Upper Mass Cutoff Research Articles

Mass Functions of Giant Molecular Clouds and Young Star Clusters in Six Nearby Galaxies

Abstract We compare the mass functions of young star clusters (ages ≤ 10 Myr) and giant molecular clouds (GMCs) in six galaxies that cover a large range in mass, metallicity, and star formation rate (LMC, M83, M51, NGC 3627, the Antennae, and NGC 3256). We perform maximum-likelihood fits of the Schechter function, , to both populations. We find that most of the GMC and cluster mass functions in our sample are consistent with a pure power-law distribution ( ). M51 is the only galaxy that shows some evidence for an upper cutoff (M *) in both populations. Therefore, physical upper mass cutoffs in populations of both GMCs and clusters may be the exception rather than the rule. When we perform power-law fits, we find a relatively small range of indices β PL = −2.3 ± 0.3 for our GMC sample and β PL = −2.0 ± 0.3 for the cluster sample. This result, that , is consistent with theoretical predictions for cluster formation and suggests that the star formation efficiency is largely independent of mass in the GMCs.

A Galaxy-targeted Search for the Optical Counterpart of the Candidate NS–BH Merger S190814bv with Magellan

Abstract On 2019 August 14 the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo gravitational wave interferometer announced the detection of a binary merger, S190814bv, with a low false alarm rate of about 1 in 1.6 × 1025 yr, a distance of 267 ± 52 Mpc, a 90% (50%) localization region of about 23 (5) deg2, and a probability of being a neutron star–black hole (NS–BH) merger of >99%. The LIGO/Virgo Collaboration (LVC) defines NS–BH such that the lighter binary member has a mass of <3 M ⊙ and the more massive one has >5 M ⊙, and this classification is in principle consistent with a BH–BH merger depending on the actual upper mass cutoff for neutron stars. Additionally, the LVC designated a probability that the merger led to matter outside the final BH remnant of <1%, suggesting that an electromagnetic (EM) counterpart is unlikely. Here we report our optical follow-up observations of S190814bv using the Magellan Baade 6.5 m telescope to target all 96 galaxies in the Galaxy List for the Advanced Detector Era catalog within the 50% localization volume (representing about 70% of the integrated luminosity within this region). No counterpart was identified to a median 3σ limiting magnitude of i = 22.2 (M i ≈ −14.9 mag), comparable to the brightness of the optical counterpart of the binary neutron star merger GW170817 at the distance of S190814bv; similarly, we can rule out an on-axis jet typical of short GRBs. However, we cannot rule out other realistic models, such as a kilonova with only ∼0.01 M ⊙ of lanthanide-rich material, or an off-axis jet with a viewing angle of θ obs ≳ 15°.

Constraining the Black Hole Initial Mass Function with LIGO/Virgo Observations

Abstract Prior to the detection of black holes (BHs) via the gravitational waves (GWs) that they generate at merger, the presence of BHs was inferred in X-ray binaries, mostly via dynamical measurements, with masses in the range between ∼5 and 20 M ⊙. The Laser Interferometer Gravitational-Wave Observatory (LIGO) discovery of the first BHs via GWs was surprising in that the two BHs that merged had masses of and , which are both above the range inferred from X-ray binaries. With 20 BH detections from the first/second observing (O1/O2) runs, the distribution of masses remains generally higher than the X-ray inferred one, while the effective spins are generally lower. This suggests that, at least in part, the GW-detected population might be of dynamical origin rather than produced by the common evolution of field binaries. Here we perform high-resolution N-body simulations of a cluster of isolated BHs with a range of initial mass spectra and upper mass cutoffs, and study the resulting binary mass spectrum resulting from the dynamical interactions. Our clusters have properties that are similar to those of the massive remnants in an OB association ∼10 Myr after formation. We perform a likelihood analysis for each of our dynamically formed binary population against the data from the O1 and O2 LIGO/Virgo runs. We find that an initial mass spectrum M BH ∝ M −2.35 with an upper mass cutoff M max ∼ 50M ⊙ is favored by the data, together with a slight preference for a merger rate that increases with redshift.

Star cluster catalogues for the LEGUS dwarf galaxies

We present the star cluster catalogs for 17 dwarf and irregular galaxies in the $HST$ Treasury Program "Legacy ExtraGalactic UV Survey" (LEGUS). Cluster identification and photometry in this subsample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogs with accurate fluxes due to low cluster statistics. The colors and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogs are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of $-2$ independent of age and global SFR binning. The age distribution declines as a power law, with an index of $\approx-0.80\pm0.15$, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cutoff ($<2\sigma$) in the composite cluster mass function, and can rule out a truncation mass below $\approx10^{4.5}$M$_{\odot}$ but cannot rule out the existence of a truncation at higher masses.

