Search

The validity of the emission-line luminosity versus ionized gas velocity dispersion (L–σ) correlation for H ii galaxies (HIIGx) and its potential as an accurate distance estimator are assessed. For a sample of 128 local (0.02 ≲ z ≲ 0.2) compact HIIGx with high equivalent widths of their Balmer emission lines, we obtained the ionized gas velocity dispersion from high signal-to-noise ratio (S/N) high-dispersion spectroscopy (Subaru High Dispersion Spectrograph (HDS) and European Southern Observatory (ESO) Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (VLT–UVES)) and integrated Hβ fluxes from low-dispersion wide aperture spectrophotometry. We find that the L(Hβ)–σ relation is strong and stable against restrictions in the sample (mostly based on the emission-line profiles). The ‘Gaussianity’ of the profile is important for reducing the root-mean-square (rms) uncertainty of the distance indicator, but at the expense of substantially reducing the sample. By fitting other physical parameters into the correlation, we are able to decrease the scatter significantly without reducing the sample. The size of the star-forming region is an important second parameter, while adding the emission-line equivalent width or the continuum colour and metallicity produces the solution with the smallest rms scatter=δlog L(Hβ) = 0.233. The derived coefficients in the best L(Hβ)–σ relation are very close to what is expected from virialized ionizing clusters, while the derived sum of the stellar and ionized gas masses is similar to the dynamical mass estimated using the Hubble Space Telescope (HST) corrected Petrosian radius. These results are compatible with gravity being the main mechanism causing the broadening of the emission lines in these very young and massive clusters. The derived masses range from about 2 × 106 M⊙ to 109 M⊙ and their ‘corrected’ Petrosian radius ranges from a few tens to a few hundred pc. [ABSTRACT FROM PUBLISHER]

ABSTRACT We explore the possibility of setting stringent constraints to the dark energy equation of state using alternative cosmic tracers like (a) the Hubble relation using H ii galaxies, which can be observed at much higher redshifts ( z≲ 3.5) than those currently traced by Type Ia supernovae (SNeIa) samples, and (b) the large-scale structure using the clustering of X-ray selected active galactic nuclei (AGN), which have a redshift distribution peaking at z∼ 1. In this paper we use extensive Monte Carlo simulations to define the optimal strategy for the recovery of the dark energy equation of state using the high-redshift ( z≳ 2) Hubble relation, but accounting also for the effects of gravitational lensing, which for such high redshifts can significantly affect the derived cosmological constraints. We investigate the size of the sample of high- z H ii galaxies needed to provide useful constraints in the dark energy equation of state. Based on a 'figure of merit' analysis, we provide estimates for the number of 2 ≲ z≲ 3.5 tracers needed to reduce the cosmological solution space, presently provided by the Constitution SNIa set, by a desired factor. The analysis is given for any level of rms distance modulus uncertainty and we find that an expected reduction (i.e. by ∼20-40 per cent) of the current level of H ii-galaxy-based distance modulus uncertainty does not provide a significant improvement in the derived cosmological constraints. It is much more efficient to increase the number of tracers than to reduce their individual uncertainties. Finally, we propose a framework to put constraints on the dark energy equation of state by using the joint likelihood of the X-ray AGN clustering and of the Hubble relation cosmological analyses. A preliminary joint analysis using the X-ray AGN clustering of the 2 XMM survey and the Hubble relation of the Constitution SNIa set provide Ωm= 0.31 ± 0.01 and w=−1.06 ± 0.05. We also find that the joint SNIa-2 XMM analysis provides significantly more stringent cosmological constraints, increasing the figure of merit by a factor of ∼2, with respect to that of the joint SNIa-baryonic acoustic oscillation analysis. [ABSTRACT FROM AUTHOR]

We present a study of outflow (OF) and broad absorption line (BAL) systems in Mrk 231, and in similar infrared (IR) quasi-stellar objects (QSOs). This study is based mainly on one-dimensional and two-dimensional spectroscopy (obtained at La Palma/William Herschel Telescope,Hubble Space Telescope,International Ultraviolet Explorer, European Southern Observatory/New Technology Telescope, Kitt Peak National Observatory, Apache Point Observatory and Complejo Astronomico El Leoncito observatories) plusHubble Space Telescopeimages. For Mrk 231, we report evidence that the extreme nuclear OF process has at least three main components on different scales, which are probably associated with: (i) the radio jet, at parsec scale; (ii) the extreme starburst at parsec and kiloparsec scale. This OF has generated at least four concentric expanding superbubbles and the BAL systems.Specifically, inside and very close to the nucleus the two-dimensional spectra show the presence of an OF emission bump in the blend Hα+[N ii], with a peak at the same velocity of the main BAL-I system. This bump was more clearly detected in the area located at 0.6–1.5 arcsec (490–1220 pc), to the south-west of the nucleus core, showing a strong and broad peak. In addition, in the same direction[at position angle (PA), i.e. close to the PA of the small-scale radio jet] at 1.7–2.5 arcsec, we also detected multiple narrow emission-line components, with‘greatly’ enhanced[N ii]/Hα ratio (very similar to the spectra of jets bow shocks). These results suggest that the BAL-I system is generated in OF clouds associated with the parsec-scale jet.TheHubble Space Telescopeimages show four (or possibly five) nuclear superbubbles or shells with radiiand 0.2 kpc. For these bubbles, the two-dimensional Hα velocity field map and two-dimensional spectra show the following. (i) At the border of the more extended bubble (S1), a clear expansion of the shell with blueshifted velocities (with circular shape and at a radius). This bubble shows a rupture arc– to the south– suggesting that the bubble is in the blowout phase. The axis of this rupture or ejection (at PA∼ 00°) is coincident with the axis of the intermediate and large-scale structures detected at radio wavelengths. (ii) In addition, in the three more external bubbles (S1, S2, S3), the two-dimensional William Herschel Telescope spectra show multiple emission-line components with OF velocities, ofandand. (iii) In the whole circumnuclear region, the[N ii]/Hα and[S ii]/Hα narrow emission-line ratios show high values (>0.8), which are consistent with low-ionization nuclear emission-line region/OF processes associated with fast velocity shocks. Therefore, we suggest that these giant bubbles are associated with the large-scale nuclear OF component, which is generated– at least in part– by the extreme nuclear starburst: giant supernova/hypernova explosions.The variability of the short-lived BAL-III Na iD system was studied, covering almost all the period in which this system appeared (between∼1984 and 2004). We have found that the BAL-III light curve is clearly asymmetric with a steep increase, a clear maximum and an exponential fall (similar to the shape of a supernova light curve). The origin of this BAL-III system is discussed, mainly in the framework of an extreme explosive event, probably associated with giant supernova/hypernova explosions.Finally, the IR colour diagram and the ultraviolet BAL systems of IR+ GW/OF+ Fe iiQSOs are analysed. This study shows two new BAL IR QSOs and suggests/confirms that these objects could be nearby young BAL QSOs, similar to those detected recently at. We propose that the phase of young QSOs is associated with accretion of a large amount of gas (by the supermassive black hole)+ extreme starbursts+ extreme composite OFs/BALs. [ABSTRACT FROM AUTHOR]

