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1.
We have conducted UBVRI and Hα CCD photometry of five barred galaxies (NGC 2523, NGC 2950, NGC 3412, NGC 3945 and NGC 5383),along with SPH simulations, in order to understand the origin of young stellar populations in the nuclei of barred galaxies. The Hα emission, which is thought to be emitted by young stellar populations, is either absent or strongly concentrated in the nuclei of early-type galaxies (NGC 2950, NGC 3412 and NGC 3945),while they are observed in the nuclei and circumnuclear regions of intermediate-type galaxies with strong bars (NGC 2523 and NGC 5383).SPH simulations of realistic mass models for these galaxies show that some disc material can be driven into the nuclear region by a strong bar potential. This implies that the young stellar populations in the circumnuclear regions of barred galaxies can be formed out of such gas. The existence of nuclear dust lanes is an indication of an ongoing gas inflow and extremely young stellar populations in these galaxies, because nuclear dust lanes such as those in NGC 5383 are not long-lasting features according to our simulations. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

2.
Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for more than 0.6 million stars in the interacting galaxies NGC 5194 and NGC 5195 of the M51 system. Stars of various ages have been identified on the constructed Hertzsprung-Russell diagram: blue and red supergiants, AGB stars, and red giants. The distance to M51 has been measured from the position of the tip of the red giant branch, D = 9.9 ± 0.7 Mpc. We have determined the change in the metallicity of red supergiants along the galactic radius in NGC 5194. Despite the gravitational interaction, the distribution of stars in NGC 5194 does not differ from that in isolated galaxies. The asymmetric stellar structures of NGC5195 (the so-called “feathers”) formed through the interaction of two galaxies have been found to consist mostly of AGB stars.  相似文献   

3.
Using the method of two-dimensional spectroscopy, we have investigated the kinematics and distribution of the gas and stars at the center of the early-type spiral galaxy NGC 7177 with a mediumscale bar as well as the change in the mean age of the stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius R = 1″.5−2″, where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate—basically, it has already turned into a pseudo-bulge as a result of secular dynamical evolution. The mean stellar age inside the star formation ring, in the galactic nucleus, is old, ∼10 Gyr.Outside, at a distance R = 6″−8″ from the nucleus, the mean age of the stellar population is ∼2 Gyr. If we agree that the bar in NGC 7177 is old, then, obviously, the star formation ring has migrated radially inward in the last 1–2 Gyr, in accordance with the predictions of some dynamical models.  相似文献   

4.
Based on deep Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for the spiral and lenticular galaxies NGC 4921 and NGC 4923 in the Coma cluster. The derived Hertzsprung-Russell diagrams show that, apart from the stellar population, NGC 4921 has several thousand globular clusters. The asymmetry of their apparent distribution proves that NGC 4921 is in close spatial proximity to NGC 4923. We have determined the positions of the peaks for the luminosity functions of globular clusters in the two galaxies, which has allowed us to measure the distance to this pair (D = 97±5 Mpc). In many cases, compact star-forming regions at the periphery of NGC 4921 are located at the ends of extended (up to 1.5 kpc) radial gas and dust filaments. The metallicity of young stars in these star-forming regions is close to that of the Sun (Z = 0.02).  相似文献   

5.
We examine the thermal and chemical evolution of gravitationally collapsing protostellar clouds with metallicity 0≤Z/Z ⊙≤1.During the first collapse stage, the temperatures are higher for lower metallicity clouds. However, in the course of the adiabatic contraction of transient cores, the evolutionary trajectories of the clouds converge to a curve that is determined only by fundamental physical constants. The trajectories coincide each other thereafter. The size of the stellar core at formation is the same regardless of metallicity. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

