arXiv subjects

Roberto Terlevich

Publications and source records attributed to Roberto Terlevich.

64 records · Page 4Linked to original sources

arXiv2000

Evolution of the X-ray luminosity and metallicity of starburst blown superbubbles

We calculate the time-dependent metal production expected from starbursts and use them as boundary conditions in our 2D simulations of evolving superbubbles. We assume that the produced metals (oxygen and iron) thoroughly mix with the ejected stellar envelopes, and/or with the matter thermally evaporated from the superbubble cold outer shell. The metal production process determines the time-dependent metallicity in hot superbubble interiors, and leads to values of Z greater or equal than solar, when oxygen is used as tracer, and under-solar when the metallicity is measured with respect to iron. In either case, the enhanced metallicity boosts the X-ray emissivity of superbubbles, bringing theory and observations closer together.

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arXiv1999

V, R, I and Halpha photometry of circumnuclear star forming regions in four galaxies with different levels of nuclear activity

V,R,I continuum bands and Halpha + [NII] photometry for circumnuclear star forming regions (CNSFR) is presented, for galaxies with different kinds of nuclear activity: NGC 7469 (Sy 1), NGC 1068 (Sy 2), NGC 7177 (LINER) and NGC 3310 (Starburst). Their Halpha luminosities (0.02 - 7x10**40 erg/s; uncorrected for internal extinction), are comparable to those observed in other galaxies. No systematic differences in the broad band colours are found for the CNSFR in the different galaxies, except for those in NGC 3310 which are considerably bluer, partially due to a younger stellar population. Using theoretical evolutionary synthesis models some colours can be reproduced by single populations (between 7 and 300 Myr) and extinction 0.5-1.5 mag). However, in many cases, the observed equivalent width of Halpha, require a younger population. NGC 1068, 7177 and 7469, are fitted by a two-burst population model at solar metallicity; the age difference between both populations is around 5-7 Myr and the younger burst involves 3 - 61% of the total mass of the cluster,consistent with the younger burst being originated by supernova activity from the previous one. NGC 3310, requires younger ionizing population ages and lower metallicity. An excess (R-I) observed/predicted, not consistent with a normal reddening law can be atributted to red supergiants present in the older population, and not properly accounted for by the models. The two-population scenario seems to show a trend of the evolutionary state of the CNSFR with the nuclear type of the parent galaxy that remains to be explored.

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arXiv1999

HII Galaxies as deep cosmological probes

We re-investigated the use of the Hubble diagram to measure the cosmological constant ($Λ$) and the mass density of the Universe ($Ω_M$). We find an important focusing effect in $Λ$ for redshifts 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 $Ω_M>0.2$. This means that $Ω_M$ can be measured independently of $Ω_Λ$ by targeting the redshift range according to an estimate of the value of $Ω_M$. We explore the evidence in support of the suggestion that extreme starburst galaxies also known as HII galaxies can be used as distance estimators over a wide range of redshifts and reaching very high values. We have compiled literature data of HII galaxies up to $z\sim 3$ 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 HII 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.

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arXiv1999

The evolution of superbubbles and the detection of Lyman alpha in star-forming galaxies

The detection of Ly alpha 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 Ly alpha emission from a nuclear H II region, and the variety of profiles that may be expected as a function of time.

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arXiv1998

Premature dismissal of high-redshift elliptical galaxies

It has recently been argued that single-collapse high-redshift models for elliptical galaxy formation can be rejected because they predict large numbers of very red galaxies at intermediate redshifts which are not seen in deep optical-infrared surveys. We argue, however, that this conclusion is premature since, while much effort has been invested in refining the predictions of hierarchical CDM models, only very simplistic models have been used to study the evolution of galaxies in other cosmogonies (e.g. isocurvature models). We demonstrate that the use of a more realistic multi-zone chemo-dynamical single-collapse model, yields colours at intermediate redshifts which are much bluer than inferred from the one-zone model, and indeed are comparable to those predicted by hierarchical merging despite still allowing $> 90%$ of the final stellar mass of elliptical galaxies to be formed in the first Gyr of their evolution. We, therefore, conclude that the one-zone model should be avoided to predict the colours of high-redshift galaxies and that the use of realistic multi-zone models allows the existence of ellipticals at high redshift, being their dismissal premature.

