Optimum merger time-scales and binary evolution

A study of galaxy mergers, on the basis of the collisional theory, using galaxy models without halos and considering the evolution of the proginator galaxies only from a time when the gravitational interaction between them is physically significant, indicates that most of the mergers are affected in 2 to 3 orbital periods for progenitors of comparable mass: shorter and longer time-scales being underabundant. These results have a bearing on the evolution of binary galaxies; indicating that once the relative orbit of a binary is circularized, the components will merge during the subsequent orbit or the next one (in a time-scale ~ 10⁸ years). These results are also indicative of the fact that binary evolution is very likely to cause a gradual evolution of the fundamental plane occupied by paired ellipticals from that of isolated ellipticals. After the merger, the remnant is very likely to define a fundamental plane with a slightly different slope.     

Dynamical friction and galaxy merging time-scales

The time-scale for galaxies within merging dark matter haloes to merge with each other is an important ingredient in galaxy formation models. Accurate estimates of merging time-scales are required for predictions of astrophysical quantities such as black hole binary merger rates, the build-up of stellar mass in central galaxies and the statistical properties of satellite galaxies within dark matter haloes. In this paper, we study the merging time-scales of extended dark matter haloes using N -body simulations. We compare these results to standard estimates based on the Chandrasekhar theory of dynamical friction. We find that these standard predictions for merging time-scales, which are often used in semi-analytic galaxy formation models, are systematically shorter than those found in simulations. The discrepancy is approximately a factor of 1.7 for M _sat/ M _host≈ 0.1 and becomes larger for more disparate satellite-to-host mass ratios, reaching a factor of ∼3.3 for M _sat/ M _host≈ 0.01. Based on our simulations, we propose a new, easily implementable fitting formula that accurately predicts the time-scale for an extended satellite to sink from the virial radius of a host halo down to the halo's centre for a wide range of M _sat/ M _host and orbits. Including a central bulge in each galaxy changes the merging time-scale by ≲10 per cent. To highlight one concrete application of our results, we show that merging time-scales often used in the literature overestimate the growth of stellar mass by satellite accretion by ≈40 per cent, with the extra mass gained in low mass ratio mergers.     

The role of magnetic fields in extragalactic astronomy

The evidence is reviewed for a universal magnetic field of strength 10^–9–10^–8G; it has been extended to include the diffuse fields of galactic clusters and the extensive magnetic halos of spiral galaxies. Some likely effects of the universal fieldB ₀ are as follows: (1) As suggested previously,B ₀ is coupled to protogalaxies and evolves into magnetic structures which depend on the angle between the field and the gas rotational axis. These provide the blueprints for the various types of the Hubble sequence, (ii) The relatively few grand-design spiral galaxies result from tidal interactioon (M51-type), but normal spirals form as a result of the spiral oblique field) magnetic blueprint acting on sheared gravitational instabilities (Goldreich and Lynden-Bell). (iii) The model explains the prevalent warped galactic disks and perhaps their flat H1 rotation curves. (iv) A variety of puzzling H1 concentrations may have hydromagnetic explanations; they include the high-velocity clouds, streamers, rings and central systems. (v) Clusters of galaxies are known to have diffuse magnetic fields, and these are likely to explain the absence of spiral galaxies and the nature of the intracluster gas. (vi) Spiral galaxies are now known to have extensive magnetic halos. These appear explicable only in terms of the universal magnetic field model.     

The formation of superdense gaseous cores in the nuclei of disk galaxies: II

In a previous paper (hereafter referred to as Paper I) we have tried to show that superdense cores in the nuclei of disk galaxies can be formed by accretion of gas ejected by the evolved stars which populate the central bulge of these galaxies. Solving the equations for radial flow of a magnetized gas, we found that the accretion of an explodable mass at the core can be achieved over a time-scale ranging from a few times 10⁷ and a few times 10⁸ yr. It was shown, however, that the accretion process is seriously inhibited if the gas possesses sufficient rotational velocity but lacks any dissipative, mechanism within the system. Since rotational velocity is an observed parameter of the stars which shed the gas to be accreted, one must consider the existence of some dissipative force in it in order that the accretion process may be efficient. In the present paper, therefore, we have solved the problem of the flow of a rotating, viscous (variable), magnetized gas. With plausible assumptions regarding some of the parameters involved, the time-scale for the accretion of an explodable mass (10⁹ M ) at the core again turns out to be ranging between a few times 10⁷ and a few times 10⁸yr. Such time-scale has been proposed by several authors as that for repeated explosions in nuclei of these galaxies. It has also been proposed by many authors that the spiral arms are generated and destroyed in disk galaxies over the same time-scale. Our solution also yields a nearly linear rotational velocity law which is usually observed in the central regions of these galaxies.     

