arXiv subjects

Alvio Renzini

Publications and source records attributed to Alvio Renzini.

At least 163 records · Page 9Linked to original sources

arXiv2002

Future Opportunities for Globular Cluster Astronomy at Ground Based Facilities

In spite of great progress over the last $\sim 10$ years, especially thanks to HST, a number of exciting open problem still puzzle astronomers working on globular clusters in our own and other galaxies. These problems range from determining more accurate ages to assess whether massive black holes hide at the center of some clusters, from identifying the physical origin of red giant winds to assess whether there is an influence of cluster structure and dynamics on the evolution of individual cluster stars, from demonstrating whether or not some clusters possess a dark matter halo on their own to eventually understand the formation of globular clusters in the context of galaxy formation, and more. In this review, I briefly sketch how ground based telescopes and their instrumentation can help solving these problems through the present decade, 2001-2010. A glimpse to the next decade is also given.

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arXiv2002

Rotation of Hot Horizontal Branch Stars in the Globular Clusters NGC 1904, NGC 2808, NGC 6093 and NGC 7078

We present high resolution UVES+VLT spectroscopic observations of 56 stars in the extended horizontal branch (EHB) of the Galactic globular clusters NGC 1904, NGC 2808, NGC 6093, and NGC 7078. Our data reveal for the first time the presence in NGC 1904 of a sizable population of fast (vsini > 20 km/s) horizontal branch (HB) rotators, confined to the cool end of the EHB, similar to that found in M13. We also confirm the fast rotators already observed in NGC 7078. The cooler stars (Teff < 11,500 K) in these three clusters show a range of rotation rates, with a group of stars rotating at ~ 15 km/s or less, and a fast rotating group at ~ 30 km/s. Apparently, the fast rotators are relatively more abundant in NGC 1904 and M13, than in NGC 7078. No fast rotators have been identified in NGC 2808 and NGC 6093. All the stars hotter than Teff ~ 11,500 K have projected rotational velocities vsini < 12 km/s, but less than 20% have vsini < 2 km/s. The connection between photometric gaps in the HB and the change in the projected rotational velocities is not confirmed by the new data. However, our data are consistent with a relation between this discontinuity and the HB jump. We discuss a number of possibilities for the origin of the stellar rotation distribution along the HB. We conclude that none of them can yet provide a satisfactory explanation of the observations.

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arXiv2001

An Empirical View to the Early Chemical Evolution of Bulge/Disk Galaxies

Scaling from the empirical metal yield as measured in clusters of galaxies, it is inferred that early in the evolution of the Galaxy the bulge stellar population has produced $\sim 10^9\msun$ of metals, at least 5 times more than the total metal content of the bulge today. It is argued that an early galactic wind from the starbursting bulge has pre-enriched a vast region around it, with these metals being able to enrich to $\sim 1/10$ solar of order of $5\times 10^{11}\msun$ of pristine material. From the empirical evidence that bulges come before disks, it is inferred that the Milky Way disk formed out of this pre-enriched material, which accounts for the scarcity of metal poor stars in the solar neighborhood, the so-called `G-Dwarf Problem'. High redshift observations are now becoming able to efficiently explore the $1.2\lsim z\lsim 3$ region of the universe, when disk formation and morphological differentiation may have taken place.

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arXiv2000

The Cuspy LINER Nucleus of the S0/a Galaxy NGC 2681

The nucleus of the bulge-dominated, multiply-barred S0/a galaxy NGC 2681 is studied in detail, using high resolution Hubble Space Telescope FOC and NICMOS imaging and FOS spectroscopy. The ionised gas central velocity dispersion is found to increase by a factor ~2 when narrowing the aperture from R~1.5" (ground) to R~0.1" (FOS). Dynamical modeling of these velocity dispersions suggests that NGC 2681 does host a supermassive black hole (BH) for which one can estimate a firm mass upper limit M_BH < 6*10^7 Solar Masses. This upper limit is consistent with the relation between the central BH mass and velocity dispersion M_BH - sigma known for other galaxies. The emission line ratios place the nucleus of NGC 2681 among LINERs. It is likely that the emission line region comes from a rather mild, but steady, feeding of gas to the central BH in this galaxy. The inner stellar population lacks any measurable color gradient (to a radius of 0.6 kpc) from the infrared to the ultraviolet, consistently with FOC, FOS and IUE data, all indicating that this system underwent a starburst ~1 Gyr ago that encompassed its whole interior, down to its very center. The most likely source of such a widely-distributed starburst is the dumping of tidally-extruded gas from a galaxy neighbor. If so, then NGC 2681 can be considered as the older brother of M82, seen face-on as opposed to the edge-on view we have for M82.

