arXiv subjects

Cristina C. Popescu

Publications and source records attributed to Cristina C. Popescu.

48 records · Page 3Linked to original sources

arXiv2003

The first detection of Far-Infrared emission associated with an extended HI disk. The case of NGC 891

Spiral galaxies in the Local Universe are commonly observed to be embedded in extended disks of neutral hydrogen - the so called ``extended HI disks''. Based on observations made using the ISOPHOT instrument on board the Infrared Space Observatory, we report the first detection of cold dust in the extended HI disk of a spiral galaxy. The detection was achieved through a dedicated deep Far-Infrared observation of a large field encompassing the entire HI disk of the edge-on spiral galaxy NGC 891. Our discovery indicates that the extended HI disk of NGC 891 is not primordial in origin.

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arXiv2002

Far-infrared Emission from Dust in Normal Galaxies

We review the morphological and spectral energy distribution characteristics of the dust continuum emission (emitted in the 40-200 micron spectral range) from normal galaxies, as revealed by detailed ISOPHOT mapping observations of nearby spirals and by ISOPHOT observations of the integrated emissions from representative statistical samples in the local universe.

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arXiv2002

Modelling the spectral energy distribution of galaxies from the ultraviolet to submillimeter

We present results from a new modelling technique which can account for the observed optical/NIR - FIR/submm spectral energy distributions (SEDs) of normal star-forming galaxies in terms of a minimum number of essential parameters specifying the star-formation history and geometrical distribution of stars and dust. The model utilises resolved optical/NIR images to constrain the old stellar population and associated dust, and geometry-sensitive colour information in the FIR/submm to constrain the spatial distributions of young stars and associated dust. It is successfully applied to the edge-on spirals NGC891 and NGC5907. In both cases the young stellar population powers the bulk of the FIR/sub-mm emission. The model also accounts for the observed surface brightness distribution and large-scale radial brightness profiles in NGC891 as determined using the Infrared Space Observatory (ISO) at 170 and 200 mcrions and at 850 micron using SCUBA.

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arXiv2002

Far-Infrared photometry of a statistical sampleof late-type Virgo Cluster galaxies

We present deep diffraction-limited far-infrared (FIR) strip maps of a sample of 63 galaxies later than S0 and brighter than B_T 16.8, selected from the Virgo Cluster Catalogue of Binggeli, Sandage & Tammann. The ISOPHOT instrument on board the Infrared Space Observatory was used to achieve sensitivities typically an order of magnitude deeper than IRAS in the 60 and 100 micron bands and to reach the confusion limit at 170 microns. The averaged 3 sigma upper limits for integrated flux densities of point sources at 60, 100 and 170 microns are 43, 33 and 58 mJy, respectively. 54 galaxies (85.7 percent) are detected at least at one wavelength, and 40 galaxies (63.5 percent) are detected at all three wavelengths. The highest detection rate (85.7 percent) is in the 170 micron band. In many cases the galaxies are resolved, allowing the scale length of the infrared disks to be derived from the oversampled brightness profiles in addition to the spatially integrated emission. The data presented should provide the basis for a variety of statistical investigations of the FIR spectral energy distributions of gas rich galaxies in the local universe spanning a broad range in star-formation activity and morphological types, including dwarf systems and galaxies with rather quiescent star formation activity.

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arXiv2001

Cold dust in late-type Virgo Cluster galaxies

We have statistically analyzed the spatially integrated far-infrared (FIR) emissions of the complete volume- and luminosity-limited sample of late-type (later than S0) Virgo cluster galaxies measured using the Infrared Space Observatory (ISO) by Tuffs et al. (2002) in bands centered on 60, 100 and 170 microns. 30 out of 38 galaxies detected at all three wavelengths contain a cold dust emission component, present within all morphological types of late-type systems ranging from early giant spirals to Blue Compact Dwarfs (BCDs), and which could not have been recognized by IRAS. The temperatures of the cold dust were found to be broadly distributed, with a median of 18 K, some 8-10 K lower than would have been predicted from IRAS. The derived total dust masses are correspondingly increased by factors of typically 6 - 13. A particularly significant result concerns the low dust temperatures (ranging down to less than 10 K) and large dust masses associated with the Im and BCD galaxies in our sample. We propose two scenarios to account for the FIR characteristics of these systems.

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arXiv2000

Modelling the spectral energy distribution of galaxies. I. Radiation fields and grain heating in the edge-on spiral NGC891

We describe a new tool for the analysis of the UV to the sub-millimeter (sub-mm) spectral energy distribution (SED) of spiral galaxies. We use a consistent treatment of grain heating and emission, solve the radiation transfer problem for a finite disk and bulge, and self-consistently calculate the stochastic heating of grains placed in the resulting radiation field. We use this tool to analyse the well-studied nearby edge-on spiral galaxy NGC891. First we investigate whether the old stellar population in NGC891, along with a reasonable assumption about the young stellar population, can account for the heating of the dust and the observed far-infrared (FIR) and sub-mm emission. The dust distribution is taken from the model of Xilouris et al. (1999), who used only optical and near-IR observations to determine it. We have found that such a simple model cannot reproduce the SED of NGC891, especially in the sub-mm range. It underestimates by a factor of 2-4 the observed sub-mm flux. A number of possible explanations exist for the missing sub-mm flux. We investigate a few of them and demonstrate that one can reproduce the observed SED in the FIR and the sub-mm quite well, as well as the observed radial profile at 850 microns. For the models calculated we give the relative proportion of the dust radiation powered by the old and young stellar populations as a function of FIR/sub-mm wavelength. In all models we find that the dust is predominantly heated by the young stellar population.

