Dwarf Satellite Galaxies Research Articles

A Tale of Giants Stealing from Dwarfs

Cold Dark Matter simulations predict 10-100 x more dwarf satellite galaxies than are observed. Some of these ‘missing satellites’ may have been accreted, along with their globular clusters (GCs), by giant galaxies (Cote et al. 1998). But examples of dwarfs in the early stages of disruption have remained elusive.

The star-forming environment of an ultraluminous X-ray source in NGC 4559: an optical study

We have studied the candidate optical counterparts and the stellar population in the star-forming complex around the bright ultraluminous X-ray source (ULX) in the western part of the spiral galaxy NGC 4559, using the HST Wide Field Planetary Camera 2 (WFPC2), XMM-Newton/Optical Monitor and ground-based data. We find that the ULX is located near a small group of OB stars, but is not associated with any massive young clusters nor with any extraordinary massive stars. The brightest point source in the Chandra error circle is consistent with a single blue supergiant (BSG) of mass ≈20 M⊚ and age ≈10 Myr. A few other stars are resolved inside the error circle: mostly BSGs and red supergiants (RSGs) with inferred masses ≈10-15 M⊚ and ages ≈20 Myr. This is consistent with the interpretation of this ULX as a black hole (BH) accreting from a high-mass donor star in its supergiant phase, with mass transfer occurring via Roche-lobe overflow. The observed optical colours and the blue-to-red supergiant ratio suggest a low metal abundance for the stellar population: 0.2 ≲Z/Z⊚≲ 0.4 (using the Padua tracks), or 0.05 ≲Z/Z⊚≲ 0.2 (using the Geneva tracks). The age of the star-forming complex is ≲30 Myr. Ha images show that this star-forming region has a ring-like appearance. We propose that it is an expanding wave of star formation, triggered by an initial density perturbation, in a region where the gas was only marginally stable to gravitational collapse. We also suggest that the most likely trigger was a collision with a satellite dwarf galaxy going through the gas-rich outer disc of NGC 4559 less than 30 Myr ago. The culprit could be the dwarf galaxy visible a few arcsec north-west of the complex. If this is the case, this system is a scaled-down version of the Cartwheel galaxy. The X-ray data favour a BH more massive (M > 50 M⊚) than typical Milky Way BH candidates. The optical data favour a young BH originating in the recent episode of massive star formation; however, they also rule out an association with young massive star clusters (none are present in the X7 field). We speculate that other mechanisms may lead to the formation of relatively massive BHs (perhaps M∼ 50-100 M⊚) from stellar evolution processes in low-metallicity environments, or when star formation is triggered by galactic collisions.

Star Clusters in the Galactic Anticenter Stellar Structure and the Origin of Outer Old Open Clusters

The Galactic anticenter stellar structure (GASS) has been identified with excess surface densities of field stars in several large-area sky surveys and with an unusual, stringlike grouping of five globular clusters. At least two of these are diffuse, young clusters between open and globular types. Here we call attention to the fact that four younger open or transitional clusters extend the previously identified, stringlike cluster grouping, with at least one having a radial velocity consistent with the previously found GASS velocity-longitude trend. All nine clusters lie close to a plane tipped 17° to the Galactic plane. This planar orientation is used to forage for additional potential cluster members in the inner Galaxy, and a number are found along the same plane and stringlike sequence, including almost all 15 known outer, old open clusters. Tidal accretion of a dwarf satellite galaxy on a low-inclination orbit—perhaps the GASS system—appears to be a plausible explanation for the origin of the outer, old open and transitional clusters of the Milky Way. We use these clusters to explore the age-metallicity relation of the putative accreted GASS progenitor. Finally, we provide the first radial velocity of a star in the cluster BH 176 and discuss its implications.

The Canis Major Dwarf Galaxy

AbstractRecent observational evidence suggests that the Sagittarius dwarf galaxy represents the only major ongoing accretion event in the Galactic halo, accounting for the majority of stellar debris identified there. This paper summarises the recent discovery of another potential Milky Way accretion event, the Canis Major dwarf galaxy. This dwarf satellite galaxy is found to lie just below the Galactic plane and appears to be on an equatorial orbit. Unlike Sagittarius, which is contributing to the Galactic halo, the location and eventual demise of Canis Major suggests that it represents a building block of the thick disk.

A ULX in NGC 4559: A “Mini-Cartwheel” Scenario?

