arXiv subjects

P. -A. Duc

Publications and source records attributed to P. -A. Duc.

At least 91 records · Page 5Linked to original sources

arXiv2004

Halpha Kinematics of Tidal Tails in Interacting Systems: Projection Effects and Dark Matter in TDGs

Several interacting systems exhibit at the tip of their long tidal tails massive condensations of atomic hydrogen, which may be the progenitors of Tidal Dwarf Galaxies. Because, quite often, these tails are observed edge-on, projection effects have been claimed to account for the large HI column densities measured there. Here we show that determining the velocity field all along the tidal features, one may disentangle projection effects along the line of view from real bound structures. Due to its large field of view, high spectral and 2D spatial resolutions, Fabry-Perot observations of the ionized gas are well adapted to detect a kinematical signature of either streaming motions along a bent tidal tail or of infalling/rotating material associated with a forming TDG. Spectroscopic observations also allow to measure the dynamical masses of the TDGs that are already relaxed and check their dark matter content.

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arXiv2004

Molecular Gas in Tidal Dwarf Galaxies: On-going Galaxy Formation

We investigate the process of galaxy formation as can be observed in the only currently forming galaxies -- the so-called Tidal Dwarf Galaxies, hereafter TDGs -- through observations of the molecular gas detected via its CO (Carbon Monoxide) emission. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a {\it bona fide} galaxy. We have now detected CO in 9 TDGs with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few $10^8 M_\odot$. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H$_2$. Although uncertainties are still large for individual objects as the geometry is unknown, we find that the "dynamical" masses of TDGs, estimated from the CO line widths, do not seem to be greater than the "visible" masses (HI + H$_2$ + a stellar component), i.e., TDGs require no dark matter. We provide evidence that TDGs are self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component.

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arXiv2004

Colliding molecular clouds in head-on galaxy collisions

We present further observations of molecular gas in head-on collisions of spiral galaxies, this time of the CO(J=1-0) and CO(J=2-1) lines in the UGC 813 -- UGC 816 system. UGC 813/6 are only the second known example of head-on spiral-spiral collisions, the first example being the UGC 12914/5 pair. Strong CO emission is present in the bridge between UGC 813 and 816, unassociated with stellar emission, just as in UGC 12914/5. The CO emission from the UGC 813/6 bridge, not counting the emission from the galaxies themselves, is at least that of the entire Milky Way. Collisions of gas-rich spirals are really collisions between the interstellar media (ISMs) of the galaxies. We show that collisions between molecular clouds bring H2 into the bridge region. Although the dense clouds are ionized by the collisions, they cool and recombine very quickly and become molecular again even before the galactic disks separate. Because the clouds acquire an intermediate velocity post-collision, they are left in the bridge between the separating galaxies. The star formation efficiency appears low in the molecular clouds in the bridges. We speculate that the pre-stellar cores in the molecular clouds may expand during the cloud collisions, thus retarding future star formation. Because the ISM-ISM collisions discussed here require a very small impact parameter, they are rare among field spirals. In clusters, however, these collisions should be an important means of ejecting enriched gas from the inner parts of spirals.

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arXiv2003

ce-61: a Tidal Dwarf Galaxy in the Hercules cluster?

A candidate Tidal Dwarf Galaxy, ce-61, was identified in the merger system IC 1182 in the Hercules supercluster. The multi-wavelength data we obtained so far do not prove, however, that it is kinematically detached from the IC 1182 system and gravitationally bound.

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arXiv2003

The large extent of dark matter haloes probed by the formation of tidal dwarf galaxies

In several interacting systems, gas accumulations as massive as 10^9 solar masses are observed near the tip of tidal tails, and are thought to be possible progenitors of Tidal Dwarf Galaxies. Using N-body simulations of galaxy interactions, we show that the existence of such features requires that dark matter haloes around spiral galaxies extend at least ten times further than the stellar disks. The massive gas clouds formed in our simulations have a kinematical origin and gravitationally collapse into dwarf galaxies that often survive for a few billion years.

