We aim to identify the stellar populations (mostly red giants and young stars) detected in the ISOGAL survey at 7 and 15micron towards a field (LN45) in the direction l=-45, b=0.0. The sources detected in the survey of the Galactic plane by the Infrared Space Observatory are characterized based on colour-colour and colour-magnitude diagrams. We combine the ISOGAL catalog with the data from surveys such as 2MASS and GLIMPSE. Interstellar extinction and distance are estimated using the red clump stars detected by 2MASS in combination with the isochrones for the AGB/RGB branch. Absolute magnitudes are thus derived and the stellar populations are identified based on their absolute magnitudes and their infrared excess. A standard approach to the analysis of ISOGAL disk observations has been established. We identify several hundred RGB/AGB stars and 22 candidate young stellar objects in the direction of this field in an area of 0.16 deg^2. An over-density of stellar sources is found at distances corresponding to the distance of the Scutum-Crux spiral arm. In addition, we determine mass-loss rates of AGB-stars using dust radiative transfer models from the literature.
The ISOGAL project is a survey of the stellar populations, structure, and recent star formation history of the inner disk and bulge of the Galaxy. ISOGAL combines 15 and 7micron ISOCAM observations with DENIS IJKs data to determine the nature of a source and the interstellar extinction. In this paper we report an ISOGAL study of a small field in the inner Galactic Bulge (l=0deg, b=1.0deg, area=0.035 sq. deg) as a prototype of the larger area ISOGAL survey of the inner Galaxy. The five wavelengths of ISOGAL+DENIS, together with the relatively low and constant extinction in front of this specifi…
Context: The stellar populations in the central region of the Galaxy are poorly known because of the high visual extinction and very great source density in this direction. Aims: To use recent infrared surveys for studying the dusty stellar objects in this region. Methods: We analyse the content of a 20x20 arcmin^2 field centred at (l,b)=(-0.27,-0.06) observed at 7 and 15 microns as part of the ISOGAL survey. These ISO observations are more than an order of magnitude better in sensitivity and spatial resolution than the IRAS observations. The sources are cross-associated with other catalogues …
The DENIS/ISOGAL near/mid-IR survey of the Milky Way for the first time probes stellar populations in the innermost obscured regions of our galaxy. Ages, metallicities and extinction-corrected luminosities are derived for these stars individually. An old metal-rich population dominates in the inner galactic Bulge, but there are also indications for the presence of a younger population. The inner Bulge has a tri-axial shape, as traced by depth effects on the observed luminosity distributions.
The 7 and 15 um observations of selected fields in the Galactic Plane obtained with ISOCAM during the ISOGAL program offer an unique possibility to search for previously unknown YSOs, undetected by IRAS because of lower sensitivity or confusion problems. In a previous paper (Felli et al. 2000) we established criteria of general validity to select YSOs from the much larger population of Post Main Sequence (Post-MS) stars present in the ISOGAL fields by comparing radio and IR observations of five fields located at l +45 deg. The selection was based primarily on the position of the point sources …
The DENIS/ISOGAL near/mid-IR survey of the Milky Way for the first time probes stellar populations in the innermost obscured regions of our galaxy. Ages, metallicities and extinction-corrected luminosities are derived for these stars individually by isochrone fitting to the preliminary IR photometry. A few hundred AGB stars with thick circumstellar dust envelopes are identified. An old metal-rich population dominates in the inner galactic Bulge, but there are indications for the presence of a younger population. The inner Bulge has a tri-axial shape, as traced by depth effects on the observed …
An analysis of low resolution spectra and infared data of 29 ISOGAL-DENIS sources with mid-IR excess is presented. Eight ISOGAL sources from our sample with 7-15 micron excess are found to be B and A-type stars, some of them with emission lines. Two ISOGAL sources, J175614.4-240831 (B3-4IIIe) and J173845.3-312403 (B7III), show a bump between 5000 and 6000 Angstroem which maybe attributed to extended red emission (ERE). Some of the B,A and F-type giants with a large infrared excess might be in the post-AGB phase. For about 50% of the sources in this preliminary study, a nearby second (or even m…
