首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 875 毫秒
1.
We analyzed a sample of 44 candidates for Tidal Dwarf Galaxies(TDGs) previously selected on optical imaging with low resolution spectroscopy. Most of the TDG candidates have the same redshift as the interacting system, 3 of them very likely are low metallicity dwarf galaxies in projection, and 13 show signs of kinematics decoupled from their hosting tidal tail. Subsequent NIR photometry in combination with evolutionary synthesis modeling allows us to derive stellar masses, star formation rates, and the ratio of old to young stars within each TDG candidate. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

2.
Summary. Hubble's (1936, p. 125) view that the Local Group (LG) is “a typical, small group of nebulae which is isolated in the general field” is confirmed by modern data. The total number of certain and probable Group members presently stands at 35. The half-mass radius of the Local Group is found to be kpc. The zero-velocity surface, which separates the Local Group from the field that is expanding with the Hubble flow, has a radius Mpc. The total mass of the LG is . Most of this mass appears to be concentrated in the Andromeda and Milky Way subgroups of the LG. The total luminosity of the Local Group is found to be :. This yields a mass-to-light ratio (in solar units) of . The solar motion with respect to the LG is \,km s, directed towards an apex at , and . The velocity dispersion within the LG is km s. The galaxies NGC 3109, Antlia, Sextans A and Sextans B appear to form a distinct grouping with kpc relative to the LG, that is located beyond the LG zero-velocity surface at a distance of 1.7 Mpc from the Local Group centroid. The luminosity distribution of the LG has a slope . This value is significantly less negative than that which is found in rich clusters of galaxies. The luminosity distribution of the dwarf spheroidal galaxies is steeper than that for dwarf irregulars. Furthermore the dSph galaxies are strongly concentrated within the Andromeda and Milky Way subclusters of the Local Group, whereas the majority of dIr galaxies appear to be free-floating members of the LG as a whole. With the possible exception of Leo I and Leo A, most LG members appear to have started forming stars simultaneously Gyr ago. Many of the galaxies, for which evolutionary data are available, appear to have shrunk with time. This result is unexpected because Hubble Space Telescope observations appear to show galaxies at to be smaller than they are at . In the Large Magellanic Cloud the rate of cluster formation was low for a period that extended from Gyr to Gyr ago. The rate of cluster formation may have increased more rapidly 3–5 Gyr ago, than did the rate of star formation. The reason for the sudden burst of cluster formation in the LMC Gyr ago remains obscure. None of the dwarf galaxies in the LG appears to have experienced a starburst strong enough to have produced a “boojum”. Received 14 April 1999  相似文献   

3.
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   

4.
Summary. Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial r?le in the cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields, outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metal-poor galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and the physical processes that are at work in these objects. The question on how they are related and may evolve from one class of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe, we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in the universe. We discuss what do QSO absorption lines and known distant galaxies tell us already? We illustrate the importance of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator and discuss the possibility to detect nearly metal-free galaxies at high redshift from Ly emission. Received 19 August 1999 / Published online: 15 February 2000  相似文献   

5.
We investigate the process of galaxy formation as can be observed in the only currently forming galaxies - the so-called Tidal Dwarf Galaxies, hereafter TDGs - through observations of the molecular gas detected via its CO (Carbon Monoxide) emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (Braine, Lisenfeld, Duc and Leon, 2000: Nature 403, 867; Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks: 2001, A&A 378, 51), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 108 M ⊙. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H2. Although TDGs share many of the properties of small irregulars, their CO luminosity is much greater (factor ∼ 100) than that of standard dwarf galaxies of comparable luminosity. This is most likely a consequence of the higher metallicity (≳sim 1/3 solar) of TDGs which makes CO a good tracer of molecular gas. This allows us to study star formation in environments ordinarily inaccessible due to the extreme difficulty of measuring the molecular gas mass. The star formation efficiency, measured by the CO luminosity per Hα flux, is the same in TDGs and full-sized spirals. CO is likely the best tracer of the dynamics of these objects because some fraction of the HI near the TDGs may be part of the tidal tail and not bound to the TDG. Although uncertainties are large for individual objects, as the geometry is unknown, our sample is now of eight detected objects and we find that the ‘dynamical’ masses of TDGs, estimated from the CO line widths, seem not to be greater than the ‘visible’ masses (HI + H2 + a stellar component). Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin. We provide strong evidence that TDGs are self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

6.
Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved ‘star-like’ objects with red-shifts appropriate to the Fornax-Cluster. These objects have intrinsic sizes of ≈ 100 pc and absolute B-band magnitudes in the range - 14 < MB < -11.5 mag and lower limits for the central surface brightness μB ≥ 23 mag/arcsec2 (Phillipps et al., 2001, Hilker et al., 1999), and so appear to constitute a new population of ultra-compact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (= clusters of star clusters; not to confuse with super stellar clusters (SSC)) by P. Kroupa (1998), which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of super-clusters in a tidal field. The YMCs merge on a few super-cluster crossing times. Super-clusters that are initially as concentrated and massive as Knot S in the interacting Antennae galaxies (Whitmore et al., 1999) evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