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arXiv2003

A study of HI-selected galaxies in the Hercules cluster

The present study is aimed at a sample of 22 galaxies detected in the blind VLA HI survey of the Hercules cluster by Dickey (1997), 18 of which were selected on an HI line width smaller than 270 km/s and 4 others with only tentative optical counterparts on the Palomar Sky Survey. Sensitive single-dish HI line spectra were obtained for 20 of them, and for one (47-154) the VLA detection was not confirmed. Optical surface photometry was obtained of 10 objects, for 8 of which optical spectroscopy was obtained as well. Based on various selection criteria, two (ce-143 and ne-204) can be classified as dwarfs. The objects of which optical observations were made show star formation properties similar to those of published samples of actively star forming galaxies, and approximately half of them have properties intermediate between those of dwarf galaxies and low-luminosity disc galaxies. No optical redshifts could be obtained for two of the galaxies (sw-103 and sw-194) and their physical association with the HI clouds detected at their positions therefore remains uncertain. Unique among the objects is the Tidal Dwarf Galaxy ce-061 in a tail of the IC 1182 merger system.

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arXiv2003

Are the hosts of Gamma-Ray Bursts sub-luminous and blue galaxies?

We present K-band imaging observations of ten Gamma-Ray Burst (GRB) host galaxies. We compare their observed and absolute K magnitudes as well as their R-K colours with those of other distant sources detected in various optical, near-infrared, mid-infrared and submillimeter deep surveys. We find that the GRB host galaxies, most of them lying at 0.5<z<1.5, exhibit very blue colours, comparable to those of the faint blue star-forming sources at high redshift. They are sub-luminous in the K-band, suggesting a low stellar mass content. We do not find any GRB hosts harbouring R- and K-band properties similar to those characterizing the luminous infrared/submillimeter sources and the extremely red starbursts. Should GRBs be regarded as an unbiased probe of star-forming activity, this lack of luminous and/or reddened objects among the GRB host sample might reveal that the detection of GRB optical afterglows is likely biased toward unobscured galaxies. It would moreover support the idea that a large fraction of the optically-dark GRBs occur within dust-enshrouded regions of star formation. On the other hand, our result might also simply reflect intrinsic properties of GRB host galaxies experiencing a first episode of very massive star formation and characterized by a rather weak underlying stellar population. Finally, we compute the absolute B magnitudes for the whole sample of GRB host galaxies with known redshifts and detected at optical wavelengths. We find that the latter appear statistically even less luminous than the sub-luminous blue sources which mostly contributed to the B-band light emitted at high redshift. This indicates that the formation of GRBs could be favoured in particular systems with very low luminosities and, therefore, low metallicities. (Abridged)

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arXiv2002

Non-confirmation of reported HI clouds without optical counterparts in the Hercules Cluster

21 cm HI line observations were made with the Arecibo Gregorian telescope of 9 HI clouds in the Hercules Cluster which were reported as tenative detections in a VLA HI study of the cluster (Dickey 1997) and for which our deep CCD imaging failed to find any optical counterparts. No sensitive observations could be made of one of these (sw-174) due to the presence of a close-by strong continuum source. The other 8 tentative HI detections were not reconfirmed by the Arecibo HI measurements. The CCD images did reveal faint, low surface brightness counterparts near the centres of two other VLA HI sources invisible on the Palomar Sky Survey, sw-103 and sw-194.

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arXiv2002

The blue colors of Gamma-ray burst host galaxies

There is now an increasing number of evidence supporting the idea that the cosmic Gamma-ray Bursts (GRBs) originate from the collapse of massive stars in distant star-forming galaxies. Because GRBs are likely detectable up to very high redshift, and because the gamma-rays are not attenuated by intervening columns of gas and dust, these phenomena thus offer a unique perspective to probe the star formation in the early Universe independently of the biases associated with dust extinction. We report on the R-K colors of the GRB host galaxies. We find that they are very blue sources, with colors comparable to those of the faint blue star-forming galaxies at high redshift.

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arXiv2002

Deriving the redshift of distant galaxies with Gamma-Ray Burst transient edges

Cosmological Gamma-Ray Bursts (GRBs) offer a unique perspective to probe the evolution of distant galaxies. We discuss one of the multiple benefits of this approach, i.e. the detection of transient edges in the GRB prompt phase emission. These absorption features can be used to directly derive the redshift of GRBs and their host galaxies without the need of any optical spectroscopic follow-up.