6.
Based on archival Hubble Space Telescope images, we have performed stellar photometry of several fields in the isolated spiral galaxies NGC 6503 and NGC 6946 with high peculiar velocities. Based on the TRGB method, we have determined the distances to the galaxies: D = 6.30 ± 0.10 Mpc for NGC 6503 and D = 6.72 ± 0.15 Mpc for NGC 6946. The current stellar content of the galaxies does not differ from that of other similar galaxies. The metallicity for young stars in NGC 6503 is Z = 0.02 (corresponding to the solar metallicity), while the metallicity for stars in NGC 6946 reaches Z = 0.05. Very few old globular clusters have been found in NGC 6946, while they have not been found at all in NGC 6503. The number density distribution of stars with different ages in NGC 6503 does not differ from the analogous distributions in other galaxies. The large sizes of the thick disk in NGC 6503, which is clearly seen up to 6 kpc from the galactic disk plane and whose possible extension is noticeable up to 8.6 kpc from the plane, are a difference. The sizes of the region occupied by red giants of the disk in NGC 6503 are 51 × 17 kpc, which are not much larger than the sizes of this galaxy from H I radio observations.  相似文献   

7.
Formation paradigms for massive galaxies have long centered around two antipodal hypotheses – the monolithic-collapse and the accretion/merger scenarios. Empirical data on the stellar contents of galaxy halos is crucial in order to develop galaxy formation and assembly scenarios which have their root in observations, rather than in numerical simulations. The Hubble Space Telescope (HST) has enabled us to study directly individual stars in the nearby E/S0 galaxies Cen A, NGC 3115, NGC 5102, and NGC 404. We here present and discuss HST single-star photometry in V and I bands. Using color-magnitude diagrams and stellar luminosity functions, we gauge the galaxies' stellar contents. This can be done at more than one position in the halo, but data with deeper limiting magnitudes are desired to quantify the variation of metallicity with galactocentric radius. We here compare the color distributions of red giant stars with stellar isochrones, and we intercompare the galaxies' halo populations, noting that their total absolute V magnitudes cover the range from about –21.5 to –17.5. In the future, we plan to model the stellar metallicity distributions with the aim to constrain chemical enrichment scenarios, a step towards unravelling the evolutionary history of elliptical and lenticular galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

8.
High spatial resolution visible and NIR observations of the bar of NGC 5850, a prototype of double barred spirals, together with visible intermediate and high dispersion spectra along the primary bar, are being used, with the aid of simple stellar population synthesis models, to investigate the mean age and metallicity of the different stellar components of the central part of the galaxy. The determination of stellar ages and metallicities would constrain theoretical scenarios for secondary bar formation and the evolution of barred spirals. Unfortunately, we cannot obtain a good fit with simple stellar populations (SSPs) to the spectral indices, so it can not give us, by now, insight into the mean stellar age and metallicity of the real populations in the central region of the galaxy. These preliminary results show a relatively old primary bar with metallicity about solar, although absolute values must be taken with care. The nucleus has a young stellar component, and is very dusty. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

9.
Low metallicity (−3 <=[Fe/H] < = −1) halo field giants exhibit the expected correlation of Na and Mg abundances, based on the assumption that Na is produced in the same nucleosynthetic sites as are the alpha elements, confirming a result noted by Sneden (1998). On the other hand, giants in at least some globular clusters (especially M13, but also M15 and NGC 6752) do not exhibit the Mg vs Na correlation found among halo field giants (Hanson et al., 1998). The very large [Na/Fe]-ratios and widely scattered [Mg/Fe]-ratios found among M13 giants depend, on the average, on evolutionary state and are probably induced by deep mixing of stellar envelopes through the CNO hydrogen-burning shell. Why M13 (and M15 and NGC 6752) giants should experience deep mixing whereas field halo giants in the same evolutionary state mix not at all is an anomaly unexplained by current theories of stellar evolution. By contrast, giants in the outer halo cluster NGC 7006 show little evidence of deep mixing (Kraft et al., 1998). These differences in the degree of deep mixing among stars in related, but different, stellar populations may be connected to the so-called `second parameter effect'. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

10.
Using multi-band photometric images of M51 and its companion NGC 5195 from ultraviolet to optical and infrared,we investigate spatially resolved stellar population properties of this interacting system with stellar population synthesis models.The observed infrared excess(IRX)is used to constrain dust extinction.Stellar mass is also inferred from the model fitting.By fitting observed spectral energy distributions(SEDs)with synthetical ones,we derive two-dimensional distributions of stellar age,metallicity,dust extinction and stellar mass.In M51,two grand-designed spiral arms extending from the bulge show young age,rich metallicity and abundant dust.The inter-arm regions are filled with older,metalpoorer and less dusty stellar populations.Except for the spiral arm extending from M51 into NGC 5195,the stellar population properties of NGC 5195 are quite featureless.NGC 5195 is much older than M51,and its core is very dusty with AV up to 1.67 mag and dense in stellar mass surface density.The close encounters might drive the dust in the spiral arm of M51 into the center of NGC 5195.  相似文献   

