Ultraviolet images of galaxies from the Optical Monitor on XMM-Newton

A large number of galaxies, both normal and active, have been observed in ultraviolet light by the Optical Monitor on XMM-Newton. These are some of the deepest wide-field ultraviolet images of these galaxies yet obtained, and in many cases the first collected in this waveband. We present images of five active galaxies, and discuss the potential uses of the ultraviolet surface brightness distribution and morphology, in association with X-ray data, for Active Galactic Nuclei, star formation and galaxy evolution studies.     

Observations of stars with the Hopkins Ultraviolet Telescope

The Hopkins Ultraviolet Telescope (HUT) was flown aboard the space shuttle Columbia as part of the Astro-1 mission during December 1990. During the nine-day flight, HUT carried out 3 Å resolution spectrophotometry of a wide variety of astronomical objects, including a number of stellar targets, in the 912–1860 Å range of the far ultraviolet. A few nearby stars were observed in the 415–912 Å range of the extreme ultraviolet as well. For nearly all of these targets, the spectra obtained by HUT are the first ever obtained in the spectroscopically rich region between Lyman (1216 Å) and the Lyman limit (912 Å). Here, we present highlights of the results obtained by HUT in a variety of areas of stellar astronomy.     

Active galactic nuclei imaging programs of the RadioAstron mission

Imaging relativistic jets in active galactic nuclei (AGN) at angular resolution significantly surpassing that of the ground-based VLBI at centimetre wavelengths is one of the key science objectives of the RadioAstron space-VLBI mission. There are three RadioAstron imaging key science programs that target both nearby radio galaxies and blazars, with one of the programs specifically focusing on polarimetry of the jets. The first images from these programs reach angular resolution of a few tens of microarcseconds and reveal unprecedented details about the jet collimation profile, magnetic field configuration, and Kelvin-Helmholtz instabilities along the flow in some of the most studied AGN (3C 84, BL Lac, 3C 273, S5 0836 + 710). Here we give an overview of the goals and strategy of these three ongoing programs, highlight their early results, and discuss the challenges of space-VLBI imaging.     

LINERs as low-luminosity active galactic nuclei

Many nearby galaxies contain optical signatures of nuclear activity in the form of LINER nuclei. LINERs may be the weakest and most common manifestation of the quasar phenomenon. The physical origin of this class of objects, however, has been ambiguous. I draw upon a number of recent observations to argue that a significant fraction of LINERs are low-luminosity active galactic nuclei. The evidence in favor of this interpretation includes the similarity in the properties of the host galaxies of LINERs and Seyferts, the detection of broad-line regions, the detection of black holes in a handful of nearby galaxies that are spectroscopically recognized as LINERs, the presence of a compact nuclear source seen in the radio, ultraviolet, and X-rays, and the detection of nonthermal spectra in the hard X-ray band. The spectral energy distributions of LINERs differ dramatically from those of luminous AGNs, most notably in the absence of the “big blue bump.” This and other characteristics suggest that the central engine in LINERs is fed by a very low accretion rate.     

A future Japanese program in far-ultraviolet astronomy and solar physics

A design study of an ultraviolet-telescope satellite (UVSAT) has been carried out by the Japanese astronomical community. The main purpose of this satellite mission would be to investigate (i) the distribution and nature of ultraviolet sources in star clusters, galaxies, and clusters of galaxies, (ii) the physical structures of galactic nebulae, and (iii) the dynamics of stellar and galactic activity. A 60 cm, f/4 Cassegrain telescope will be launched into a semi-circular orbit of ～500 km altitude and of ～30° inclination, by an ISAS/Japan M3S-III rocket. An intensified CCD camera and/or a concave-grating spectrograph will be operated for the wavelength range λ?1200 Å. Various possible auxiliary instruments and sub-telescopes are considered.     

