arXiv subjects

M. Pettini

Publications and source records attributed to M. Pettini.

At least 55 records · Page 3Linked to original sources

arXiv2002

Topological Signature of First Order Phase Transitions

We show that the presence and the location of first order phase transitions in a thermodynamic system can be deduced by the study of the topology of the potential energy function, V(q), without introducing any thermodynamic measure. In particular, we present the thermodynamics of an analytically solvable mean-field model with a k-body interaction which -depending on the value of k- displays no transition (k=1), second order (k=2) or first order (k>2) phase transition. This rich behavior is quantitatively retrieved by the investigation of a topological invariant, the Euler characteristic, of some submanifolds of the configuration space. Finally, we conjecture a direct link between the Euler characteristic and the thermodynamic entropy.

cond-mat.stat-mech↗

arXiv2002

The Population of Faint Optically-Selected AGN at z~3

We discuss a sample of 29 AGN (16 narrow-lined and 13 broad-lined) discovered in a spectroscopic survey of ~1000 star-forming Lyman-break galaxies (LBGs) at z~3. Reaching apparent magnitudes of R_{AB}=25.5, the sample includes broad-lined AGN approximately 100 times less UV-luminous than most surveys to date covering similar redshifts, and the first statistical sample of UV/optically-selected narrow-lined AGN at high redshift. The fraction of objects in our survey with clear evidence for AGN activity is ~3%. A substantial fraction, perhaps even most, of these objects would not have been detected in even the deepest existing X-ray surveys. We argue that these AGN are plausibly hosted by the equivalent of LBGs. The UV luminosities of the broad-lined AGN in the sample are compatible with Eddington-limited accretion onto black holes that satisfy the locally determined M_{BH} versus M_{bulge} relation given estimates of the stellar masses of LBGs. The clustering properties of the AGN are compatible with their being hosted by objects similar to LBGs. The implied lifetime of the active AGN phase in LBGs, if it occurs some time during the active star-formation phase, is ~10^7 years.

astro-ph↗

arXiv2002

The Kinematic Connection Between QSO-Absorbing Gas and Galaxies at Intermediate Redshift

We present complementary data on 5 intermediate redshift (0.44 < z < 0.66) MgII absorbing galaxies, combining high spatial resolution imaging from HST, high--resolution QSO spectroscopy from Keck/HIRES, and galaxy kinematics from intermediate resolution spectroscopy using Keck/LRIS. These data allow a direct comparison of the kinematics of gas at large galactocentric impact parameters with the galaxy kinematics obtained from the faint galaxy spectroscopy. All 5 galaxies appear to be relatively normal spirals, with measured rotation curves yielding circular velocities in the range 100 < v_c < 260 km/s. We find that in 4 of the 5 cases examined, the velocities of all of the Mg II absorption components lie entirely to one side of the galaxy systemic redshift. These observations are consistent with rotation being dominant for the absorbing gas kinematics; however, the total range of velocities observed is inconsistent with simple disk rotation in every case. Simple kinematic models that simultaneously explain both the systemic offset of the absorbing material relative to the galaxy redshifts, and the total velocity width spanned by the absorption, require either extremely thick rotating gas layers, rotation velocities that vary with height above the extrapolation of the galactic plane, or both. In any case, our small sample suggests that rotating ``halo'' gas is a common feature of intermediate redshift spiral galaxies, and that the kinematic signature of rotation dominates over radial infall or outflow even for gas well away from the galactic plane. We discuss possible explanations for this behavior, and compare our observations to possible local analogs.