7.
Our knowledge about the dynamics, the chemical abundances and the evolutionary histories of the more luminous dwarf spheroidal (dSph) galaxies is constantly growing. However, very little is known about the enrichment of the ultra‐faint systems recently discovered in large numbers in large sky surveys. Current low‐resolution spectroscopy and photometric data indicate that these galaxies are highly dark matter dominated and predominantly metal poor. On the other hand, recent high‐resolution abundance analyses indicate that some dwarf galaxies experienced highly inhomogeneous chemical enrichment, where star formation proceeds locally on small scales. In this article, I will review the kinematic and chemical abundance information of the Milky Way satellite dSphs that is presently available from low‐ and high resolution spectroscopy. Moreover, some of the most peculiar element and inhomogeneous enrichment patterns will be discussed and related to the question of to what extent the faintest dSph candidates could have contributed to the Galactic halo, compared to more luminous systems (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
Using HST and ground-based optical and NIR imaging data, we investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 possesses an extended low-surface brightness (LSB) old stellar population underlying its star-forming (SF) regions. We show that the exponential intensity decrease observed in the filamentary LSB envelope of the BCD out to 18″ (1.3 kpc at the adopted distance of 15 Mpc) is not due to an evolved stellar disc, but rather due to extended ionized gas emission. Broad-band images reveal, after subtraction of nebular line emission, a compact stellar LSB component extending slightly beyond the SF regions. This stellar host, being blue over a radius range of 5 exponential scale lengths and showing little colour contrast to the SF component, differs strikingly from the red LSB host of standard BCDs. This fact, in connection with the blue colours of component I Zw 18 C (see discussion in Papaderos et al. 2002), suggests that most of the stellar mass in I Zw 18 has formed within the last 0.5 Gyr. Furthermore, we show that the exponential intensity fall-off in the filamentary ionized envelope of I Zw 18 is not particular to this system but a common property of the ionized halo of many SF dwarf galaxies on galactocentric distances of several kpc. In the absence of an appreciable underlying stellar background, extended ionized gas emission dominates in the periphery of I Zw 18, superficially resembling an exponential stellar disc on optical surface brightness profiles. The case of I Zw 18 suggests caution in the search of more distant young galaxy candidates. Intense SF activity in the early phase of dwarf galaxy formation may result in an extended ionized gas halo which can be mistaken for an evolved stellar disc by studying only its exponential surface brightness profile. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

9.
We show that simple models of the chemical and spectrophotometric evolution of galaxies can be used to explore the properties of present-day galaxies and especially the causes of the observed variety among disc galaxies. We focus on the link between `classical' spirals and Low Surface Brightness galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

10.
Luminous and Ultraluminous infrared galaxies (ULIRGs) contain the most intense regions of star formation in the local universe. Because molecular gas is the fuel for current and future star formation, the physical properties and distribution of the warm, dense molecular gas are key components for understanding the processes and timescales controlling star formation in these merger and merger remnant galaxies. We present new results from a legacy project on the Submillimeter Array which is producing high resolution images of a representative sample of galaxies with log L FIR >11.4 and D<200 Mpc.  相似文献   

11.
Galactic winds are important in recycling energy and metals in galaxies. They are also suspected to be the primary mechanism in the enrichment of the intergalactic medium. New observations are revealing the ubiquity of galactic winds, particularly at high redshift. We review the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe.  相似文献   

12.
We study the spatial orientation of 5 169 galaxies that have radial velocity 3 000 to 5 000 km s−1. The ‘position angle–inclination’ method is used to find the spin vector and the projections of spin vector of the galaxy rotation axes. The spatial isotropic distribution is assumed to examine the non-random effects. For this, we have performed chi-square, Fourier, and auto-correlation tests. We found a random alignment of spin vectors of total galaxies with respect to the equatorial coordinate system. The spin vector projections of total galaxies is found to be oriented tangentially with respect to the equatorial center. The spiral galaxies show a similar orientation as shown by the total sample. Five subsamples of barred spiral (late-type) galaxies show a preferred alignment. However, early-type barred spirals show a random orientation. A weak morphological dependence is noticed in the subsamples of late type barred spirals. A comparison with the previous works and the possible explanation of the results will be presented.  相似文献   

13.
At high redshift the ubiquity of outflows and winds in strongly star‐forming galaxies has been demonstrated using rest frame UV absorption lines. In the cases with optical emission lines, the studies mostly had to rely on low and intermediate dispersion spectra. This implies that for detailed studies of galactic wind physics we have to use local objects. In particular, dwarf galaxies are well suited to extrapolation to high redshift protogalaxies. Several kinematic studies of strongly starforming dwarf galaxies using Fabry‐Pérot and IFU spectrographs exist. Unfortunately, similar as for high redshift galaxies the employed spectral resolution is often significantly higher that the thermal line width. As a result faint high velocity features and details of the turbulent motion are hidden or unresolved. Here we will present an analysis of the ionized gas kinematics of the prototypical star‐forming irregular galaxy NGC 4449 using long‐slit, high‐dispersion échelle spectra. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
Compact relativistic jets in active galactic nuclei offer an effective tool for investigating the physics of nuclear regions in galaxies. The emission properties, dynamics, and evolution of jets in AGN are closely connected to the characteristics of the central supermassive black hole, accretion disk and broad-line region in active galaxies. Recent results from studies of the nuclear regions in several active galaxies with prominent outflows are reviewed in this contribution.  相似文献   

