Massimo Stiavelli

Spiral Galaxies with WFPC2. II. The Nuclear Properties of 40 Objects

We report the analysis of Hubble Space Telescope Wide Field Planetary Camera 2 F606W images of 40 spiral galaxies belonging to the sample introduced in Paper I, where 35 other targets were discussed. We describe the optical morphological properties of the new 40 galaxies, derive the surface brightness profiles for 25 of them, and present the results of photometric decompositions of these profiles into a bulge (R1/4 or exponential) and a disk component. The analysis of the enlarged sample of 75 galaxies puts on a statistically more solid ground the main results presented in Paper I: (1) In ≈30% of the galaxies, the inner, morphologically distinct structures have an irregular appearance. Some of these irregular bulges are likely to be currently forming stars. (2) Resolved, central compact sources are detected in about 50% of the galaxies. (3) The central compact sources in galaxies with nuclear star formation are brighter, for similar sizes, than those in non–star-forming galaxies. (4) The luminosity of the compact sources correlates with the total galactic luminosity. Furthermore, the analysis of the enlarged sample of 75 objects shows the following: (a) Several of the nonclassical inner structures are well fitted by an exponential profile. These exponential bulges are typically fainter than R1/4 bulges, for a given total galaxy luminosity and (catalog) Hubble type later than Sab. (b) Irregular/exponential bulges typically host central compact sources. (c) The central sources are present in all types of disk galaxies, starting with systems as early as S0a. About 60% of Sb to Sc galaxies host a central compact source. Many of the galaxies that host compact sources contain a barred structure. (d) Galaxies with apparent nuclear star formation, which also host the brightest compact sources, are preferentially the early- and intermediate-type (S0a–Sb) systems. (e) None of the features depend on environment: isolated and nonisolated galaxies show indistinguishable properties. Independent of the physical nature of the nonclassical inner structures, our main conclusion is that a significant fraction of galaxies classified from the ground as relatively early-type spirals show a rich variety of central properties and little or no morphological/photometric evidence for a smooth, R1/4 law bulge.

Spiral Galaxies with WFPC2. III. Nuclear Cusp Slopes

In this paper, the third of a series dedicated to the investigation of the nuclear properties of spiral galaxies, we (1) model the Wide Field Planetary Camera 2 F606W nuclear surface brightness profiles of 41 spiral galaxies presented in Papers I and II with the analytic law introduced by Lauer et al. and (2) deconvolve these surface brightness profiles and their analytic fits, so as to estimate the nuclear stellar densities of bulges of spiral galaxies. We find that the nuclear stellar cusps (quantified by the average logarithmic slope of the surface brightness profiles within 01–05) are significantly different for R1/4 law and exponential bulges. The former have nuclear properties similar to those of early-type galaxies, i.e., similar values of nuclear cusps for comparable luminosities, and increasingly steeper stellar cusps with decreasing luminosity. By contrast, exponential bulges have (underlying the light contribution from photometrically distinct, central compact sources) comparatively shallower stellar cusps, and likely lower nuclear densities, than R1/4 law bulges.

A NICMOS Search for High-Redshift Elliptical Galaxy Candidates*

We have collected optical follow-up observations from the ground and the Hubble Space Telescope (HST) for 13.74 arcmin2 of archival images taken with the HST Near-Infrared Camera and Multiobject Spectrograph (NICMOS). We use two criteria to select E/S0 galaxy candidates at z 1 and z 1.5 (high-redshift elliptical candidates [HizECs]) based on colors and infrared morphology. The observed surface density of HizECs is significantly smaller than what is predicted assuming the local luminosity function (LF) of E/S0, constant comoving density, and pure passive evolution with a high redshift of formation of the stellar populations (z 5). On the other hand, assuming a low redshift (z 2) of formation, not enough HizECs are predicted. The data are very well matched assuming that a substantial fraction of present-day E/S0s (10%-66%, depending on cosmology and on the local LF of E/S0s) formed at z = 3 or higher and then evolved passively. The rest of the current E/S0 population may have formed at lower redshift or may have been rejected by the selection criteria because of interactions or more recent episodes of star formation (disturbed morphology and/or bluer colors).

Does the fine structure constant really vary in time

We discuss how laboratory experiments can be used to place constraints on possible variations of the fine-structure constant α in the observationally relevant redshift interval z0-5, within a rather general theoretical framework. We find a worst case upper limit for Δα/α of 8×10−6 for z≤5 and Δα/α of 0.9×10−6 for z≤1.6. The derived limits are at variance with the recent findings by Webb et al., who claim an observed variation of Δα/α=-2.6±0.4×10−5 at 1

The UDF05 Follow-up of the HUDF: II. Constraints on Reionization from z-dropout Galaxies

