arXiv subjects

M. Stiavelli

Publications and source records attributed to M. Stiavelli.

At least 55 records · Page 3Linked to original sources

arXiv2010

The clustering properties of the first galaxies

We study the clustering properties of the first galaxies formed in the Universe. We find that, due to chemical enrichment of the inter-stellar medium by isolated Population III stars formed in mini-halos at redshift z>30, the (chronologically) first galaxies are composed of metal-poor Population II stars and are highly clustered on small scales. In contrast, chemically pristine galaxies in halos with mass M~10^8 M_sun may form at z<20 in relatively underdense regions of the Universe. This occurs once self-enrichment by Population III in mini-halos is quenched by the build-up of an $H_2$ photo-dissociating radiative background in the Lyman-Werner bands. We find that these chemically pristine galaxies are spatially uncorrelated. Thus, we expect that deep fields with the James Webb Space Telescope may detect clusters of chemically enriched galaxies but individual chemically pristine objects. We predict that metal-free galaxies at 10 <= z <= 15$ have surface densities of about 80 per square arcmin and per unit redshift but most of them will be too faint even for JWST. However, the predicted density makes these objects interesting targets for searches behind lensing clusters.

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arXiv2010

Star formation rates and stellar masses of z=7-8 galaxies from IRAC observations of the WFC3/IR ERS and the HUDF field

We investigate the Spitzer/IRAC properties of 36 z~7 z-dropout galaxies and 3 z~8 Y-dropout galaxies derived from deep/wide-area WFC3/IR data of the Early Release Science, the ultradeep HUDF09, and wide-area NICMOS data. We fit stellar population synthesis models to the SEDs to derive mean redshifts, stellar masses, and ages. The z~7 galaxies are best characterized by substantial ages (>100 Myr) and M/L_V~0.2. The main trend with decreasing luminosity is that of bluing of the far-UV slope from beta~-2.0 to beta~-3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1 L*{z=3}), where low metallicity and smooth SFHs fail to match the blue far-UV and moderately red H-[3.6] color. Such colors may require episodic SFHs with short periods of activity and quiescence ("on-off" cycles) and/or a contribution from emission lines. The stellar mass of our sample of z~7 star forming galaxies correlates with SFR according to log M* = 8.70 (+-0.09) + 1.06(+-0.10) log SFR, implying star formation may have commenced at z>10. No galaxies are found with SFRs much higher or lower than the past averaged SFR suggesting that the typical star formation timescales are probably a substantial fraction of the Hubble time. We report the first IRAC detection of Y-dropout galaxies at z~8. The average rest-frame U-V=0.3 (AB) of the 3 galaxies are similar to faint z~7 galaxies, implying similar M/L. The stellar mass density to M_{UV,AB}<-18 is rho*(z=8) = 1.8^{+0.7}_{-1.0} x 10^6 M_sun Mpc^-3, following log rho*(z)= 10.6(+-0.6) - 4.4(+-0.7) log(1+z) [M_sun Mpc^-3] over 3<z< 8.

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arXiv2010

The Galaxy Luminosity Function during the Reionization Epoch

The new Wide Field Camera 3/IR observations on the Hubble Ultra-Deep Field started investigating the properties of galaxies during the reionization epoch. To interpret these observations, we present a novel approach inspired by the conditional luminosity function method. We calibrate our model to observations at z=6 and assume a non-evolving galaxy luminosity versus halo mass relation. We first compare model predictions against the luminosity function measured at z=5 and z=4. We then predict the luminosity function at z>=7 under the sole assumption of evolution in the underlying dark-matter halo mass function. Our model is consistent with the observed z>6.5 galaxy number counts in the HUDF survey and suggests a possible steepening of the faint-end slope of the luminosity function: alpha(z>8)< -1.9 compared to alpha=-1.74 at z=6. Although we currently see only the brightest galaxies, a hidden population of lower luminosity objects (L/L_{*}> 10^{-4}) might provide >75% of the total reionizing flux. Assuming escape fraction f_{esc}~0.2, clumping factor C~5, top heavy-IMF and low metallicity, galaxies below the detection limit produce complete reionization at z>8. For solar metallicity and normal stellar IMF, reionization finishes at z>6, but a smaller C/f_{esc} is required for an optical depth consistent with the WMAP measurement. Our model highlights that the star formation rate in sub-L_* galaxies has a quasi-linear relation to dark-matter halo mass, suggesting that radiative and mechanical feedback were less effective at z>6 than today.