Dissipative properties and isothermal compressibility of hot and dense hadron gas using non-extensive statistics

We evaluate the transport properties such as shear viscosity (eta ), bulk viscosity (zeta ) and their ratios over entropy density (s) for hadronic matter using relativistic non-extensive Boltzmann transport equation (NBTE) in relaxation time approximation (RTA). In NBTE, we argue that the system far from equilibrium may not reach to an equilibrium described by extensive (Boltzmann-Gibbs (BG)) statistics but to a q-equilibrium defined by Tsallis non-extensive statistics after subsequent evolution, where q denotes the degree of non-extensivity. We observe that eta /s and zeta /s decrease rapidly with temperature (T) for various q-values. As q increases, the magnitudes of eta /s and zeta /s decrease with T. We also show the upper mass cutoff dependence of these ratios for a particular q and find that they decrease with the increase in mass cutoff of hadrons. Further, we present the first estimation of isothermal compressibility (kappa _T) using non-extensive Tsallis statistics at finite baryon chemical potential (mu _B). It is observed that, kappa _T changes significantly with the degree of non-extensivity. We also study the squared speed of sound (c_{s}^{2}) as a function of temperature at finite baryon chemical potential for various q and upper mass cutoffs. It is noticed that there is a strong impact of q and mass cutoff on the behaviour of c_{s}^{2}.

Does the Black Hole Merger Rate Evolve with Redshift?

Abstract We explore the ability of gravitational-wave detectors to extract the redshift distribution of binary black hole (BBH) mergers. The evolution of the merger rate across redshifts 0 &lt; z ≲ 1 is directly tied to the formation and evolutionary processes, providing insight regarding the progenitor formation rate together with the distribution of time delays between formation and merger. Because the limiting distance to which BBHs are detected depends on the masses of the binary, the redshift distribution of detected binaries depends on their underlying mass distribution. We therefore consider the mass and redshift distributions simultaneously, and fit the merger rate density, dN/dm 1 dm 2 dz. Our constraints on the mass distribution agree with previously published results, including evidence for an upper mass cutoff at ∼40 M ⊙. Additionally, we show that the current set of six BBH detections are consistent with a merger rate density that is uniform in comoving volume. Although our constraints on the redshift distribution are not yet tight enough to distinguish between BBH formation channels, we show that it will be possible to distinguish between different astrophysically motivated models of the merger rate evolution with ∼100–300 Laser Interferometer Gravitational-Wave Observatory/Virgo detections (to be expected within 2–5 years). Specifically, we will be able to infer whether the formation rate peaks at higher or lower redshifts than the star formation rate, or the typical time delay between formation and merger. Meanwhile, with ∼100 detections, the inferred redshift distribution will place constraints on more exotic scenarios such as modified gravity.

Constraints for the Progenitor Masses of Historic Core-collapse Supernovae

Abstract We age-date the stellar populations associated with 12 historic nearby core-collapse supernovae (CCSNe) and two supernova impostors; from these ages, we infer their initial masses and associated uncertainties. To do this, we have obtained new Hubble Space Telescope imaging covering these CCSNe. Using these images, we measure resolved stellar photometry for the stars surrounding the locations of the SNe. We then fit the color–magnitude distributions of this photometry with stellar evolution models to determine the ages of any young existing populations present. From these age distributions, we infer the most likely progenitor masses for all of the SNe in our sample. We find ages between 4 and 50 Myr, corresponding to masses from 7.5 to 59 solar masses. There were no SNe that lacked a local young population. Our sample contains four SNe Ib/c; their masses have a wide range of values, suggesting that the progenitors of stripped-envelope SNe are binary systems. Both impostors have masses constrained to be ≲7.5 solar masses. In cases with precursor imaging measurements, we find that age-dating and precursor imaging give consistent progenitor masses. This consistency implies that, although the uncertainties for each technique are significantly different, the results of both are reliable to the measured uncertainties. We combine these new measurements with those from our previous work and find that the distribution of 25 core-collapse SNe progenitor masses is consistent with a standard Salpeter power-law mass function, no upper mass cutoff, and an assumed minimum mass for core-collapse of 7.5 M ⊙. The distribution is consistent with a minimum mass &lt;9.5 M ⊙.