[Abridged] We detect three (plus one less certain) z-dropout sources in two separate fields of our UDF05 HST NICMOS images. These z~7 Lyman-Break Galaxy (LBG) candidates allow us to constrain the Luminosity Function (LF) of the star forming galaxy population at those epochs. By assuming a change in only M* and adopting a linear evolution in redshift, anchored to the measured values at z~6, the best fit evolution coefficient is found to be 0.43+-0.19 mag per unit redshift (0.36+-0.18, if including all four candidates), which provides a value of M*(z=7.2)=-19.7+-0.3. This implies a steady evolution for the LBG LF out to z~7, at the same rate that is observed throughout the z~3 to 6 period. This puts a strong constraint on the star-formation histories of z~6 galaxies, whose ensemble star-formation rate density must be lower by a factor 2 at ~170 Myr before the epoch at which they are observed. In particular, a large fraction of stars in the z~6 LBG population must form at redshifts well above z~7. Extrapolating this steady evolution of the LF out to higher redshifts, we estimate that galaxies would be able to reionize the universe by z~6, provided that the faint-end slope of the z>7 LF steepens to alpha~-1.9, and that faint galaxies, with luminosities below the current detection limits, contribute a substantial fraction of the required ionizing photons. This scenario gives however an integrated optical depth to electron scattering that is ~2sigma below the WMAP-5 measurement. Therefore, altogether, our results indicate that, should galaxies be the primary contributors to reionization, either the currently detected evolution of the galaxy population slows down at z>7, or the LF evolution must be compensated by a decrease in metallicity and a corresponding increase in ionization efficiency at these early epochs.

The L[CLC]y[/CLC]α Forest of the Quasar in the Hubble Deep Field South

The quasar in the Hubble Deep Field South (HDF-S), J2233-606 (zem=2.23), has been observed exhaustively by ground-based telescopes and by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope at low, medium, and high resolution in the spectral interval from 1120 to 10000 A. The combined data give continuous coverage of the Lyα forest from redshift 0.9 to 2.24. This very large baseline represents a unique opportunity to study in detail the distribution of clouds associated with emitting structures in the field of the quasar and in nearby fields already observed as part of the HDF-S campaign. Here we report on the main properties obtained from the large spectroscopic data set that is available for the Lyα clouds in the intermediate-redshift range of 1.20-2.20, where our present knowledge has been complicated by the difficulty in producing good data. The number density is shown to be higher than what is expected by extrapolating the results from both lower and higher redshifts: 63±8 lines with logNH I≥14.0 are found (including metal systems) at z=1.7, compared with the ~40 lines predicted by extrapolating from previous studies. The redshift distribution of the Lyα clouds shows a region spanning z1.383-1.460 (comoving size of 94 h−165 Mpc, Ω0=1) with a low density of absorption lines; we detect five lines in this region, compared with the 16 expected from an average density along the line of sight. The two-point correlation function shows a positive signal up to scales of about 3 h−165 Mpc and an amplitude that is larger for larger H I column densities. The average Doppler parameter is about 27 km s−1, which is comparable to the mean value found at z>3, thus casting doubts on the temperature evolution of the Lyα clouds.

WFPC2 Observations of NGC 454: An Interacting Pair of Galaxies*

We present WFPC2 images in the F450W, F606W and F814W filters of the interacting pair of galaxies NGC 454. Our data indicate that the system is in the early stages of interaction. A population of young star-clusters has formed around the late component, and substantial amounts of gas have sunk into the center of the earlier component, where it has not yet produced significant visible star formation or nuclear activity. We have photometric evidence that the star-clusters have strong line emission, which indicate the presence of a substantial component of hot, massive stars which formed less than 5-10 Myrs ago.

The Hubble Deep Field South Flanking Fields

As part of the Hubble Deep Field South program, a set of shorter two-orbit observations were obtained of the area adjacent to the deep fields. The WFPC2 flanking fields cover a contiguous solid angle of 48 arcmin2. Parallel observations with the STIS and NICMOS instruments produce a patchwork of additional fields with optical and near-infrared (1.6 μm) response. Deeper parallel exposures with WFPC2 and NICMOS were obtained when STIS observed the NICMOS deep field. These deeper fields are offset from the rest, and an extended low surface brightness object is visible in the deeper WFPC2 flanking field. In this data paper, which serves as an archival record of the project, we discuss the observations and data reduction and present SExtractor source catalogs and number counts derived from the data. Number counts are broadly consistent with previous surveys from both ground and space. Among other things, these flanking field observations are useful for defining slit masks for spectroscopic follow-up over a wider area around the deep fields, for studying large-scale structure that extends beyond the deep fields, for future supernova searches, and for number counts and morphological studies, but their ultimate utility will be defined by the astronomical community.

The Lyα Forest of the Quasar in the Hubble Deep Field South

The quasar in the Hubble Deep Field South (HDF-S), J2233-606 (zem=2.23), has been observed exhaustively by ground-based telescopes and by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope at low, medium, and high resolution in the spectral interval from 1120 to 10000 A. The combined data give continuous coverage of the Lyα forest from redshift 0.9 to 2.24. This very large baseline represents a unique opportunity to study in detail the distribution of clouds associated with emitting structures in the field of the quasar and in nearby fields already observed as part of the HDF-S campaign. Here we report on the main properties obtained from the large spectroscopic data set that is available for the Lyα clouds in the intermediate-redshift range of 1.20-2.20, where our present knowledge has been complicated by the difficulty in producing good data. The number density is shown to be higher than what is expected by extrapolating the results from both lower and higher redshifts: 63±8 lines with logNH I≥14.0 are found (including metal systems) at z=1.7, compared with the ~40 lines predicted by extrapolating from previous studies. The redshift distribution of the Lyα clouds shows a region spanning z1.383-1.460 (comoving size of 94 h−165 Mpc, Ω0=1) with a low density of absorption lines; we detect five lines in this region, compared with the 16 expected from an average density along the line of sight. The two-point correlation function shows a positive signal up to scales of about 3 h−165 Mpc and an amplitude that is larger for larger H I column densities. The average Doppler parameter is about 27 km s−1, which is comparable to the mean value found at z>3, thus casting doubts on the temperature evolution of the Lyα clouds.