X-ray source populations in nearby star-forming galaxies

We present results on the X-ray source populations of nearby star-forming galaxies based on Chandra observations. First we discuss the monitoring campaigns on the Antennae and M82 galaxies. In both cases we find that the majority of the X-ray sources exhibit intensity and/or spectral variability. However, despite of this variability, we do not find any statistically significant variations of their X-ray luminosity functions (XLFs). We also find that the majority of the X-ray sources are associated with star-forming regions, although we do not always identify optical counterparts to the X-ray sources. Especially in the case of M82 we find that the most luminous sources are clustered in the central region of the galaxy. Finally, we present the first results from a study of a sample of nearby star-forming galaxies which form a starburst age sequence: although their XLFs to first order are represented by power-laws with consistent slopes, there is an indication for small variations, suggesting a change of their X-ray binary populations.     

Observations of ultraviolet spectra of H II regions and galaxies with IUE

The ultraviolet spectra, obtained with the International Ultraviolet Explorer, of a sample of H II regions and the nuclear regions of spiral and elliptical galaxies are described. The star formation rates in the nuclei of spiral galaxies are similar to the star formation rate in the solar neighbourhood. The data indicate that the current thinking on the synthesis of carbon and nitrogen in galaxies has to be revised and the K-corrections determined from the ultraviolet spectra of galaxies when compared with the photometry of distant galaxies suggests colour evolution of galaxies at z > 0.3.     

Galaxy evolution in groups. USGC U268 and USGC U376 in the Leo cloud

With the aim of investigating galaxy evolution in nearby galaxy groups, we analyzed the spectral energy distribution of 24 galaxies, members of two groups in the Leo cloud, USGC U268 and USGC U376. We estimated the ages and stellar masses of the galaxies by fitting their total apparent magnitudes from far-ultraviolet to near-infrared with population synthesis models. The comparison of the results for a subsample of galaxies with smooth particle hydrodynamic (SPH) simulations with chemo-photometric implementation, shows that in most cases the estimated stellar masses obtained with the two different approaches are in good agreement. The kinematical and dynamical analysis indicates that USGC U268 is in a pre-virial collapse phase while USGC U376 is likely in a more evolved phase towards virialization.     

The first results from the Herschel-HIFI mission

This paper contains a summary of the results from the first years of observations with the HIFI instrument onboard ESA’s Herschel space observatory. The paper starts by outlining the goals and possibilities of far-infrared and submillimeter astronomy, the limitations of the Earth’s atmosphere, and the scientific scope of the Herschel-HIFI mission. The presentation of science results from the mission follows the life cycle of gas in galaxies as grouped into five themes: Structure of the interstellar medium, First steps in interstellar chemistry, Formation of stars and planets, Solar system results and Evolved stellar envelopes. The HIFI observations paint a picture where the interstellar medium in galaxies has a mixed, rather than a layered structure; the same conclusion may hold for protoplanetary disks. In addition, the HIFI data show that exchange of matter between comets and asteroids with planets and moons plays a large role. The paper concludes with an outlook to future instrumentation in the far-infrared and submillimeter wavelength ranges.     

Compact X-ray sources in nearby galaxy nuclei

We have found compact, near-nuclear X-ray sources in 21 (54%) of a complete sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT HRI data. ROSAT X-ray luminosities (0.2 – 2.4 keV) of these compact X-ray sources are ∼10³⁷ – 10⁴⁰ erg s⁻¹. The mean displacement between the location of the compact X-ray source and the optical photometric center of the galaxy is ∼390 pc. ASCA spectra of six of the 21 galaxies show the presence of a hard component with relatively steep (Γ ≈ 2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the data from the spiral galaxies well, suggesting that the X-ray objects in these galaxies may be similar to a black hole candidate (BHC) in its soft (high) state. ASCA data from the elliptical galaxies indicate that hot (kT ≈ 0.7 keV) gas dominates the emission. The fact that the spectral slope of the spiral galaxy sources is steeper than in normal type 1 active galactic nuclei (AGNs) and that relatively low absorbing columns (N_H ≈ 10²¹ cm⁻²) were found to the power-law component indicates that these objects are somehow geometrically and/or physically different from AGNs in normal active galaxies. The X-ray sources in the spiral galaxies may be BHCs, low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs or AGNs) to be ∼10²–10³ M_⊙. The X-ray sources in the elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.     

