Central regions of the early-type galaxies in the NGC 3169 group

We have investigated the central regions of the galaxies in the NGC 3169/NGC 3166/NGC 3156 group with the multipupil fiber spectrograph of the 6-m telescope; the first (central) galaxy in the group is a spiral (Sa) one and the other two galaxies are lenticular ones. The group is known to have an extended HI cloud with a size of more than 100 kpc that is associated in its position, orientation, and rotation with the central galaxy NGC 3169. The mean age of the stellar populations in the centers of all three galaxies has been found to be approximately the same, ～1 Gyr. Since the galaxies are early-type ones and since NGC 3166 and NGC 3156 show no global star formation, we are dealing here with a synchronous star formation burst in the centers of all three galaxies.     

Environmental effects on satellite galaxies: the link between concentration, size and colour profile

Using the Sloan Digital Sky Survey Data Release 4 group catalogue of Yang et al., we investigate sizes, concentrations, colour gradients and surface brightness profiles of central and satellite galaxies. We compare central and satellite galaxies at fixed stellar mass, in order to disentangle environmental from stellar mass dependencies. Early- and late-type galaxies are defined according to concentration. We find that at fixed stellar mass, late-type satellite galaxies have smaller radii and larger concentrations than late-type central galaxies. No such differences are found for early-type galaxies. We have also constructed surface brightness and colour profiles for the central and satellite galaxies in our sample. We find that late-type satellite galaxies have a lower surface brightness and redder colours than late-type central galaxies. We show that all observed differences between satellite and central galaxies can be explained by a simple fading model, in which the star formation in the disc decreases over time-scales of 2–3 Gyr after a galaxy becomes a satellite. Processes that induce strong morphological changes (e.g. harassment) and processes that strip the galaxy of its entire interstellar medium need not to be invoked in order to explain the environmental dependencies we find.     

Stellar subsystems of different ages in spiral and irregular galaxies

Based on archival Hubble Space Telescope images, we have performed stellar photometry for eight edge-on spiral and irregular galaxies. We have identified stars of three ages in the derived Hertzsprung-Russell diagrams and constructed their number density distribution perpendicularly to the plane of the galactic disk. The sizes of the stellar subsystems of young (up to 100 Myr), middle (0.1–1.0 Gyr), and old (up to 12 Gyr) ages have been determined. A relationship between the age of a subsystem and its size has been found in all of the galaxies studied. Our results can be explained by the model of galactic thick-disk formation through thin-disk expansion. In this case, the middle-age stellar subsystem is a transitional stage from the thin disk to the thick one.     

Star formation history in the central region of the barred galaxy NGC 7177

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.     

Young nuclei in dwarf elliptical galaxies

We report the discovery of young embedded structures in three diffuse elliptical galaxies (dE) in the Virgo cluster: IC 783, IC 3468, and IC 3509. We performed 3D spectroscopic observations of these galaxies with the MPFS spectrograph at the 6-m Special Astrophysical Observatory telescope and obtained spatially resolved distributions of kinematic and stellar population parameters by fitting high-resolution PEGASE. HR synthetic single stellar populations (SSP) in pixel space. In all three galaxies, the luminosity-weighted age of the nuclei (∼4 Gyr) is considerably younger than that of the population in the outer regions of the galaxies. We discuss two possibilities for the formation of such structures—a dissipative merger event and a different ram pressure stripping efficiency during two consecutive crossings of the Virgo cluster centre. The article was translated by the authors.     

Stellar population and evolution of galaxies in groups: the NGC2300 group

Using panoramic and long-slit spectroscopy at the 6-m telescope of SAO RAS, we studied the stellar population and kinematics of five early-type disc galaxies—members of the NGC2300 group. The evolution of galaxies appears to be absolutely out of synch: while the average age of the stars in the central regions of the galaxies located close to the center of the group ranges from 2 to 7 Gyr, the peripheral spiral galaxies have old nuclei and bulges, with the ages of 10–15 Gyr. The brightest galaxy of the NGC2300 group, which up to now has been considered to be lenticular, of the SA0 type, turned out to be extremely hot dynamically: its bulge rotates slowly, v/σ = 0.06, and the outer parts do not rotate at all.We conclude that the kinematics of the stellar component of NGC2300 indicates that it is not a disc galaxy, but a triaxial spheroid.     