We present VLA radio continuum observations at 3.6 and 6 cm of a ~0.65 sq.deg. field in the galactic plane at l=+45deg . These observations are meant to be used in a comparison with ISO observations at 7 and 15 um of the same region. In this paper we compare the radio results with other radio surveys and with the IRAS-PSC. At 3.6 and/or 6 cm we detect a total of 34 discrete sources, 13 of which are found in five separate extended complexes. These are all multiple or single extended thermal ultra-compact HII (UCHII) regions. While for each of these complexes an IRAS counterpart could be reliabl…
We present a study of ISOGAL sources in the "intermediate" galactic bulge ($|$$l$$|$ $<$ 2$^\circ$, $|$$b$$|$ $\sim$ 1$^\circ$--4$^\circ$), observed by ISOCAM at 7 and 15 $μm$. In combination with near-infrared (I, J, K$_{\rm s}$) data of DENIS survey, complemented by 2MASS data, we discuss the nature of the ISOGAL sources, their luminosities, the interstellar extinction and the mass-loss rates. A large fraction of the 1464 detected sources at 15 $μm$ are AGB stars above the RGB tip, a number of them show an excess in ([7]-[15])$_{\rm 0}$ and (K$_{\rm s}$-[15])$_{\rm 0}$ colours, characteristi…
In this work we present near-IR spectra (HK-band) of a sample of 107 sources with mid-IR excesses at 7 and 15 $\rm μ$m detected during the ISOGAL survey. Making use of the DENIS interstellar extinction map from Schultheis et al. (1999) we derive luminosities and find that the $\rm M_{bol}$ vs.~$\rm ^{12}CO$ and $\rm M_{bol} vs. H_{2}O$ diagrams are powerful tools for identifying supergiants, AGB stars, giants and young stellar objects. The majority of our sample are AGB stars (~ 80%) while we find four good supergiant candidates, nine young stellar objects and 12 RGB candidates. We have used t…
The Baade's Windows of low obscuration towards the inner parts of the Galactic bulge represent ideal places in which to develop an understanding of the ISOGAL colour-magnitude diagrams. Unlike the case for the solar neighbourhood, their contents are at a uniform distance from the Sun, affected only by the finite thickness of the Bulge. The objects detected in the ISOGAL survey are found to be late-type M-giants at the red giant tip or on the Asymptotic Giant Branch (AGB). The ISOGAL colour-magnitude diagrams show that mass-loss starts at about M4 and increases towards later types. Many non-Mir…
The ISOGAL project is an infrared survey of specific regions sampling the Galactic Plane selected to provide information on Galactic structure,stellar populations,stellar mass-loss and the recent star formation history of the inner disk and Bulge of the Galaxy. ISOGAL combines 7 and 15 micron ISOCAM observations - with a resolution of 6'' at worst - with DENIS IJKs data to determine the nature of the sources and theinterstellar extinction. We have observed about 16 square degrees with a sensitivity approaching 10-20mJy, detecting ~10^5 sources,mostly AGB stars,red giants and young stars. The m…
We present version 1.0 of the ISOGAL-DENIS Point Source Catalogue (PSC), containing more than 100,000 point sources detected at 7 and/or 15 micron in the ISOGAL survey of the inner Galaxy with the ISOCAM instrument on board the Infrared Space Observatory (ISO). These sources are cross-identified, wherever possible, with near-infrared (0.8-2.2 micron) data from the DENIS survey. The overall surface covered by the ISOGAL survey is about 16 square degrees, mostly (95%) distributed near the Galactic plane (|b| < 1 deg), where the source extraction can become confusion limited and perturbed by the …
The extinction laws at 7um and 15um are derived for more than 120 sightlines in the inner Galactic plane based on the ISOGAL survey data and the near-infrared data from DENIS and 2MASS. The tracers are the ISOGAL point sources with [7]-[15]<0.4 which are RGB tip stars or early AGB stars with moderate mass loss. They have well-defined intrinsic color indices (J-Ks)_0, (Ks-[7])_0 and (Ks-[15])_0. By a linear fitting of the observed color indices Ks-[7] and Ks-[15] to the observed J-Ks, we obtain the ratio between the E(Ks-[7]) and E(Ks-[15]) color excesses and E(J-Ks). We infer the selective ext…
