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1.
Possible orbital histories of the Sgr dwarf galaxy are explored. A special-purpose N -body code is used to construct the first models of the Milky Way–Sgr dwarf system in which both the Milky Way and the Sgr dwarf are represented by full N -body systems and followed for a Hubble time. These models are used to calibrate a semi-analytic model of the Sgr dwarf's orbit that enables us to explore a wider parameter space than is accessible to the N -body models. We conclude that the extant data on the Sgr dwarf are compatible with a wide range of orbital histories. At one extreme the Sgr dwarf initially possesses ∼1011 M⊙ and starts from a Galactocentric distance R D(0)≳200 kpc. At the other extreme the Sgr dwarf starts with ∼109 M⊙ and R D(0)∼60 kpc, similar to its present apocentric distance. In all cases the Sgr dwarf is initially dark matter dominated and the current velocity dispersion of the Sgr dwarf's dark matter is tightly constrained to be 21±2 km s−1. This number is probably compatible with the smaller measured dispersion of the Sgr dwarf's stars because of (i) the dynamical difference between dark and luminous matter, and (ii) velocity anisotropy.  相似文献   

2.
The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy that is surrounded by ∼160 dwarf satellites. The projected number density profile of this dwarf population deviates within ∼1/3 of the virial radius from a projected Navarro, Frenk and White (NFW) profile, which is assumed to approximate the underlying total matter distribution. By means of a semi-analytic model, we demonstrate that the interplay between gravitation, dynamical friction and tidal mass loss and destruction can explain the observed number density profile. We use only two parameters in our models: the total to stellar mass fraction of the satellite haloes and the disruption efficiency. The disruption efficiency is expressed by a minimum radius. If the tidal radius of a galaxy (halo) falls below this radius, it is assumed to become unobservable. The preferred parameters are an initial total to stellar mass fraction of ∼20 and a disruption radius of  4 kpc  . In that model, about 20 per cent of all the satellites are totally disrupted on their orbits within the group environment. Dynamical friction is less important in shaping the inner slope of the number density profile because the reduction in mass by tidal forces lowers the impact of the friction term. The main destruction mechanism is tide. In the preferred model, the total B -band luminosity of all disrupted galaxies is about twice the observed luminosity of the central elliptical galaxy, indicating that a significant fraction of stars are scattered into the intragroup medium. Dwarf galaxy satellites closer to the centre of the NGC 5044 group may exhibit optical evidence of partial tidal disruption. If dynamical friction forces the satellite to merge with the central elliptical, the angular momentum of the satellite tends to be removed at the apocentre passage. Afterwards, the satellite drops radially towards the centre.  相似文献   

3.
Published maps of red giant stars in the halo region of M31 exhibit a giant stellar stream to the south of this galaxy, as well as a giant 'shelf' to the northeast of M31's centre. Using these maps, we find that there is a fainter shelf of comparable size on the western side as well. By choosing appropriate structural and orbital parameters for an accreting dwarf satellite within the accurate M31 potential model of Geehan et al., we produce a very similar structure in an N -body simulation. In this scenario, the tidal stream produced at pericentre of the satellite's orbit matches the observed southern stream, while the forward continuation of this tidal stream makes up two orbital loops, broadened into fan-like structures by successive pericentric passages; these loops correspond to the north-eastern and western shelves. The tidal debris from the satellite also reproduces a previously observed 'stream' of counterrotating planetary nebulae and a related stream seen in red giant stars. The debris pattern in our simulation resembles the shell systems detected around many elliptical galaxies, though this is the first identification of a shell system in a spiral galaxy and the first in any galaxy close enough to allow measurements of stellar velocities and relative distances. We discuss the physics of these partial shells, highlighting the role played by spatial and velocity caustics in the observations. We show that kinematic surveys of the tidal debris will provide a sensitive measurement of M31's halo potential, while quantifying the surface density of debris in the shelves will let us reconstruct the original mass and time of disruption of the progenitor satellite.  相似文献   

4.
We use numerical simulations to investigate the cusp at the centre of elliptical galaxies caused by the slow growth of a supermassive black hole. We study this problem for axisymmetric models of galaxies with or without rotation. The numerical simulations are based on the 'perturbation particles' method, and use GRAPEs to compute the force arising from the cusp. We study the way in which the density cusp is affected by the initial flattening of the model, as well as the role played by initial rotation. The logarithmic slope of the density cusp is found to be very much insensitive to flattening; as a consequence, we deduce that tangential velocity anisotropy – which supports the flattening – is also of little influence on the final cusp. We investigate, via two different kinds of rotating models, the efficiency with which a rotation velocity component builds within the cusp. A cusp in rotation develops only for models where a net rotation component is initially present at high energy levels. The eventual observation of a central rotational velocity peak in elliptical galaxies has, therefore, some implications for the galaxy dynamical history.  相似文献   