ABSTRACT We report, as a part of a long-term study of infrared (IR) mergers and IR quasi-stellar objects (QSOs), detailed spectroscopic evidence for outflow (OF) and Wolf–Rayet (WR) features in the nearby mergers NGC 4038/39 and IRAS 23128-5919 (with low-velocity OF); and the nearby QSOs IRAS 01003-2238 and 13218+0552 (with extreme velocity OF, EVOF). We also study the presence of EVOF in a complete sample of ultraluminous IR galaxies and QSOs (‘The IRAS 1-Jy Survey’, 118 objects). We found EVOF in IRAS 11119+3257, 14394+5332, 15130-1958 and 15462-0450. The low-velocity OF components were detected mainly in objects with starburst processes, i.e. OF associated with galactic winds generated in multiple type II supernova (SN) explosions and massive stars. Meanwhile the EVOF were detected mainly in objects with strong starburst plus obscured IR QSOs; which suggests that the coexistence of both processes could generate EVOF. Hubble Space Telescope (HST) images of IR+BAL+Fe iiQSOs show in practically all of these objects ‘arc or shell’ features probably associated with galactic winds [i.e. with multiple type II SN explosions or with starburst+active galactic nuclei (AGN)] or merger processes. In addition, we analyse the presence of Wolf–Rayet features in part of the large sample of bright PG-QSOs. We found possible WR features in the Fe iiPG-QSOs PG 1244+026, 1444+407, 1448+273 and 1535+547. The results are discussed mainly within the framework of the composite scenario: starburst+AGN. We analyse the presence of extreme starburst and galactic winds as a possible link between IR mergers and IR QSOs. Finally, we discuss the probable role of mergers, extreme starburst and galactic winds processes in BAL-QSOs and galaxies in formation. [ABSTRACT FROM AUTHOR]

We re-investigated the use of the Hubble diagram to measure the cosmological constant (Λ) and the mass density of the Universe (Ω[sub M]). We find an important focusing effect in Λ for redshifts of about 3. This effect implies that the apparent magnitude of a standard candle at redshifts z=2–3 has almost no dependence on Λ for Ω[sub M]>0.2. This means that Ω[sub M] can be measured independently of Ω[sub Λ] by targeting the redshift range according to an estimate of the value of Ω[sub M]. We explore the evidence in support of the suggestion that extreme starburst galaxies, also known as H ii galaxies, can be used as distance estimators over a wide range of redshifts and reaching very high values. We have compiled literature data of H ii galaxies up to z∼3 and found a good correlation between their luminosity and velocity dispersion measured from their strong emission lines, thus confirming the correlation already known to exist for H ii galaxies in the nearby Universe. Several systematic effects, such as age, extinction, kinematics and metallicity, are discussed, as well as the effects of different cosmologies. [ABSTRACT FROM AUTHOR]

We present a spatially resolved spectroscopic study for the metal poor H  ii galaxy J084220+115000 using MEGARA Integral Field Unit observations at the Gran Telescopio Canarias. We estimated the gas metallicity using the direct method for oxygen, nitrogen, and helium and found a mean value of 12 + log (O/H) = 8.03 ± 0.06, and integrated electron density and temperature of ∼161 cm−3 and ∼15400 K, respectively. The metallicity distribution shows a large range of Δ(O/H) = 0.72 dex between the minimum and maximum (7.69 ± 0.06 and 8.42 ± 0.05) values, unusual in a dwarf star-forming galaxy. We derived an integrated log (N/O) ratio of −1.51 ± 0.05 and found that both N/O and O/H correspond to a primary production of metals. Spatially resolved maps indicate that the gas appears to be photoionized by massive stars according to the diagnostic line ratios. Between the possible mechanisms to explain the starburst activity and the large variation of oxygen abundance in this galaxy, our data support a possible scenario where we are witnessing an ongoing interaction triggering multiple star-forming regions localized in two dominant clumps. [ABSTRACT FROM AUTHOR]

We describe a simple step-by-step guide to qualitative interpretation of galaxy spectra. Rather than an alternative to existing automated tools, it is put forward as an instrument for quick-look analysis and for gaining physical insight when interpreting the outputs provided by automated tools. Though the recipe is for general application, it was developed for understanding the nature of the Automatic Spectroscopic K-means-based (ASK) template spectra. They resulted from the classification of all the galaxy spectra in the Sloan Digital Sky Survey data release 7, thus being a comprehensive representation of the galaxy spectra in the local universe. Using the recipe, we give a description of the properties of the gas and the stars that characterize the ASK classes, from those corresponding to passively evolving galaxies, to H II galaxies undergoing a galaxy-wide starburst. The qualitative analysis is found to be in excellent agreement with quantitative analyses of the same spectra. We compare the mean ages of the stellar populations with those inferred using the code STARLIGHT. We also examine the estimated gas-phase metallicity with the metallicities obtained using electron-temperature-based methods. A number of byproducts follow from the analysis. There is a tight correlation between the age of the stellar population and the metallicity of the gas, which is stronger than the correlations between galaxy mass and stellar age, and galaxy mass and gas metallicity. The galaxy spectra are known to follow a one-dimensional sequence, and we identify the luminosity-weighted mean stellar age as the affine parameter that describes the sequence. All ASK classes happen to have a significant fraction of old stars, although spectrum-wise they are outshined by the youngest populations. Old stars are metal-rich or metal-poor depending on whether they reside in passive galaxies or in star-forming galaxies. [ABSTRACT FROM AUTHOR]