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arXiv2002

VLT and HST Observations of the Host Galaxy of GRB990705

We present VLT spectroscopic observations of Gamma-Ray Burst (GRB) 990705 host galaxy and highlight the benefits provided by the prompt-phase features of GRBs to derive the redshifts of the latter. In the host spectrum, we indeed detect an emission feature which we attribute to the [OII] 3726/3729 Angstrom doublet and derive an unambiguous redshift z=0.8424+/-0.0002 for this galaxy. This is in full agreement with the value z~0.86+/-0.17 previously derived using a transient absorption edge discovered in the X-ray spectrum of GRB990705. This burst is therefore the first GRB for which a reliable redshift was derived from the prompt phase emission itself, as opposed to redshift determinations performed using putative host galaxy emission lines or intestellar absorption lines in the GRB afterglows. Deep and high resolution images of the host of GRB990705 with the HST/STIS camera reveal that the burst occured in a nearly face-on Sc spiral galaxy typical of disk-dominated systems at 0.75<z<1. Assuming a cosmology with Ho=65km/s/Mpc, Omega_m=0.3 and Omega_lambda=0.7, we derive an absolute B magnitude M_B=-21.75 for this galaxy and a star formation rate SFR ~ 5-8~Msol/yr. Finally, we discuss the implications of using X-ray transient features to derive GRB redshifts with larger burst samples, and especially examine the case of short and dark-long GRBs.

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arXiv2002

Tidal Dwarf Candidates in a Sample of Interacting Galaxies. II. Properties and Kinematics of the Ionized Gas

We present low-resolution spectroscopy of the ionized gas in a sample of optical knots located along the tidal features of 14 interacting galaxies and previously selected as candidates of Tidal Dwarf Galaxies (TDGs). From redshift measurements, we are able to confirm their physical association with the interacting system in almost all cases. For most knots, the oxygen abundance does not depend on the blue luminosity. The average, 12+log(O/H)=8.34+-0.20, is typical of TDGs and comparable to that measured in the outer stellar disk of spirals from which they were formed. A few knots showing low metallicities are probably pre-existing low-mass companions. The estimated Ha luminosity of the TDG candidates is higher than the one of typical individual HII regions in spiral disks and comparable to the global Ha luminosity of dwarf galaxies. We find several instances of velocity gradients with amplitudes apparently larger than 100 km/s in the ionized gas in the tidal knots and discuss various possible origins for the large velocity amplitudes. While we can exclude tidal streaming motions and outflows, we cannot rule out projection effects with the current resolution. The velocity gradients could be indicative of the internal kinematics characteristic of self-gravitating objects. Higher resolution spectra are required to confirm whether the tidal knots in our sample have already acquired their dynamical independence and are therefore genuine Tidal Dwarf Galaxies.

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arXiv2002

An unusual iron Lo-BAL quasar detected by ISOCAM

We report the discovery of an unusual low--ionization broad absorption line quasar at z=1.776 which exhibits absorption lines from many excited states of Feii. This member of the rare class of 'FeLoBAL' quasars was serendipitously found in a mid-infrared (MIR) survey of distant clusters carried out with the ISOCAM camera. ISO J005645.1-273816 has a high MIR to UV luminosity ratio, suggesting a strong dust obscuration plus emission from very hot dust. This characteristic makes MIR surveys particularly efficient at detecting LoBAL quasars.

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arXiv2001

Near Infrared Observations of Galactic Black Hole Candidates

We report on several European Southern Observatory (ESO) near-infrared (NIR) observational campaigns aimed at understanding the nature of galactic black hole candidates. Our results, including NIR photometry of the sources GRO J1655-40, GRS 1739-278, GRS 1716-249, GRS 1121-68 and GX 339-4, show that all the sources but GRO J1655-40 are consistent with low-mass stars as the companion star of the binary system. By locating the counterparts on a colour-magnitude diagram (CMD), we better constrain the spectral type of the companion star of three of the systems considered here, and confirm a fourth one. The spectral types are respectively: M0-5 V for GRS 1716-249, F8-G2 III for GX 339-4 and later than F5 V for GRS 1739-278. We confirm the already known spectral type of the companion in GRS 1121-68 (K0-5 V). The location of GRO J1655-40 on the CMD is consistent with the sub-giant luminosity class and with this source crossing the Hertzsprung gap. However, a non-stellar emission seems to contribute to the NIR flux of this source.