astro-ph↗

arXiv2001

The Rest-Frame Optical Properties of z~3 Galaxies

We present the results of a near-infrared imaging survey of z~3 Lyman Break Galaxies (LBGs). The survey covers a total of 30 arcmin^2 and includes 118 photometrically selected LBGs with K_s band measurements, 63 of which also have J band measurements, and 81 of which have spectroscopic redshifts. Using the distribution of optical {\cal R} magnitudes from previous work and {\cal R}-K_s colors for this sub-sample, we compute the rest-frame optical luminosity function of LBGs. At the brightest magnitudes, where it is fairly well constrained, this luminosity function strikingly exceeds locally determined optical luminosity functions. The V-band luminosity density of only the observed bright end of the z~3 LBG luminosity function already approaches that of all stars in the local universe. For the 81 galaxies with measured redshifts, we investigate the range of LBG stellar populations implied by the photometry which generally spans the range 900--5500 AA in the rest-frame. While there are only weak constraints on the parameters for most of the individual galaxies, there are strong trends in the sample as a whole. A unified scenario which accounts for the observed trends in bright LBGs is one in which a relatively short period of very rapid star-formation (hundreds of M_sun/yr) lasts for roughly 50--100 Myr, after which both the extinction and star-formation rate are considerably reduced and stars are formed at a more quiescent, but still rapid, rate for at least a few hundred Myr. In our sample, a considerable fraction (~20%) of the LBGs have best-fit star-formation ages ~> 1 Gyr, implied stellar masses of ~> 10^10 M_sun, and are still forming stars at \~30 M_sun/yr.

astro-ph↗

arXiv2000

Optical and Infrared Spectroscopy of the type IIn SN 1998S : Days 3-127

We present contemporary infrared and optical spectroscopic observations of the type IIn SN 1998S for the period between 3 and 127 days after discovery. In the first week the spectra are characterised by prominent broad emission lines with narrow peaks superimposed on a very blue continuum(T~24000K). In the following two weeks broad, blueshifted absorption components appeared in the spectra and the temperature dropped. By day 44, broad emission components in H and He reappeared in the spectra. These persisted to 100-130d, becoming increasingly asymmetric. We agree with Leonard et al. (2000) that the broad emission lines indicate interaction between the ejecta and circumstellar material (CSM) and deduce that progenitor of SN 1998S appears to have gone through at least two phases of mass loss, giving rise to two CSM zones. Examination of the spectra indicates that the inner zone extended to <90AU, while the outer CSM extended from 185AU to over 1800AU. Analysis of high resolution spectra shows that the outer CSM had a velocity of 40-50 km/s. Assuming a constant velocity, we can infer that the outer CSM wind commenced more than 170 years ago, and ceased about 20 years ago, while the inner CSM wind may have commenced less than 9 years ago. During the era of the outer CSM wind the outflow was high, >2x10^{-5}M_{\odot}/yr corresponding to a mass loss of at least 0.003M_{\odot} and suggesting a massive progenitor. We also model the CO emission observed in SN 1998S. We deduce a CO mass of ~10^{-3} M_{\odot} moving at ~2200km/s, and infer a mixed metal/He core of ~4M_{\odot}, again indicating a massive progenitor.

astro-ph↗

arXiv2000

Lyman Continuum Emission from Galaxies at z~3.4

We report the detection of significant Lyman continuum flux in the composite spectrum of 29 Lyman break galaxies (LBGs) with redshifts = 3.40+/-0.09. After correction for opacity due to intervening absorption using a new composite QSO spectrum evaluated at the same redshift, the ratio of emergent flux density at 1500 Åin the rest frame to that in the Lyman continuum is L(1500)/L(900) = 4.6 +/- 1.0. If the relative intensity of the inferred escaping Lyman continuum radiation is typical of LBGs at z ~ 3 (the galaxies in this sample are drawn from the bluest quartile of LBG spectral energy distributions due to known selection effects), then observed LBGs produce about 5 times more H-ionizing photons per unit co-moving volume than QSOs at z ~ 3. The associated contribution to the metagalactic ionizing radiation field is J_ν(912) = 1.2+/-0.3 x 10^{-21} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1} at z ~ 3, very close to most estimates of the radiation field background based on the ``proximity effect''. A preliminary analysis of the density of faint QSOs in our Lyman break galaxy survey indicates that the standard extrapolated QSO luminosity function may slightly over-predict the QSO contribution to J_ν(912) at z ~ 3. We briefly discuss the implications of a galaxy-dominated UV background at high redshifts.