AbstractWe have studied the peculiar environment around a ULX in NGC 4559 (with Lx ≈ 2 x 1040 erg s–1 and MBH ≳ 50M⊙). The X-ray source is located near the rim of a young (age < 30 Myr), large (diameter ≈ 700 pc) ring-like star forming complex possibly triggered by the impact of a dwarf satellite galaxy through the gas-rich outer disk of NGC 4559. We speculate that galaxy interactions (including the infall of high-velocity clouds and satellites on a galactic disk) and low-metallicity environments offer favourable conditions for the formation of compact remnants more massive than “standard” X-ray binaries, and accreting from a massive Roche-lobe filling companion.

Solar Neighbourhood Age–Metallicity Relation Based on Hipparcos Data

AbstractWe derive age–metallicity relations (AMRs) and orbits for the 1658 solar neighbourhood stars for which accurate distances are measured by the Hipparcos satellite. The sample comprises 1382 thin disk stars, 229 thick disk stars, and 47 halo stars according to their orbital parameters. We find a considerable scatter for thin disk AMRs along the one-zone Galactic chemical evolution (GCE) model. Orbits and metallicities of thin disk stars show no clear relation to each other. The scatter along the AMR exists even if stars with the same orbits are selected. We examine simple extensions of one-zone GCE models which account for inhomogeneity in the effective yield and inhomogeneous star formation rate in the Galaxy. Both extensions of the one-zone GCE model cannot account for the scatter in the age–[Fe/H]–[Ca/Fe] relation simultaneously. We conclude, therefore, that the scatter along the thin disk AMR is an essential feature in the formation and evolution of the Galaxy. The AMR for thick disk stars shows that star formation terminated 8 Gyr ago in the thick disk. As previously reported, thick disk stars are more Ca-rich than thin disk stars with the same [Fe/H]. We find that thick disk stars show a vertical abundance gradient. These three facts — AMR, vertical gradient, and [Ca/Fe]–[Fe/H] relation — support monolithic collapse and/or accretion of satellite dwarf galaxies as likely thick disk formation scenarios.

Galaxy disruption in a halo of dark matter.

The relics of disrupted satellite galaxies have been found around the Milky Way and Andromeda, but direct evidence of a satellite galaxy in the early stages of disruption has remained elusive. We have discovered a dwarf satellite galaxy in the process of being torn apart by gravitational tidal forces as it merges with a larger galaxy's dark matter halo. Our results illustrate the morphological transformation of dwarf galaxies by tidal interaction and the continued buildup of galaxy halos.

A New Method of Estimating the Mass-to-Light Ratio of the Ursa Minor Dwarf Spheroidal Galaxy

Dwarf satellite galaxies undergo strong tidal forces produced by the main galaxy potential. These forces disturb the satellite and produce asymmetries in its stellar distribution, tidal tail formation, and modifications of the velocity dispersion profiles. Most of these features are observed in the Ursa Minor (UMi) dwarf spheroidal galaxy, which is one of the closest satellites of the Milky Way. These features show that UMi is being tidally disrupted and is probably not in virial equilibrium. The high-velocity dispersion of UMi would also be a reflection of this tidal disruption and is not the signature of the large dark matter content that would be deduced if virial equilibrium is assumed. In order to avoid the uncertainty produced when virial equilibrium is assumed in systems in strong tidal fields, we present a new method of calculating the mass-to-luminosity ratio of disrupted dwarf galaxies. This method is based on numerical simulations and only takes into account the shape of the dwarf density profile and the tidal tail brightness, but it does not depend on the kinematics of the dwarf. Applying this method to UMi, we obtain a mass-to-luminosity relation of 12, which is lower than the value obtained assuming virial equilibrium (M/L = 60). In addition, if UMi has a large dark matter content, it will be impossible to reproduce a tidal tail as luminous as the one observed.

${\it HIPPARCOS}$ age-metallicity relation of the solar neighbourhood disc stars

We derive age-metallicity relations (AMRs) and orbital parameters for the 1658 solar neighbourhood stars to which accurate distances are measured by the HIPPARCOS satellite. The sample stars comprise 1382 thin disc stars, 229 thick disc stars, and 47 halo stars according to their orbital parameters. We find a considerable scatter for thin disc AMR along the one-zone Galactic chemical evolution (GCE) model. Orbits and metallicities of thin disc stars show now clear relation each other. The scatter along the AMR exists even if the stars with the same orbits are selected. We examine simple extension of one-zone GCE models which account for inhomogeneity in the effective yield and inhomogeneous star formation rate in the Galaxy. Both extensions of the one-zone GCE model cannot account for the scatter in age – [Fe/H] – [Ca/Fe] relation simultaneously. We conclude, therefore, that the scatter along the thin disc AMR is an essential feature in the formation and evolution of the Galaxy. The AMR for thick disc stars shows that the star formation terminated 8 Gyr ago in the thick disc. As already reported by Gratton et al. ([CITE]) and Prochaska et al. ([CITE]), thick disc stars are more Ca-rich than thin disc stars with the same [Fe/H]. We find that thick disc stars show a vertical abundance gradient. These three facts, the AMR, vertical gradient, and [Ca/Fe]–[Fe/H] relation, support monolithic collapse and/or accretion of satellite dwarf galaxies as likely thick disc formation scenarios.