The ISOGAL mid-infrared survey of areas close to the Galactic Plane aims to determine their stellar content and its possible bearing on the history of the Galaxy. The NGC6522 and Sgr I Baade's Windows of low obscuration towards the inner parts of the Bulge represent ideal places in which to calibrate and understand the ISOGAL colour-magnitude diagrams. The survey observations were made with the ISOCAM instrument of the ISO satellite. The filter bands chosen were LW2(~7 microns) and LW3(~15 microns). The results presented here show that most of the detected objects are late M-type giants on the…
To study the nature of Bulge AGB stars and in particular their circumstellar dust, we have analysed mid-infrared spectra obtained with the ISOCAM CVF spectrometer in three Bulge fields. The ISOCAM 5-16.5 micron CVF spectra were obtained as part of the ISOGAL infrared survey of the inner Galaxy. A classification of the shape of the 10 micron dust feature was made for each case. The spectra of the individual sources were modelled using a radiative transfer model. Different combinations of amorphous silicates and aluminium-oxide dust were used in the modelling. Spectra were obtained for 29 source…
Asymptotic Giant Branch (AGB) stars are good tracers of the Galactic structure. They are bright in the infrared and can therefore be detected even in the most obscured regions of the Galaxy. Maser emission from their circumstellar envelopes can be detected throughout the Galaxy, and reveals the stellar line-of-sight velocities with an accuracy of a few km/s. Our SiO maser survey of AGB stars was able to detect 255 new maser sources, thereby doubling the number of known stellar line-of-sight velocities toward the inner Galaxy. The longitude-velocity diagram of the SiO maser stars clearly reveal…
Recent observations demonstrate that the thin and thick disks of the Galaxy have different chemical abundance trends and evolution timescales. The relative abundances of $α$-elements in the thick Galactic disk are increased relative to the thin disk. Our goal is to investigate the cause of such differences in thick and thin disk abundances. We investigate the chemical evolution of the Galactic disk in the framework of the open two-zone model with gas inflow. The Galactic abundance trends for $α$-elements (Mg, Si, O) and Fe are predicted for the thin and thick Galactic disks. The star formation…
Detailed models of galactic disk formation and evolution require knowledge about the initial conditions under which disk galaxies form, the boundary conditions that affect their secular evolution and the micro-physical processes that drive the multi-phase interstellar medium and regulate the star formation history. Unfortunately, up to now, most of these ingredients are still poorly understood. The challenge therefore is to, despite this caveat, construct realistic models of galactic disks with predictive power. This short review will summarize some problems related to numerical simulations of…
This review addresses recent developments in the field of disk galaxy warps. Both results from a new HI survey of edgeon disk galaxies, and of simulations of the interaction between a disk+halo and an orbiting satelite, will be discussed.
High resolution spectra data of red clump stars towards the NGP have been obtained with the high resolution spectrograph Elodie at OHP for Tycho-2 selected stars. Combined with Hipparcos local analogues, we determine both the gravitational force law perpendicaular to the Galactic plane, and the total surface mass density and thickness of the Galactic disk. The surface mass density of the Galactic disk within 800 pc derived from this analysis is Sigma(|z|<800pc)=76 Msol.pc-2 and, removing the dark halo contribution, the total disk mass density is Sigma0=67 Msol.pc-2 at solar radius. The thickne…
We have extended previous analytical studies of the interaction of dark halos with galactic disks by introducing for the halo particles anisotropic distribution functions in phase space. For this purpose we have employed the shearing sheet model of a patch of a galactic disk embedded in a homogeneous halo. We find that velocity anisotropy increases considerably the maximum growth factor of perturbations in the disk.
The contributions of different radiation mechanisms to the diffuse gamma-ray emission of the galactic disk are studied in a broad energy region from X-rays to very high energy gamma rays.
Here, we present our on-going work on the determination of stellar parameters of giants in the Galactic Disks and Bulge observed with UVES on the VLT. We present some preliminarily results.