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arXiv2010

Very blue UV-continuum slopes of low luminosity z~7 galaxies from WFC3/IR: Evidence for extremely low metallicities?

We use the ultra-deep WFC3/IR data over the HUDF and the Early Release Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-forming galaxies at z~7. We determine the UV-continuum slope beta in these galaxies, and compare the slopes with galaxies at later times to measure the evolution in beta. For luminous L*(z=3) galaxies, we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to the beta~-2 derived at similar luminosities at z~5-6. However, for the lower luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is substantially bluer than is found for similar luminosity galaxies at z~4, just 800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can be matched by a very young, dust-free stellar population, but when nebular emission is included the expected beta becomes >~-2.7. To produce these very blue beta's (i.e., beta~-3), extremely low metallicities and mechanisms to reduce the red nebular emission are likely required. For example, a large escape fraction (i.e., f_{esc}>~0.3) could minimize the contribution from this red nebular emission. If this is correct and the escape fraction in faint z~7 galaxies is >~0.3, it may help to explain how galaxies reionize the universe.

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arXiv2009

Discovery of z~8 Galaxies in the HUDF from ultra-deep WFC3/IR Observations

We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDF to search for star-forming galaxies at z~8-8.5, only 600 million years from recombination, using a Y_{105}-dropout selection. The new 4.7 arcmin**2 WFC3/IR observations reach to ~28.8 AB mag (5 sigma) in the Y_{105}J_{125}H_{160} bands. These remarkable data reach ~1.5 AB mag deeper than the previous data over the HUDF, and now are an excellent match to the HUDF optical ACS data. For our search criteria, we use a two-color Lyman-Break selection technique to identify z~8-8.5 Y_{105}-dropouts. We find 5 likely z~8-8.5 candidates. The sources have H_{160}-band magnitudes of ~28.3 AB mag and very blue UV-continuum slopes, with a median estimated beta of <~-2.5 (where f_λ\propto λ^β). This suggests that z~8 galaxies are not only essentially dust free but also may have very young ages or low metallicities. The observed number of Y_{105}-dropout candidates is smaller than the 20+/-6 sources expected assuming no evolution from z~6, but is consistent with the 5 expected extrapolating the Bouwens et al. 2008 LF results to z~8. These results provide evidence that the evolution in the LF seen from z~7 to z~3 continues to z~8. The remarkable improvement in the sensitivity of WFC3/IR has enabled HST to cross a threshold, revealing star-forming galaxies at z~8-9.

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arXiv2009

Ultradeep Infrared Array Camera Observations of sub-L* z~7 and z~8 Galaxies in the Hubble Ultra Deep Field: the Contribution of Low-Luminosity Galaxies to the Stellar Mass Density and Reionization

We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z=7 z_{850}-dropout galaxies and 5 z=8 Y_{105}-dropout galaxies. The WFC3/IR depth and spatial resolution allow accurate removal of contaminating foreground light, enabling reliable flux measurements at 3.6 micron and 4.5 micron. None of the galaxies are detected to [3.6]=26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detection for the z_{850}-dropouts and an upper limit for the Y_{105}-dropouts. We construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. For the z_{850}-dropouts, we find z=6.9^{+0.1}_{-0.1}, (U-V)_{rest}=0.4, reddening A_V=0, stellar mass M*=1.2^{+0.3}_{-0.6} x 10^9 M_sun (Salpeter IMF). The best-fit ages ~300Myr, M/L_V=0.2, and SSFR=1.7Gyr^{-1} are similar to values reported for luminous z=7 galaxies, indicating the galaxies are smaller but not younger. The sub-L* galaxies observed here contribute significantly to the stellar mass density and under favorable conditions may have provided enough photons for sustained reionization at 7<z<11. In contrast, the z=8.3^{+0.1}_{-0.2} Y_{105}-dropouts have stellar masses that are uncertain by 1.5 dex due to the near-complete reliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), the stellar mass density to M_{UV,AB} < -18 declines from rho*(z=7)=3.7^{+1.0}_{-1.8} x 10^6 M_sun Mpc^{-3} to rho*(z=8) < 8 x 10^5 M_sun Mpc^{-3}, following (1+z)^{-6} over 3<z<8. Lower masses at z=8 would signify more dramatic evolution, which can be established with deeper IRAC observations, long before the arrival of the James Webb Space Telescope.