The observational determination of the behaviour of the star formation rate (SFR) with look-back time or redshift has two main weaknesses: (i) the large uncertainty of the dust/extinction corrections, and (ii) that systematic errors may be introduced by the fact that the SFR is estimated using different methods at different redshifts. Most frequently, the luminosity of the Hα emission line, that of the forbidden line [O ii] λ3727 and that of the far-ultraviolet continuum are used with low-, intermediate- and high-redshift galaxies, respectively. To assess the possible systematic differences among the different SFR estimators and the role of dust, we have compared SFR estimates using Hα, [O ii] λ3727 Å, ultraviolet (UV) and far-infrared (FIR) luminosities [SFR(Hα), SFR(O ii), SFR(UV) and SFR(FIR), respectively] of a sample comprising the 31 nearby star-forming galaxies that have high-quality photometric data in the UV, optical and FIR. We review the different ‘standard’ methods for the estimation of the SFR and find that while the standard method provides good agreement between SFR(Hα) and SFR(FIR), both SFR(O ii) and SFR(UV) are systematically higher than SFR(FIR), irrespective of the extinction law. We show that the excess in the SFR(O ii) and SFR(UV) is mainly due to an overestimation of the extinction resulting from the effect of underlying stellar Balmer absorptions in the measured emission line fluxes. Taking this effect into consideration in the determination of the extinction brings the SFR(O ii) and SFR(UV) in line with the SFR(FIR), and simultaneously reduces the internal scatter of the SFR estimations. Based on these results, we have derived ‘unbiased’ SFR expressions for the SFR(UV), SFR(O ii) and SFR(Hα). We have used these estimators to recompute the SFR history of the Universe using the results of published surveys. The main results are that the use of the unbiased SFR estimators brings into agreement the results of all surveys. Particularly important is the agreement achieved for the SFR derived from the FIR/millimetre and optical/UV surveys. The ‘unbiased’ star formation history of the Universe shows a steep rise in the SFR from z=0 to z=1 with SFR∝(1+z)[sup 4.5], followed by a decline for z>2 where SFR∝(1+z)[sup -1.5]. Galaxy formation models tend to have a much flatter slope from z=0 to z=1. [ABSTRACT FROM AUTHOR]

We have obtained accurate CCD narrow-band Hβ and Hα photometry of giant H ii regions (GEHRs) in M33, NGC 6822 and M101. Comparison with previous determinations of emission-line fluxes shows large discrepancies; their probable origins are discussed. Combining our new photometric data with global velocity dispersion (σ) derived from emission linewidths, we review the L(Hβ)–σ relation. A re-analysis of the properties of the GEHRs included in our sample shows that age spread and the superposition of components in multiple regions introduce a considerable spread in the regression. Combining the information available in the literature regarding ages of the associated clusters, evolutionary footprints on the interstellar medium, and kinematical properties of the knots that build up the multiple GEHRs, we find that a subsample – which we refer to as young and single GEHRs – do follow a tight relation in the L–σ plane. [ABSTRACT FROM AUTHOR]

We have modelled the evolution of hot superbubbles in starbursts, taking into account the rapid changes in the chemical composition of the interior of the superbubbles resulting from the large stellar mass loss, i.e. stellar winds from massive young stars and type II supernovae. We have followed in detail the time-dependent production and mixing of oxygen and iron in the interior of the hot superbubbles and showed that while the oxygen abundance rapidly climbs to over solar values in less than 10 Myr, iron abundance remains always under solar. This highly enhanced oxygen metallicity boosts the early X-ray luminosity of superbubbles while keeping the iron abundance subsolar. This brings theory and X-ray observations of the luminosity and metal content of young starbursts closer together. [ABSTRACT FROM AUTHOR]

We present spectra of six type 1 and two type 2 Seyfert galaxies, a starburst galaxy and a compact narrow-line radio galaxy, taken in two spectral ranges centred around the near-infrared Ca ii triplet (∼8600 Å), and the Mgb stellar feature at 5180 Å. We measured the equivalent widths (EWs) of these features and the Fe[sub 52] and Fe[sub 53] spectral indices. We found that the strength of the infrared Ca ii triplet (CaT) in type 1 Seyfert galaxies with prominent central point sources is larger than what would be expected from the observed strength of the blue indices. This could be explained by the presence of red supergiants in the nuclei of Seyfert 1 galaxies. On the other hand, the blue indices of these galaxies could also be diluted by the strong Fe ii multiplets that can be seen in their spectra. We have also measured the stellar- and gas-velocity dispersions of the galaxies in the sample. The stellar velocity dispersions were measured using both the Mgb and CaT stellar features. The velocity dispersion of the gas in the narrow-line region (NLR) was measured using the strong emission lines [O iii] λλ5007, 4959 and [S iii] λ9069. We compare the gas- and star-velocity dispersions and find that the magnitudes of both are correlated in Seyfert galaxies. Most of the Seyfert 1 galaxies that we observe have stellar-velocity dispersions somewhat greater than that of the gas in the NLR. [ABSTRACT FROM AUTHOR]

Broad-band V, R and I and narrow-band Hα CCD surface photometry of a sample of 24 active and three normal nearby galaxies is presented, in order to study their morphological and structural properties. This first paper presents the observations, data reduction and processing techniques used; the main characteristics of the central regions of the sample galaxies are derived from broad-band and Hα images; broad-band surface brightness and colour profiles are presented; based on the inspection of individual images and the examination of the radial variation of ellipticity and polar angle, the morphological class of the sample objects is revised: several objects previously classified as non-barred or unclassified are found to be barred galaxies. A high fraction (∼71 per cent) of the objects that present any kind of nuclear activity are barred. This proportion is even higher (∼87 per cent) if only Seyfert galaxies are considered. This result can reinforce the clues about the connection between bars and nuclear activity as a fuelling mechanism that can trigger and sustain nuclear activity. [ABSTRACT FROM AUTHOR]

We present high signal-to-noise ratio, long-slit spectra of the central region of the 30 Doradus nebula taken with unprecedented spatial (∼ 0.1 pc) and spectral (∼ 3 km s[sup -1]) resolution. We use these observations to investigate the dominant line-broadening mechanisms for the global nebula. We find no evidence for significant gravitational broadening of the lines. Thus, broadening appears to be dominated by stellar winds and thermal motions. However, we identify a new broad component to the lines that explains the integrated profile wings identified in previous observations. The broad component appears at all positions observed, and is possibly related to the disruption through photo-evaporation of high-density condensations within the nebula. [ABSTRACT FROM AUTHOR]

Extragalactic very-high-energy (VHE; E > 100 GeV) sources are unique objects to study the most powerful particle accelerators in nature, as active galactic nuclei are likely sources of ultra-high-energy cosmic rays. BL Lacertae blazars are the most frequent extragalactic objects found in the VHE gamma-ray catalogs. It is very difficult to estimate their redshifts, considering they have no strong enough optical spectral features, hence ∼20% of them have unknown or poorly constrained redshifts. KUV 00311–1938 is a VHE BL Lacertae blazar, with an uncertain redshift in the range of 0.5 < z < 0.98. We have obtained deep spectroscopy using Gemini and the GTC telescopes of KUV 00311–1938 and its surroundings with high signal-to-noise ratio in 2016–2017. The lack of features did not allow us to determine the spectroscopic redshift of KUV 00311–1938; nevertheless, we obtain a lower limit of z ≳ 0.475. We determined the redshifts of 41 galaxies observed in the field-of-view of the blazar and through a population study, identified three pairs and four groups consisting of 3 or 4 members with redshifts in the range z = 0.1468–0.4756. Due to the absence of a large group of galaxies, we could not associate KUV 00311–1938 with any of the groups detected. [ABSTRACT FROM AUTHOR]

