arXiv subjects

L. Vanzi

Publications and source records attributed to L. Vanzi.

54 records · Page 3Linked to original sources

arXiv2008

Metallicity Effects on Dust Properties in Starbursting Galaxies

We present infrared observations of 66 starburst galaxies over a wide range of oxygen abundances, to measure how metallicity affects their dust properties. The data include imaging and spectroscopy from the Spitzer Space Telescope, supplemented by groundbased near-infrared imaging. We confirm a strong correlation of aromatic emission with metallicity, with a threshold at a metallicity [12+log(O/H)]~8. The large scatter in both the metallicity and radiation hardness dependence of this behavior implies that it is not due to a single effect, but to some combination. We show that the far-infrared color temperature of the large dust grains increases towards lower metallicity, peaking at a metallicity of 8 before turning over. We compute dust masses and compare them to HI masses from the literature to derive the gas to dust ratio, which increases by nearly 3 orders of magnitude between solar metallicity and a metallicity of 8, below which it flattens out. The abrupt change in aromatic emission at mid-infrared wavelengths thus appears to be reflected in the far-infrared properties, indicating that metallicity changes affect the composition of the full range of dust grain sizes that dominate the infrared emission. In addition, we find that the ratio L(8 micron)/L(TIR), important for calibrating 24 micron measurements of high redshift galaxies, increases slightly as the metallicity decreases from ~solar to ~50% of solar, and then decreases by an order of magnitude with further decreases in metallicity. Although the great majority of galaxies show similar patterns of behavior as described above, there are three exceptions, SBS 0335-052E, Haro 11, and SHOC 391. Their infrared SEDs are dominated energetically by the mid-IR near 24 micron rather than by the 60 - 200 micron region. (Abridged)

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arXiv2006

The Wide Field Imager Lyman-Alpha Search (WFILAS) for Galaxies at Redshift ~5.7: II. Survey Design and Sample Analysis

Context: Wide-field narrowband surveys are an efficient way of searching large volumes of high-redshift space for distant galaxies. Aims: We describe the Wide Field Imager Lyman-Alpha Search (WFILAS) over 0.74 sq. degree for bright emission-line galaxies at z~5.7. Methods: WFILAS uses deep images taken with the Wide Field Imager (WFI) on the ESO/MPI 2.2m telescope in three narrowband (70 A), one encompassing intermediate band (220 A) and two broadband filters, B and R. We use the novel technique of an encompassing intermediate band filter to exclude false detections. Images taken with broadband B and R filters are used to remove low redshift galaxies from our sample. Results: We present a sample of seven Lya emitting galaxy candidates, two of which are spectroscopically confirmed. Compared to other surveys all our candidates are bright, the results of this survey complements other narrowband surveys at this redshift. Most of our candidates are in the regime of bright luminosities, beyond the reach of less voluminous surveys. Adding our candidates to those of another survey increases the derived luminosity density by ~30%. We also find potential clustering in the Chandra Deep Field South, supporting overdensities discovered by other surveys. Based on a FORS2/VLT spectrum we additionally present the analysis of the second confirmed Lya emitting galaxy in our sample. We find that it is the brightest Lya emitting galaxy (1 x 10^-16 erg s^-1 cm^-2) at this redshift to date and the second confirmed candidate of our survey. Both objects exhibit the presence of a possible second Lya component redward of the line.

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arXiv2004

The Star Cluster population of NGC 5253

We present a detailed analysis of the star cluster population in the starburst galaxy NGC 5253. Our work is based on HST optical and VLT Ks images. We detect more than 300 clusters, and for all of them we derive the photometry in the V, I and Ks bands and built a color-color V-I/V-Ks diagram. After correction for star contamination, we matched 115 sources in all the three bands V, I and Ks. Comparison of the data points with models shows how most of the sources are affected by a red excess. The ages of the younger clusters are obtained from a HST Halpha image which allows to partially remove the degeneracy between age and extinction and to calculate the screen optical depth toward the clusters. The V and Ks luminosity functions show a marked turnover at M(V)=-7.3 and M(K)=-10.2, with a shape roughly similar to the gaussian distribution of old globular clusters. We convert the luminosity function into a mass function, and find that the clusters younger than 10 Myr show a power law distribution with slope -1.6, while the distribution of the older clusters has a maximum at about 4x10^4 Msun.