15.
Research on two-dimensional (2-D) properties of galaxies is a significant component of the study of galaxy formation and evolution. Through the spatial distribution of physical properties (derived from integrated luminosity and spectroscopy) of galaxies, we are allowed to realize the inner environment and evolution history of each individual galaxy and finally answer how galaxies were assembled. In this paper, with reviewing previous work, we present a proposal for study on 2-D properties of nearby galaxies. In our prospective work, we will make use of multi-wavelength data covering a range from ultraviolet to far-infrared to determine the distributions of properties such as age, metallicity and dust-reddening in nearby galaxies, and try to remove the degeneracy among them. Combining with surface photometry and spectroscopy, we will also analyze the distribution of HII regions and star formation properties in galaxies. In our future plan, the World Space Observatory for Ultraviolet (WSO/UV) will be applied to our research and allow detail diagnosis of nearby galaxies at ultraviolet band.  相似文献   

16.
利用SDSS光谱,研究了IRAS卫星亮红外源星表中的盘状星系中的恒星形成性质,并着重探讨了棒对星系核区恒星形成活动的影响.利用星族合成的方法得到了每个样本星系核区的恒星组成性质、恒星形成活动的强度等信息,并比较了星系整体和核区恒星形成性质的差异.得到的结论:除去相互作用,样本中的棒星系显示出比非棒旋星系更强的核区恒星形成活动和更多的年轻星族成分.  相似文献   

17.
This is a study concerning the investigation of galaxy formation and evolution in small-scale structures and the influence of the environment on the properties of galaxies. The environment plays a key role in the evolution of galaxies since it governs the type of encounters. We present results from low-resolution spectroscopy and R-band surface photometry of multiplets of galaxies found in low-density environments and compare them to cluster environments. Properties such as induced galaxy activity, star formation enhancements, AGN activity and the connection between merging and galaxy morphology are investigated. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

18.
We discuss the properties of the galaxies in about 60 rich, nearby clusters, using kinematic data from the ESO Nearby Abell Cluster Survey, combined with new imaging data. The images were used to classify the galaxies, and to recalibrate the galaxy types derived from the ENACS spectra; this yields galaxy-type estimates for about4800 galaxies. For about 1200 galaxies, a bulge/disk decomposition could be made, which yields sizes and luminosities of bulges and disks. From the projected radial distances and relative l.o.s.-velocities we derived the galaxy ensembles with significantly different phase-space distributions. We find that galaxies in and outside substructure must be distinguished. The morphological composition of the substructures appears to vary with projected radius. Outside substructures, 4 galaxy ensembles must be defined: viz. the brighest Es, the other early-type galaxies, the early spirals(Sa-Sb), and the late spirals (including the emission-line galaxies). We also study the morphology-density relation, and we find that the segregation of the late spirals is driven mostly by global factors, while the segregation of Es, S0s and early spirals is driven mostly by local density. The properties of early spirals and S0ssupport the picture in which early spirals transform into S0s, while the properties of the late spirals do not support such a relation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
Interacting galaxies like the famous Antennae (NGC 4038/4039) or Stephan's Quintet (HCG 92) show considerable star forming activity in their tidal arms. High resolution images (e.g. from HST-observations) indicate that these regions consist of up to hundreds of massive stellar clusters or tidal dwarf galaxies (TDG). In this paper we want to investigate the future fate of these clusters of massive star clusters (in this work called super-clusters). We simulate compact super-clusters in the tidal field of a host-galaxy and investigate the influence of orbital and internal parameters on the rate and timescale of the merging process. We show that it is possible that such configurations merge and build a dwarf galaxy, which could be an important mechanism of how long-lived dwarf satellite galaxies form. A detailed study of the merger object will appear in a follow-up paper.This revised version was published online in October 2005 with corrections to the Cover Date.  相似文献   

20.
Results are presented from a detailed spectrophotometric study of a unique system of physically coupled galaxies whose spectra contain identical emission lines. This system consists of four galaxies, three of which have a double or multiple structure in their central portions and are, by definition, galaxies with two or multiple nuclei. Two of these objects are the galaxies with an ultraviolet excess (UVE), Mark 261 and Mark 262, while one is a galaxy made up of two identical starlike nuclei that are referred to as the “twin objects.” In the DSS2 charts, the latter show up as two adjacent stars without any surroundings. However, there are two condensations between these starlike nuclei. The fourth object has a triplet structure. It is shown here that all these objects have the same emission (line) spectra and red shifts. All the physical characteristics that can be determined from the spectra are determined: red shifts, relative intensities of emission lines, their equivalent widths, distances to the galaxies, etc. It is concluded that the members of this system have a common origin, more or less consistent with Ambartsumyan's idea of the fragmentation of an isolated, high density body. __________ Translated from Astrofizika, Vol. 48, No. 4, pp. 503–514 (November 2005).  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号