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arXiv1997

The cosmological evolution of the QSO luminosity density and of the star formation rate

We demonstrate that the evolution of the QSO luminosity density with epoch displays a striking similarity to the cosmological evolution of the field galaxy star formation rate, recently derived from a number of independent surveys. The QSO luminosity density at 2800A is approximately one-fortieth that implied by the star formation rate in galaxies throughout the past 11 Gigayears (z<4). This similarity suggests that a substantial fraction of the QSO luminosity may be closely linked to the star formation process and its evolution with cosmic time.

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arXiv1997

The Morphology of HII Galaxies

We present CCD images of a sample of 39 HII galaxies taken at the Danish 1.54m telescope on La Silla. The images are used to analyse the morphology of these emission line dwarfs, and the structural properties of the knots of star formation and of the underlying galaxy. The sizes of the starbursts are measured. We propose a morphological classification based on the presence or absence of signs of tails, extensions, or distorted outer isophotes. This criterion segregates the objects into two broad morphological types with different physical properties: the more disturbed and extended (type I) HII galaxies having larger luminosities and velocity dispersions than the more compact and regular (type II) objects. The relative position of HII galaxies and of a sample of dwarf elliptical galaxies in the [R -- sigma] diagram support the hypothesis of a possible evolutionary link between the two types of galaxy.

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arXiv1996

The V R I colours of HII galaxies

We present a high spatial resolution CCD surface photometry study in the optical V, R and I broadband filters of a sample of 15 HII galaxies. The colours of the starburst continuum and of the underlying galaxy are measured. The distribution of colours of the underlying galaxy in HII galaxies is similar to the colours of other late type low surface brightness galaxies which suggests a close kinship of these with the quiescent phases of HII galaxies. However, comparison with recent evolutionary population synthesis models shows that the observational errors and the uncertainties in the models are still too large to put strict constraints on their past star formation history.

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arXiv1995

The Colours of HII Galaxies

We present high spatial resolution CCD surface photometry study in the optical V, R and I filters of 15 HII galaxies from the Nordic Optical Telescope and the Jacobus Kapteyn Telescope at Canary Islands. The colours of the starburst continuum and of the underlying galaxy are measured. The distribution of colours of the underlying galaxy in HII galaxies is similar to the colours of other late type low surface brightness galaxies which suggests a close kinship of these with the quiescent phases of HII galaxies. However, comparison with recent evolutionary population synthesis models show that the observational errors and the uncertainties in the models are still too large to put strict constraints on their past star formation history.

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arXiv1995

The environment of HII galaxies

Recent morphological studies (Telles \& Terlevich 1994) of HII galaxies, i.e. dwarf galaxies dominated by a very luminous starburst, have indicated that luminous HII galaxies tend to show distorted morphology suggestive of tidal interactions triggering the present starburst while low luminosity HII galaxies tend to be instead symmetric and regular. To check the tidal origin of the starburst in HII galaxies, we have searched for companions in the neighbourhood of a sample of 51 HII galaxies. We found that only 12 HII galaxies have a neighbour within a projected distance of 1 Mpc and 250 \kmsec~ in velocity difference, and of these 12, only 4 have a luminous (M$_B <$ --19) neighbour. Surprisingly, isolated HII galaxies tend to be of high luminosity and disturbed morphology while HII galaxies with neighbours tend to be low luminosity regular HII galaxies. Furthermore, the metal abundance and the equivalent width of the emission lines in HII galaxies do not depend on the presence of a companion. These results are opposed to simple expectations if interaction with a bright companion is the main mechanism triggering the starbursts. We have also found a loose group of HII galaxies with no luminous companion. For this, there is the additional difficulty of understanding how these starbursts are synchronized on time scales of less than $10^7$ yrs in systems separated by $\sim$ 1-2 Mpc.

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