Recent HI emission observations of the Southern Galactic hemisphere have revealed a mushroom-like structure extending from z=-70 to -450 pc, composed of a stem and a cap. Similar structures occur in three-dimensional simulations of a dynamic galactic disk driven by isolated and clustered supernovae. Using these simulations, we show that hot gas in the Galactic disk that is not evacuated through chimneys expands into the cooler gas of the thick disk, forming mushroom-shaped structures. This new class of objects traces buoyant flow of hot gas into the thick disk.
In the present paper, we introduce a two-component model of the Galactic disk to investigate its chemical evolution. The formation of the thick and thin disks occur in two main accretion episodes with both infall rates to be Gaussian. Both the pre-thin and post-thin scenarios for the formation of the Galactic disk are considered. The best-fitting is obtained through $χ^2$-test between the models and the new observed metallicity distribution function of G dwarfs in the solar neighbourhood (Hou et al 1998). Our results show that post-thin disk scenario for the formation of the Galactic disk shou…
The hypothesis that the Damped Ly-alpha systems (DLAs) are large, galactic disks (Milky Way sized) is tested by confronting predictions of models of the formation and evolution of (large) disk galaxies with observations, in particular the Zinc abundance distribution with neutral hydrogen column density found for DLAs. A pronounced mismatch is found strongly hinting that the majority of DLAs may not be large, galactic disks.
We review the observational evidences and the possible theoretical explanations for the abundance gradients in the Galactic disk. In particular, we discuss the implications of abundance gradients and gradients of abundance ratios on the mechanism for the formation of the Galaxy. We conclude that an {\bf inside-out} formation of the Galaxy, and in particular of the Galactic disk, where the innermost regions are assumed to have formed much faster than the outermost ones, represents the most likely explanation for abundance gradients and we predict that the abundance gradients along the Galactic …
The results of a three-dimensional model for disk-halo interaction are presented here. The model considers explicitly the input of energy and mass by isolated a nd correlated supernovae in the disk. Once disrupted by the explosions, the disk never returns to its initial state. Instead it approaches a state where a thin HI disk is formed in the Galactic plane overlayed by thick HI and HII gas disk w ith scale heights of 500 pc and of 1 to 1.5 kpc, respectively. The upper parts o f the thick HII disk (the diffuse ionized medium) act as a disk-halo interface a nd its formation and stability are d…
I review the results of recent cosmological simulations of galaxy formation that highlight the importance of satellite accretion in the formation of galactic disks. Tidal debris of disrupted satellites may contribute to the disk component if they are compact enough to survive the decay and circularization of the orbit as dynamical friction brings the satellite into the disk plane. This process may add a small but non-negligible fraction of stars to the thin and thick disks, and reconcile the presence of very old stars with the protracted merging history expected in a hierarchically clustering …
We investigate the Jeans instability of a galactic disk embedded in a dynamically responsive dark halo. It is shown that the disk-halo system becomes nominally Jeans unstable. On small scales the instability is suppressed, if the Toomre stability index Q_T is higher than a certain threshold, but on large scales the Jeans instability sets invariably in. However, using a simple self-consistent disk-halo model it is demonstrated that this occurs on scales which are much larger than the system so that this is indeed only a nominal effect. From a practical point of view the Jeans instability of gal…
We show that collisions between the outer Galactic HI disk and the leading arms (LAs) of the Magellanic stream (MS) can create giant HI holes and chimney-like structures in the disk. Based on the results of our N-body simulations on the last 2.5 Gyr evolution of the Large and Small Magellanic Clouds (LMC and SMC, respectively) interacting with the Galaxy, we investigate when and where the LAs can pass through the Galactic plane after the MS formation. We then investigate hydrodynamical interaction between LAs and the Galactic HI disk (``the Magellanic impact'') by using our new hydrodynamical …
We used red clump stars to measure the surface mass density of the Galactic disk in the solar neighbourhood. High resolution spectra of red clump stars towards the NGP have been obtained with the ELODIE spectrograph at OHP for Tycho-2 selected stars, and nearby Hipparcos counterparts were also observed. We determined their distances, velocities, and metallicities to measure the gravitational force law perpendicular to the Galactic plane. As in most previous studies, we applied one-parameter models of the vertical gravitational potential. We obtained a disk surface mass density within 1.1kpc of…
A series of arguments is presented for heavy galaxy disks not only in the optical regions, but also in the dark matter dominated regions of spirals. We are testing this possibility with extreme maximum disk N-body models without any conventional spheroidal dark halo.