astro-ph↗

arXiv2000

Hamiltonian dynamics and geometry of phase transitions in classical XY models

The Hamiltonian dynamics associated to classical, planar, Heisenberg XY models is investigated for two- and three-dimensional lattices. Besides the conventional signatures of phase transitions, here obtained through time averages of thermodynamical observables in place of ensemble averages, qualitatively new information is derived from the temperature dependence of Lyapunov exponents. A Riemannian geometrization of newtonian dynamics suggests to consider other observables of geometric meaning tightly related with the largest Lyapunov exponent. The numerical computation of these observables - unusual in the study of phase transitions - sheds a new light on the microscopic dynamical counterpart of thermodynamics also pointing to the existence of some major change in the geometry of the mechanical manifolds at the thermodynamical transition. Through the microcanonical definition of the entropy, a relationship between thermodynamics and the extrinsic geometry of the constant energy surfaces $Σ_E$ of phase space can be naturally established. In this framework, an approximate formula is worked out, determining a highly non-trivial relationship between temperature and topology of the $Σ_E$. Whence it can be understood that the appearance of a phase transition must be tightly related to a suitable major topology change of the $Σ_E$. This contributes to the understanding of the origin of phase transitions in the microcanonical ensemble.

cond-mat.stat-mech↗

arXiv1999

Lyman Alpha Imaging of a Proto-Cluster Region at =3.09

We present very deep imaging observations, through a narrow-band filter tuned to Lyman alpha at , of a volume containing a significant over-density of galaxies at this redshift previously discovered in our survey for continuum-selected Lyman break galaxies (LBGs). The new observations are used in conjunction with our spectroscopic results on LBGs to compare the effectiveness of continuum and emission line searches for star forming galaxies at high redshift, and to extend the search for members of the structure at = 3.09 to much fainter continuum luminosities. We find that only 20-25 percent of all galaxies at a given UV continuum luminosity would be flagged as narrow-band excess objects subject to the typical limits W_Lya > 80 Angstroms in the observed frame. The density enhancement of strong Lyman alpha emitters in this field is consistent with that inferred from the analysis of the spectroscopic Lyman break galaxy sample in the same region (6+/-1), but extends to continuum luminosities up to 2 magnitudes fainter. There is no evidence for a significantly higher fraction of large Lyman alpha line equivalent widths at faint continuum luminosities. By combining the 24 spectroscopic members of the z=3.09 ``spike'' with the narrow-band candidates, we are able to produce a sample of 162 objects which are either known or likely members of this large structure. We have also discovered two extremely large and diffuse Lyman alpha emitting ``blobs'', which have physical extents >100/h kpc and Lyman alpha line fluxes 20-40 times larger than the typical line emitters at the same redshifts in the field. The possible nature of the ``blobs'' is discussed. (abstract abridged)

astro-ph↗

arXiv1999

Lyman Break Galaxies at z>4 and the Evolution of the UV Luminosity Density at High Redshift

We present initial results of a survey for star-forming galaxies in the redshift range 3.8 < z < 4.5. This sample consists of a photometric catalog of 244 galaxies culled from a total solid angle of 0.23 square degrees to an apparent magnitude of I_{AB}=25.0. Spectroscopic redshifts in the range 3.61 < z < 4.81 have been obtained for 48 of these galaxies; their median redshift is =4.13. Selecting these galaxies in a manner entirely analogous to our large survey for Lyman break galaxies at smaller redshift (2.7 < z < 3.4) allows a relatively clean differential comparison between the populations and integrated luminosity density at these two cosmic epochs. Over the same range of UV luminosity, the spectroscopic properties of the galaxy samples at z~4 and z~3 are indistinguishable, as are the luminosity function shapes and the total integrated UV luminosity densities (rho_{UV}(z=3)/rho_{UV}(z=4) = 1.1 +/-0.3). We see no evidence at these bright magnitudes for the steep decline in the star formation density inferred from fainter photometric Lyman-break galaxies in the Hubble Deep Field (HDF). If the true luminosity density at z~4 is somewhat higher than implied by the HDF, as our ground-based sample suggests, then the emissivity of star formation as a function of redshift is essentially constant for all z>1 once internally consistent corrections for dust are made. This suggests that there is no obvious peak in star formation activity, and that the onset of substantial star formation in galaxies occurs at z > 4.5. [abridged abstract]

astro-ph↗

arXiv1998

Lyman Break Galaxies at z~3 and Beyond

We report on the status of large surveys of photometrically selected star forming galaxies at z~3 and z~4, with particular emphasis on both the advantages and the limitations of selecting objects using the ``Lyman break'' technique. Current results on the luminosity functions, luminosity densities, color distribution, star formation rates, clustering properties, and the differential evolution of the population as a function of redshift are summarized.