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arXiv2009

z~7 Galaxies in the HUDF: First Epoch WFC3/IR Results

We present a sample of 16 robust z~7 z-drop galaxies detected by the newly installed WFC3/IR camera on the Hubble Space Telescope. Our analysis is based on the first epoch data of the HUDF09 program covering the Hubble Ultra Deep Field with 60 orbits of Y, J, and H observations. These remarkable data cover 4.7 arcmin^2 and are the deepest NIR images ever taken, reaching to ~29 mag AB (5 sigma). The 16 z~6.5-7.5 galaxies have been identified based on the Lyman Break technique utilizing (z-Y) vs. (Y-J) colors. They have magnitudes J = 26.0-29.0 (AB), an average apparent half-light radius of ~0.16 arcsec (<~1 kpc), and show very blue colors (some even beta<~-2.5), in particular at low luminosities. The WFC3/IR data confirms previous NICMOS detections indicating that the dropout selection at z~7 is very reliable. Our data allow a first determination of the faint end slope of the z~7 luminosity function, reaching down to M_UV ~ -18, a full magnitude fainter than previous measurements. When fixing phi*=1.4e-3/Mpc^3/mag to the value previously measured at z~6, we find a best-fit value of alpha=-1.77+-0.20, with a characteristic luminosity of M_*=-19.91+-0.09. This steep slope is similar to what is seen at z~2-6 and indicates that low luminosity galaxies could potentially provide adequate flux to reionize the universe. The remarkable depth and resolution of these new images provide insights into the coming power of JWST.

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arXiv2009

Structure and Morphologies of z~7-8 Galaxies from ultra-deep WFC3/IR Imaging of the HUDF

We present a first morphological study of z~7-8 Lyman Break galaxies (LBGs) from Oesch et al. 2009 and Bouwens et al. 2009 detected in ultra-deep near-infrared imaging of the Hubble Ultra Deep field (HUDF) by the HUDF09 program. With an average intrinsic size of 0.7+-0.3 kpc these galaxies are found to be extremely compact having an average observed surface brightness of mu_J ~= 26 mag arcsec^(-2), and only two out of the full sample of 16 z~7 galaxies show extended features with resolved double cores. By comparison to lower redshift LBGs it is found that only little size evolution takes place from z~7 to z~6, while galaxies between z~4-5 show more extended wings in their apparent profiles. The average size scales as (1+z)^(-m) with m=1.12+-0.17 for galaxies with luminosities in the range (0.3-1)L*_{z=3} and with m=1.32+-0.52 for (0.12-0.3)L*_{z=3}, consistent with galaxies having constant comoving sizes. The peak of the size distribution changes only slowly from z~7 to z~4. However, a tail of larger galaxies (>~ 1.2 kpc) is gradually built up towards later cosmic times, possibly via hierarchical build-up or via enhanced accretion of cold gas. Additionally, the average star-formation surface density of LBGs with luminosities (0.3-1)L*_{z=3} is nearly constant at Sigma_{SFR}=1.9 Msun/yr/kpc^2 over the entire redshift range z~4-7 suggesting similar star-formation efficiencies at these early epochs. The above evolutionary trends seem to hold out to z~8 though the sample is still small and possibly incomplete.