Optical Identification of IRAS Point Sources. Galaxies. IX

The ninth list of objects from the BIG (Byurakan-IRAS Galaxies) sample is presented; 87 galaxies are identified with 60 point sources from the IRAS PSC catalog in the region +69° +73° and 03^h50^m 09^h50^m with an area of 117 deg². Most of the identified galaxies are compact with bright bulges and faint peripheries. The identified objects include 7 Sy candidates, 8 interacting pairs (of which one is a merger candidate), and 6 LSB galaxies. The optical coordinates and their deviations from the IR coordinates, the V magnitudes, morphological types, angular dimensions, and position angles are given. The identified galaxies are of morphological types Sa-Sc and have optical stellar magnitudes in the range 15.5^m-21.5^m with angular sizes in the range 3-26. Finding charts from DSS2 are provided for these objects.     

An application of sphere of activity in three-body model of interacting galaxies

The sphere of activity which be considered in the test stars of both central galaxies and satellite galaxies is used in three-body model to study the orbital decay of interacting galaxies. It can take account of both semi-restrictedN-body program (Lin and Tremaine, 1983) and multiple three-body algorithm (Borne, 1984) at the same time. The merger time is calculated accurately. The orbital decay of satellite galaxy is characterized. Energy and angular momentum which are carried away by escaping star are computed, too.     

A note on the post-explosion gas depletion in the central region of normal spiral galaxies

The problem of post-explosive gas depletion in the central region of normal spiral galaxies like our own has been investigated. It has been calculated that with a plausible density law and with some restriction on gas temperature, a sufficient quantity of gas will be depleted from the central region over a time-scale ranging from a few times 10⁷ to a few times 10⁸ years, the exact time depending on the particular density law. Such a time-scale has been suggested by many authors as the period for one phase of activity in the nuclei of these galaxies. A similar time-scale has also been proposed by several authors as that for the formation and destruction of the spiral patterns of these galaxies.     

Molecular gas distribution and dynamics in the luminous merger NGC 6240

We present the results of our subarcsecond resolution interferometricobservations of the 1.3 mm CO J = 21 line in the luminous merger NGC6240. Roughly half of the CO flux is contained in a rotating and highlyturbulent thick disk centered between the two radio and near-infrarednuclei. In this disk the molecular gas has velocity widths which reachFWZP line widths of up to 1000 km s^-1. The mass of this gasconcentration makes up between 30%–70% of the dynamical mass in thisregion. NGC 6240 may be in an earlier merging stage than typical ULIRGssuch as Arp 220. We compare these results from NGC 6240 with thoseof other luminous, gas-rich interacting galaxies and mergers.     

On the formation of superdense gaseous cores in the nuclei of disk galaxies — I

A model for the formation of superdense gaseous cores by accretion in the nuclei of disk galaxies has been proposed. Equations for radial flow of gas into the nucleus in the presence of aweak galactic magnetic field have been solved, and time scales for the accretion of an exploding mass in the nucleus (10⁹ M ) have been obtained under several different situations in the absence of any rotation. The time scales are found to lie in the range between a few times 10⁷ yr and 10⁸ yr. Such time scales have been proposed by some authors for repeated explosions in the nuclei of galaxies; they have also proposed that spiral arms in disk galaxies are repeatedly formed and destroyed over such time scales. It is shown that the presence of rotational velocities in the infalling gas practically destroys the efficiency of the accretion process unless such velocities are dissipated by frictional forces within the system. Viscosity of gas is the most obvious dissipative agent. The problem of accretion of a rotating viscous gas will be discussed in a subsequent paper.     