We present results of numerical modeling made for the galactic stellar disk embedded in the spherical halo. The non-linear dynamics of bending instabilities developed in the disk is studied. The axisymmetrical bending mode is considered as a main factor increasing the disk thickness. The model dependence between the disk thickness and the halo to disk mass ratio $M_h/M_d$ is inferred. The modeling results enable us to estimate the dark halo mass for several edge-on galaxies. The obtained values of $M_h/M_d$ for the galaxies considered are of order 1 - 2.
We present the analysis of a CCD survey of 31 nearby (<= 110 Mpc) edge-on spiral galaxies. The three-dimensional one-component best fit models provide their disk-scalelengths h and for the first time their disk cut-off radii R_{co}. We confirm for this sample the existence of such sharp truncations, and find a significantly lower mean value of the distance independent ratio R_{co}/h =2.9 +- 0.7 than the standard value of 4.5 often used in the literature. Our data show no correlation of these parameters with Hubble type, whereas we report a correlation between R_{co}/h and the distance based sc…
A new idea is proposed for the origin of bulges in spiral galaxies. Numerical simulations for the protogalactic collapse suggest strongly that galactic bulges have been assembled from massive clumps formed in the galactic disks in their early evolutionary phase. These clumps result from the gravitational instability of the gas-rich disks of young galaxies. Those massive clumps, individual masses of which can be as large as one billion solar masses, are able to spiral toward the galactic center owing to dynamical frictions within a few Gyr. Inward transport of disk matter by this process leads …
We raise the question whether in the past a disk could have existed in our Galactic Center which has disappeared now. Our model for the interaction of a cool disk and a hot corona above (Liu et al. 2004) allows to estimate an upper limit for the mass that might have been present in a putative accretion disk after a last star forming event, but would now have evaporated by coronal action.
We propose that the mushroom-shaped structure of the Galactic worm GW 123.4--1.5 is created by a cloud collision with the Galactic gas disk. A hydrodynamic simulation shows that a mushroom-shaped structure is created after the cloud crosses the Galactic midplane. The lifetime of the mushroom-shaped structure is of order the dynamical time scale of the disk, \sim 10^7 years. We find that the velocities across the cap of the mushroom-shaped structure in the simulation are consistent with the observed values. The simulation also predicts a structure on the opposite side of the Galactic plane whic…
By cross-correlating the results of two recent large-scale surveys, the general properties of a well defined sample of semi-regular variable stars have been determined. ISOGAL mid-infrared photometry and MACHO lightcurves are assembled for approximately 300 stars in the Baade's Windows of low extinction towards the Galactic bulge. These stars are mainly giants of late M spectral type, evolving along the asymptotic giant branch (AGB). They are found to possess a wide and continuous distribution of pulsation periods and to obey an approximate log~period -- bolometric magnitude relation or set of…
We show that the Galactic thick disk reaches at least solar metallicities, and that it experienced strong chemical enrichment during a period of ~3 Gyr, ending around 8-9 Gyr ago. This finding puts further constraints on the relation and interface between the thin and thick disks, and their formation processes. Our results are based on a detailed elemental abundance analysis of 261 kinematically selected F and G dwarf stars in the solar neighborhood: 194 likely members of the thick disk and 67 likely members of the thin disk, in the range -1.3<[Fe/H]<+0.4.
We calculate upper bounds on the inner radii of geometrically thin accretion disks in galactic black-hole systems by relating their rapid variability properties to those of neutron stars. We infer that the inner disk radii do not exhibit large excursions between different spectral states, in contrast with the concept that the disk retreats significantly during the soft-to-hard state transition. We find that, in the hard state, the accretion disks extend down to radii less than 6-25 GM/c^2 and discuss the implications of our results for models of black-hole X-ray spectra.