Gamma astrometric measurement experiment (GAME) – Science case

We report here on the science case of a concept for a satellite orbiting at 1 AU from the Sun and using a baffled Fizeau interferometer to look as close as possible to its limb. This configuration, and the need for looking nearby the Sun, is required for the main scientific driver of the mission, namely the measure of the γ parameter of the Parameterized Post-Newtonian formulation to the 10⁻⁶–10⁻⁷ level at least. This would lead to an accurate test of the General Theory of Relativity against other alternative theories of gravity, and set stringent constraints on some of the most significant issues of Astrophysics like those involving exotic forms of dark matter and dark energy. Exploiting the possibilities offered by the observation strategy, it is also possible to target other interesting scientific goals. One is, again, in the realm of General Relativity and aims at measuring the light deflection nearby the Giant Planets to detect asymmetric effects induced by their quadrupoles, predicted by GR but never measured so far. Others can be found in the observation of selected extrasolar systems where, e.g., the astrometric and photometric capabilities of GAME will help to improve on the knowledge of the brown-dwarf regime and on the search for exo-planets with the transit method, respectively.     

NMA survey of CO and HCN emission from nearby active galaxies

The Nobeyama Millimeter Array has been used to map CO(1-0) and HCN(1-0) emission in nearby Seyfert galaxies. A wide variety of molecular gas distributions are found, and there appears to be no “typical” gas distribution either in type-1 Seyferts or type-2s. All the gas distributions and kinematics in the observed Seyferts can be understood as a response to a non-axisymmetric potential in the central regions, suggesting that a small scale (a few 100 pc — a few kpc) distortion of the underlying potential is necessary for Seyfert activity, although it is not a sufficient condition. Circumnuclear star formation in the host of the observed Seyferts can occur via gravitational instabilities of the molecular gas, as in the case of star forming regions in non-Seyfert galaxies. Our results may support the idea that the host galaxies of Seyferts (both type-1s and 2s) and non-Seyferts are the same in terms of the fuel and trigger of star formation. Near the center of Seyfert nuclei (r < a few 100 pc), we find that the molecular gas tends to be gravitationally stable. We find that the R_HCN/CO value ranges over an order of magnitude, from 0.086 to 0.6 among Seyfert galaxies. It seems that the presence of kpc scale jet/outflows is related to the extremely high R_HCN/CO values.     

Diffuse thermal emission from very hot gas in starburst galaxies: Spatial results

New BeppoSAX observations of the nearby prototypical starburst galaxies NGC 253 and M82 are presented. A companion paper (Cappi et al. 1998) shows that the hard (2–10 keV) spectrum of both galaxies, extracted from the source central regions, is best described by a thermal emission model with kT ∼ 6–9 keV and abundances ∼ 0.1-0.3 solar. The spatial analysis yields clear evidence that this emission is extended in NGC 253, and possibly also in M82. This quite clearly rules out a LLAGN as the main responsible for their hard X-ray emission. Significant contribution from point-sources (i.e. X-ray binaries (XRBs) and Supernovae Remnants (SNRs)) cannot be excluded; neither can we at present reliably estimate the level of Compton emission. However, we argue that such contributions shouldn't affect our main conclusion, i.e., that the BeppoSAX results show, altogether, compelling evidence for the existence of a very hot, metal-poor interstellar plasma in both galaxies.     

The X-ray properties of normal galaxies

An extensive program to study nearby normal galaxies was carried out by various observers using the imaging instruments on the $\text{Einstein}$ Observatory; more than 50 such galaxies were detected with 0.5 – 3.0 keV luminosities ranging from 2 × 10³⁸ ergs s^?1 to 3 × 10⁴¹ergs s^?1. The X-ray luminosity of normal galaxies is ~2 × 10^?4 of the optical luminosity and shows no strong correlation with morphological type. For the nearest galaxies, (the Large and Small Magellanic Clouds, M31 and M33,) studies, performed with the Observatory, were comparable to the $\text{Uhuru}$ survey of the Galaxy. Approximately 30 new SNR were recognized in the Magellanic Clouds as a result. Over 90 sources were detected in M31 of which at least 20 are identified with globular cluster. The numbers of luminous (>10³⁷ ergs s^?1) sources detected in the nearest galaxies per unit mass are similar to that found in our own galaxy. Individual X-ray sources in the arms of nearby spirals can be very luminous; seven with luminosities in excess of 10³⁹ergs s^?1 have been discovered. The nuclei of some, but not all, normal galaxies are luminous X-ray sources; X-ray activity is not presently predictable from the radio or optical properties of the nucleus.     