11.
Using a sample of 57 VLT FORS spectra in the redshift range 1.37< z < 3.40 and a comparison sample with 36 IUE spectra of local ( ) starburst galaxies we derive CIV equivalent width values and estimate metallicities of starburst galaxies as a function of redshift. Assuming that a calibration of the CIV equivalent widths in terms of the metallicity based on the local sample of starburst galaxies is applicable to high-z objects, we find a significant increase of the average metallicities from about 0.16 Z ⊙ at the cosmic epoch corresponding to z ≈ 3.2 to about 0.42 Z ⊙ at z ≈ 2.3. A significant further increase in metallicity during later epochs cannot be detected in our data. Compared to the local starburst galaxies our high-redshift objects tend to be overluminous for a given metallicity. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

12.
The goal of this work is to assess the expected scientific output from the photometric studies of globular clusters in the Large Magellanic Cloud with ESA's astrometric space mission GAIA. For this purpose we simulate GAIA photometry of individual stars in synthetic cluster populations, covering a large range of cluster ages and metallicities. We find that accurate effective temperatures (Δ T eff<10%) can be obtained from GAIA photometry down to V ∼ 18 for stars in populations within the studied metallicity range ([M/H] = -0.4 ... -1.7). GAIA will also provide photometric metallicities (Δ [M/H] ≲ 0.3 dex) for the cluster giants brighter than V ∼ 17.5. The knowledge of the effective temperature sand metallicities will allow to obtain accurate ages of stellar populations younger than about 1 Gyr using the usual procedure of main sequence turn-off point fitting. Ages of older stellar populations (≳ 1 Gyr) may be constrained from the isochrone fits to the giant branches in the observed CMDs. We conclude that GAIA will provide excellent opportunities for studying star formation histories far beyond the Milky Way, providing means for better understanding of stellar and galactic evolution in different astrophysical environments. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

13.
bdAbstract The open cluster NGC 2818 containing a planetary nebula has been observed inVRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope. The study extending to starsV ∼ 21 magnitude establishes the distance modulus as(m-M) 0 = 12.9 ±0.1 for the cluster. Based on the fitting of theoretical isochrones computed for solar metallicity, an age of 5(±1) × 108 years has been assigned to the cluster. Association of the planetary nebula with the cluster indicates that the progenitor mass of the planetary nebula on the main sequence is ≥2.5M⊙ Based on observations obtained with the Vainu Bappu Telescope.  相似文献   

14.
Blue Supergiants (BSGs) are the brightest stars in the universe at visual light with absolute magnitudes up to M V =−10 mag. They are ideal stellar objects for the determination of extragalactic distances, in particular, because the perennial uncertainties troubling most of the other stellar distance indicators, interstellar extinction and metallicity, do not affect them. The quantitative spectral analysis of low resolution spectra of individual BSGs provides accurate stellar parameters and chemical composition, which are then used to determine accurate reddening and extinction from photometry for each individual object. Accurate distances can be determined from stellar gravities and effective temperatures using the “Flux Weighted Gravity–Luminosity Relationship (FGLR)”.  相似文献   

15.
The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M≥100M ⊙. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main-Sequence stars for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300M ⊙ galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10−6 and 10−9, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously. For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations. Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low-metallicity massive stars are hotter and more compact and luminous than their metal-enriched counterparts. Due to their high temperatures, pregalactic stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics of the first stars have significant influence in predictions of the ionizing photon yields from the first luminous objects; also they develop large convective cores with important helium core masses which are important for explosion calculations.  相似文献   