The measured rotation velocity profiles of mature spiral galaxies are successfully described with a gravitational model consisting of a thin axisymmetric disk of finte radius. The disk is assumed uniformly thin but with variable radial mass density. The governing integral equation is based on mechanical balance between Newtonian gravitational and centrifugal forces (due to galaxy rotation) at each and every point in a finite set of concentric rings. The nondimensionalized mathematical system contains a dimensionless parameter we call ``galactic rotation parameter'' which concisely crystallizes…
Recent observations of the Galactic center revealed a nuclear disk of young OB stars, in addition to many similar outlying stars with higher eccentricities and/or high inclinations relative to the disk (some of them possibly belonging to a second disk). Binaries in such nuclear disks, if they exist in non-negligible fractions, could have a major role in the evolution of the disks through binary heating of this stellar system. We suggest that interactions with/in binaries may explain some (or all) of the observed outlying young stars in the Galactic center. Such stars could have been formed in …
We show that the observational data of extragalactic radio sources tend to support the theoretical relationship between the jet precession period and the optical luminosity of the sources, as predicted by the model in which an accretion disk causes the central black hole to precess.
Observations reveal that mature spiral galaxies consist of stars, gases and plasma approximately distributed in a thin disk of circular shape, usually with a central bulge. The rotation velocities quickly increase from the galactic center and then achieve a constant velocity from the core to the periphery. The basic dynamic behavior of a mature spiral galaxy, such as the Milky Way, is well described by simple models balancing Newtonian gravitational forces against the centrifugal forces associated with a rotating thin axisymmetric disk. In this research, we investigate the effects of adding ce…
The formation and evolution of disk galaxies in the cosmological context is studied. We consider the observable properties of disk galaxies and treat the disk formation and galactic evolutionary processes in a self-consistent fashion. We find the matter accretion regime to be the dominant ingredient in establishing the Hubble sequence. The accretion regime is a phenomenon directly related to the statistical properties of the primordial density fluctuations from which disk galaxies emerged. In this paper we assumed a Gaussian density field and a standard CDM model.
Two disks of young stars have recently been discovered in the Galactic Center. The disks are rotating in the gravitational field of the central black hole at radii r=0.1-0.3 pc and thus open a new opportunity to measure the central mass. We find that the observed motion of stars in the clockwise disk implies M=4.3+/-0.5 million solar masses for the fiducial distance to the Galactic Center R_0=8 kpc and derive the scaling of M with R_0. As a tool for our estimate we use orbital roulette, a recently developed method. The method reconstructs the three-dimensional orbits of the disk stars and chec…
We present chemical evolution models for the Galactic disk. We also present a new determination of X, Y, and Z for M17 a Galactic metal-rich Hii region. We compare our models for the Galactic disk with the Galactic Hii regions abundances. The $ΔY/ΔO$ ratio predicted from the galactic chemical evolution model is in very good agreement with the $ΔY/ΔO$ value derived from M17 and the primordial helium abundance, Yp, taking into account the presence of temperature variations in this Hii region. From the M17 observations we obtain that $ΔY/ΔZ = 1.97 \pm 0.41$, in excellent agreement with two $ΔY/ΔZ…
Hydrodynamical calculations undertaken to simulate the collisional interaction between the Sgr dwarf galaxy and the Galactic outer HI disk are presented, constrained by recently derived orbital and mass parameters for this dwarf galaxy. It is found that a significant distortion to the structure of the Galactic HI disk will be induced by the collision if the mass of the dwarf exceeds ~10^9 M_Sun; this value is consistent with an estimate derived by requiring that the dwarf galaxy is sufficiently robust to survive tidal disruption until the present time. Though the precise details of the interac…
Many disk galaxies are lopsided: their brightest inner parts are displaced from the center of the outer isophotes, or the outer contours of the HI disk. This asymmetry is particularly common in small, low-luminosity galaxies. We argue here that long-lived lopsidedness is a consequence of the disk lying off-center in the potential of the galaxy's extended dark halo, and spinning in a sense retrograde to its orbit about the halo center. The stellar velocity field predicted by our gravitational N-body simulations is clearly asymmetric.