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arXiv2009

The Growth of Supermassive Black Holes Across Cosmic Time

One of the main themes in extragalactic astronomy for the next decade will be the evolution of galaxies over cosmic time. Many future observatories, including JWST, ALMA, GMT, TMT and E-ELT will intensively observe starlight over a broad redshift range, out to the dawn of the modern Universe when the first galaxies formed. It has, however, become clear that the properties and evolution of galaxies are intimately linked to the growth of their central black holes. Understanding the formation of galaxies, and their subsequent evolution, will therefore be incomplete without similarly intensive observations of the accretion light from supermassive black holes (SMBH) in galactic nuclei. To make further progress, we need to chart the formation of typical SMBH at z>6, and their subsequent growth over cosmic time, which is most effectively achieved with X-ray observations. Recent technological developments in X-ray optics and instrumentation now bring this within our grasp, enabling capabilities fully matched to those expected from flagship observatories at longer wavelengths.

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arXiv2009

The Formation Rates of Population III Stars and Chemical Enrichment of Halos during the Reionization Era

[abridged] The First Stars in the Universe form out of pristine primordial gas clouds that have been radiatively cooled to a few hundreds of degrees Kelvin either via molecular or atomic (Lyman-Alpha) hydrogen lines. This primordial mode of star formation is eventually quenched once radiative and/or chemical (metal enrichment) feedbacks mark the transition to Population II stars. In this paper we present a model for the formation rate of Population III stars based on Press-Schechter modeling coupled with analytical recipes for gas cooling and radiative feedback. Our model also includes a novel treatment for metal pollution based on self-enrichment due to a previous episode of Population III star formation in progenitor halos. With this model we derive the star formation history of Population III stars, their contribution to the re-ionization of the Universe and the time of the transition from Population III star formation in minihalos to that in more massive halos where atomic hydrogen cooling is also possible. We consider a grid of models highlighting the impact of varying the values for the free parameters used, such as star formation and feedback efficiency. The most critical factor is the assumption that only one Population III star is formed in a halo. In this scenario, metal free stars contribute only to a minor fraction of the total number of photons required to re-ionize the universe. In addition, metal free star formation is primarily located in minihalos and chemically enriched halos become the dominant locus of star formation very early in the life of the Universe, at redshift z~25. If instead multiple metal free stars are allowed to form out of a single halo, then there is an overall boost of Population III star formation, with a consequent significant contribution to the re-ionizing radiation budget.

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arXiv2008

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.

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arXiv2008

Early-type galaxies in the near-infrared: 1.5-2.4\mum spectroscopy

Near-infrared (hereafter NIR) data may provide complementary information to the traditional optical population synthesis analysis of unresolved stellar populations because the spectral energy distribution of the galaxies in the 1-2.5\mum range is dominated by different types of stars than at optical wavelengths. Furthermore, NIR data are subjected to less absorption and hence could constrain the stellar populations in dust-obscured galaxies. We want to develop observational constraints on the stellar populations of unresolved stellar systems in the NIR. To achieve this goal we need a benchmark sample of NIR spectra of ``simple'' early-type galaxies, to be used for testing and calibrating the outputs of population synthesis models. We obtained low-resolution (R~1000) long-slit spectra between 1.5 and 2.4\mum for 14 nearby early-type galaxies using SofI at NTT and higher resolution (R~3000) long-slit spectra, centered at the MgI at ~1.51\mum for a heterogeneous sample of 5 nearby galaxies observed with ISAAC at VLT. We defined spectral indices for CO, NaI, CaI and MgI features and measured the strengths of these features in the sample galaxies. We defined a new global NIR metallicity index, suitable for abundance measurements in low-resolution spectra. Finally, we present an average NIR spectrum of an early-type galaxy, built from a homogenized subset of our sample. The NIR spectra of the sample galaxies show great similarity and the strength of some features does correlate with the iron abundance [Fe/H] and optical metal features of the galaxies. The data suggest that the NIR metal features, in combination with a hydrogen absorption feature may be able to break the age-metallicity degeneracy just like the Mg and Fe features in the optical wavelength range.

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arXiv2008

Where can we really find the First Stars' Remnants today?