Metallicity and Kinematics of M31’s Outer Stellar Halo from a Keck Spectroscopic Survey

We present first results from a spectroscopic survey designed to examine the metallicity and kinematics of individual red giant branch stars in the outer halo of the Andromeda spiral galaxy (M31). This study is based on multislit spectroscopy with the Keck II 10 m telescope and Low Resolution Imaging Spectrograph of the Ca II near-infrared triplet in 99 M31 halo candidates in a field at R = 19 kpc on the southeast minor axis with brightnesses from 20 < I < 22. The spectra are used to isolate M31 halo red giants from foreground Milky Way dwarf stars, faint compact background galaxies, and M31 disk giants. The observed distribution of radial velocities is well fitted by an equal mix of foreground Milky Way dwarf stars, drawn from a standard Galactic model and with velocities v 0 km s-1, and M31 halo giants represented by a Gaussian of width σ ~ 150 km s-1 centered on its systemic velocity of v ≈ -300 km s-1. A secure sample of 29 M31 red giant stars is identified on the basis of radial velocity (v < -220 km s-1) and, in the case of four intermediate-velocity stars (-160 < v < -220 km s-1), broadband B-I color. For this sample of objects, there is rough agreement between the metallicities derived in independent ways: two different calibrations of the Ca II absorption-line strength and a photometric estimate based on fitting model stellar isochrones to an object's location in a (B-I, I) color-magnitude diagram. The [Fe/H] distribution of M31 halo giants has an rms spread of at least 0.6 dex and spans the 2 dex range over which the abundance measurement methods are calibrated. The mean/median metallicity of the M31 halo is about [Fe/H] = -1.9 to -1.1 dex (depending on the details of metallicity calibration and sample selection) and possibly higher: the high-metallicity end of the distribution is poorly constrained by our data since the selection function for the secure M31 sample excludes over 80% of the giants in solar/supersolar metallicity range. Possible reasons are explored for the apparent discrepancy between the mean [Fe/H] found in our spectroscopic survey (corrected for metallicity selection bias) and the slightly higher mean values found in earlier photometric studies. Field halo red giants in M31 appear to be somewhat more metal-rich on average than their Milky Way counterparts. The M31 halo [Fe/H] distribution is comparable to that of M31 globular clusters, Galactic globular clusters, and Local Group dwarf satellite galaxies. The data in this 19 kpc outer halo field are broadly consistent with a scenario in which the halo is built from the accretion of small stellar subsystems. There are four stars in the secure M31 sample that have particularly strong Ca II lines, indicating solar metallicity, at a common velocity of ≈-340 km s-1 close to the galaxy's systemic velocity, similar to what might be expected for M31 disk giants on the minor axis. An extrapolation of the inner disk brightness profile, however, falls far short of accounting for these four stars—the disk would instead have to be very large (Rdisk 80 kpc) and/or warped. More likely, these four stars represent a metal-rich debris trail from a past accretion event in the halo.

Satellite decay in flattened dark matter haloes

We carry out a set of self-consistent N-body calculations to compare the decay rates of satellite dwarf galaxies orbiting a disc galaxy embedded in a dark matter halo (DMH). We consider both spherical and oblate axisymmetric DMHs of aspect ratio q_h=0.6. The satellites are given different initial orbital inclinations, orbital periods and mass. The live flattened DMHs with embedded discs and bulges are set-up using a new fast algorithm, MaGalie (Boily, Kroupa and Pe\~{n}arrubia 2001). We find that the range of survival times of satellites within a flattened DMH becomes of the order of 100% larger than the same satellites within a spherical DMH. In the oblate DMH, satellites on polar orbits have the longest survival time, whereas satellites on coplanar prograde orbits are destroyed most rapidly. The orbital plane of a satellite tilts as a result of anisotropic dynamical friction, causing the satellite's orbit to align with the plane of symmetry of the DMH. Polar orbits are not subjected to alignment. Therefore the decay of a satellites in an axisymmetric DMH may provide a natural explanation for the observed lack of satellites within (0-30) degrees of their host galaxy's disc (Holmberg 1969; Zaritsky and Gonz\'alez 1999). The computations furthermore indicate that the evolution of the orbital eccentricity $e$ is highly dependent of its initial value e(t=0) and the DMH's shape. We also discuss some implications of flattened DMHs for satellite debris streams.