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arXiv2004

The Origin of Massive O-type Field Stars. Part I: A Search for Clusters

We present a study aimed at clarifying the birthplace for 43 massive O-type field stars. In this first paper we present the observational part: a search for stellar clusters near the target stars. We derive stellar density maps at two different resolving scales, viz. 0.25pc and 1.0pc from NTT and TNG imaging and the 2MASS catalogue. These scales are typical for cluster sizes. The main result is that the large majority of the O-type field population are isolated stars: only 12% (5 out of 43) of the O-type field stars is found to harbour a small-scale stellar cluster. We review the literature and aim at characterizing the stellar field of each O-type field star with the emphasis on star formation and the presence of known young stellar clusters. An analysis of the result of this paper and a discussion of the O-type field population as products of a dynamical ejection event is presented in an accompanying paper.

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arXiv2003

Low-Metallicity Blue Compact Dwarfs as Templates for Primordial Star Formation

Understanding how galaxies formed their first stars is a vital cosmological question, but the study of high-redshift objects, caught in the act of forming their first stars, is difficult. Here we argue that two extremely low-metallicity Blue Compact Dwarf galaxies (BCDs), IZw18 and SBS0335-052, could be local templates for primordial star formation, since both lack evolved ($> $1 Gyr) stellar populations; but they form stars differently.

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arXiv2003

The infrared supernova rate in starburst galaxies

We report the results of our ongoing search for extincted supernovae (SNe) at near-infrared wavelengths. We have monitored at 2.2 micron a sample of 46 Luminous Infrared Galaxies and detected 4 SNe. The number of detections is still small but sufficient to provide the first estimate of supernova rate at near-infrared wavelengths. We measure a SN rate ofv 7.6+/-3.8 SNu which is an order of magnitude larger than observed in quiescent galaxies. On the other hand, the observed near-infrared rate is still a factor 3-10 smaller than that estimated from the far-infrared luminosity of the galaxies. Among various possibilities, the most likely scenario is that dust extinction is so high (Av>30) to obscure most SNe even in the near-IR. The role of type Ia SNe is also discussed within this context. We derive the type Ia SN rate as a function of the stellar mass of the galaxy and find a sharp increase toward galaxies with higher activity of star formation. This suggests that a significant fraction of type Ia SNe are associated with young stellar populations. Finally, as a by-product, we give the average K-band light curve of core-collapse SNe based on all the existing data, and review the relation between SN rate and far-infrared luminosity.

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arXiv2002

Near-infrared properties of blue compact dwarf galaxies: the link between solar and low metallicity

We have obtained near-infrared images and spectra of three blue compact dwarf galaxies of intermediate sub-solar metallicity Tol~35, Tol~3 and UM 462. This work is part of a larger project aimed to study the star formation and the stellar populations of low metallicity galaxies in the near-infrared. In this frame work galaxies of intermediate metallicity represent an important step in understanding the most extreme cases filling the gap between solar and very low metallicity galaxies. We have observed HII region like spectra in all three galaxies; in all cases the star formation episodes are only a few Myr old. Consistently with a young age our spectra show no evidence for stellar absorption features typical of supergiants, nor of [FeII] emission typical of supernovae. The K-band gas fraction ranges from 20 to 40 % showing that gas emission can significantly contaminate broadband near-infrared colors in young metal-poor starbursts. We have detected molecular hydrogen in emission in all three objects. All sources show bright knots superimposed on a lower surface brighness envelope. The knots are identified with Super Star Clusters; six of them are present in UM 462. In all galaxies we detect the presence of an old stellar population.

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arXiv2002

Discovery of two infrared supernovae: a new window on the SN search

We report the discovery of two supernovae (SN 1999gw and SN 2001db) obtained within the framework of an infrared monitoring campaign of Luminous Infrared Galaxies, aimed at detecting obscured supernovae. SN 2001db, extinguished by Av~5.5 mag, is the first supernova discovered in the infrared which has received the spectroscopic confirmation. This result highlights the power of infrared monitoring in detecting obscured SNe and indicates that optical surveys are probably missing a significant fraction of SNe, especially in obscured systems such as starburst galaxies. The preliminary estimate of SN rate in LIRG galaxies is about an order of magnitude higher than that expected from optical surveys.

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arXiv2002

Optical and Infrared Spectroscopy of SN 1999ee and SN 1999ex

We report optical and infrared spectroscopic observations of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex, both of which were hosted by the galaxy IC 5179. For SN 1999ee we obtained a continuous sequence with an unprecedented wavelength and temporal coverage beginning 9 days before maximum light and extending through day 42. Before maximum light SN 1999ee displayed a normal spectrum with a strong Si II 6355 absorption, thus showing that not all slow-declining SNe are spectroscopically peculiar at these evolutionary phases. A comparative study of the infrared spectra of SN 1999ee and other Type Ia supernovae shows that there is a remarkable homogeneity among the Branch-normal SNe Ia during their first 60 days of evolution. SN 1991bg-like objects, on the other hand, display spectroscopic peculiarities at infrared wavelengths. SN 1999ex was characterized by the lack of hydrogen lines, weak optical He I lines, and strong He I 10830,20581, thus providing an example of an intermediate case between pure Ib and Ic supernovae. We conclude therefore that SN 1999ex provides first clear evidence for a link between the Ib and Ic classes and that there is a continuous spectroscopic sequence ranging from the He deficient SNe Ic to the SNe Ib which are characterized by strong optical He I lines.