A number of recent numerical investigations concluded that the remnants of rare structures formed at very high redshift, such as the very first stars and bright redshift z~6 QSOs, are preferentially located at the center of the most massive galaxy clusters at redshift z=0. In this paper we readdress this question using a combination of cosmological simulations of structure formation and extended Press-Schechter formalism and we show that the typical remnants of Population III stars are instead more likely to be found in a group environment, that is in dark matter halos of mass ~2x10^{13} h^{-1}M_sun. Similarly, the descendants of the brightest z~6 QSOs are expected to be in medium-sized clusters (mass of a few 10^{14} h^{-1}M_sun), rather than in the most massive superclusters (M>10^{15} h^{-1}M_sun) found within the typical 1 Gpc^3 cosmic volume where a bright z~6 QSO lives. The origin of past claims that the most massive clusters preferentially host these remnants is rooted in the numerical method used to initialize their numerical simulations: Only a small region of the cosmological volume of interest was simulated with sufficient resolution to identify low-mass halos at early times, and this region was chosen to host the most massive halo in the cosmological volume at late times. The conclusion that the earliest structures formed in the entire cosmological volume evolve into the most massive halo at late times was thus arrived at by construction. We demonstrate that, to the contrary, the first structures to form in a cosmological region evolve into relatively typical objects at later times. We propose alternative numerical methods for simulating the earliest structures in cosmological volumes.

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arXiv2008

SPACE: the SPectroscopic All-sky Cosmic Explorer

We describe the scientific motivations, the mission concept and the instrumentation of SPACE, a class-M mission proposed for concept study at the first call of the ESA Cosmic-Vision 2015-2025 planning cycle. SPACE aims to produce the largest three-dimensional evolutionary map of the Universe over the past 10 billion years by taking near-IR spectra and measuring redshifts for more than half a billion galaxies at 0<z<2 down to AB~23 over 3πsr of the sky. In addition, SPACE will also target a smaller sky field, performing a deep spectroscopic survey of millions of galaxies to AB~26 and at 2<z<10+. These goals are unreachable with ground-based observations due to the ~500 times higher sky background. To achieve the main science objectives, SPACE will use a 1.5m diameter Ritchey-Chretien telescope equipped with a set of arrays of Digital Micro-mirror Devices (DMDs) covering a total field of view of 0.4 deg2, and will perform large-multiplexing multi-object spectroscopy (e.g. ~6000 targets per pointing) at a spectral resolution of R~400 as well as diffraction-limited imaging with continuous coverage from 0.8mum to 1.8mum.

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arXiv2007

The very first Pop III stars and their relation to bright z~6 quasars

We discuss the link between dark matter halos hosting the first PopIII stars formed at redshift z > 40 and the rare, massive, halos that are generally considered to host bright z~6 quasars. We show that within the typical volume occupied by one bright high-z QSO the remnants of the first several thousands PopIII stars formed do not end up in the most massive halos at z~6, but rather live in a large variety of environments. The black hole seeds planted by these very first PopIII stars can easily grow to M > 10^{9.5} Msun by z=6 assuming Eddington accretion with radiative efficiency epsilon~0.1. Therefore quenching of the accretion is crucial to avoid an overabundance of supermassive black holes. We implement a simple feedback model for the growth of the seeds planted by PopIII stars and obtain a z~6 BH mass function consistent with the observed QSO luminosity function.

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arXiv2007

The UDF05 Follow-up of the HUDF: I. The Faint-End Slope of the Lyman-Break Galaxy Population at z~5