Constraining the star formation histories of spiral bulges

Stellar populations in spiral bulges are investigated using the Lick system of spectral indices. Long-slit spectroscopic observations of line strengths and kinematics made along the minor axes of four spiral bulges are reported. Comparisons are made between central line strengths in spiral bulges and those in other morphological types [elliptical, spheroidal (Sph) and S0]. The bulges investigated are found to have central line strengths comparable to those of single stellar populations of approximately solar abundance or above. Negative radial gradients are observed in line strengths, similar to those exhibited by elliptical galaxies. The bulge data are also consistent with correlations between Mg₂, Mg₂ gradient and central velocity dispersion observed in elliptical galaxies. In contrast to elliptical galaxies, central line strengths lie within the loci defining the range of 〈Fe〉 and Mg₂ achieved by Worthey's solar abundance ratio, single stellar populations (SSPs). The implication of solar abundance ratios indicates significant differences in the star formation histories of spiral bulges and elliptical galaxies. A 'single zone with infall' model of galactic chemical evolution, using Worthey's SSPs, is used to constrain the possible star formation histories of our sample. We show that the 〈Fe〉, Mg₂ and H β line strengths observed in these bulges cannot be reproduced using primordial collapse models of formation but can be reproduced by models with extended infall of gas and star formation (2–17 Gyr) in the region modelled. One galaxy (NGC 5689) shows a central population with a luminosity-weighted average age of ∼5 Gyr, supporting the idea of extended star formation. Kinematic substructure, possibly associated with a central spike in metallicity, is observed at the centre of the Sa galaxy NGC 3623.     

A multiwavelength study of the early evolution of the classical nova LMC 1988 1

We study star-formation-inducing mechanisms in galaxies through multiwavelength measurements of a sample of dwarf galaxies in the Virgo cluster described in Paper I. Our main goal is to test how star-formation-inducing mechanisms depend on several parameters of the galaxies, such as morphological type and hydrogen content. We derive the star formation rate and star formation histories of the galaxies, and check their dependence on other parameters.   Comparison of the sample galaxies with population synthesis models shows that these objects have significantly lower metallicity than the solar value. The colours can generally be explained as a combination of two different stellar populations: a young (3–20 Myr) metal-poor population which represents the stars currently forming presumably in a starburst, and an older (0.1–1 Gyr) population of previous stellar generations. There is evidence that the older stellar population was also formed in a starburst. This is consistent with the explanation that star formation in this type of objects takes place in short bursts followed by long quiescent periods.   No significant correlation is found between the star formation properties of the sample galaxies and their hydrogen content. Apparently, when star formation occurs in bursts, other parameters influence the star formation properties more significantly than the amount of atomic hydrogen. No correlation is found between the projected Virgocentric distance and the rate of star formation in the galaxies, suggesting that tidal interactions are not significant in triggering star formation in cluster dwarf galaxies.     

The Stellar Populations of Early-Type Galaxies in the Fornax Cluster

We have measured central line strengths for a complete sample of early-type galaxies in the Fornax Cluster comprising 11 elliptical and 11 lenticular galaxies, more luminous than M _B=–17. We find that the centres of Fornax ellipticals follow the locus of galaxies of fixed age in Worthey's models and have metallicities varying roughly from half to 2.5 times solar. The centres of (relatively low luminosity) lenticular galaxies, however, exhibit a substantial spread to younger luminosity-weighted ages indicating amore extended star formation history. Our conclusions are based on two age/metallicity diagnostic diagrams in the Lick/IDS system comprising established indices such as [MgFe]and H_β as well as new and more sensitive indices, such as Fe3and H . The inferred difference in the age distribution between lenticular and elliptical galaxies is a robust conclusion as the models generate consistent relative ages using different age and metallicity indicators even though the absolute ages remain uncertain. The absolute age uncertainty is caused mainly by the effects of non-solar abundance ratios, which are not accounted for in the stellar population models. We find that Es are generally overabundant in magnesium and the most luminous galaxies show stronger overabundances. The luminosity-weighted stellar populations of young S0s are consistent with solar abundance ratios or a weak Mg under abundance. Two of the faintest lenticular galaxies in our sample have blue continua and extremely strong Balmer-line absorptions suggesting starbursts <2 Gyr ago. This revised version was published online in July 2006 with corrections to the Cover Date.     

Population synthesis of H ii galaxies

We study the stellar population of galaxies with active star formation, determining ages of the stellar components by means of spectral population synthesis of their absorption spectra. The data consist of optical spectra of 185 nearby ( z 0.075) emission-line galaxies . They are mostly H  ii galaxies, but we also include some starbursts and Seyfert 2s, for comparison purposes. They were grouped into 19 high signal-to-noise ratio template spectra, according to their continuum distribution, absorption- and emission-line characteristics. The templates were then synthesized with a star cluster spectral base.
The synthesis results indicate that H  ii galaxies are typically age-composite stellar systems, presenting important contributions from generations up to as old as 500 Myr. We detect a significant contribution of populations with ages older than 1 Gyr in two groups of H  ii galaxies. The age distributions of stellar populations among starbursts can vary considerably despite similarities in the emission-line spectra. In the case of Seyfert 2 groups we obtain important contributions from the old population, consistent with a bulge.
From the diversity of star formation histories, we conclude that typical H  ii galaxies in the local Universe are not systems presently forming their first stellar generation.     