5.
We present results for a galaxy formation model that includes a simple treatment for the disruption of dwarf galaxies by gravitational forces and galaxy encounters within galaxy clusters. This is implemented a posteriori in a semi-analytic model by considering the stability of cluster dark matter subhaloes at   z = 0  . We assume that a galaxy whose dark matter substructure has been disrupted will itself disperse, while its stars become part of the population of intracluster stars responsible for the observed intracluster light. Despite the simplicity of this assumption, our results show a substantial improvement over previous models and indicate that the inclusion of galaxy disruption is indeed a necessary ingredient of galaxy formation models. We find that galaxy disruption suppresses the number density of dwarf galaxies by about a factor of 2. This makes the slope of the faint end of the galaxy luminosity function shallower, in agreement with observations. In particular, the abundance of faint, red galaxies is strongly suppressed. As a result, the luminosity function of red galaxies and the distinction between the red and the blue galaxy populations in colour–magnitude relationships are correctly predicted. Finally, we estimate a fraction of intracluster light comparable to that found in clusters of galaxies.  相似文献   

6.
The results obtained from a study of the mass distribution of 36 spiral galaxies are presented. The galaxies were observed using Fabry–Perot interferometry as part of the GHASP survey. The main aim of obtaining high-resolution Hα 2D velocity fields is to define more accurately the rising part of the rotation curves which should allow to better constrain the parameters of the mass distribution. The Hα velocities were combined with low resolution H  i data from the literature, when available. Combining the kinematical data with photometric data, mass models were derived from these rotation curves using two different functional forms for the halo: an isothermal sphere (ISO) and a Navarro–Frenk–White (NFW) profile. For the galaxies already modelled by other authors, the results tend to agree. Our results point at the existence of a constant density core in the centre of the dark matter haloes rather than a cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all types the result already obtained by other authors studying dwarf and low surface brightness galaxies but would necessitate a larger sample of galaxies to conclude more strongly. Whatever model is used (ISO or NFW), small core radius haloes have higher central densities, again for all morphological types. We confirm different halo scaling laws, such as the correlations between the core radius and the central density of the halo with the absolute magnitude of a galaxy: low-luminosity galaxies have small core radius and high central density. We find that the product of the central density with the core radius of the dark matter halo is nearly constant, whatever the model and whatever the absolute magnitude of the galaxy. This suggests that the halo surface density is independent from the galaxy type.  相似文献   

7.
We recover the luminosity distributions over a wide range of absolute magnitude (−24.5 <  M R  < −16.5) for a sample of seven rich southern galaxy clusters. We find a large variation in the ratio of dwarf to giant galaxies, DGR: 0.8 ≤ DGR ≤ 3.1. This variation is shown to be inconsistent with a ubiquitous cluster luminosity function. The DGR shows a smaller variation from cluster to cluster in the inner regions ( r  ≲ 0.56 Mpc). Outside these regions we find the DGR to be strongly anticorrelated with the mean local projected galaxy density, with the DGR increasing towards lower densities. In addition, the DGR in the outer regions shows some correlation with Bautz–Morgan type. Radial analysis of the clusters indicates that the dwarf galaxies are less centrally clustered than the giants, and they form a significant halo around clusters. We conclude that measurements of the total cluster luminosity distribution based on the inner core alone are likely to be severe underestimates of the dwarf component, the integrated cluster luminosity and the contribution of galaxy masses to the cluster's total mass. Further work is required to quantify this. The observational evidence that the unrelaxed, lower density outer regions of clusters are dwarf-rich adds credence to the recent evidence and conjecture that the field is a predominantly dwarf-rich environment, and that the dwarf galaxies are under-represented in measures of the local field luminosity function.  相似文献   