Distribution of the disrupted satellite galaxies as a function of metallicity

Recent observations show that some dwarf satellite galaxies fall into the gravitational well of the Galaxy while being disrupted by the tidal force. As a consequence, the Galactic halo suers contamination from this process, the stellar populations of the tidal disrupted satellites make an additional contribution and alter the intrinsic metallicity distribution of the halo stars. The distribution of this kind of disrupted satellite galaxy as a function of metallicity is investigated in this paper. The model is limited to one class of dwarf spherical satellite galaxy, that is assumed to have similar abundance patterns to the Galactic eld halo stars. We discuss their distributions through the links between minor merger processes in the history of the Galaxy and the observed distribution of metal-poor eld halo stars. The upper limit of 35% of the metal-poor halo eld stars was established by merging of this kind of satellite galaxy with characteristic mass Msat, and adopting the mass-metallicity relation among dwarf spheroidal galaxy, the distribution of the disrupted satellite galaxy as a function of metallicity is derived to reproduce the observed metallicity distribution of the extremely metal-poor halo eld stars in the Galaxy. The problem of missing metal-free halo stars in the Galaxy is explained in this model as well.

On the Distribution of Orbital Poles of Milky Way Satellites

In numerous studies of the outer Galactic halo some evidence for accretion has been found. If the outer halo did form in part or wholly through merger events, we might expect to find coherent streams of stars and globular clusters following orbits similar to those of their parent objects, which are assumed to be present or former Milky Way dwarf satellite galaxies. We present a study of this phenomenon by assessing the likelihood of potential descendant "dynamical families" in the outer halo. We conduct two analyses: one that involves a statistical analysis of the spatial distribution of all known Galactic dwarf satellite galaxies (DSGs) and globular clusters, and a second, more specific analysis of those globular clusters and DSGs for which full phase space dynamical data exist. In both cases our methodology is appropriate only to members of descendant dynamical families that retain nearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) is considered a paradigm for the type of merger/tidal interaction event for which we are searching, we also undertake a case study of the Sgr system and identify several globular clusters that may be members of its extended dynamical family. In our first analysis, the distribution of possible orbital poles for the entire sample of outer (Rgc > 8 kpc) halo globular clusters is tested for statistically significant associations among globular clusters and DSGs. Our methodology for identifying possible associations is similar to that used by Lynden-Bell & Lynden-Bell, but we put the associations on a more statistical foundation. Moreover, we study the degree of possible dynamical clustering among various interesting ensembles of globular clusters and satellite galaxies. Among the ensembles studied, we find the globular cluster subpopulation with the highest statistical likelihood of association with one or more of the Galactic DSGs to be the distant, outer halo (Rgc > 25 kpc), second-parameter globular clusters. The results of our orbital pole analysis are supported by the great circle cell count methodology of Johnston, Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC 6229, NGC 7006, and Pyxis are predicted to be among those most likely to show the clusters to be following stream orbits, since these clusters are responsible for the majority of the statistical significance of the association between outer halo, second-parameter globular clusters and the Milky Way DSGs. In our second analysis, we study the orbits of the 41 globular clusters and six Milky Way-bound DSGs having measured proper motions to look for objects with both coplanar orbits and similar angular momenta. Unfortunately, the majority of globular clusters with measured proper motions are inner halo clusters that are less likely to retain memory of their original orbit. Although four potential globular cluster/DSG associations are found, we believe three of these associations involving inner halo clusters to be coincidental. While the present sample of objects with complete dynamical data is small and does not include many of the globular clusters that are more likely to have been captured by the Milky Way, the methodology we adopt will become increasingly powerful as more proper motions are measured for distant Galactic satellites and globular clusters, and especially as results from the Space Interferometry Mission (SIM) become available.