We here report the detection of extended He  ii  λ4686 nebular emission in the central region of NGC 1569 using the integral field spectrograph MEGARA at the 10.4 m Gran Telescopio Canarias. The observations cover a field of view (FoV) of 12.5 arcsec × 11.3 arcsec at a seeing-limited spatial resolution of ∼15 pc and at a spectral resolution of R  = 6000 in the wavelength range 4330–5200 Å. The emission extends over a semicircular arc of ∼40 pc width and ∼150 pc diameter around the superstar cluster A (SSC-A). The A V derived using Balmer decrement varies from the Galactic value of 1.6 mag to a maximum of ∼4.5 mag, with a mean value of 2.65 ± 0.60 mag. We infer 124 ± 11 Wolf–Rayet (WR) stars in SSC-A using the He  ii  λ4686 broad feature and A V = 2.3 mag. The He+ ionizing photon rate from these WR stars is sufficient to explain the luminosity of the He  ii nebula. The observationally determined total He+ and H0 ionizing photon rates, their ratio, and the observed number of WR stars in SSC-A are all consistent with the predictions of simple stellar population models at an age of 4.0 ± 0.5 Myr and a mass of (5.5 ± 0.5) × 105 M⊙. Our observations reinforce the absence of WR stars in SSC-B, the second most massive cluster in the FoV. None of the other locations in our FoV where He  ii  λ4686 emission has been reported from narrow-band imaging observations contain WR stars. [ABSTRACT FROM AUTHOR]

We present new spectroscopic data of the BL Lac RGB 2243 + 203, and its surroundings, obtained with the OSIRIS Multi Object Spectrograph (MOS) mounted in the Gran Telescopio Canarias (GTC). The spectra of neither the BL Lac nor its host galaxy show any spectral feature, thus hindering direct determination of its redshift. The spectroscopic redshift distribution of objects in the MOS field of view shows four galaxies with redshift between 0.5258 and 0.5288. We make use of a statistical analysis to test the possibility that the targeted BL Lac may be a member of that group. By using the spectroscopic redshifts obtained with our GTC observations, we found that this probability is between 86  and 93 per cent. [ABSTRACT FROM AUTHOR]

The main goal of this study is to characterize the stellar populations in very low-metallicity galaxies. We have obtained broad U, B, R, I, J, H, K, intermediate Str¨omgren y and narrow Hα and [O III] deep images of the Wolf–Rayet, blue compact dwarf, HII galaxy Tol 02. We have analysed the low surface brightness component, the stellar cluster complexes and the HII regions. The stellar populations in the galaxy have been characterized by comparing the observed broad-band colours with those of single stellar population models. The main results are consistent with Tol 02 being formed by a 1.5 Gyr old disc component at the centre of which a group of eight massive (>104M⊚) stellar cluster clumps is located. Six of these clumps are 10 Myr old and their near-infrared colours suggest that their light is dominated by Red Supergiant (RSG) stars, the other two are young Wolf–Rayet cluster candidates of ages 3 and 5 Myr, respectively. 12 HII regions in the star-forming region of the galaxy are also identified. These are immersed in a diffuse Hα and [O III] emission that spreads towards the north and south covering the old stellar disc. Our spatial-temporal analysis shows that star formation is more likely stochastic and simultaneous within short time-scales. The mismatch between observations and models cannot be attributed alone to a mistreat of the RSG phase and still needs to be further investigated. [ABSTRACT FROM AUTHOR]

We present the results of our search near the end of the Reionization Epoch for faint galaxies. This has been done using very deep OSIRIS images obtained at the Gran Telescopio Canarias (GTC). Our observations focus around two close, massive Lyman a emitters (LAEs) at redshift 6.5, discovered in the SXDS field within a large-scale overdense region. The total GTC observing time in three medium band filters (F883w35, F913w25 and F941w33) is over 34 h covering 7.0 × 8.5 arcmin² (or ~30 000 Mpc³ at z = 6.5). In addition to the two spectroscopically confirmed LAEs in the field, we have identified 45 other LAE candidates. The preliminary luminosity function derived from our observations, assuming a spectroscopic confirmation success rate of 2/3 as in previous surveys, suggests this area is about 2 times denser than the general field galaxy population at z = 6.5. If confirmed spectroscopically, our results will imply the discovery of one of the earliest protoclusters in the Universe, which will evolve to resemble the most massive galaxy clusters today. [ABSTRACT FROM AUTHOR]

We use the H II galaxies L-σ relation and the resulting Hubble expansion cosmological probe of a sample of just 25 high-z (up to z ~ 2.33) H II galaxies, in a joint likelihood analysis with other well tested cosmological probes (cosmic microwave background, CMB, Baryon Acoustic Oscillations, BAOs) in an attempt to constrain the dark energy equation of state (EoS). The constraints, although still weak, are in excellent agreement with those of a similar joint analysis using the well established SNIa Hubble expansion probe. Interestingly, even with the current small number of available high redshift H II galaxies, the H II/BAO/CMB joint analysis gives a 13 per cent improvement of the quintessence dark energy cosmological constraints compared to the BAO/CMB joint analysis. We have further performed extensive Monte Carlo simulations, with a realistic redshift sampling, to explore the extent to which the use of the L-σ relation, observed in H II galaxies, can constrain effectively the parameter space of the dark energy EoS. The simulations predict substantial improvement in the constraints when increasing the sample of high-z H II galaxies to 500, a goal that can be achieved in reasonable observing times with existing large telescopes and state-of-the-art instrumentation. [ABSTRACT FROM AUTHOR]

