arXiv subjects

D. Rigopoulou

Publications and source records attributed to D. Rigopoulou.

176 records · Page 10Linked to original sources

arXiv2001

The extended counterpart of submm source Lockman850.1

The IRAM Plateau de Bure mm interferometer and deep K-band imaging have been used to identify the brightest submm source detected in the Lockman field of the UK 8mJy SCUBA survey. The near infrared counterpart is an extended (20-30kpc), clumpy, and extremely red object. The spectral energy distribution suggests it to be a dusty star forming object at a redshift of about 3 (2-4). Its star formation rate and near-infrared properties are consistent with Lockman850.1 being a massive elliptical in formation.

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arXiv2001

Ultra-Luminous Infrared Mergers: Elliptical Galaxies in Formation?

We report high quality near-infrared spectroscopy of 12 ultra-luminous infrared galaxy mergers (ULIRGs). Our new VLT and Keck data provide ~0.5" resolution, stellar and gas kinematics of these galaxies most of which are compact systems in the last merger stages. We confirm that ULIRG mergers are 'ellipticals-in-formation'. Random motions dominate their stellar dynamics, but significant rotation is common. Gas and stellar dynamics are decoupled in most systems. ULIRGs fall on or near the fundamental plane of hot stellar systems, and especially on its less evolution sensitive, r(eff)-sigma projection. The ULIRG velocity dispersion distribution, their location in the fundamental plane and their distribution of v(rot)*sin(i)/sigma closely resemble those of intermediate mass (~L*), elliptical galaxies with moderate rotation. As a group ULIRGs do not resemble giant ellipticals with large cores and little rotation. Our results are in good agreement with other recent studies indicating that disky ellipticals with compact cores or cusps can form through dissipative mergers of gas rich, disk galaxies while giant ellipticals with large cores have a different formation history.

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arXiv2001

The European Large Area ISO Survey V: a BeppoSAX hard X-ray survey of the S1 region

We present BeppoSAX observations of the Southern S1 region in the European Large Area ISO Survey (ELAIS). These observations cover an area of ~1.7 sq. deg. and reach an on-axis (~0.7 sq. deg) 2-10 keV (HX) sensitivity of ~10E-13 cgs. This is the first HX analysis of an ISOCAM survey. We detect 9 sources with a signal to noise ratio SNR_HX>3, 4 additional sources with a 1.3 to 10 keV (T) SNR_T>3 and 2 additional sources which appear to be associated with QSOs with SNR_T>2.9. The number densities of the SNR_HX>3 sources are consistent with the ASCA and BeppoSAX logN-logS functions. Six BeppoSAX sources have reliable ISOCAM 15 micron counterparts within ~60 arcsec. All these ISOCAM sources have optical counterparts of R<20 mags. Five of these sources have been previously optically classified giving 4 QSOs and 1 BALQSO at z=2.2. The remaining unclassified source has X-ray and photometric properties consistent with that of a nearby Seyfert galaxy. One further HX source has a 2.6sigma ISOCAM counterpart associated with a galaxy at z=0.325. If this ISOCAM source is real, the HX/MIR properties suggest either an unusual QSO or a cD cluster galaxy. We have constructed MIR and HX spectral energy distributions to compute the expected HX/MIR ratios for these classes of objects up to z=3.2 and assess the HX/MIR survey depth. The BALQSO has an observed X-ray softness ratio and HX/MIR flux ratio similar to QSOs but different to those found for low redshift BALQSOs. This difference can be explained in terms of absorption, and suggests that high redshift BALQSOs should be comparatively easy to detect in the HX band, allowing their true fraction in the high redshift QSO population to be determined (ABRIDGED).

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arXiv2001

ISOCAM-CVF 5-12 mum Spectroscopy of Ultraluminous Infrared Galaxies

We present low resolution mid infrared spectra of 16 ultraluminous infrared galaxies (ULIRGs) obtained with the CVF spectroscopy mode of ISOCAM on board the Infrared Space Observatory ISO. Our sample completes previous ISO spectroscopy of ultra- and hyperluminous infrared galaxies towards higher luminosities. The combined samples cover an infrared luminosity range of \~10^{12 -13.1} Lo. For about half of the high luminosity ULIRGs studied here, strong aromatic emission bands suggest starburst dominance. Other spectra are dominated by a strong AGN-related continuum with weak superposed emission features of uncertain nature. An improved method to quantitatively characterize the relative contribution of star formation and AGN activity to the mid-infrared emission of ULIRGs is presented. As dominant source of the bolometric luminosity, starbursts prevail at the lower end and AGNs at the higher end of this range. The transition between mostly starburst and mostly AGN powered occurs at ~10^{12.4}$ to $10^{12.5} Lo, and individual luminous starbursts are found up to ~10^{12.65} Lo.