8.
We report the results of a survey of the H  i environment of NGC 6946 in a search for gas-rich companions to the galaxy. Such gas-rich companions could include material left over from the galaxy assembly process which could persist into the current day around an isolated galaxy such as NGC 6946. NGC 6946 is prolifically forming stars, has a nuclear starburst, and has widespread high-velocity clouds associated with the disc. All of these features could be explained by the accretion of low-mass H  i clouds by NGC 6946. Our survey recovered two previously detected dwarf galaxies associated with NGC 6946, but otherwise found no signatures of interactions in the NGC 6946 system. The companions are small enough, and distant enough from NGC 6946 that they should have minimal effect on the main galaxy. Some tidal debris may be expected due to interaction between the two dwarf galaxies, but none is observed. This could be because it is at low column densities, or because the dwarf galaxies are more separated than they appear on the sky. This study of the system suggests that NGC 6946 is a gravitationally bound system with two dwarf galaxies in stable orbits about the larger primary galaxy.  相似文献   

9.
In this paper, the second in a series of two, we justify two important assumptions on which the result is based that in the course of a galaxy merger, the slingshot ejection of bound stars is sufficiently efficient to allow a supermassive black hole binary (BHB) to merge. A steep cusp with a power-law index of 2.5–3 is required which is as massive as the binary and surrounds the BHs when the binary becomes hard. This cusp is probably formed when both clusters, surrounding each BH, merge and combine with the matter funnelled into the centre. We find this profile to be in agreement with observed post-merger distributions after the cusp has been destroyed. The time dependency we derive for the merger predicts that stalled BHs, if they exist at all, will preferentially be found at less than  ∼0.2 pc  distance. To test this prediction we compute the current semimajor axis of 12 candidates of ongoing mergers. We find all binaries unambiguously to be already in the last phase when they decay due to the emission of gravitational waves. Therefore, in striking contradiction with predictions of a depleted loss-cone, the absence of even a single source in the slingshot phase strongly supports our previous and current results: binaries merge due to the slingshot ejection of stars which have been funnelled into the central regions in the course of a galaxy collision.  相似文献   

10.
A detailed study of the star formation history of the Sagittarius dwarf spheroidal galaxy is performed through the analysis of data from the Sagittarius Dwarf Galaxy Survey (SDGS). Accurate statistical decontamination of the SDGS colour–magnitude diagrams (CMDs) allows us to obtain many useful constraints on the age and metal content of the Sgr stellar populations in three different regions of the galaxy.
A coarse metallicity distribution of Sgr stars is derived, ranging from [Fe/H]∼−2.0 to [Fe/H]∼−0.7, the upper limit being somewhat higher in the central region of the galaxy. A qualitative global fit to all the observed CMD features is attempted, and a general scheme for the star formation history of the Sgr dSph is derived. According to this scheme, star formation began at a very early time from a low metal content interstellar medium and lasted for several  Gyr, coupled with progressive chemical enrichment. The star formation rate (SFR) had a peak from 8 to 10  Gyr ago, when the mean metallicity was in the range −1.3≤[Fe/H]≤−0.7. After that maximum, the SFR rapidly decreased and a very low rate of star formation took place until ∼1–0.5  Gyr ago.  相似文献   

11.
With the help of a statistical parameter derived from optical spectra, we show that the current star formation rate of a galaxy, falling into a cluster along a supercluster filament, is likely to undergo a sudden enhancement before the galaxy reaches the virial radius of the cluster. From a sample of 52 supercluster-scale filaments of galaxies joining a pair of rich clusters of galaxies within the two-degree Field Redshift Survey region, we find a significant enhancement of star formation, within a narrow range between ∼2 and  3  h −170 Mpc  of the centre of the cluster into which the galaxy is falling. This burst of star formation is almost exclusively seen in the fainter dwarf galaxies  ( M B ≥−20)  . The relative position of the peak does not depend on whether the galaxy is a member of a group or not, but non-group galaxies have on average a higher rate of star formation immediately before falling into a cluster. From the various trends, we conclude that the predominant process responsible for this rapid burst is the close interaction with other galaxies falling into the cluster along the same filament, if the interaction occurs before the gas reservoir of the galaxy gets stripped off due to the interaction with the intracluster medium.  相似文献   

12.
If supermassive black holes in centres of galaxies form by merging of black hole remnants of massive Population III stars, then there should be a few black holes of mass one or two orders of magnitude smaller than that of the central ones, orbiting around the centre of a typical galaxy. These black holes constitute a weak perturbation in the gravitational potential, which can generate wave phenomena in gas within a disc close to the centre of the galaxy. Here, we show that a single orbiting black hole generates a three-arm spiral pattern in the central gaseous disc. The density excess in the spiral arms in the disc reaches values of 3–12 per cent when the orbiting black hole is about 10 times less massive than the central black hole. Therefore, the observed density pattern in gas can be used as a signature in detecting the most massive orbiting black holes.  相似文献   