The Morphological Evolution of Field Galaxies

I review two observational programs which, together, promise to unravel the detailed astrophysical evolution of normal field galaxies over the last 5–7 Gyr. Systematic ground-based spectroscopy of faint galaxies have revealed an increasing faint end slope for the luminosity function with redshift. The trend is strongest for galaxies undergoing intense star-formation. Deep images taken with the repaired HST can be used to count galaxies as a function of morphological type. Regular "Hubble sequence" galaxies follow the no-evolution prediction, but irregular/peculiar sources have a steeper count slope and provide the excess population. Although the overlap between the spectral and HST samples is currently small, plans to merge similar datasets should reveal the physical explanation for the demise of star formation in faint blue galaxies since z ⋍0.5–1. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinematics of disc galaxies using photometrically calibrated N-body models

We have studied the evolution of isolated galaxies over several Gyr using a self-consistent N-body code including stars, gas and star formation. The results of our simulations are calibrated using spectrophotometric evolution models. We thus simultaneously analyse kinematical and photometrical evolution of the various stellar populations born during the successive bursts of star formation. Our calibrated simulations show that the properties of stellar velocity dispersion drops observed in the centre of three barred active galaxies by Emsellem et al. (2001) could depend on the observational wavelength. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stellar Populations at Large Redshifts

It is now possible to probe stellar populations at large lookback times. The Butcher-Oemler Effect in distant clusters is un-ambiguous evidence for unexpectedly recent evolution of disk galaxies in dense environments, probably through starbursts induced by interactions. Recent applications of the "red envelope" test at z ≲ 1 indicate that some luminous galaxies formed ∼ 10–13 Gyr ago. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Effect of Bar on Star-forming Activities in Nuclear Regions of Nearby Spiral Galaxies

By using the SDSS spectra, we have studied the star formation properties of the nearby spiral galaxies selected from the Revised Bright Galaxy Sample, and tried to find the effect of bar structure on the star formation activity in the nuclear regions of nearby galaxies. The stellar population composition and the intensity of star formation activities of each sample galaxy are acquired by using the stellar population synthesis code—STARLIGHT, and the star formation properties of nuclear regions are compared with those of integral sample galaxies. We find that the star formation in barred spiral galaxies is more active than that of unbarred spirals, and that barred spirals have younger stellar populations.     

The correlation of star formation quenching with internal galaxy properties and environment

We investigate the correlation of star formation quenching with internal galaxy properties and large-scale environment (halo mass) in empirical data and theoretical models. We make use of the halo-based group catalogue of Yang and collaborators, which is based on the Sloan Digital Sky Survey. Data from the Galaxy evolution explorer are also used to extract the recent star formation rate. In order to investigate the environmental effects, we examine the properties of 'central' and 'satellite' galaxies separately. For central galaxies, we are unable to conclude whether star formation quenching is primarily connected with halo mass or stellar mass, because these two quantities are themselves strongly correlated. For satellite galaxies, a nearly equally strong dependence on halo mass and stellar mass is seen. We make the same comparison for five different semi-analytic models based on three independently developed codes. We find that the models with active galactic nuclei feedback reproduce reasonably well the dependence of the fraction of central red and passive galaxies on halo mass and stellar mass. However, for satellite galaxies, the same models badly overproduce the fraction of red/passive galaxies and do not reproduce the empirical trends with stellar mass or halo mass. This satellite overquenching problem is caused by the too-rapid stripping of the satellites' hot gas haloes, which leads to rapid strangulation of star formation.     

Dense gas in normal and active galaxies

Dense molecular medium plays essential roles in galaxies. As demonstrated by the tight and linear correlation between HCN(1–0) and FIR luminosities among star-forming galaxies, from very nearby to high-z ones, the observation of a dense molecular component is indispensable to understand the star formation laws in galaxies. In order to obtain a general picture of the global distributions of dense molecular medium in normal star-forming galaxies, we have conducted an extragalactic CO(3–2) imaging survey of nearby spiral galaxies using the Atacama Submillimeter Telescope Experiment (ASTE). From the survey (ADIoS; ASTE Dense gas Imaging of Star-forming galaxies), CO(3–2) images of M 83 and NGC 986 are presented. Emphasis is placed on the correlation between the CO(3–2)/CO(1–0) ratio and the star formation efficiency in galaxies. In the central regions of some active galaxies, on the other hand, we often find enhanced or overluminous HCN(1–0) emission. The HCN(1–0)/CO(1–0) and HCN(1–0)/HCO⁺(1–0) intensities are often enhanced up to ∼0.2–0.3 and ∼2–3, respectively. Such elevated ratios have never been observed in the nuclear starburst regions. One possible explanation for these high HCN(1–0)/CO(1–0) and HCN(1–0)/HCO⁺(1–0) ratios is X-ray induced chemistry in X-ray dominated regions (XDRs), i.e., the overabundance of the HCN molecule in the X-ray irradiated dense molecular tori. If this view is true, the known tight correlation between HCN(1–0) and the star-formation rate breaks in the vicinity of active nuclei. Although the interpretation of these ratios is still an open question, these ratios have a great potential for a new diagnostic tool for the energy sources of dusty galaxies in the ALMA era because these molecular lines are free from dust extinction.     