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arXiv2000

The European Large Area ISO Survey IV: the preliminary 90 micron luminosity function

We present the luminosity function of 90um selected galaxies from the European Large Area ISO Survey (ELAIS), extending to z=0.3. Their luminosities are in the range 10^9 < h_65^-2 L/Lsun < 10^12, i.e. non-ultraluminous. From our sample of 37 reliably detected galaxies in the ELAIS S1 region from the Efstathiou et al. (2000) S_90 >= 100mJy database, we found optical, 15um or 1.4GHz identifications for 24 (65%). We have obtained 2dF and UK Schmidt FLAIR spectroscopy of 89% of IDs to rigid multivariate flux limits. We construct a luminosity function assuming (a) our spectroscopic subset is an unbiased sparse sample, and (b) there are no galaxies which would not be represented in our spectroscopic sample at {\it any} redshift. We argue that we can be confident of both assumptions. We find the luminosity function is well-described by the local 100um luminosity function of Rowan-Robinson, Helou & Walker (1987). {\it Assuming} this local normalisation, we derive luminosity evolution of (1+z)^{2.45\pm0.85} (95% confidence). We argue that star formation dominates the bolometric luminosities of these galaxies and we derive comoving star formation rates in broad agreement with the Flores et al. (1999) and Rowan-Robinson et al. (1997) mid-IR-based estimates.

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arXiv2000

Gas Flow and Star Formation in the `Antennae' Galaxies NGC 4038/9

The prominent interacting galaxy pair NGC 4038/9 contains many active star-forming regions and is continuously forming new star clusters. We present a self-consistent n-body model for this system which includes an SPH gas component. The model qualitatively explains the apparent concentration of gas in the so-called overlap region between the two nuclei as a bridge of gas connecting the two galaxies. Projected on the sky, the bridge appears as a dense spot of gas. We discuss some implications for the evolution of Ultra-luminous infrared galaxies.

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arXiv2000

The European Large Area ISO Survey III: 90micron extragalactic source counts

We present results and source counts at 90micron extracted from the Preliminary Analysis of the European Large Area ISO Survey (ELAIS). The survey covered about 11.6 square degrees of the sky in four main areas and was carried out with the PHOT instrument onboard the Infrared Space Observatory (ISO). The survey is at least an order of magnitude deeper than the IRAS 100micron survey and is expected to provide constraints on the formation and evolution of galaxies. The majority of the detected sources are associated with galaxies on optical images. In some cases the optical associations are interacting pairs or small groups of galaxies suggesting the sample may include a significant fraction of luminous infrared galaxies. The source counts extracted from a reliable subset of the detected sources are in agreement with strongly evolving models of the starburst galaxy population.

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arXiv2000

VLT-ISAAC near-IR Spectroscopy of ISO selected Hubble Deep Field South Galaxies

We report the results of near-infrared VLT-ISAAC spectroscopy of a sample of 12 galaxies at z = 0.4-1.4, drawn from the ISOCAM survey of the Hubble Deep Field South. We find that the rest frame R-band spectra of the ISOCAM galaxies resemble those of powerful dust-enshrouded starbursts. Halpha emission is detected in 11 out of 12 objects down to a flux limit of 7x10^(-17) erg/cm^2/s, corresponding to a luminosity limit of 10^41 erg/s at z = 0.6, (for an Ho = 50 and Omega = 0.3 cosmology). From the Halpha luminosities in these galaxies we derive estimates of the star formation rate in the range 2--50 Mo/yr for stellar masses 1--100 Mo. The raw Halpha-based star formation rates are an order of magnitude or more lower than SFR(FIR) estimates based on ISOCAM LW3 fluxes. If the Halpha emission is corrected for extinction the median offset is reduced to a factor of 3. The sample galaxies are part of a new population of optically faint but infrared--luminous active starburst galaxies, which are characterized by an extremely high rate of evolution with redshift up to z~1.5 and expected to contribute significantly to the cosmic far-IR extragalactic background.

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arXiv2000

NIR Spectroscopy with the VLT of a sample of ISO selected Hubble Deep Field South Galaxies

A new population of faint galaxies characterized by an extremely high rate of evolution with redshift up to z~1.5 has recently been discovered by ISO. These sources are likely to contribute significantly to the cosmic far-IR extragalactic background. We have carried out near-infrared VLT-ISAAC spectroscopy of a sample of ISOCAM galaxies from the Hubble Deep Field South. The rest-frame R-band spectral properties of the ISO population resembles that of powerful dust-enshrouded active starburst galaxies.