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arXiv2000

Synthetic post-Asymptotic Giant Branch evolution: basic models and applications to disk populations

We explore the realm of post-Asymptotic Giant Branch (post-AGB) stars from a theoretical viewpoint, by constructing synthetic population of transition objects, proto-Planetary Nebulae, Planetary Nebulae Nuclei, and post-Planetary Nebulae objects. We use the Montecarlo procedure to filter out the populations accordingly to a given set of assumptions. We explore the parameter space by studying the effects of the Initial Mass Function (IMF), the Initial Mass-Final Mass Relation (IMFMR), the transition time (t_tr), the envelope mass at the end of the envelope ejection (Me_r), the planetary nebula lifetime t_PN, the hydrogen- and helium-burning phases of the central stars. The results are discussed on the basis of the HR diagram distributions, on the Mv-t plane, and with mass histograms. We found that: (1) the dependence of the synthetic populations on the assumed IMF and IMFMR is generally mild; (2) the Me_r indetermination produces very high indeterminations on the t_tr and thus on the resulting post-AGB populations; (3) the synthetic models give a test check for the ratio of He- to H-burning PNNi. In this paper, disk post-AGB populations are considered. Future applications will include Magellanic Clouds PNe, and populations of bulges and elliptical galaxies.

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arXiv2000

WWhat Heavy Elements in Clusters of Galaxies Tell About Clusters and Galaxies

Clusters of galaxies allow a direct estimate of the metallicity and metal production yield on the largest scale so far. The ratio of the total iron mass in the ICM to the total optical luminosity of the cluster (the iron mass-to-light-ratio) is the same for all clusters which ICM is hotter than $\sim 2$ keV, and the elemental proportions (i.e. the [$α$/Fe] ratio) appear to be solar. The simplest interpretation of these evidences is that both the IMF as well the relative contributions of SN types are universal. Currently available abundances in cooler clusters and groups are much more uncertain, possibly due to insufficiently accurate atomic physics data for multi-electron ions, or to the ICM being multi-phase, or to a combination thereof. This uncertainty automatically extends to the reality of radial abundance gradients so far reported in cool clusters. It is emphasized that most metals reside in the ICM rather than in galaxies, which demonstrates that energetic winds operated early in the evolution of massive galaxies, the likely producers of most metals now in the ICM. The ICM metallicity is also used to set a semiempirical constraint of $\sim 0.1$ keV per particle to the ICM {\it preheating} due to supernova driven galactic winds. A lower limit of the universe global metallicity at $z=3$ is also mentioned.

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arXiv2000

Supermassive Black Holes Can Hardly Be "Silent"

There is now ample evidence that most - perhaps all - galactic spheroids host a supermassive BH at their center. This has been assessed using a variety of observational techniques, from stellar and/or gas dynamics to megamasers. Yet another promising technique is offered by the case of the Virgo elliptical NGC 4552, in which early HST/FOC observations revealed a central low-luminosity flare. Subsequent HST/FOS observations with a 0.21 arcsec aperture have revealed a rich emission-line spectrum with broad and narrow components with FWHM of 3000 and 700 km/s, respectively. This variable, mini-AGN at the center of NGC 4552 is most naturally the result of a sporadic accretion event on a central BH. It has a H$α$ luminosity of only $\sim 10,000 \lsun$, making it the likely, intrinsically faintest AGN known today. Only thanks to the superior resolution of HST such a faint object has been discovered and studied in detail, but adaptive optics systems on large ground-based telescopes may reveal in the future that a low level of accretion onto central massive BHs is an ubiquitous phenomenon among galactic spheroids. FOC/FOS observations of a central spike in NGC 2681 reveal several analogies with the case of NGC 4552, while yet another example is offered by a recent exciting finding with STIS by R.W. O'Connell in NGC 1399, the third galaxy in our original program. garchingbh

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arXiv1999

HST-NICMOS Color Transformations and Photometric Calibrations

This paper presents color transformations for HST NICMOS camera 2 observations to the ground-based CIT/CTIO photometric system, using observations of nineteen moderately bright, red stars in Baade's window in the color range 0.7 < (J-K) < 1.6. We estimate an extension of the transformation to 0.4 < (J-K) < 3.0 with five standards observed by STScI. Convolving near-IR spectra taken above the atmosphere with different filter transmission profiles, we simulate both NICMOS and ground-based photometry, obtaining results which are consistent with our transformation and its extension.