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arXiv2001

Hidden star-formation in the cluster of galaxies Abell 1689

At a redshift of 0.18, Abell 1689 is so far the most distant cluster of galaxies for which substantial mid-infrared (MIR) data have been published. We present here the follow-up optical photometric and spectroscopic observations which were used to study the individual properties of the galaxies members of A1689. Sources with a low [15 micron] / [6.75 micron] flux ratio typically consist of luminous passive early-type galaxies while those with a high MIR color index are mainly luminous, blue, emission-line, morphologically disturbed spirals, i.e. the star-forming galaxies usually associated with the 'Butcher-Oemler' effect. On the other hand, at least 30% of the 15 micron sources have optical counterparts showing no evidence of current star-formation activity, while their 15 micron emission is most likely due to obscured star formation. We argue that the LW3 luminosity measured in the cluster members is a reliable tracer of the total infrared luminosity, L(IR). We derive from L(IR) a star-formation rate free of dust extinction, SFR(IR), which we compare with that determined in the optical from the flux of the [OII] emission line, SFR(opt). The highest total star formation rates (11 Msun/yr) and dust extinction are measured in those galaxies exhibiting in their optical spectrum a signature of a dusty starburst. In contrast, none of the galaxies with post-starburst optical spectra has been detected by ISOCAM. We find a median SFR(IR) of the LW3-detected galaxies of 2 Msun/yr, that is ten times higher than the median SFR(opt) of the [OII] detected galaxies. The ratio SFR(IR)/SFR(opt) is in fact very high, ranging between 10 and 100 for LW3-detected galaxies with [OII] emission. We conclude that a major part, at least 90%, of the star formation activity taking place in Abell 1689 is hidden.

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arXiv2001

Abundant Molecular Gas in Tidal Dwarf Galaxies: On-going Galaxy Formation

[Abridged...] We investigate the process of galaxy formation as can be observed in the only currently forming galaxies -- the so-called Tidal Dwarf Galaxies, hereafter TDGs -- through observations of the molecular gas detected via its CO emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (two of them have already been published in Braine et al. 2000), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 10^8 M_sun. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H2.... Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin....

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arXiv2001

Star Formation in Tidal Dwarf Galaxies

Tidal Dwarf Galaxies (TDGs) are objects presently forming from gas which has been expelled from their parent galaxies during an interaction. We observed the CO emission of a sample of 11 TDGs, of which 8 were detected. The CO is found at the peak of the HI observations and has the same line velocity and width, indicating that the molecular gas is forming in situ instead of being torn from the parent galaxies. The presence of H$α$ emission furthermore shows that stars are forming from this molecular gas. In order to investigate star formation in TDGs further, we compared their molecular gas content and star formation rate (SFR), traced by H$α$, to those of spiral galaxies and classical dwarfs. The major difference between TDGs and classical dwarfs is the lower metallicity of the latter. The star formation efficiency (SFR per molecular gas mass) of TDGs lies in the range typical of spiral galaxies indicating that star formation is proceeding in a normal fashion from molecular gas.

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arXiv2000

An excess of mid-IR luminous galaxies in Abell 1689?

We present the results of infrared observations of Abell 1689 which was observed with ISOCAM, at 6.7\mic and 15\mic, and ISOPHOT at 200\mic. The cluster galaxies detected above a sensitivity limit of 0.15 mJy in the 6.7\mic band, whose emission is mostly dominated by their stellar component, show optical colors similar to the overall cluster population and are gathered in the center of the cluster, following the distribution of the cluster early-types. In the 15\mic band, above a sensitivity limit of 0.3 mJy, the galaxies spectroscopically confirmed to be cluster members are blue outliers of the cluster color-magnitude relation and become brighter going from the center to the outer parts of the cluster. We compare the 6.7\mic and 15\mic fluxes and the cumulative distributions of the B-[6.75] and B-[15] colors of the A1689 galaxies, above our 90% completeness limits of 0.2 and 0.4 mJy for 6.7\mic and 15\mic respectively, to the galaxies of two nearby clusters, Virgo and Coma, and to the field galaxies at the same redshift of the cluster. Although the B-[6.7] color distributions of the three clusters are compatible, we find a systematic excess of B-[15] color distribution for the galaxies located in Abell 1689 with respect to Coma or Virgo galaxies. This result proves the existence of a mid-infrared equivalent of the Butcher-Oemler effect measured in the optical. The comparison of 15\mic flux and B-[15] color distributions of A1689 and field galaxies does not show strong differences between the population of starburst galaxies in the cluster and in the field.

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