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arXiv1999

A large mid-infrared spectroscopic and near-IR imaging survey of ULIRGs: their nature and evolution

We present a low resolution mid-infrared spectroscopic survey of an unbiased sample of 62 Ultraluminous Infrared Galaxies (ULIRGs) using ISOPHOT-S on board ISO. For comparison we also present ISOPHOT-S spectra for 23 AGN and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 $μ$m PAH emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We find that the majority of ULIRGs are predominantly powered by starbursts. The ratio L$_{PAH}/$L$_{IR}$ for starburst dominated ULIRGs is similar to the one found for template starbursts.The shapes of the PAH features are sometimes unusual.Extinction has a noticeable effect on the PAH spectra of ULIRG starbursts. We have obtained high resolution near-infrared imaging for the Southern ISOPHOT-S ULIRGs to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems and all show distorted morphologies. Using separations for our work and from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity.We also find that the total mass of interstellar gas, as estimated from the CO (1-0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we find that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.

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arXiv1999

Proposed identification of Hubble Deep Field submillimeter source HDF 850.1

The IRAM interferometer has been used to detect the submm source HDF 850.1 found by Hughes et al. (1998) in the Hubble Deep Field. The flux density measured at 1.3mm is 2.2 mJy, in agreement with the flux density measured at the JCMT. The flux densities and upper limits measured at 3.4, 2.8, 1.3, 0.85, and 0.45 mm show that the emission is from dust. We suggest that the 1.3mm dust source is associated with the optical arc-like feature, 3-593.0, that has a photometric redshift of about 1.7. If HDF 850.1 is at this redshift and unlensed, its spectral energy distribution, combined with that of 3-593.0, matches closely that of the ultraluminous galaxy VII Zw 31. Another possibility is that the dust source may be gravitationally lensed by the elliptical galaxy 3-586.0 at a redshift of 1.

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arXiv1998

An ISO survey of PAH features in Ultraluminous Infrared Galaxies

We have obtained ISOPHOT-S low resolution mid-infrared spectra of a sample of 60 Ultraluminous Infrared Galaxies (ULIRGs). We use the strength of the `PAH' mid-infrared features as a discriminator between starburst and AGN activity, and to probe for evolutionary effects. We focus on the fact that observed ratios of PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. We suggest that such PAH ratio changes relate to the conditions in the interstellar medium in these galaxies, and in particular to extinction.

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arXiv1998

The nature and evolution of Ultraluminous Infrared Galaxies: A mid-infrared spectroscopic survey

We report the first results of a low resolution mid-infrared spectroscopic survey of an unbiased, far-infrared selected sample of 60 ultraluminous infrared galaxies, using ISOPHOT-S on board ISO. We use the ratio of the 7.7um `PAH' emission feature to the local continuum as a discriminator between starburst and AGN activity. About 80% of all the ULIRGs are found to be predominantly powered by star formation but the fraction of AGN powered objects increases with luminosity. Observed ratios of the PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. This can be plausibly explained by the higher extinction and/or different physical conditions in the interstellar medium of ULIRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio of feature-free 5.9um continuum to the IRAS 60um continuum, confirming suggestions that strong mid-IR continuum is a prime AGN signature. The location of starburst-dominated ULIRGs in such a diagram is consistent with previous ISO-SWS spectroscopy which implies significant extinction even in the mid-infrared. We have searched for indications that ULIRGs which are advanced mergers might be more AGN-like, as postulated by the classical evolutionary scenario. No such trend has been found amongst those objects for which near infrared images are available to assess their likely merger status.

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arXiv1997

What Powers Ultra-luminous IRAS Galaxies?

We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultra-luminous IRAS galaxies. We combine the survey results with a detailed case study, based on near-IR and mm imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultra-luminous galaxies to ISO and literature data of thirty starburst and active galactic nuclei (AGN), template galaxies. We find that 1) 70-80% of the ultra-luminous IRAS galaxies in our sample are predominantly powered by recently formed massive stars. 20-30% are powered by a central AGN. These conclusions are based on a new infrared 'diagnostic diagram' involving the ratio of high to low excitation mid-IR emission lines on the one hand, and on the strength of the 7.7um PAH feature on the other hand. 2) at least half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circum-nuclear disk/ring. 3) the mid-infrared emitting regions are highly obscured. After correction for these extinctions, we estimate that the star forming regions in ULIRGs have ages between 10^7 and 10^8 years, similar to but somewhat larger than those found in lower luminosity starburst galaxies. 4) in the sample we have studied there is no obvious trend for the AGN component to dominate in the most compact, and thus most advanced mergers. Instead, at any given time during the merger evolution, the time dependent compression of the circum-nuclear interstellar gas, the accretion rate onto the central black hole and the associated radiation efficiency may determine whether star formation or AGN activity dominates the luminosity of the system.

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