astro-ph↗

arXiv1998

Galaxy Clustering at z~3

Galaxies at very high redshift (z~3 or greater) are now accessible to wholesale observation, making possible for the first time a robust statistical assessment of their spatial distribution at lookback times approaching ~90% of the age of the Universe. This paper summarizes recent progress in understanding the nature of these early galaxies, concentrating in particular on the clustering properties of photometrically selected ``Lyman break'' galaxies. Direct comparison of the data to predictions and physical insights provided by galaxy and structure formation models is particularly straightforward at these early epochs, and results in critical tests of the ``biased'', hierarchical galaxy formation paradigm.

astro-ph↗

arXiv1998

Galaxies and Large Scale Structure at z~3

We summarize the status of a ``targeted'' redshift survey aimed at establishing the properties of galaxies and their large scale distribution in the redshift range 2.5 < z < 3.5. At the time of this writing, we have obtained spectra of more than 400 galaxies in this redshift range, all identified using the ``Lyman break'' color-selection technique. We present some of the first results on the general clustering properties of the Lyman break galaxies. The galaxies are very strongly clustered, with co-moving correlation length similar to present-day galaxies, and they are evidently strongly biased relative to the mass distribution at these early epochs, which is consistent with hierarchical galaxy formation models if Lyman break galaxies trace the most massive halos at z ~ 3. Prospects for large surveys for galaxies beyond z ~ 4 are discussed.

astro-ph↗

arXiv1998

A Counts-in-Cells Analysis of Lyman-break Galaxies at z~3

We have measured the counts-in-cells fluctuations of 268 Lyman-break galaxies with spectroscopic redshifts in six 9 arcmin by 9 arcmin fields at z~3. The variance of galaxy counts in cubes of comoving side length 7.7, 11.9, 11.4 h^{-1} Mpc is σ_{gal}^2 ~ 1.3\pm0.4 for Ω_M=1, 0.2 open, 0.3 flat, implying a bias on these scales of σ_{gal} / σ_{mass} = 6.0\pm1.1, 1.9\pm0.4, 4.0\pm0.7. The bias and abundance of Lyman-break galaxies are surprisingly consistent with a simple model of structure formation which assumes only that galaxies form within dark matter halos, that Lyman-break galaxies' rest-UV luminosities are tightly correlated with their dark masses, and that matter fluctuations are Gaussian and have a linear power-spectrum shape at z~3 similar to that determined locally (Γ~0.2). This conclusion is largely independent of cosmology or spectral normalization σ_8. A measurement of the masses of Lyman-break galaxies would in principle distinguish between different cosmological scenarios.

astro-ph↗

arXiv1997

A Large Structure of Galaxies At Redshift z~3 and its Cosmological Implications

We report the discovery of a highly significant concentration of galaxies at a redshift of =3.090. The structure is evident in a redshift histogram of photometrically selected ``Lyman break'' objects in a 9' by 18' field in which we have obtained 78 spectroscopic redshifts in the range 2.0 < z <3.4. The dimensions of the structure projected on the plane of the sky are at least 11'by 8', or 14h_{70}^{-1} by 10h_{70}^{-1} Mpc (comoving; Ω_M=1). The concentration contains 15 galaxies and one faint (R=21.7) QSO. We consider the structure in the context of a number of cosmological models and argue that Lyman-break galaxies must be very biased tracers of mass, with an effective bias on mass scale M~10^{15}M_{\sun} ranging from b~2 for Ω_M=0.2 to b >~6 for Ω_M=1. In a Cold Dark Matter scenario the large bias values suggest that individual Lyman-break galaxies are associated with dark halos of mass M~10^{12} M_{\sun}, reinforcing the interpretation of these objects as the progenitors of massive galaxies at the present epoch. Preliminary results of spectroscopy in additional fields suggest that such large structures are common at z~3, with about one similar structure per survey field. The implied space density is consistent with the possibility that we are observing moderately rich clusters of galaxies in their early non-linear evolution. Finally, the spectrum of one of the QSOs discovered in our survey (z_{em} = 3.356) exhibits metal line absorption systems within the 3 redshift bins having the largest number of galaxies in field, z = 2.93, 3.09, and 3.28. These results are the first from an ongoing ``targeted'' redshift survey designed to explore the nature and distribution of star-forming galaxies in the redshift range 2.7 <~ z <~ 3.4.

astro-ph↗

arXiv1996

Circumstellar H-alpha from SN 1994D and future Type Ia supernovae: an observational test of progenitor models