Dynamical evolution in clusters of galaxies with low-frequency radio emission

Clusters of galaxies in which radio emission at low frequencies (178 MHz) has been detected were classified on the Bautz-Morgan (BM) system according to the dominance of the brightest galaxy. Radio sources with steep low-frequency spectra occur in clusters of all BM types but more often in rich clusters; the distributions of BM types for clusters with high and low spectral indices between 38 and 178 MHz are similar. Glass copies of Mount Palomar Sky Survey plates were measured to determine the distribution of the ten brightest galaxies in clusters without dominant galaxies. Some clusters were found to have central cores of bright galaxies which may reflect mass segregation of galaxies due to dynamical friction. The bright galaxies in such cores may later merge to form dominant cD galaxies. The positions of the cD galaxies and cores of bright galaxies are often at projected distances <200 kpc from the low-frequency radio emission. The low-frequency spectrum of radio emission associated with a cD galaxy may be either steep or normal, but the low-frequency spectrum from a core of bright galaxies is usually steep. A steep spectrum may develop when a radio source is confined by hot gas in a cluster over a long period (10⁹ yr). Confinement would probably occur for radio sources associated with bright galaxies in the cores of clusters and cD galaxies in clusters. However, cD galaxies may have recurrent radio outbursts so that steep spectra are not always observed.     

Stellar Mass—Halo Mass Relation and Star Formation Efficiency in High-Mass Halos

We study relation between stellar mass and halo mass for high-mass halos using a sample of galaxy clusters with accurate measurements of stellar masses from optical and ifrared data and total masses from X-ray observations. We find that stellar mass of the brightest cluster galaxies (BCGs) scales as M_*,BCG ∝ M ₅₀₀ ^αBCG with the best fit slope of α_BCG ≈ 0.4 ± 0.1. We measure scatter of M_*,BCG at a fixed M₅₀₀ of ≈0.2 dex. We show that stellar mass-halo mass relations from abundance matching or halo modelling reported in recent studies underestimate masses of BCGs by a factor of ～2?4. We argue that this is because these studies used stellar mass functions (SMF) based on photometry that severely underestimates the outer surface brightness profiles of massive galaxies. We show that M_*?M relation derived using abundance matching with the recent SMF calibration by Bernardi et al. (2013) based on improved photometry is in a much better agreement with the relation we derive via direct calibration for observed clusters. The total stellar mass of galaxies correlates with total mass M₅₀₀ with the slope of ≈0.6 ± 0.1 and scatter of 0.1 dex. This indicates that efficiency with which baryons are converted into stars decreases with increasing cluster mass. The low scatter is due to large contribution of satellite galaxies: the stellar mass in satellite galaxies correlates with M₅₀₀ with scatter of ≈0.1 dex and best fit slope of α_sat ≈ 0.8 ± 0.1. We show that for a fixed choice of the initial mass function (IMF) total stellar fraction in clusters is only a factor of 3?5 lower than the peak stellar fraction reached in M ≈ 10¹²M_⊙ halos. The difference is only a factor of ～1.5?3 if the IMF becomes progressively more bottom heavy with increasing mass in early type galaxies, as indicated by recent observational analyses. This means that the overall efficiency of star formation in massive halos is only moderately suppressed compared to L_* galaxies and is considerably less suppressed than previously thought. The larger normalization and slope of the M_*?M relation derived in this study shows that feedback and associated suppression of star formation in massive halos should be weaker than assumed in most of the current semi-analytic models and simulations.     

The maximum and minimum masses of galaxies

Galaxies may have formed by fragmentation in a collapsing cloud of very large mass. The most massive galaxies were formed from fragments which were nearly but not quite opaque: the least massive galaxies were formed from fragments about as large as the Jeans mass. If the maximum mass of galaxies is 10¹³ M , then the minimum mass should be 10⁶ M .     

The activity of the nuclei of spiral galaxies and IR-emission

It is shown that the brightness of the spiral galactic nuclei at 10 depends on the Byurakan type of galaxies: the brightness is higher in the case of galaxies of types 5, 2s, 4, and 2 with optical signs of activity in comparison with galaxies of types 3 and 1 without optical signs of activity. The noted difference is larger when nonstellar emission at 10 is considered. The obtained results confirm conclusions made earlier on the activity of the nuclei of galaxies of types 5, 2s, 4, and 2.     