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arXiv2002

Multi-wavelength Study of IRAS 19254-7245 - The Superantennae

We present observations in the optical, near-infrared and millimetre bands of the Ultraluminous Infrared galaxy IRAS 19254-7245, also known as "The Superantennae". This galaxy is an interacting system with a double nucleus and long tails extending for about 350 kpc. We studied in detail the southern component of the system which is optically classified as a Sy2 galaxy. We have developed a method to determine the parameters of the emission lines in a spectrum in the case of multiple components and severe blending. Our data allow us to build a picture of the environment around the nucleus of the galaxy with unprecedented detail. The optical lines show a complex dynamical structure that is not observable in the near-infrared. In addition we find typical features of AGN such as the coronal lines of [FeVII]5721 and [SiVI]1.96. We also detect strong emission from [FeII]1.64 and $H_2$.

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arXiv2001

The deeply embedded starburst in SBS 0335-052

We present 4-micron ISAAC imaging and spectroscopy of the extremely metal-poor dwarf galaxy SBS 0335-052, aimed at a better understanding of the dust in this low-metallicity galaxy. The 4-micron emission turns out to be very compact, confined to the brightest pair of Super Star Clusters (SSCs). The Ks-L$^\prime$ color is extremely red, and the $L^\prime$ emission is consistent with the extrapolation of the ISO mid-infrared spectral energy distribution (SED). From hydrogen recombination lines and a fit to the near-/mid-infrared SED, we confirm a visual extinction of >= 15 mag. Our data suggest that the sites of the optical and infrared emission are distinct: the optical spectral lines come from an almost dustless region with a high star formation rate and a few thousand OB stars. This region lies along the line-of-sight to a very dusty central star cluster in which there are more than three times as many massive stars, completely hidden in the optical. From the extinction, we derive an upper limit for the dust mass of 10^5 M$_\odot$ which could be produced by recent supernovae.

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arXiv2001

Near-infrared template spectra of normal galaxies: k-corrections, galaxy models and stellar populations

We have observed 28 local galaxies in the wavelength range between 1 and 2.4 mic in order to define template spectra of the normal galaxies along the Hubble sequence. Five galaxies per morphological type were observed in most cases, and the resulting RMS spread of the normalized spectra of each class, including both intrinsic differences and observational uncertainties, is about 1% in K, 2% in H and 3% in J. Many absorption features can be accurately measured. The target galaxies and the spectroscopic aperture (7"x53") were chosen to be similar to those used by Kinney et al. (1996) to define template UV and optical spectra. The two data sets are matched in order to build representative spectra between 0.1 and 2.4 mic. The continuum shape of the optical spectra and the relative normalization of the near-IR ones were set to fit the average effective colours of the galaxies of the various Hubble classes. The resulting spectra are used to compute the k-corrections of the normal galaxies in the near-IR bands and to check the predictions of various spectral synthesis models: while the shape of the continuum is generally well predicted, large discrepancies are found in the absorption lines. Among the other possible applications, here we also show how these spectra can be used to place constraints on the dominant stellar population in local galaxies. Spectra and k-corrections are publicly available and can be downloaded from the web site http://www.arcetri.astro.it/~filippo/spectra.

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arXiv2000

Dust in active nuclei. I. Evidence for "anomalous" properties

We present observational evidences that dust in the circumnuclear region of AGNs has different properties than in the Galactic diffuse interstellar medium. By comparing the reddening of optical and infrared broad lines and the X-ray absorbing column density we find that the E(B-V)/N_H ratio is nearly always lower than Galactic by a factor ranging from ~3 up to ~100. Other observational results indicate that the Av/N_H ratio is significantly lower than Galactic in various classes of AGNs including intermediate type 1.8-1.9 Seyferts, hard X-ray selected and radio selected quasars, broad absorption line QSOs and grism selected QSOs. The lack of prominent absorption features at 9.7um (silicates) and at 2175A (carbon dip) in the spectra of Seyfert 2s and of reddened Seyfert 1s, respectively, add further evidence for dust in the circumnuclear region of AGNs being different from Galactic. These observational results indicate that the dust composition in the circumnuclear region of AGNs could be dominated by large grains, which make the extinction curve flatter, featureless and are responsible for the reduction of the E(B-V)/N_H and Av/N_H ratios. Regardless of the physical origin of these phenomena, the reduced dust absorption with respect to what expected from the gaseous column density should warn about a mismatch between the optical and the X-ray classification of the active galactic nuclei in terms of their obscuration.