We present 8.5 arcsec resolution 1.1-mm continuum imaging and CO spectroscopic redshift measurements of eight extremely bright submillimetre galaxies identified from the Planck and Herschel surveys, taken with the Large Millimeter Telescope's AzTEC and Redshift Search Receiver instruments. We compiled a candidate list of high-redshift galaxies by cross-correlating the Planck Surveyor mission's highest frequency channel (857 GHz, full width at half-maximum = 4.5 arcmin) with the archival Herschel Spectral and Photometric Imaging Receiver imaging data, and requiring the presence of a unique, single Herschel counterpart within the 150-arcsec search radius of the Planck source positions with 350-μm flux density larger than 100 mJy, excluding known blazars and foreground galaxies. All eight candidate objects observed are detected in 1.1 mm continuum by AzTEC bolometer camera, and at least one CO line is detected in all cases with a spectroscopic redshift between 1.3 < zCO < 3.3. Their infrared (IR) spectral energy distributions (SEDs) mapped using the Herschel and AzTEC photometry are consistent with cold dust emission with characteristic temperature between Td = 43 and 84 K. With apparent IR luminosity of up to LIR = 3 × 1014μ−1 L⊙, they are some of the most luminous galaxies ever found (with yet unknown gravitational magnification factor μ). The analysis of their SEDs suggests that star formation is powering the bulk of their extremely large IR luminosities. Derived molecular gas masses of MH2=(0.6−7.8)×1011M⊙ (for μ ≈ 10) also make them some of the most gas-rich high-redshift galaxies ever detected. [ABSTRACT FROM AUTHOR]

This work investigates the main mechanism(s) that regulate the specific star formation rate (SSFR) in nearby galaxies, cross-correlating two proxies of this quantity -- the equivalent width of the Hα line and the (u - r) colour -- with other physical properties (mass, metallicity, environment, morphology, and the presence of close companions) in a sample of ~82 500 galaxies extracted from the Sloan Digital Sky Survey. The existence of a relatively tight 'ageing sequence' in the colour--equivalent width plane favours a scenario where the secular conversion of gas into stars (i.e. nature) is the main physical driver of the instantaneous SSFR and the gradual transition from a 'chemically primitive' (metal-poor and intensely star-forming) state to a 'chemically evolved' (metal-rich and passively evolving) system. Nevertheless, environmental factors (i.e. nurture) are also important. In the field, galaxies may be temporarily affected by discrete 'quenching' and 'rejuvenation' episodes, but such events show little statistical significance in a probabilistic sense, and we find no evidence that galaxy interactions are, on average, a dominant driver of star formation. Although visually classified mergers tend to display systematically higher EW(Hα) and bluer (u - r) colours for a given luminosity, most galaxies with high SSFR have uncertain morphologies, which could be due to either internal or external processes. Field galaxies of early and late morphological types are consistent with the gradual 'ageing' scenario, with no obvious signatures of a sudden decrease in their SSFR. In contrast, star formation is significantly reduced and sometimes completely quenched on a short time-scale in dense environments, where many objects are found on a 'quenched sequence' in the colour--equivalent width plane. [ABSTRACT FROM AUTHOR]

This work investigates the main mechanism(s) that regulate the specific star formation rate (SSFR) in nearby galaxies, cross-correlating two proxies of this quantity - the equivalent width of the Ha line and the (u - r) colour - with other physical properties (mass, metallicity, environment, morphology, and the presence of close companions) in a sample of ~82 500 galaxies extracted from the Sloan Digital Sky Survey. The existence of a relatively tight 'ageing sequence' in the colour-equivalent width plane favours a scenario where the secular conversion of gas into stars (i.e. nature) is the main physical driver of the instantaneous SSFR and the gradual transition from a 'chemically primitive' (metal-poor and intensely star-forming) state to a 'chemically evolved' (metal-rich and passively evolving) system. Nevertheless, environmental factors (i.e. nurture) are also important. In the field, galaxies may be temporarily affected by discrete 'quenching' and 'rejuvenation' episodes, but such events show little statistical significance in a probabilistic sense, and we find no evidence that galaxy interactions are, on average, a dominant driver of star formation. Although visually classified mergers tend to display systematically higher EW(Ha) and bluer (u - r) colours for a given luminosity, most galaxies with high SSFR have uncertain morphologies, which could be due to either internal or external processes. Field galaxies of early and late morphological types are consistent with the gradual 'ageing' scenario, with no obvious signatures of a sudden decrease in their SSFR. In contrast, star formation is significantly reduced and sometimes completely quenched on a short time-scale in dense environments, where many objects are found on a 'quenched sequence' in the colour-equivalent width plane. [ABSTRACT FROM AUTHOR]

ABSTRACT We use the self-consistent model technique developed by Martín-Manjón et al. that combines the chemical evolution with stellar population synthesis and photoionization codes, to study the star formation scenarios capable of reproducing the observed properties of star-forming galaxies. The comparison of our model results with a data base of H ii galaxies shows that the observed spectra and colours of the present burst and the older underlying population are reproduced by models in a bursting scenario with star formation efficiency involving close to 20 per cent of the total mass of gas, and interburst times longer than 100 Myr, and more probably around 1 Gyr. Other modes like gasping and continuous star formation are not favoured. [ABSTRACT FROM AUTHOR]

Integral field spectroscopy obtained with the Potsdam Multi-Aperture Spectrophotometer Fiber Package (PPak) and the 3.5-m telescope at the Calar Alto Observatory has been used to study an outer H region complex in the well-studied galaxy NGC 6946. This technique provides detailed maps of the region in different emission lines, yielding spatially resolved information about the physical properties of the gas. The configuration was chosen to cover the whole spectrum from 3600 up to 10 000 Å, allowing the measurement of the near-infrared [S ] lines. We selected four luminous knots to perform a detailed integrated spectroscopic analysis of these structures and of the whole PPak field of view (FOV). For all the knots the electron density has been found to be very similar and below 100 cm. The [O ] electron temperature was measured in knots A, B, C and in the integrated PPak field, and was found to be around 8000 K. The temperatures of [O ] and [S ] were estimated in the four cases. The elemental abundances computed from the 'direct method' are typical of high-metallicity disc H regions, with a mean value of 12+log(O/H) = 8.65, comparable to what has been found in this galaxy by other authors for regions at similar galactocentric distance. Therefore a remarkable abundance uniformity is found despite the different excitations found throughout the nebula. However, due to the quality of the data, the electron temperatures and metallicities obtained have associated errors comparable to the typical dispersion found in empirical calibrations. Wolf-Rayet (WR) features have been detected in three of the knots, leading to a derived total number of WR stars of 125, 22 and 5 for knots A, C and B, respectively. The ratios of the numbers of WR to O stars are consistent with the prediction of Starburst99 for individual bursts with an age about 4 Myr. Knot D, with no WR features, shows weak Hα emission, low excitation and the lowest Hβ equivalent width, all of which points to a more evolved state. The integrated spectrum of the whole PPak FOV shows high excitation and a relatively evolved age that does not correspond to the individual knot evolutionary stages. Some effects associated with the loss of spatial resolution are also evidenced by the higher ionizing temperature that is deduced from the parameter measured in the integrated PPak spectrum with respect to that of the individual knots. [ABSTRACT FROM AUTHOR]