16.
We have obtained Keck spectra for 16 globular clusters (GCs) associated with the merger remnant elliptical NGC 1052, as well as a long-slit spectrum of the galaxy. We derive ages, metallicities and abundance ratios from simple stellar population models using the recently published methods of Proctor & Sansom , applied to extragalactic GCs for the first time. A number of GCs indicate the presence of strong blue horizontal branches that are not fully accounted for in the current stellar population models. We find all of the GCs to be ∼13 Gyr old according to simple stellar populations, with a large range of metallicities. From the galaxy spectrum we find NGC 1052 to have a luminosity-weighted central age of ∼2 Gyr and metallicity of  [Fe/H]∼+0.6  . No strong gradients in either age or metallicity were found to the maximum radius measured  (0.3  r e≃ 1 kpc)  . However, we do find a strong radial gradient in α-element abundance, which reaches a very high central value. The young central starburst age is consistent with the age inferred from the H  i tidal tails and infalling gas of ∼1 Gyr. Thus, although NGC 1052 shows substantial evidence for a recent merger and an associated starburst, it appears that the merger did not induce the formation of new GCs, perhaps suggesting that little recent star formation occurred. This interpretation is consistent with 'frosting' models for early-type galaxy formation.  相似文献   

17.
We present the theoretical analysis of the Al I line formation in the spectra of late-type stars ignoring the assumption of local thermodynamic equilibrium (LTE). The calculations were based on the 39-level aluminum atom model for one-dimensional hydrostatic stellar atmosphere models with the parameters: T eff from 4000 to 9000 K, log g = 0.0–4.5, and metallicity [A] = 0.0;–1.0;–2.0;–3.0;–4.0. The aluminum atom model and the method of calculations were tested by the study of line profiles in the solar spectrum. We refined the oscillator strengths and Van-der-Vaals broadening constants C 6 of the investigated transitions. We conclude that the Al I atom is in the overionization state: the 3p level is underpopulated in the line formation region. This leads to the line weakening, as compared with the LTE results. The overionization effect becomes more pronounced with increasing temperature and decreasing metallicity. We show that the use of various atomic data (ionization cross-sections) for the low levels of Al I does not change the behavior of non-LTE deviations, whereas the value of these deviations varies essentially. For nine selected Al I lines we calculated the grids of theoretical non-LTE corrections (ΔX NLTE = logɛ NLTE − log ɛ LTE) to the Al abundances determinedwith the LTE assumption. The non-LTE corrections are positive and significant for the stars with temperatures T eff > 6000 K. These corrections weakly depend on log g, and increase with declining stellar metallicity.  相似文献   

18.
We investigate numerically the chemodynamical evolution of major disc–disc galaxy mergers in order to explore the origin of the mass-dependent chemical, photometric and spectroscopic properties observed in elliptical galaxies. We investigate especially the dependence of the fundamental properties on merger progenitor disc mass (M d). Three main results are obtained in this study:– More massive (luminous) ellipticals formed by galaxy mergers between more massive spirals have higher metallicity (Z) and thus show redder colours; the typical metallicity ranges from ∼ 1.0 solar abundance (Z∼ 0.02) for ellipticals formed by mergers with M d = 1010 M ⊙to ∼ 2.0 solar (Z∼ 0.04) for those with M d= 1012 M ⊙.– Both the Mg2 line index in the central part of ellipticals (R ≤ 0.1 R e) and the radial gradient of Mg2 (δ Mg2 / δ log R) are more likely to be larger for massive ellipticals. δ Mg2 / δ log R correlates reasonably well with the central Mg2 in ellipticals. For most of the present merger models, ellipticals show a positive radial gradient of the Hβ line index. – Both M/L B and M/L K (where M, L B, and L K are the total stellar mass of galaxy mergers, the B-band and the K-band luminosities, respectively) depend on galactic mass in such a way that more massive ellipticals have larger M/L B and smaller M/L K. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
Integrated broad-band colors and metallic lines cannot discriminate clearly between the effects of age and Z in old stellar populations. Such data are more sensitive to Z than to age. The Hβ feature provides a way to break this degeneracy. New measurements indicate that the mean stellar ages of typical E galaxy nuclei are fairly young, ranging from 2 to ∼12 Gyr. The outer parts of E galaxies are both older and more metal-poor than nuclei, consistent with the formation of E galaxies via mergers and starbursts. Age effects contribute strongly to the classic color-line strength sequence of E galaxies. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

20.
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