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arXiv2000

The near-infrared view of SBS 0335-052

We present near-infrared (NIR) spectra, deep J, H and K low-resolution ima ges, and high-resolution images in J and Ks of the low-metallicity blue dwarf gal axy SBS 0335-052. These new NIR data, together with optical and mid-infrared data from the literature, are used to constrain the properties and the star formation history of the galaxy. The NIR spectral characteristics and broadband colors of this galaxy are those of an extremely young starburst. We find that the NIR emission is dominated by star formation occuring in clusters younger than 5 Myr; no optically hidden star formation is revealed in our data. We also find that a warm-dust component is necessary to explain the H-K color, and is consistent with the mid-infrared spectral energy distribution. Finally, we quantify the possible contribution from an evolved stellar populatio n.

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arXiv1999

Northern JHK Standard Stars for Array Detectors

We report J, H and K photometry of 86 stars in 40 fields in the northern hemisphere. The fields are smaller than or comparable to a 4x4 arcmin field-of-view, and are roughly uniformly distributed over the sky, making them suitable for a homogeneous broadband calibration network for near-infrared panoramic detectors. K magnitudes range from 8.5 to 14, and J-K colors from -0.1 to 1.2. The photometry is derived from a total of 3899 reduced images; each star has been measured, on average, 26.0 times per filter on 5.5 nights. Typical errors on the photometry are about 0.012.

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arXiv1999

Optical observations of the black hole candidate XTE J1550-564 during the Sep/Oct 1998 outburst

We report here optical observations during the September/October 1998 outburst of the black hole candidate XTE J1550-564. CCD photometry was obtained for the optical counterpart with the 0.9m Dutch telescope at La Silla since the onset of this event. We analysed 211 U, V and i frames, from Sep 10 to Oct 23. Stochastic flaring activity was observed on Sep 11-16, but no evidence for a superhump period, as seen in other soft X-ray transients, was found. An optical flare nearly simultaneous to an X-ray flare that occurred on Sep 21 was observed in the V-band. A reddened optical spectrum showed the typical emission lines corresponding to X-ray transients in outburst. From the interstellar absorption lines, we derive E(B-V) = 0.70 +/- 0.10 and suggest a distance of about 2.5 kpc. In such case, M_{B} = +7 mag. for the progenitor, which is consistent with the spectral type of a low-mass K0-K5 main-sequence companion.

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arXiv1999

GRB 990510: linearly polarized radiation from a fireball

Models for gamma-ray burst afterglows envisage an hyper-relativistic fireball that is decelerated in the ambient medium around the explosion site. This interaction produces a shock wave which amplifies the magnetic field and accelerates electrons to relativistic energies, setting the conditions for an efficient production of synchrotron photons. If produced in a region of large-scale ordered magnetic field, synchrotron radiation can be highly polarized. The optical transient associated with GRB 990510 was observed ~18.5 hr after the event and linear polarization in the R band was measured at a level of 1.7 +/- 0.2 %. This is the first detection of linear polarization in the optical afterglow of a gamma-ray burst. We exclude that this polarization is due to dust in the interstellar material, either in our Galaxy or in the host galaxy of the gamma-ray burst. These results provide important new evidence in favor of the synchrotron origin of the afterglow emission, and constrains the geometry of the fireball and/or magnetic field lines.

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arXiv1997

LonGSp: the Gornergrat Longslit Infrared Spectrometer

We present a near-infrared cooled grating spectrometer that has been developed at the Arcetri Astrophysical Observatory for the 1.5 m Infrared Telescope at Gornergrat (TIRGO). The spectrometer is equipped with cooled reflective optics and a grating in Littrow configuration. The detector is an engineering grade Rockwell NICMOS3 array (256x256 pixels of 40 $μ$m). The scale on the focal plane is 1.73 arcsec/pixel and the field of view along the slit is 70 arcsec. The accessible spectral range is 0.95-2.5 $μ$m with a dispersion, at first order, of about 11.5 angstroms/pixel. This paper presents a complete description of the instrument, including its optics and cryo-mechanical system, along with astronomical results from test observations, started in 1994. Since January 1996, LonGSp is offered to TIRGO users and employed in several Galactic and extragalactic programs.

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