13.
The smallest dwarf galaxies are the most straight forward objects in which to study star formation processes on a galactic scale. They are typically single cell star forming entities, and as small potentials in orbit around a much larger one they are unlikely to accrete much (if any) extraneous matter during their lifetime (either intergalactic gas, or galaxies) because they will typically lose the competition with the much larger galaxy. We can utilise observations of stars of a range of ages to measure star formation and enrichment histories back to the earliest epochs. The most ancient objects we have ever observed in the Universe are stars found in and around our Galaxy. Their proximity allows us to extract from their properties detailed information about the time in the early Universe into which they were born. A currently fashionable conjecture is that the earliest star formation in the Universe occurred in the smallest dwarf galaxy sized objects. Here I will review some recent observational highlights in the study of dwarf galaxies in the Local Group and the implications for understanding galaxy formation and evolution. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

14.
We present 21-cm H  i line observations of the blue compact dwarf galaxy NGC 1705. Previous optical observations show a strong outflow powered by an ongoing starburst dominating the H  ii morphology and kinematics. In contrast, most of the H  i lies in a rotating disc. An extraplanar H  i spur accounts for ∼8 per cent of the total H  i mass, and is possibly associated with the H  ii outflow. The inferred mass loss rate out of the core of the galaxy is significant, ∼0.2 − 2 M⊙ yr−1, but does not dominate the H  i dynamics. Mass model fits to the rotation curve show that the dark matter (DM) halo is dominant at nearly all radii and has a central density ρ0 ≈ 0.1 M⊙ pc−3: ten times higher than typically found in dwarf irregular galaxies, but similar to the only other mass-modelled blue compact dwarf, NGC 2915. This large difference strongly indicates that there is little evolution between dwarf irregular and blue compact dwarf types. Instead, dominant DM haloes may regulate the morphology of dwarf galaxies by setting the critical surface density for disc star formation. Neither our data nor catalogue searches reveal any likely external trigger to the starburst in NGC 1705.  相似文献   

15.
We confirm and extend the recent finding that the central surface density  μ0D≡ r 0ρ0  of galaxy dark matter haloes, where r 0 and  ρ0  are the halo core radius and central density, is nearly constant and independent of galaxy luminosity. Based on the co-added rotation curves (RCs) of ∼1000 spiral galaxies, the mass models of individual dwarf irregular and spiral galaxies of late and early types with high-quality RCs, and the galaxy–galaxy weak-lensing signals from a sample of spiral and elliptical galaxies, we find that  log μ0D= 2.15 ± 0.2  in units of  log(M⊙ pc−2)  . We also show that the observed kinematics of Local Group dwarf spheroidal galaxies are consistent with this value. Our results are obtained for galactic systems spanning over 14 mag, belonging to different Hubble types and whose mass profiles have been determined by several independent methods. In the same objects, the approximate constancy of  μ0D  is in sharp contrast to the systematical variations, by several orders of magnitude, of galaxy properties, including  ρ0  and central stellar surface density.  相似文献   

16.
We present and discuss optical, near-infrared and H  i measurements of the galaxy Markarian 1460 at a distance of 19 Mpc in the Ursa Major Cluster. This low-luminosity ( M B =−14) galaxy is unusual because (i) it is blue ( B − R =0.8) and has the spectrum of an H  ii galaxy, (ii) it has a light profile that is smooth and well fitted by an r 1/4 and not an exponential function at all radii larger than the seeing, and (iii) it has an observed central brightness of about μ B =20 mag arcsec−2 , intermediate between those of elliptical galaxies (on the bright μ B side) and normal low-luminosity dwarf irregular (on the low μ B side) galaxies. No other known galaxy exhibits all these properties in conjunction. On morphological grounds this galaxy looks like a normal distant luminous elliptical galaxy, since the Fundamental Plane tells us that higher luminosity normal elliptical galaxies tend to have lower surface-brightnesses. Markarian 1460 has 2×107 M⊙ of H  i and a ratio M (H  i )/ L B of 0.2, which is low compared to the typical values for star-forming dwarf galaxies. From the high surface-brightness and r 1/4 profile, we infer that the baryonic component of Markarian 1460 has become self-gravitating through dissipative processes. From the colours, radio continuum, H  i and optical emission line properties, and yet smooth texture, we infer that Markarian 1460 has had significant star formation as recently as ∼1 Gyr ago but not today.  相似文献   