ISO Spectroscopy of Ultraluminous Infrared Galaxies

Spectroscopy from the Infrared Space Observatory ISO has for the first timeprovided the sensitivity to exploit the diagnostic power ofmid-infrared fine structure lines and PAH features for the study ofultraluminous infrared galaxies (L_IR >10¹² L ). We report on observations obtainedwith SWS, ISOPHOT-S, and the CVF option of ISOCAM. From both fine structure lines and PAH features, we find that the majority of ULIRGs is predominantlypowered by star formation. Our total sample of about 75 ULIRGs allows tosearch for trends within the class of ULIRGs: The fraction of AGNs increaseswith luminosity above 3 × 10¹²L but there is no obvioustrend for ULIRGs to be more AGN-like with more advanced merger phase.     

Evolution of Intergalactic Gas in the Neighborhood of Dwarf Galaxies and Its Manifestations in the HI 21 cm Line

Low-mass galaxies are known to have played the crucial role in the hydrogen reionization in the Universe. In this paper we investigate the contribution of soft x-ray radiation (E ~ 0.1–1 keV) from dwarf galaxies to hydrogen ionization during the initial reionization stages. The only possible sources of this radiation in the process of star formation in dwarf galaxies during the epochs preceding the hydrogen reionization epoch are hot intermediate-mass stars (M ~ 5–8 M_⊙) that entered the asymptotic giant branch (AGB) stage and massive x-ray binaries. We analyze the evolution of the intergalactic gas in the neighborhood of a dwarf galaxy with a total mass of 6 × 10⁸M_⊙ formed at the redshift of z ~ 15 and having constant star-formation rate of 0.01–0.1 M_⊙ yr^?1 over a starburst with a duration of up to 100 Myr. We show that the radiation from AGB stars heats intergalactic gas to above 100 K and ensures its ionization x_e ? 0.03 within about 4–10 kpc from the galaxy in the case of a star-formation rate of star formation 0.03–0.1 M_⊙ yr^?1, and that after the end of the starburst this region remains quasi-stationary over the following 200–300 Myr, i.e., until z ~ 7.5. Formation of x-ray binaries form in dwarf galaxies at z ~ 15 results in a 2–3 and 5–6 times greater size of the ionized and heated region compared to the case where ionization is produced by AGB stars exclusively, if computed with the “x-ray luminosity–star-formation rate” dependence (L_X ~ f_XSFR) factor f_X = 0.1 and f_X ~ 1, respectively. For f_X ? 0.03 the effect of x-ray binaries is smaller that that of AGB star population. Lyα emission, heating, and ionization of the intergalactic gas in the neighborhood of dwarf galaxies result in the excitation of the 21 cm HI line. We found that during the period of the starburst end at z ~11.5–12.5 the brightness temperature in the neighborhood of galaxies is 15–25 mK and the region where the brightness temperature remains close to its maximum has a size of about 12–30 kpc. Hence the epoch of the starburst end is most favorable for 21 cm HI line observations of dwarf galaxies, because at that time the size of the region of maximum brightness temperature is the greatest over the entire evolution of the dwarf galaxy. In the case of the sizes corresponding to almost 0.’1 for z ~ 12 regions with maximum emission can be detected with the Square Kilometre Array, which is currently under construction.     

Probing the Baryon Density of the Universe using Light Echoes of High Redshift Radio Sources