In an effort to understand the correlation between X-ray emission and present star formation rate, we obtained XMM–Newton data to estimate the X-ray luminosities of a sample of actively star-forming H ii galaxies. The obtained X-ray luminosities are compared to other well-known tracers of star formation activity such as the far-infrared and the ultraviolet luminosities. We also compare the obtained results with empirical laws from the literature and with recently published analysis applying synthesis models. We use the time delay between the formation of the stellar cluster and that of the first X-ray binaries, in order to put limits on the age of a given stellar burst. We conclude that the generation of soft X-rays, as well as the Hα or infrared luminosities is instantaneous. The relation between the observed radio and hard X-ray luminosities, on the other hand, points to the existence of a time delay between the formation of the stellar cluster and the explosion of the first massive stars and the consequent formation of supernova (SN) remnants and high-mass X-ray binaries, which originate the radio and hard X-ray fluxes, respectively. When comparing hard X-rays with a star formation indicator that traces the first million years of evolution (e.g. Hα luminosities), we found a deficit in the expected X-ray luminosity. This deficit is not found when the X-ray luminosities are compared with infrared luminosities, a star formation tracer that represents an average over the last 108 yr. The results support the hypothesis that hard X-rays are originated in X-ray binaries which, as SN remnants, have a formation time delay of a few mega years after the star-forming burst. [ABSTRACT FROM AUTHOR]

From a study of broad absorption line (BAL) + infrared (IR) + Fe ii quasi-stellar objects (QSOs) [using deep Gemini Multi-Object Spectrograph Integral Field Unit (GMOS-IFU) spectroscopy], new results are presented for IRAS 04505−2958. Specifically, we have studied in detail the outflow (OF) process at two large galactic scales: (i) two blobs/shells at radius 1.1 and 2.2 kpc, and (ii) an external hypergiant shell at . In addition, the presence of two very extended hypergiant shells at is also discussed. From this GMOS study the following main results were obtained. (i) For the external hypergiant shell, the kinematics GMOS maps of the ionized gas ([O ii], [Ne iii], [O iii], Hβ) show a small-scale bipolar OF, with similar properties to those observed in the prototype of exploding external supershells: NGC 5514. (ii) Three main knots – of this hypershell S3 – show the presence of a young starburst. (iii) The two internal shells show OF components with typical properties of nuclear shells. (iv) The two blobs and the hypershell are aligned at PA ∼ 131° showing bipolar OF shape at ∼10–15 kpc scale. In addition, the more external shells (at ∼60–80 kpc scale) are aligned at PA ∼ 40° also with bipolar OF shape (perpendicular to the more internal OF). (v) A strong blue continuum and multiple emission-line components were detected in all the GMOS fields. The new GMOS data show a good agreement with an extreme + explosive OF scenario for IRAS 04505−2958, in which part of the interstellar medium (ISM) of the host galaxy was ejected (in multiple shells). This extreme OF could also be associated with two main processes in the evolution of QSOs: (i) the formation of companion/satellite galaxies by giant explosions; and (ii) to define the final mass of the host galaxy, and even if the explosive nuclear OF is extremely energetic, this process could disrupt an important fraction of the host galaxy. Finally, the generation of ultra-high-energy cosmic rays and neutrino/dark matter – associated with HyNe in explosive BAL + IR + Fe ii QSOs – is discussed. [ABSTRACT FROM AUTHOR]

In this paper we present the first results of a study of BAL QSOs (at low and high redshift), based on very deep Gemini GMOS integral field spectroscopy. In particular, the results obtained for the nearest BAL IR–QSO Mrk 231 are presented. For the nuclear region of Mrk 231, the QSO and host galaxy components were modelled, using a new technique of decoupling 3D spectra. From this study, the following main results were found: (i) in the pure host galaxy spectrum an extreme nuclear starburst component was clearly observed, as a very strong increase in the flux, at the blue wavelengths; (ii) the BAL system I is observed in the spectrum of the host galaxy; (iii) in the clean/pure QSO emission spectrum, only broad lines were detected. 3D GMOS individual spectra (specially in the near-infrared Ca ii triplet) and maps confirm the presence of an extreme and young nuclear starburst (8 < age < 15 Myr), which was detected in a ring or toroid with a radius , around the core of the nucleus. The extreme continuum blue component was detected only to the south of the core of the nucleus. This area is coincident with the region where we previously suggested that the galactic wind is cleaning the nuclear dust. Very deep 3D spectra and maps clearly show that the BAL systems I and II – in the strong ‘ absorption lines’ Na iDλ5889–95 and Ca ii Kλ3933 – are extended (reaching ∼1.4–1.6 arcsec ∼ 1.2–1.3 kpc, from the nucleus) and clearly elongated at the position angle (PA) close to the radio jet PA, which suggest that the BAL systems I and II are ‘both’ associated with the radio jet. The physical properties of the four expanding nuclear bubbles were analysed, using the GMOS 3D spectra and maps. In particular, we found strong multiple LINER/OF emission-line systems and Wolf–Rayet features in the main knots of the more external super bubble S1 . The kinematics of these knots – and the internal bubbles – suggest that they are associated with an area of rupture of the shell S1 (at the south-west). In addition, in the more internal superbubble S4 and close to the core of the nucleus (for ), two similar narrow emission-line systems were detected, with strong [S ii] and [O i] emission and . These results suggest that an important part of the nuclear NLR is generated by the OF process and the associated low-velocity ionizing shocks. Finally, the nature of the composite BAL systems and very extended OF process – of 50 kpc – in Mrk 231 (and similar QSOs) are discussed. In addition, the ‘ composite hyperwind scenario’ (already proposed for BALs) is suggested for the origin of giant Lyα blobs. The importance of study the end phases of Mrk 231, and similar evolving elliptical galaxies and QSOs (i.e. galaxy remnants) is discussed. [ABSTRACT FROM AUTHOR]

We have computed a series of realistic and self-consistent models of the emitted spectra of H ii galaxies. Our models combine different codes of chemical evolution, evolutionary population synthesis and photoionization. The emitted spectrum of H ii galaxies is reproduced by means of the photoionization codecloudy, using as ionizing spectrum the spectral energy distribution of the modelled H ii galaxy, which in turn is calculated according to a star formation history and a metallicity evolution given by a chemical evolution model that follows the abundances of 15 different elements. The contribution of emission lines to the broad-band colours is explicitly taken into account. The results of our code are compared with photometric and spectroscopic data of H ii galaxies. Our technique reproduces observed diagnostic diagrams, abundances, equivalent width–colour and equivalent width–metallicity relations for local H ii galaxies. [ABSTRACT FROM AUTHOR]