We present the UDF05 project, a HST Large Program of deep ACS (F606W, F775W, F850LP) and NICMOS (F110W, F160W) imaging of three fields, two of which coincide with the NICP1-4 NICMOS parallel observations of the Hubble Ultra Deep Field (HUDF). In this first paper we use the ACS data for the NICP12 field, as well as the original HUDF ACS data, to measure the UV Luminosity Function (LF) of z~5 Lyman Break Galaxies (LBGs) down to very faint levels. Specifically, based on a V-i, i-z selection criterion, we identify a sample of 101 and 133 candidate z~5 galaxies down to z_{850}=28.5 and 29.25 magnitudes in the NICP12 and in the HUDF fields, respectively. Using an extensive set of Monte Carlo simulations we derive corrections for observational biases and selection effects, and construct the rest-frame 1400 A LBG LF over the range M_{1400}=[-21.4, -17.1], i.e. down to ~0.04 L* at z~5, and complement it with data from the Subaru Deep Field (SDF) from Yoshida et al. (2006) to extend it to the brighter end (M_{1400}>-22.2). We show that: (i) Different assumptions regarding the SED distribution of the LBG population, dust properties and intergalactic absorption result in a 25% variation in the number density of LBGs at z~5; (ii) Under consistent assumptions for dust properties and intergalactic absorption, the HUDF is about 30% under-dense in z~5 LBGs relative to the NICP12 field, a variation which is well explained by cosmic variance; (iii) The faint-end slope of the LF is independent of the specific assumptions for the input physical parameters, and has a value of alpha ~ -1.6, similar to the faint-end slope of the LF that has been measured for LBGs at z~3 and z~6.

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arXiv2007

Distribution of the very first PopIII stars and their relation to bright z~6 quasars

We discuss the link between dark matter halos hosting the first PopIII stars and the rare, massive, halos that are generally considered to host bright quasars at high redshift z~6. The main question that we intend to answer is whether the super-massive black holes powering these QSOs grew out from the seeds planted by the first intermediate massive black holes created in the universe. This question involves a dynamical range of 10^13 in mass and we address it by combining N-body simulations of structure formation to identify the most massive halos at z~6 with a Monte Carlo method based on linear theory to obtain the location and formation times of the first light halos within the whole simulation box. We show that the descendants of the first ~10^6 Msun virialized halos do not, on average, end up in the most massive halos at z~6, but rather live in a large variety of environments. The oldest PopIII progenitors of the most massive halos at z~6, form instead from density peaks that are on average one and a half standard deviations more common than the first PopIII star formed in the volume occupied by one bright high-z QSO. The intermediate mass black hole seeds planted by the very first PopIII stars at z>40 can easily grow to masses m_BH>10^9.5 Msun by z=6 assuming Eddington accretion with radiative efficiency ε~0.1. Quenching of the black hole accretion is therefore crucial to avoid an overabundance of supermassive black holes at lower redshift. This can be obtained if the mass accretion is limited to a fraction η~6*10^{-3} of the total baryon mass of the halo hosting the black hole. The resulting high end slope of the black hole mass function at z=6 is α~ -3.7, a value within the 1σerror bar for the bright end slope of the observed quasar luminosity function at z=6.

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arXiv2006

Old and young bulges in late-type disk galaxies

ABRIDGED: We use HSTACS and NICMOS imaging to study the structure and colors of a sample of nine late-type spirals. We find: (1) A correlation between bulge and disks scale-lengths, and a correlation between the colors of the bulges and those of the inner disks. Our data show a trend for bulges to be more metal-enriched than their surrounding disks, but otherwise no simple age-metallicity connection between these systems; (2) A large range in bulge stellar population properties, and, in particular, in stellar ages. Specifically, in about a half of the late-type bulges in our sample the bulk of the stellar mass was produced recently. Thus, in a substantial fraction of the z=0 disk-dominated bulged galaxies, bulge formation occurs after the formation/accretion of the disk; (3) In about a half of the late-type bulges in our sample, however, the bulk of the stellar mass was produced at early epochs; (4) Even these "old" late-type bulges host a significant fraction of stellar mass in a young(er) c component; (5) A correlation for bulges between stellar age and stellar mass, in the sense that more massive late-type bulges are older than less massive late-type bulges. Since the overall galaxy luminosity (mass) also correlates with the bulge luminosity (mass), it appears that the galaxy mass regulates not only what fraction of itself ends up in the bulge component, but also "when" bulge formation takes place. We show that dynamical friction of massive clumps in gas-rich disks is a plausible disk-driven mode for the formation of "old" late-type bulges. If disk evolutionary processes are responsible for the formation of the entire family of late-type bulges, CDM simulations need to produce a similar number of initially bulgeless disks in addition to the disk galaxies that are observed to be bulgeless at z=0.

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