Searching for the presence of circumstellar material is currently the only direct way to discriminate between the different types of possible progenitor systems for Type Ia supernovae. We have therefore looked for narrow H-alpha in a high-resolution spectrum of the normal Type Ia supernova 1994D taken 10 days before maximum and only 6.5 days after explosion. We derive an upper limit of 2.0E-16 erg cm^{-2} s^{-1} for an unresolved emission line at the local H II region velocity. To estimate the limit this puts on wind density, we have made time-dependent photoionization calculations. Assuming spherical symmetry we find an upper limit of the mass loss rate which is roughly 1.5E-5 solar masses per year for a wind speed of 10 km s^{-1}. This limit can exclude only the highest-mass-loss-rate symbiotic systems as progenitors. We discuss the effect of asymmetry and assess the relative merits of early optical, radio and X-ray limits in constraining mass loss from Type Ia progenitors. We find that X-ray observations can probably provide the most useful limits on the progenitor mass loss, while high-resolution optical spectroscopy offers our only chance of actually identifying circumstellar hydrogen.

astro-ph↗

arXiv1996

Spectroscopic Confirmation of a Population of Normal Star-Forming Galaxies at Redshifts z>3

We report the discovery of a substantial population of star--forming galaxies at $3.0 \simlt z \simlt 3.5$. These galaxies have been selected using color criteria sensitive to the presence of a Lyman continuum break superposed on an otherwise very blue far-UV continuum, and then confirmed with deep spectroscopy on the W. M. Keck telescope. The surface density of galaxies brighter than R=25 with 3 < z < 3.5 is $0.4\pm0.07$ galaxies arcmin$^{-2}$, approximately 1.3\% of the deep counts at these magnitudes; this value applies both to ``random'' fields and to fields centered on known QSOs. The corresponding co-moving space density is approximately half that of luminous ($L \simgt L^{\ast}$) present--day galaxies. Our sample of $z > 3$ galaxies is large enough that we can begin to detail the spectroscopic characteristics of the population as a whole. The spectra of the $z>3$ galaxies are remarkably similar to those of nearby star-forming galaxies, the dominant features being strong low--ionization interstellar absorption lines and high--ionization stellar lines, often with P-Cygni profiles characteristic of Wolf-Rayet and O--star winds. Lyman $α$ emission is generally weak ($< 20$ Å rest equivalent width) and is absent for >50% of the galaxies. The star formation rates, measured directly from the far-UV continua, lie in the range 4-25 $h_{50}^{-2}$ M$\sun$ yr$^{-1}$ for $q_0=0.5$. Together with the morphological properties of the $z>3$ galaxy population, which we discuss in a companion paper (Giavalisco \et 1996), all of these findings strongly suggest that we have identified the high-redshift counterparts of the spheroid component of present--day luminous galaxies. In any case, it is clear that massive galaxy formation was already well underway by $z \sim 3.5$. (shortened abstract). arch-ive/yymmnnn

astro-ph↗

arXiv1996

The chemical evolution of galaxies at high redshift

Observations of absorption lines in the spectra of distant QSOs offer a new approach for tracking the evolution of normal galaxies from early epochs to the present day. The damped Ly alpha systems are particularly suitable for measuring the properties of what are likely to be the progenitors of present-day luminous galaxies. We have recently concluded a long-term survey of 30 damped absorbers (including eight from the literature) aimed at measuring the metallicity and dust content of the universe from redshift z = 3.39 to 0.69. The major conclusions are that the epoch of chemical enrichment in galaxies may have begun at z = 2.5-3--corresponding to a look-back time of 14 Gyr--and that at z = 2 the typical metallicity was 1/15 of solar. There is clear evidence for the presence of interstellar dust at z = 2, although several high-redshift galaxies, particularly the most metal-poor, appear to be essentially dust-free. We discuss the nature of the damped Ly alpha galaxies in the light of these and other new results.

astro-ph↗

arXiv1995

An Observational Limit on Circumstellar H-Alpha from Supernova 1994D

We searched for narrow H$α$ in a high-resolution spectrum of SN 1994D taken 10 days before maximum, and found none. We estimate the limit this places on the progenitor mass loss, and find that it is competitive with recent radio limits, and excludes the highest-mass-loss-rate symbiotic systems as possible progenitors of the normal Type Ia SN 1994D.

astro-ph↗