This is the third paper of a series devoted to the study of the global properties of Joguet's sample of 79 nearby galaxies observable from the southern hemisphere, of which 65 are Seyfert 2 galaxies. We use the population synthesis models of Paper II to derive ‘pure’ emission-line spectra for the Seyfert 2 galaxies in the sample, and thus explore the statistical properties of the nuclear nebular components and their relation to the stellar populations. We find that the emission-line clouds suffer substantially more extinction than the starlight, and we confirm the correlations between stellar and nebular velocity dispersions and between emission-line luminosity and velocity dispersions, although with substantial scatter. Nuclear luminosities correlate with stellar velocity dispersions, but Seyferts with conspicuous star-forming activity deviate systematically towards higher luminosities. Removing the contribution of young stars to the optical continuum produces a tighter and steeper relation, , consistent with the Faber–Jackson law. Emission-line ratios indicative of the gas excitation such as [O iii]/Hβ and [O iii]/[O ii] are statistically smaller for Seyferts with significant star formation, implying that ionization by massive stars is responsible for a substantial and sometimes even a dominant fraction of the Hβ and [O ii] fluxes. We use our models to constrain the maximum fraction of the ionizing power that can be generated by a hidden active galactic nucleus (AGN). We correlate this fraction with classical indicators of AGN photoionization (i.e. X-ray luminosity and nebular excitation), but find no significant correlations. Thus, while there is a strong contribution of starbursts to the excitation of the nuclear nebular emission in low-luminosity Seyferts, the contribution of the hidden AGN remains elusive even in hard X-rays. [ABSTRACT FROM AUTHOR]

A study of the morphology, kinematics and ionization structure of the infrared (IR) merger NGC 5514 is presented. This study is based mainly on INTEGRAL two-dimensional (2D) spectroscopy (obtained at the 4.2-m William Herschel Telescope, WHT), plus optical and near-IR images. Clear evidence of two extranuclear starbursts with young outflows (OFs) and low-ionization nuclear emission region (LINER) activity are reported. One of these OFs has generated a supergiant bubble and the other is associated with an extended complex of H ii regions. In the galactic bubble it was found that: (i) the [S ii], Hα, [N ii], [O i] and [O iii] emission-line maps show a bubble with a distorted ellipsoidal shape, with major and minor axes of ∼6.5 kpc [13.6 arcsec; at position angle (PA) = 120°± 10°] and ∼4.5 kpc (9.6 arcsec); (ii) these maps depict four main knots, a very strong one and three others more compact and located at the border; (iii) the centre of the bubble is located at ∼4.1 kpc (8.5 arcsec) to the west of the main nucleus; (iv) the WHT spectra show, in this area, two strong components: blue and red emission-line systems, probably associated with emission from the near and far side of the external shell, for which the mean OF velocities were measured as VOF blue= (−320 ± 20) km s−1 and VOF red= (+265 ± 25) km s−1; (v) these two components depict LINER properties, probably associated with large-scale OF + shocks; (vi) at the east border, the kinematics of the ionized gas and the [S ii] emission-line maps show an extended ejection of 4 kpc aligned with the PA of the major axis; (vii) three other ejections were found, two of them perpendicular to the extended one. Each ejection starts in one of the knots. These results suggest that the bubble is in the rupture phase. For the complex of giant H ii regions it was found that: (i) the Hα, [N ii] and [S ii] emission-line maps show a compact strong emission area (peaking at ∼810 pc ∼1.7 arcsec, to the east of the second nucleus) and faint extended emission with an elongated shape, and major and minor axes of ∼5.1 kpc (10.8 arcsec; at PA ∼20°) and ∼2.9 kpc (6.0 arcsec); (ii) inside this complex, the spectra show H ii region and transition LINER/H ii characteristics; (iii) at the border of this extended H ii area the spectra have outflow components and LINER properties. INTEGRAL 2D [N ii], Hα, [S ii] and [O iii] velocity fields (VFs) are presented. These VF maps show results consistent with an expansion of the bubble, plus four ejections of ionized gas. The U, B, V, I, J, H and KS images show a pre-merger morphology, from which faint filaments of emission emerge, centred on the bubble. The ionization structure and the physical conditions were analysed using the following 2D emission-line ratio and width maps: [S ii]/Hα, [N ii]/Hα, [O i]/Hα, [O iii]/Hβ and FWHM-[N ii]. In the region of the bubble, 100 per cent of the [N ii]/Hα and [S ii]/Hα ratios show very high values (>0.8) consistent with LINER processes associated with high-velocity shocks. These new results support the previous proposition that extreme nuclear and 'extranuclear' starbursts with galactic winds + shocks play an important role in the evolution of IR mergers/quasi-stellar objects. [ABSTRACT FROM AUTHOR]

We present a study of the stellar populations in the central∼200 pc of a large and homogeneous sample comprising 79 nearby galaxies, most of which are Seyfert 2s. The star formation history of these nuclei is reconstructed by means of state-of-the-art population synthesis modelling of their spectra in the 3500–5200Å interval. A quasar-like featureless continuum (FC) is added to the models to account for possible scattered light from a hidden active galactic nucleus (AGN).We find the following. (1) The star formation history of Seyfert 2 nuclei is remarkably heterogeneous: young starbursts, intermediate-age and old stellar populations all appear in significant and widely varying proportions. (2) A significant fraction of the nuclei show a strong FC component, but this FC is not always an indication of a hidden AGN: it can also betray the presence of a young, dusty starburst. (3) We detect weak broad Hβ emission in several Seyfert 2s after cleaning the observed spectrum by subtracting the synthesis model. These are most likely the weak scattered lines from the hidden broad-line region envisaged in the unified model, given that in most of these cases independent spectropolarimetry data find a hidden Seyfert 1. (4) The FC strengths obtained by the spectral decomposition are substantially larger for the Seyfert 2s which present evidence of broad lines, implying that the scattered non-stellar continuum is also detected. (5) There is no correlation between the star formation in the nucleus and either the central or overall morphology of the parent galaxies. [ABSTRACT FROM AUTHOR]

We have studied the extreme starburst in the infrared merger NGC 3256. We detect four galactic bubbles (using ESO New Technology Telescope andHubble Space TelescopeWFPC2 Hα images). These shells would be associated with previous supernova explosions.The first analysis of the spatial distribution of young star cluster candidates shows that more than 90 per cent of them are located in a complex starburst structure, including some of the bubble walls, three nuclei and three blue asymmetrical spiral arms.We have made a kinematic study of the ionized gas in the core of the main optical nucleus, performed withHSTSTIS spectra. The shape of the rotation curve and the emission-line profile can be explained by the presence in the core of young star clusters with outflow. Any low-luminosity active galactic nucleus associated with this core would have a mass less than. It is also probable that the compact X-ray and radio emission of ULX(7)N– the source coincident with the main optical nucleus– is the result of a few recent supernova remnants. [ABSTRACT FROM AUTHOR]