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arXiv2000

On the FIR emission from intracluster dust

We make predictions for the diffuse far-infrared (FIR) emission from dust in the intracluster medium (ICM) of the Virgo cluster using detailed information on potential dust sources, grain heating and sputtering rates available for this cluster from recent optical and X-ray studies. We show that the infrared emission from dust injected by sources inside the core region of the Virgo cluster is a factor of 10 below the detection limit of currently available telescopes. The outer regions of dynamically young clusters like the Virgo cluster have a further potential source of intracluster grains since they are still accreting freshly infalling spiral galaxies which are presumably contained in an accreting intergalactic medium (IGM). We show that cosmic ray driven winds from the infalling spirals can inject grains into a subvirial IGM that is external to the observed X-ray-emitting ICM. Sputtering during the injection process and in the IGM is weak, so that the injected grains should accumulate in the IGM until the infall brings them into contact with the hot ICM. This effect dominates the dust injection rate from known sources embedded in the hot Virgo ICM. Thus, any detection of diffuse IR emission would probe the current dust accretion rate for the cluster, acting as an indicator of the youth and the dynamical state of the cluster. The predictions for the Virgo cluster are generalised to other clusters and the possibility of detection of dynamically young clusters at cosmological distances is discussed.

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arXiv1999

A spectrophotometric catalogue of HII galaxies

We present a spectrophotometric catalogue of 90 emission-line galaxies (ELGs) discovered during an objective - prism survey that aimed to search for dwarf galaxies within the voids (Popescu et al. 1996, 1998). The paper presents line ratios, equivalent widths and absolute fluxes for the emission-lines seen in the spectra of the galaxies. A list of newly discovered Wolf-Rayet galaxies is presented. Many objects included in the catalogue have low metallicity and the extreme cases are proposed as candidates for very low metallicity galaxies.

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arXiv1999

On the star-formation properties of emission-line galaxies in and around voids

We present a study of the star formation properties of a sample of emission line galaxies (ELGs) with respect to their environment. This study is part of a bigger project that aimed to find galaxies in voids and to investigate the large scale structure of the ELGs. A survey for ELGs was therefore conducted with the result that 16 galaxies have been found in very low density environments, of which 8 ELGs were found in two very well defined nearby voids. The sample presented here contains some galaxies identified in voids, as well as in the field environment that delimited the voids. These ELGs are all Blue Compact Galaxies (BCGs), and all void galaxies are also dwarfs. Both void and field sample contain the same mixture of morphological subtypes of BCDs, from the extreme Searle-Sargent galaxies to the Dwarf-Amorphous Nuclear-Starburst galaxies. The main result of this study is that field and void galaxies seem to have similar star formation rates (SFR), similar ratios between the current SFR and their average past SFR and similar mean SFR surface densities. There is no trend in metallicity, in the sense that void galaxies would have lower metallicities than their field counterparts. The field-cluster dichotomy is also discussed using available results from the literature, since our sample does not cover the cluster environment.

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arXiv1997

SN 1995ah-the first supernova observed in a Blue Compact Dwarf galaxy

We present the properties of the supernova SN 1995ah discovered in a Blue Compact Dwarf galaxy (BCD) around 10 days after the maximum. This is the first supernova event observed in a BCD. The photometric and spectroscopic data suggest that SN 1995ah is a Type II supernova and could belong to the rare Bright SNII Linear subclass, for which ~ =-18.9+-0.6 at maximum light.

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arXiv1997

Results of a search for emission-line galaxies towards nearby voids. The spatial distribution

We present the results of a search for emission-line galaxies (ELGs) towards nearby voids, based on a sample selected on the HQS (Hamburg Quasar Survey) - III-aJ objective-prism plates. The survey was based on the presence of emission-lines and therefore has the advantage to detect very faint objects (with all the flux in the emission-lines and almost no continuum), that would be missed by an apparent magnitude limited survey. We found objects as faint as B=20.5 and M(B)=(-15.0,-12.0). The observational data are given in Popescu et al. (1996) and in the present paper we consider a complete subsample from these data. We analyse the spatial distribution of our sample of ELGs in comparison with the distribution of normal galaxies. Overall both distributions trace the same structures. Nevertheless we also found a few ELGs in voids, from which at least 8 lie in the very well defined nearby voids. The isolated galaxies seem to form fainter structures that devide the larger voids into smaller ones. {}From our estimates of the expected number of void galaxies we conclude that we did not find an underlying homogenous void population.

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arXiv1995

Search for emission-line galaxies towards nearby voids. Observational data

We present the observational results of our search for emission-line galaxies (ELG) towards nearby voids. In order to find ELG, we started a survey using the IIIa-J objective prism plates from the Hamburg QSO Survey. The plates are digitized and an automatic procedure was applied to select the candidates. Digitized direct plates were used to determine coordinates and to reject overlaps between spectra. A total area of 1248 square degrees was scanned, distributed in four different regions. All the selected objects were observed with follow-up spectroscopy. We have obtained a final sample of 203 objects, of which 196 are emission-line galaxies, four are galaxies with absorption lines and three are QSOs. Almost half of our objects are newly discovered ones and three quarters of the given redshifts are new. Our sample contains mainly high ionization galaxies and is less sensitive in the detection of low-ionization objects. The apparent magnitudes, as derived from the objective prism plates, range between 15.0<B<19.5. The sample is dominated by nearby galaxies, with a peak in the redshift distribution at cz=4500\,km/s.

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