKIRT, for a total area of 277 arcmin², roughly equally divided between four regions at zero Galactic latitude and longitudes ±4.°3 and ±2.°3. The 80 per cent completeness level for these observations is at roughly magnitude 11.0. This magnitude limit, owing to the low coefficient for interstellar extinction at this wavelength ( A _{nb L} =0.047 A_V ), allows us to observe bulge giants. We match the nb L magnitudes with DENIS survey K magnitudes, and find 95 per cent of nb L sources are matched to K sources. Constructing colour–magnitude diagrams, we deredden the magnitudes and find evidence for a longitude-dependent asymmetry in the source counts. We find that there are ∼15 per cent and ∼5 per cent more sources at the negative longitude than at the corresponding positive longitude for the fields at ±4.°3 and ±2.°3, respectively. This is compared with the predictions of some Galactic bar models. We find an asymmetry in the expected sense, which favours gas dynamical models and the recent deconvolution of surface photometry data over earlier treatments of photometric data.     

The APM survey for cool carbon stars in the Galactic halo – I

A by-product of the APM high-redshift quasar survey was the discovery of several distant (20–100 kpc) N-type carbon stars at high galactic latitude. Following on from this, we have started a systematic all-sky survey at galactic latitudes ⊢ b ⊢>30° to find further examples of these rare objects, and we report here on the results from the first season of follow-up spectroscopy. Faint, high-latitude carbon (FHLC) giants make excellent probes of the kinematic structure of the outer Galactic halo. Therefore, in addition to detailed spectrophotometry covering a wide wavelength range, we have obtained high-resolution (∼1 Å) spectra centred on the CN bands at ∼8000 Å, and have derived accurate (≲10 km s⁻¹) radial velocities for the known FHLC stars. From the initial phase of our survey covering ≈6500 deg², we find a surface density of faint N-type carbon stars in the halo of ≈1 per 200 deg², roughly a factor of 4 less than the surface density of CH-type carbon stars in the halo. Intermediate-age, N-type carbon stars seem unlikely to have formed in the halo in isolation from other star-forming regions, and one possibility that we are investigating is that they either arise from the disruption of tidally captured dwarf satellite galaxies or are a manifestation of the long-sought optical component of the Magellanic Stream.     

Kinematics of the Galactic halo from horizontal branch stars in the Hamburg/ESO survey

Large samples of field horizontal branch (FHB) stars make excellent tracers of the Galactic halo; by studying their kinematics, one can infer important physical properties of our Galaxy. Here we present the results of a medium-resolution spectroscopic survey of 530 FHB stars selected from the Hamburg/ESO survey. The stars have a mean distance of ∼7 kpc and thus probe the inner parts of the Milky Way halo. We measure radial velocities from the spectra in order to test the model of Sommer-Larsen et al., who suggested that the velocity ellipsoid of the halo changes from radially dominated orbits to tangentially dominated orbits as one proceeds from the inner to the outer halo. We find that the present data are unable to discriminate between this model and a more simple isothermal ellipsoid; we suggest that additional observations towards the Galactic Centre might help to differentiate them.