Tidal dwarf galaxies at intermediate redshifts

We present the first attempt at measuring the production rate of tidal dwarf galaxies (TDGs) and estimating their contribution to the overall dwarf population. Using HST/ACS deep imaging data from GOODS and GEMS surveys in conjunction with photometric redshifts from COMBO-17 survey, we performed a morphological analysis for a sample of merging/interacting galaxies in the Extended Chandra Deep Field South and identified tidal dwarf candidates in the rest-frame optical bands. We estimated a production rate about 1.4×10⁻⁵ per Gyr per comoving volume for long-lived TDGs with stellar mass 3×10⁸⁻⁹ M_⊙ at 0.5<z<1.1. Together with galaxy merger rates and TDG survival rate from the literature, our results suggest that only a marginal fraction (less than 10%) of dwarf galaxies in the local universe could be tidally-originated. TDGs in our sample are on average bluer than their host galaxies in the optical. Stellar population modelling of optical to near-infrared spectral energy distributions (SEDs) for two TDGs favors a burst component with age 400/200 Myr and stellar mass 40%/26% of the total, indicating that a young stellar population newly formed in TDGs. This is consistent with the episodic star formation histories found for nearby TDGs.     

The starburst-AGN connection: the role of stellar clusters in AGNs

Nuclear stellar clusters are a common phenomenon in spirals and in starburst galaxies, and they may be a natural consequence of the star formation processes in the central regions of galaxies. HST UV imaging of a few Seyfert 2 galaxies have resolved nuclear starbursts in Seyfert 2 revealing stellar clusters as the main components of the extended emission. However, we do not know whether stellar clusters are always associated with all types of nuclear activity. We present HST NUV and optical images to study the role that stellar clusters play in different types of AGNs. Also with these images, we study the circumnuclear dust morphology as a probe of the circumnuclear environment of AGNs.     

Study of the galaxy DDO 68: New evidence for its youth

DDO 68 (UGC 5340) is the second most metal-poor star-forming galaxy (12 + log(O/H) = 7.14). Its peculiar optical morphology and its HI distribution and kinematics are indicative of a merger origin. We use the u, g, r, and i photometry based on the SDSS images of DDO 68 to estimate its stellar population ages. The Hα images of DDO 68 were used to select several representative regions without nebular emission. The derived colors were analyzed by comparison with the PEGASE2 evolutionary tracks for various star formation (SF) scenarios, including the two extreme cases: (i) an instantaneous starburst and (ii) continuous SF with a constant rate. The (u − g) and (g − r) colors for all of the selected regions are consistent with the scenario of several “instantaneous” SF episodes with ages between ∼0.05 and ∼1 Gyr. The total mass of the visible stars in DDO 68 was estimated by comparing the colors and fluxes of the observed stellar subsystems with PEGASE2 models to be ∼2.4 × 10⁷ M _⊙. This accounts for ∼6% of the total baryonic mass of the galaxy. All of the available data are consistent with the fact that DDO 68 is a very rare candidate for young galaxies. The bulk of its stars were formed during the recent (with the first encounter ∼1 Gyr ago) merger of two very gas-rich disks. DDO 68 is closest in its properties to cosmologically young low-mass galaxies. This article was submitted by the authors in English.     

The cosmological evolution of colour gradients in spheroids

The analysis of the four-colour maps of galaxies in the Hubble Deep Field (HDF) has revealed, in the sample of 0.4< z <1 early-type field galaxies, the existence of ellipticals with a predominantly old coeval stellar population. However, there is another, unexpected, category of HDF early-type galaxies, in which the galaxy core is significantly bluer than the outer regions. We demonstrate that these colour gradients are predicted by the multizone chemodynamical model for the evolution of elliptical galaxies.
We suggest that the colour gradient could be used as a chronometer for the evolution of elliptical galaxies: galaxies younger than a few Gyr exhibit cores bluer than the surrounding galaxy as a result of ongoing star formation, while more evolved galaxies have redder cores because of metallicity gradients increasing toward the centre.