Hierarchical Galaxy Formation and Substructure in the Galaxy’s Stellar Halo

We develop an explicit model for the formation of the stellar halo from tidally disrupted, accreted dwarf satellites in the cold dark matter (CDM) framework, focusing on predictions testable with the Sloan Digital Sky Survey (SDSS) and other wide-field surveys. Subhalo accretion and orbital evolution are calculated using a semianalytic approach based on the extended Press-Schechter formalism. Motivated by our previous work, we assume that low-mass subhalos (vc < 30 km s-1) form significant populations of stars only if they accreted a substantial fraction of their mass before the epoch of reionization. With this assumption, the model reproduces the observed velocity function of galactic satellites in the Local Group, solving the "dwarf satellite problem" without modifying the basic tenets of the popular Λ + CDM cosmological scenario. The tidally disrupted satellites in this model yield a stellar distribution whose total mass and radial density profile are consistent with those observed for the Milky Way stellar halo. Most significantly, the model predicts the presence of many large-scale, coherent substructures in the outer halo. These substructures are remnants of individual tidally disrupted dwarf satellite galaxies. Substructure is more pronounced at large galactocentric radii because of the smaller number density of tidal streams and the longer orbital times. This model provides a natural explanation for the coherent structures in the outer stellar halo found in the SDSS commissioning data, and it predicts that many more such structures should be found as the survey covers more of the sky. The detection (or nondetection) and characterization of such structures could eventually test variants of the CDM scenario, especially those that aim to solve the dwarf satellite problem by enhancing satellite disruption.

The fate of dwarf satellite galaxies (or: Can we observe soft merging?)

AbstractDwarf satellite galaxies and globular clusters bound to luminous (in particular early type) galaxies, can be steered by interactions to orbits penetrating deep into the potential of their parent galaxy, where the outer stars are eroded and the dwarf satellite is completely destroyed in a relatively short time, resulting in very faint disturbances (soft merging) in the form of faint rings in the parent galaxy.With special image processing methods we have found faint structures in E and SO galaxies, which for many reasons (mass, statistics, etc.) can be interpreted as observational evidence of this type of soft merging. This debris of dwarf satellites can provide a considerable part of the mass of the parent galaxy.

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 A) spectra centred on the CN bands at ∼8000 A, 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.

THE RELATIVE AGES OF GALACTIC GLOBULAR CLUSTERS

ABSTRACT We discuss the present state of knowledge and thought concerning the spread in age found among Galactic globular clusters, with some discussion of the implications for what happened during the earliest stages of the formation of the Milky Way Galaxy. Differnetial observational techniques to derive the relative age differences among clusters of similar metallicity are discussed in detail. We conclude that as of the current date (mid-April 1996) the state of the field is still somewhat muddled. However, we believe that there is now a substantial body of evidence--including a particularly revealing intercomparison of the color-magnitude diagrams of NGC 1851, NGC 288, and NGC 362 presented here--indicating that age is not the dominant second parameter determining the shape of globular clusters' horizontal branches. If our assertion is correct, then apart from a handful of anomalous clusters that my well have been captured from a satellite dwarf galaxy, there is no strong evidence either for a significant spread in age among clusters of a given metal abundance or for a systematic variation of mean age with Galactocentric distance. On the question of whether there is a significant age dfference between metal-poor and metal-rich clusters, we feel compelled to fall back on the Scottish verdict: "Unproven." Data now being collected by numerous groups in various sub-disciplines may resolve the remaining controversy within a few years.

A dwarf satellite galaxy in Sagittarius

WE have detected a large, extended group of comoving stars in the direction of the Galactic Centre, which we interpret as belonging to a dwarf galaxy that is closer to our own Galaxy than any other yet known. Located in the constellation of Sagittarius, and on the far side of the Galactic Centre, it has not previously been seen because of the large number of foreground stars (in the Milky Way) in that direction. Following convention, we propose to call it the Sagittarius dwarf galaxy. Its properties are similar to those of the eight other dwarf spheroidal companions to the Milky Way, and it is comparable in size and luminosity to the largest of them— the Fornax system. The Sagittarius dwarf is elongated towards the plane of the Milky Way, suggesting that it is undergoing some tidal disruption before being absorbed by the Milky Way.

Can neutrino decay-driven mock gravity save hot dark matter?

The radiative decay of a 30 eV neutrino with a lifetime of order 10^23-24^ s has recently been shown to yield a satisfactory explanation of a wide range of problems in astrophysics. Melott in 1985 showed how such a decay can explain morphological features of dwarf satellite galaxies. Melott, McKay, and Ralston in 1988 demonstrated that such a decay process can ionize the IGM to the Gunn-Peterson limit. Finally, in a series of papers Sciama has addressed the issue of radiative neutrino decay and successfully applied it to an even broader range of problems. In this paper we investigate whether the photon flux generated by the radiative decay of a massive neutrino is capable of generating sufficient radiation pressure to cause a "mock gravitational" collapse of primordial hydrogen clouds. We show that when using neutral hydrogen as a source of opacity for mock gravity the time scale for mock gravitational collapse is significantly larger than the expansion time scale. Thus, the model fails as a source of galactic seed perturbations. Furthermore, we argue that nonlinear feedback mechanisms will be unable to increase the collapse rate of the cloud under mock gravity.