It is shown that the high redshift intergalactic gas that probably contains most of the baryonic density b = 0.05(Ho = 75kms-1 Mpc-1) in the standard Cold Dark Matter model can be detected through "true" intergalactic Thompson-scattered halos or light echoes around isotropic radio-loud quasars and radio galaxies. These cosmological halos, which form in the uniform ionized medium around "old" non-evolving sources, have a very large degree of polarization, up to pmax 44 percent at a projected distance half the light-age radius, and a plateau in the annular polarized flux density within 1/3 of the light-age radius, which is about 4 arc min for sources older than 107 years at z 4. The optimal wavelength range for halo polarimetric observations is 20 - 30 cm, depending on the Galactic rotation measure in the direction of the source, the spectral index of the source, and the specific maximized parameter of the halo. If observed with large single-dish radiotelescopes, the 21 cm polarized brightness temperatures of some inner halos, for example the halo around the quasar OQ 172 (S1400 = 2.5 Jy,z = 3.53), are of the same order of magnitude as the current total-intensity limits for cosmic background fluctuations on arc min scales. The halo test can be extended to larger volumes of space by concentric co-adding of source-normalized halos around the much more numerous isotropic radio galaxies. The expected ensemble-averaged profile of the polarized flux density around the symmetry center is calculated by integrating over the halo ages, assuming a uniform source-age distribution. In addition to the electron density or a lower limit for b based on the halo brightness, the characteristic life time of the radio emission can be derived based on its angular size. If there is a moderate deviation of the density evolution of the intergalactic gas from the conservative cubic law due to galaxy formation, high redshift radio halos may be on the rising branch of their brightness beyond its minimum, similar to the well known nonmonotonic behaviour of angular sizes.     

Ultraviolet (200 nm) observations of the nearby cluster of galaxies abell 1367

Ultraviolet (200 nm) observations of 120 galaxies are performed in a 1.8 square degree field centered on the A 1367 cluster with a 40 cm imaging telescope flown on a stabilized balloon-borne gondola. Total ultraviolet fluxes are obtained and used to derive the current Star Formation Rate (SFR) in the cluster galaxies with measured Hi masses. A good correlation is found between the SFR per unit area and the gas surface density, similar to that previously found for a sample of field galaxies (Donaset al., 1987), while the mean gas depletion time-scale (M _g/SFR) is probably shorter in the cluster than in the field.     

Tidal circularization in massive binaries

We consider the time-scale of orbital circularization for those massive binary systems that produce WR+C systems. Using the observation that such systems have acquired circular orbits on a time-scale of the order of the Main-Sequence lifetime of the initial secondary, we estimate the intrinsic time-scale of the tidal interaction ₀. We find that the tidal dissipation in such systems is very efficient and that, on the average, ₀ ~ 10³ yr.     

A unified model for the evolution of galaxies and quasars

We incorporate a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter-dominated Universe. We assume that supermassive black holes are formed and fuelled during major mergers. If two galaxies of comparable mass merge, their central black holes coalesce and a few per cent of the gas in the merger remnant is accreted by the new black hole over a time-scale of a few times 10⁷ yr. With these simple assumptions, our model not only fits many aspects of the observed evolution of galaxies, but also reproduces quantitatively the observed relation between bulge luminosity and black hole mass in nearby galaxies, the strong evolution of the quasar population with redshift, and the relation between the luminosities of nearby quasars and those of their host galaxies. The strong decline in the number density of quasars from z ∼2 to z =0 is a result of the combination of three effects: (i) a decrease in the merging rate; (ii) a decrease in the amount of cold gas available to fuel black holes, and (iii) an increase in the time-scale for gas accretion. The predicted decline in the total content of cold gas in galaxies is consistent with that inferred from observations of damped Ly α systems. Our results strongly suggest that the evolution of supermassive black holes, quasars and starburst galaxies is inextricably linked to the hierarchical build-up of galaxies.     

Optical Identification of IRAS Point Sources. Galaxies. X

The tenth list of objects from the BIG (Byurakan-IRAS Galaxies) sample, containing 104 galaxies identified with 60 point sources from the IRAS PSC catalog, is introduced. The identifications have been made in the region +69°+73° and 09^h50^m15^h20^m with an area of 107 sq.deg. The identified objects include 13 Sy candidates, 5 compact star-formation galaxies, 11 interacting pairs (including 5 merger candidates) and 1 interacting triple system, 5 LSB galaxies, and 5 groups. 21 objects are also identified with radio sources. The optical coordinates, their deviations from the IR coordinates, V stellar magnitudes, morphological types, angular sizes of the central regions, and position angles have been determined. The identified galaxies are of Sa-Sc, SB, and Irr morphological types, the optical stellar magnitudes lie in the range 15^m.0-21^m.5, and the angular sizes of the central regions lie in the range 3-32. Finding charts from the DSS2 are included for these objects.