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arXiv1999

Chemical Evolution on the Scale of Clusters of Galaxies, and Beyond

Clusters of galaxies allow a direct estimate of the metallicity and metal production yield on the largest scale so far. The ratio of the total iron mass in the ICM to the total optical luminosity of the cluster (the iron mass-to-light-ratio) is the same for all clusters which ICM is hotter than $\sim 2$ keV, and the elemental proportions (i.e. the [$α$/Fe] ratio) appear to be solar. From these evidences it is argued that both the IMF as well the relative contributions of SN types are likely to be universal. Constraints on the past SN activity in galaxy clusters are then derived, and support is given to the notion that the average SNIa rate was much higher in the past, i.e. at least 10 times more than currently observed in local ellopticals. It is also argued that cluster metallicity ($\sim 1/3$ solar) should be taken as representative of the low-z universe as a whole. There is now compelling evidence that the bulk of stars in cluster as well as in field ellipticals and bulges formed at high redshifts ($z\gsim 3$). Since such stars account for at least $\sim 30%$ of the baryons now locked into stars, it is argued that at least 30% of stars and metals formed before $z\simeq 3$. As a consequence, the metallicity of the universe at z=3 is predicted to be $\sim 1/10$ solar. This requires the cosmic star formation rate to run at least flat from $z\sim 1$ to $\sim 5$, which appears to agree with the most recent derect determinations of the star formation rate in Lyman-break galaxies.

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arXiv1999

The Mini AGN at the Center of the Elliptical Galaxy NGC 4552 with HST

The complex phenomenology shown by the UV-bright, variable spike first detected with the Hubble Space Telescope (HST) at the center of the otherwise normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near UV have been analyzed in a homogeneous fashion, showing that the central spike has brightened by a factor ~4.5 between 1991 and 1993, and has decreased its luminosity by a factor ~2.0 between 1993 and 1996. FOS spectroscopy extending from the near UV to the red side of the optical spectrum reveals a strong UV continuum over the spectrum of the underlying galaxy, along with several emission lines in both the UV and the optical ranges. In spite of the low luminosity of the UV continuum of the spike (~3*10^5 Lsolar), the spike is definitely placed among AGNs by current diagnostics based on the emission line intensity ratios, being just on the borderline between Seyferts and LINERs. Line profiles are very broad, and both permitted and forbidden lines are best modelled with a combination of broad and narrow components, with FWHM of ~3000 km s^-1 and ~700 km s^-1, respectively. This evidence argues for the variable central spike being produced by a modest accretion event onto a central massive black hole (BH), with the accreted material having possibly being stripped from a a star in a close fly by with the BH. The 1996 broad Halpha luminosity of this mini-AGN is ~5.6*10^37 erg s^-1, about a factor of two less than that of the nucleus of NGC 4395, heretofore considered to be the faintest known AGN.

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arXiv1999

Origin of Bulges

Insight into the origin of bulges is saught in this review only from the properties of their stellar populations. Evidence concerning the age of the Galactic bulge stellar population is reviewed first, then the case of the bulge of M31 is discussed. The similarity of bulges and ellipticals is then illustrated, inferring that the problems of the origin of bulges and of the origin of ellipticals may well be one and the same: i.e. the origin of galactic spheroids. In this mood, the current evidence concerning the age of the bulk stellar populations of early-type galaxies is then reviewed, both for low- as well as high-redshift galaxies, and both for cluster as well as field ellipticals. All reported evidence argues for the bulk of stars in galactic spheroids having formed at high redshift, with only minor late additions and small dependence on environment. In the final, more speculative Section an attempt is made to evaluate how current formation scenarios can account for this observational evidence. The role of spheroids in the cosmic star formation and metal enrichment history is also briefly discussed. Finally, some critical questions are asked, which answers may help our further understanding of the formation and evolution of galactic spheroids.

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arXiv1999

Central UV Spikes in two Galactic Spheroids

FOS spectra and FOC photometry of two centrally located, UV-bright spikes in the elliptical galaxy NGC 4552 and the bulge-dominated early spiral NGC 2681, are presented. These spectra reveal that such point-like UV sources detected by means of HST within a relatively large fraction ~15% of bulges can be related to radically different phenomena. While the UV unresolved emission in NGC 4552 represents a transient event likely induced by an accretion event onto a supermassive black hole, the spike seen at the center of NGC 2681 is not variable and it is stellar in nature.

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arXiv1999

Objects in NGC 205 Resolved into Stellar Associations by HST Ultraviolet Imaging

We have obtained high resolution UV images with the HST+WFPC2 of the central region of the dwarf elliptical galaxy NGC 205. Our images reveal that many of the hot UV stars previously detected and studied from the ground are actually multiple systems, open clusters and star associations. We have performed photometry of two such clusters and we find our data are consistent with stellar ages of 50 and 100 Myr, respectively. From the number of massive stars in NGC 205 we estimate that the star formation episode in this galaxy has turned ~1,000 Msolar of gas into stars over the last 100 Myr.