“Microquasars” and the X-ray background

The results of 1–20 μm infrared photometry of seven 3CR radio galaxies are discussed. The broad line galaxies all show steep infrared spectra with a power law index α −2.4 (F ∝ ν^+α) in direct continuation of the optical spectra. These spectra are far steeper than those observed in Seyfert 1 galaxies, the radio quiet counterparts of broad line radio galaxies. No infrared excesses were observed in narrow line radio galaxies. However, more sensitive observations are needed before any resemblance in the infrared between narrow line radio galaxies and Seyfert 2 galaxies can be excluded.     

Active galaxies observed by IUE

IUE has extended the grasp of ultraviolet astronomy to cover active galaxies and quasars fainter than the sixteenth magnitude. These observations have:(i) provided a diagnostic for the source of ionization in active galaxies,(ii) cast light on the excitation mechanism of the Fe II lines,(iii) shown the broad hydrogen lines in Seyferts and quasars are not in their recombination ratios,(iv) demonstrated the absence of gas in BL Lac objects,(v) supported the gravitational lens explanation of the double quasar,(vi) demonstrated the presence of a hot (30 000 K) black body in active nuclei,(vii) discovered stratification of the ionization conditions in the Broad Line Regions of active nuclei.     

Ginga observations of Seyfert galaxies

We observed twenty-eight Seyfert 2 galaxies with the Japanese X-ray satellite, Ginga, and found Seyfert 2 galaxies, in general, have the X-ray spectral characteristics of obscured Seyfert 1 nuclei. This result agrees with the predictions from Unified Seyfert model proposed by Antonucci and Miller /1/. However, among the observed Seyfert 2 galaxies, there are a few galaxies with no evidence of an obscuration, contrary to the general predictions of the unified model. We note that type 2 AGN will contribute to the Cosmic Diffuse X-ray Background, if the unified Seyfert model can be extended to the far distant AGN such as quasars.     

Superclustering and the large-scale structure of the universe

The spatial distribution of a complete redshift sample of nearby rich clusters of galaxies is studied. Strong superclustering, extending to very large scales (100h⁻¹Mpc), is observed in the cluster distribution. In particular, we determined and discuss below the following studies:I. The spatial correlation function of rich clusters of galaxies.II. The construction of a complete catalog of superclusters and a study of their growth (percolation).III. The finding of large-scale superclusters surrounding the giant galaxy void in Bootes.IV. The finding of a 300 Mpc void of rich clusters of galaxies.All the above studies provide evidence for the existence of large-scale structure in the universe, and reveal some of its characteristics. These findings are of importance to models of the formation of galaxies and structure in the universe.     

The dark matter halo structure of “fossil” groups candidates

We investigated properties of four isolated giant elliptical galaxies with extended X-ray halo using ASCA data. The derived size of X-ray halo, X-ray luminosity, and gravitational mass of the dark halo are unusually large those of X-ray halo of a single galaxy, but are typical for X-ray halos of groups and poor clusters of galaxies. The measured temperatures and abundances of the X-ray halo gas in these galaxies are also similar to those of the groups and poor clusters. Based on these results we identified these galaxies as “isolated X-ray overluminous elliptical galaxy” (IOLEG). The radial profiles of dark halo in these objects were derived from X-ray data. It is found that some are similar to those of compact groups while others are the same as those of normal ellipticals. The dark halos of lOLEGs are thus indistinguishable from those of groups (and poor clusters), which appears to be consistent with a widely believed idea that lOLEGs are a product of dynamical evolution of a compact group. However, mass-to-light ratios of IOLEGs (M₂₀₀/L_B  100–1000) are far greater than those of Hickson compact groups M₂₀₀/L_B  40–60). Since it is hard to consider that total optical luminosity of a compact group decreases by an order of magnitude in the course of dynamical evolution, such difference in the observed mass-to-light ratio between IOLEGs and Hickson compact groups strongly suggests that most IOLEGs have not evolved from compact groups which are observed at present.