Two sets of observational carbon stellar yields for low-and-intermediate mass stars are computed based on planetary nebula abundances derived from C II lambda 4267 and C III lambda lambda 1906 + 1909 lines, respectively. These observational yields are assumed in chemical evolution models for the solar vicinity and the Galactic disk. C/O values observed in stars of the solar vicinity and Galactic H II regions are compared with those predicted by chemical evolution models for the Galaxy. I conclude that the C yields derived from permitted lines are in better agreement with the observational cons…
We have re-estimated the surface density of the Galactic disk in the solar neighborhood within $\pm$ 0.4 kpc of the Sun using parallaxes and proper motions of a kinematically and spatially unbiased sample of 1476 old bright red giant stars from the Hipparcos catalog with measured radial velocities from Barbier-Brossat & Figon (2000). We determine the vertical distribution of the red giants as well as the vertical velocity dispersion of the sample, (14.4 $\pm$ 0.26 km/sec), and combine these to derive the surface density of gravitating matter in the Galactic disk as a function of the galactic c…
Correlations between stellar kinematics and chemical abundances are fossil evidence for evolutionary connections between Galactic structural components. Extensive stellar surveys show that the only tolerably clear distinction between galactic components appears in the distributions of specific angular momentum. Here the stellar metal-poor halo and the metal-rich bulge are indistinguishable from each other, as are the thick disk and the old disk. Each pair is very distinct from the other. This leads to an evolutionary model in which the metal-poor stellar halo evolves into the inner bulge, whil…
We study gravitational instabilities in disks, with special attention to the most massive clumps that form because they are expected to be the progenitors of globular-type clusters. The maximum unstable mass is set by rotation and depends only on the surface density and orbital frequency of the disk. We propose that the formation of massive clusters is related to this largest scale in galaxies not stabilized by rotation. Using data from the literature, we predict that globular-like clusters can form in nuclear starburst disks and protogalactic disks but not in typical spiral galaxies, in agree…
I review different methods devised to derive the age of the Galactic Disk, namely the Radio-active Decay (RD), the Cool White Dwarf Luminosity Function (CWDLF), old opne clusters (OOC) and the Color Magnitude Diagram (CMD) of the stars in the solar vicinity. I argue that the disk is likely to be 8-10 Gyr old. Since the bulk of globulars has an age around 13 Gyr, the possibility emerges that the Galaxy experienced a minimum of Star Formation at the end of the halo/bulge formation. This minimum might reflect the time at which the Galaxy started to acquire material to form the disk inside-out.
We report the first results of a detailed study of the star formation law in a sample of 32 nearby spiral galaxies with well-measured rotation curves, HI and H$_2$ (as traced by CO) surface density profiles, and new \Ha CCD photometry. Our results strongly support the view that the formation of gravitationally bound interstellar clouds regulates the onset of widespread star formation -- at least in the outer regions of galactic disks.
We calculate the self-consistent response of an axisymmetric galactic disk perturbed by an elliptical halo potential of harmonic number m = 2, and obtain the net disk ellipticity. Such a potential is commonly expected to arise due to a galactic tidal encounter and also during the galaxy formation process. The self-gravitational potential corresponding to the self-consistent, non-axisymmetric density response of the disk is obtained by inversion of Poisson equation for a thin disk. This response potential is shown to oppose the perturbation potential, because physically the disk self-gravity re…
We present iron abundance measurements, based on high resolution spectroscopy, and accurate distance determinations, based on near infrared photometry, for 34 Galactic Cepheids. The new data are used to constrain the Galactic iron abundance gradient in the outer disk, namely from 10 to 14 kpc. We confirm the flattening of the gradient toward the outer disk. In this region we also found an increase in the metallicity dispersion. Current data do not support the occurrence of a jump in the metallicity gradient for Galactocentric distances of the order of 10-12 kpc.
We investigate the origin of the abundance variations observed among similarly aged F and G dwarfs in the local Galactic disk. We present arguments in support of combined infall of metal-deficient gas and sequential enrichment by successive stellar generations in the local Galactic disk ISM. We show that galactic chemical evolution models which take into account these processes simultaneously are consistent with both the observed abundance variations among similarly aged dwarfs in the solar neighbourhood and the abundances observed in the local disk ISM. For reasonable choices of parameters, t…
There is apparently a widespread belief that the gravitational field (and subsequently the rotation curve) ``inside'' razor-thin, axially symmetric disks can not be determined accurately from elliptic integrals because of the singular kernel in the Poisson integral. Here, we report a simple and powerful method to achieve this task numerically using the technique of ``density splitting''.