We present the first results of a study of the morphology, kinematics and ionization structure of infrared (IR) mergers/quasi-stellar objects (QSOs) with galactic winds. This study is based mainly on integral two-dimensional (2D) fibre spectroscopy [obtained on the 4.2-m William Herschel Telescope (WHT), La Palma] combined with high-resolution Hubble Space Telescope ( HST) observations. Clear evidence of outflow (OF) from the nucleus of the luminous infrared merger NGC 2623 is reported. Specifically: (i) the integral 2D Hα, [N ii] and [S ii] emission line maps depict a cone-shaped extended nebula that emerges from the nucleus, with an aperture angle and reaching a distance of ∼3.2 kpc from the nucleus; (ii) inside the nebula and in the central region, all the emission-line WHT spectra show low velocity blue/OF components, with ; and (iii) in the OF nebula, the emission line ratios are consistent with ionization by a dusty nuclear starburst plus shock heating. These results are consistent with a galactic wind process powered mainly by a nuclear starburst. The integral 2D Hα and [N ii]λ6583 velocity field (VF) maps for the main body of show outflow motion in the nuclear and the Hα+[N ii] nebular regions superposed on a general circular motion. This circular motion prevails inside , and for larger radii we detected non-circular motions. In the central region, the average observed rotation curve was fitted with a model corresponding to a single-component Plummer spherical potential. After the subtraction of the Plummer and an axisymmetric polynomial model, the residues of the VF in both cases indicate ejection as the origin of the cone nebula. The fitted Plummer model implies a total mass of and a spherical distribution of matter in the central region. The high-resolution HST WFPC2 F555W (∼ V) and F814W (∼ I) broad-band images display a strongly obscured nucleus in the apex of a small nuclear cone, an asymmetrical clumpy spiral arm located to the east of the nucleus, a ring plus an arc to the west and several large-scale filaments of dust. A good r1/4-law fit to the HST WFPC2 I band luminosity profile was found. In 85 per cent of the integral 2D field we measure very high values (>1) of the [N ii]λ6583/Hα and [S ii]λ6717 + 31/Hα ratios, suggesting that shocks are important on large scales (in almost all the main body). Furthermore, the 2D full width at half maximum FWHM–[N ii] and VF residual maps show a good spatial correlation, suggesting that the OF shocks ionize the gas and broaden the emission lines. However, close to the nucleus, the OF nebula shows low values of the [N ii]λ6583/Hα ratio (in the range 0.1–0.4), indicating that photoionization by a dusty nuclear starburst also plays a significant role in the excitation of the nebula. The characteristics of the nucleus of NGC 2623 could be associated with a starburst-related LINER. The properties found in IR mergers/QSOs with galactic winds mainly underline the importance of studying the possible link between IR mergers with starburst + galactic wind → IR QSOs with composite nature + galactic wind, and elliptical galaxies. [ABSTRACT FROM AUTHOR]

We report on multi-epoch X-ray observations of the Type IIn (narrow emission line) Supernova (SN) 1995N with the ROSAT and ASCA satellites. The 1998 January ASCA X-ray spectrum is well fitted by a thermal bremsstrahlung (kT∼10 keV, N[sub H]∼6×10[sup 20] cm[sup -2]) or power-law (α∼1.7, N[sub H]∼10[sup 21] cm[sup -2]) model. The X-ray light curve shows evidence for significant flux evolution between 1996 August and 1998 January: the count rate from the source decreased by 30 per cent between our 1996 August and 1997 August ROSAT observations, and the X-ray luminosity most likely increased by a factor of ∼2 between our 1997 August ROSAT and 1998 January ASCA observations, although evolution of the spectral shape over this interval is not ruled out. The high X-ray luminosity, L[sub X]∼10[sup 41] erg s[sup -1], places SN 1995N in a small group of Type IIn supernovae with strong circumstellar interaction, and the evolving X-ray luminosity suggests that the circumstellar medium is distributed inhomogeneously. [ABSTRACT FROM AUTHOR]

The detection of Lyα emission in star-forming galaxies in different shapes and intensities (always smaller than predicted for case B recombination) has puzzled the astronomical community for more than a decade. Here we use two-dimensional calculations to follow the evolution of superbubbles and of the H ii regions generated by the output of UV photons from massive stars. We show the impact caused by massive star formation in the ISM of different galaxies and we look at the conditions required to detect Lyα emission from a nuclear H ii region, and the variety of profiles that may be expected as a function of time. [ABSTRACT FROM AUTHOR]

We report the detection of optical and X-ray variability in the least luminous known Seyfert galaxy, NGC 4395. Between 1996 July and 1997 January the featureless continuum changed by a factor of 2, which is typical of more luminous AGN. The largest variation was seen at shorter wavelengths, so that the spectrum became `harder' during higher activity states. During the same period, the broad emission-line flux changed by ∼ 20-30 per cent. In a one-week optical broad-band monitoring programme, a 20 per cent change was seen between successive nights. The difference in flux observed between the spectroscopy of 1996 July and the broad-band observations implies variation by a factor of 3 at 4400 Å in just one month. In the same period, the spectral shape changed from a power law with spectral index α ∼ 0 (characteristic of quasars) to a spectral index α ∼ 2 (as observed in other dwarf AGN). ROSAT HRI and PSPC archive data show a variable X-ray source coincident with the Galactic nucleus. A change in X-ray flux by a factor ∼ 2 in 15 d has been observed. When compared with more luminous AGN, NGC 4395 appears to be very X-ray quiet. The hardness ratio obtained from the PSPC data suggests that the spectrum could be absorbed. We also report the discovery of weak Ca II K absorption, suggesting the presence of a young stellar cluster providing of the order of 10 per cent of the blue light. The stellar cluster may be directly observed as a diffuse component in Hubble Space Telescope (HST ) optical imaging. Using HST UV archive data, together with the optical and X-ray observations, we examine the spectral energy distribution for NGC 4395 and discuss the physical conditions implied by the nuclear activity under the standard AGN model. When in the low state, the extrapolated UV continuum is insufficient to explain the observed broad emission lines. This could be explained by intrinsic variability or absorption or may imply an extra heating source for the broad-line region. The observations can be explained either by an accreting massive black hole emitting at about 10[sup -3]L[sub Edd] or by a single old compact supernova remnant with an age of 50 to 500 yr generated by a small nuclear starburst. [ABSTRACT FROM AUTHOR]