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arXiv1998

The UV emission of elliptical galaxies

Recent observational and theoretical developments concerning the origin of the ultraviolet emission from ellipticals are reviewed and discussed,with particular, reference to the extensive set of predictions contained in Greggio and Renzini (1990). Strong support to the preferred hot horizontal branch and AGB-manque' scenario for the origin of the UV emission has come from UIT, HUT and HST data. However, the UV sensitivity of such space telescopes has proved insufficient to unambiguously detect the predicted fading away of the UV rising branch with redshift, that we consider the ultimate check for this scenario.

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arXiv1998

Hints on Galaxy Formation from the Metal Content of Galaxy Cluster and Other Fossil Evidence

Clusters of galaxies allow a direct estimate of the metallicity and metal production yield on the largest scale so far. It is argued that cluster metallicity ($\sim 1/3$ solar) should be taken as representative of the low-$z$ universe as a whole. There is now compelling evidence that the bulk of stars not only in cluster ellipticals but also in field ellipticals and bulges formed at high redshifts ($z\gsim 3$). Since such stars account for at least $\sim 30%$ of the baryons now locked into stars, it is argued that at least 30% of stars and metals formed before $z\simeq 3$, and correspondingly the metallicity of the universe at $z=3$ is predicted to be $\sim 1/10$ solar. This requires the cosmic star formation rate to run at least flat from $z\sim 1$ to $\sim 5$, which appears to agree with the most recent derect determinations of the star formation rate in Lyman-break galaxies.

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arXiv1998

Cluster vs. Field Elliptical Galaxies and Clues on their Formation

Using new observations for a sample of 931 early-type galaxies we investigate whether the \mg2--\so relation shows any dependence on the local environment. The galaxies have been assigned to three different environments depending on the local overdensity: clusters, groups, and field, having used our completeredshift database to guide the assignment of galaxies. It is found that cluster, group and field early-type galaxies follow almost identical \mg2--\so\ relations, with the largest \mg2 zero-point difference (clusters minus field) being only $0.007\pm 0.002$ mag. No correlation of the residuals is found with the morphological type or the bulge to disk ratio. Using stellar population models in a differential fashion, this small zero-point difference implies a luminosity-weighted age difference of only $\sim 1$ Gyr between the corresponding stellar populations, with field galaxies being younger. The mass-weighted age difference could be significantly smaller, if minor events of late star formation took place preferentially in field galaxies. We combine these results with the existing evidence for the bulk of stars in cluster early-type galaxies having formed at very high redshift, and conclude that the bulk of stars in galactic spheroids had to form at high redshifts ($z\gsim 3$), no matter whether such spheroids now reside in low or high density regions. The cosmological implications of these findings are briefly discussed.

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arXiv1998

The Stellar Populations of Pixels and Frames

Derived from first physical principles, a few simple rules are presented that can help in both the planning and interpretation of CCD and IR-array camera observations of resolvable stellar populations. These rules concern the overall size of the population sampled by a frame as measured by its total luminosity, and allow to estimate the number of stars (in all evolutionary stages) that are included in the frame. The total luminosity sampled by each pixel (or resolution element) allows instead to estimate to which depth meaningful stellar photometry can be safely attempted, and below which crowding makes it impossible. Simple relations give also the number of pixels (resolution elements) in the frame that will contain an unresolved blend of two stars of any kind. It is shown that the number of such blends increases quadratically with both the surface brightness of the target, as well as with the angular size of the pixel (or resolution element). A series of examples are presented illustrating how the rules are practically used in concrete observational situations. Application of these tools to existing photometric data for the inner parts of the bulge of M31, M32 and NGC 147 indicates that no solid evidence has yet emerged for the presence of a significant intermediate age population in these objects.

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arXiv1998

The Main Epoch of Metal Production in the Universe

Clusters of galaxies allow a direct estimate of the metallicity and metal production yield on the largest scale so far. It is argued that cluster metallicity (about 1/3 solar) should be taken as representative of the low-z universe as a whole. There is now compelling evidence that the bulk of stars not only in cluster ellipticals but also in field ellipticals and bulges formed at high redshifts (z>~ 3). Since such stars account for at least 30% of the baryons now locked into stars, it is argued that at least 30% of stars and metals formed before z=~3, and correspondingly the metallicity of the universe at z=3 is predicted to be about 1/10 solar.

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