17.
One of the predictions of the standard cold dark matter model is that dark haloes have centrally divergent density profiles. An extensive body of rotation curve observations of dwarf and low surface brightness galaxies shows the dark haloes of those systems to be characterized by soft constant-density central cores. Several physical processes have been proposed to produce soft cores in dark haloes, each one with different scaling properties. With the aim of discriminating among them we have examined the rotation curves of dark-matter-dominated dwarf and low surface brightness galaxies and the inner mass profiles of two clusters of galaxies lacking a central cD galaxy and with evidence of soft cores in the centre. The core radii and central densities of these haloes scale in a well-defined manner with the depth of their potential wells, as measured through the maximum circular velocity. As a result of our analysis we identify self-interacting cold dark matter as a viable solution to the core problem, where a non-singular isothermal core is formed in the halo centre surrounded by a Navarro, Frenk & White profile in the outer parts. We show that this particular physical situation predicts core radii in agreement with observations. Furthermore, using the observed scalings, we derive an expression for the minimum cross-section ( σ ) which has an explicit dependence with the halo dispersion velocity ( v ). If m x is the mass of the dark matter particle: σ m x ≈4×10−25 (100 km s−1  v −1) cm2 GeV−1.  相似文献   

18.
Galaxy harassment has been proposed as a physical process that morphologically transforms low surface density disc galaxies into dwarf elliptical galaxies in clusters. It has been used to link the observed very different morphology of distant cluster galaxies (relatively more blue galaxies with 'disturbed' morphologies) with the relatively large numbers of dwarf elliptical galaxies found in nearby clusters. One prediction of the harassment model is that the remnant galaxies should lie on low surface brightness tidal streams or arcs. We demonstrate in this paper that we have an analysis method that is sensitive to the detection of arcs down to a surface brightness of 29 B μ and we then use this method to search for arcs around 46 Virgo cluster dwarf elliptical galaxies. We find no evidence for tidal streams or arcs and consequently no evidence for galaxy harassment as a viable explanation for the relatively large numbers of dwarf galaxies found in the Virgo cluster.  相似文献   

19.
We present the results of new radio interferometer H  i line observations for the merging galaxy pair NGC 4038/9 ('The Antennae'), obtained using the Australia Telescope Compact Array. The results improve substantially with respect to those of van der Hulst and show in detail the two merging galactic discs and the two tidal tails produced by their interaction. The small edge-on spiral dwarf galaxy ESO 572–G045 is also seen near the tip of the southern tail, but distinct from it. It shows no signs of tidal interaction. The northern tidal tail of the Antennae shows no H  i connection to the discs and has an extension towards the west. The southern tidal tail is continuous, with a prominent H  i concentration at its tip, roughly at the location of the tidal dwarf galaxy observed optically by Mirabel, Dottori & Lutz. Clear velocity structure is seen along the tidal tails and in the galactic discs. Radio continuum images at 20 and 13 cm are also presented, showing the discs in detail.  相似文献   

20.
We investigate the old globular cluster (GC) population of 68 faint  ( M V > −16 mag)  dwarf galaxies located in the halo regions of nearby (≲12 Mpc) loose galaxy groups and in the field environment based on archival Hubble Space Telescope ( HST )/Advanced Camera for Surveys (ACS) images in F606W and F814W filters. The combined colour distribution of 175 GC candidates peaks at  ( V − I ) = 0.96 ± 0.07 mag  and the GC luminosity function turnover for the entire sample is found at   M V ,TO=−7.6 ± 0.11 mag  , similar to the old metal-poor Large Magellanic Cloud (LMC) GC population. Our data reveal a tentative trend of   M V ,TO  becoming fainter from late- to early-type galaxies. The luminosity and colour distributions of GCs in dIrrs show a lack of faint blue GCs (bGCs). Our analysis reveals that this might reflect a relatively younger GC system than typically found in luminous early-type galaxies. If verified by spectroscopy, this would suggest a later formation epoch of the first metal-poor star clusters in dwarf galaxies. We find several bright (massive) GCs which reside in the nuclear regions of their host galaxies. These nuclear clusters have similar luminosities and structural parameters as the peculiar Galactic clusters suspected of being the remnant nuclei of accreted dwarf galaxies, such as M54 and ωCen. Except for these nuclear clusters, the distribution of GCs in dIrrs in the half-light radius versus cluster mass plane is very similar to that of Galactic young halo clusters, which suggests comparable formation and dynamical evolution histories. A comparison with theoretical models of cluster disruption indicates that GCs in low-mass galaxies evolve dynamically as self-gravitating systems in a benign tidal environment.  相似文献   

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