The MOSDEF Survey: Direct Observational Constraints on the Ionizing Photon Production Efficiency, ξion, at z ∼ 2

Abstract We combine Hα and Hβ spectroscopic measurements and UV photometry for a sample of 673 galaxies from the MOSDEF survey to constrain hydrogen-ionizing photon production efficiencies ( ) at z = 1.4–2.6. We find , assuming the Calzetti (SMC) curve for the UV dust correction and a scatter of 0.28 dex in the distribution. After accounting for observational uncertainties and variations in dust attenuation, we conclude that the remaining scatter in is likely dominated by galaxy-to-galaxy variations in stellar populations, including the slope and upper-mass cutoff of the initial mass function, stellar metallicity, star formation burstiness, and stellar evolution (e.g., single/binary star evolution). Moreover, is elevated in galaxies with high ionization states (high [O iii]/[O ii]) and low oxygen abundances (low [N ii]/Hα and high [O iii]/Hβ) in the ionized ISM. However, does not correlate with the offset from the z ∼ 0 star-forming locus in the BPT diagram, suggesting no change in the hardness of the ionizing radiation accompanying the offset from the z ∼ 0 sequence. We also find that galaxies with blue UV spectral slopes ( ) have elevated by a factor of ∼2 relative to the average of the sample ( ). If these blue galaxies are similar to those at z &gt; 6, our results suggest that a lower Lyman-continuum escape fraction is required for galaxies to maintain reionization, compared to the canonical predictions from stellar population models. Furthermore, we demonstrate that even with robustly dust-corrected Hα, the UV dust attenuation can cause on average a ∼0.3 dex systematic uncertainty in calculations.

Where Are LIGO’s Big Black Holes?

Abstract In LIGO’s O1 and O2 observational runs, the detectors were sensitive to stellar-mass binary black hole (BBH) coalescences with component masses up to , with binaries with primary masses above representing ≳90% of the total accessible sensitive volume. Nonetheless, of the 5.9 detections (GW150914, LVT151012, GW151226, GW170104, GW170608, and GW170814) reported by LIGO-Virgo, the most massive binary detected was GW150914 with a primary component mass of , far below the detection mass limit. Furthermore, there are theoretical arguments in favor of an upper mass gap, predicting an absence of black holes in the mass range . We argue that the absence of detected binary systems with component masses heavier than may be preliminary evidence for this upper mass gap. By allowing for the presence of a mass gap, we find weaker constraints on the shape of the underlying mass distribution of BBHs. We fit a power-law distribution with an upper mass cutoff to real and simulated BBH mass measurements, finding that the first 3.9 BBHs favor shallow power-law slopes and an upper mass cutoff . This inferred distribution is entirely consistent with the two recently reported detections, GW170608 and GW170814. We show that with ∼10 additional LIGO-Virgo BBH detections, fitting the BH mass distribution will provide strong evidence for an upper mass gap if one exists.

Black hole mass function from gravitational wave measurements

We examine how future gravitational-wave measurements from merging black holes (BHs) can be used to infer the shape of the black-hole mass function, with important implications for the study of star formation and evolution and the properties of binary BHs. We model the mass function as a power law, inherited from the stellar initial mass function, and introduce lower and upper mass cutoff parametrizations in order to probe the minimum and maximum BH masses allowed by stellar evolution, respectively. We initially focus on the heavier BH in each binary, to minimize model dependence. Taking into account the experimental noise, the mass measurement errors and the uncertainty in the redshift dependence of the merger rate, we show that the mass function parameters, as well as the total rate of merger events, can be measured to $&lt;10%$ accuracy within a few years of advanced LIGO observations at its design sensitivity. This can be used to address important open questions such as the upper limit on the stellar mass which allows for BH formation and to confirm or refute the currently observed mass gap between neutron stars and BHs. In order to glean information on the progenitors of the merging BH binaries, we then advocate the study of the two-dimensional mass distribution to constrain parameters that describe the two-body system, such as the mass ratio between the two BHs, in addition to the merger rate and mass function parameters. We argue that several years of data collection can efficiently probe models of binary formation, and show, as an example, that the hypothesis that some gravitational-wave events may involve primordial black holes can be tested. Finally, we point out that in order to maximize the constraining power of the data, it may be worthwhile to lower the signal-to-noise threshold imposed on each candidate event and amass a larger statistical ensemble of BH mergers.

The very massive star content of the nuclear star clusters in NGC 5253

AbstractThe blue compact dwarf galaxy NGC 5253 hosts a very young starburst containing twin nuclear star clusters. Calzetti et al. (2015) find that the two clusters have an age of 1 Myr, in contradiction to the age of 3–5 Myr inferred from the presence of Wolf-Rayet (W-R) spectral features. We use Hubble Space Telescope (HST) far-ultraviolet (FUV) and ground-based optical spectra to show that the cluster stellar features arise from very massive stars (VMS), with masses greater than 100 M⊙, at an age of 1–2 Myr. We discuss the implications of this and show that the very high ionizing flux can only be explained by VMS. We further discuss our findings in the context of VMS contributing to He ii λ1640 emission in high redshift galaxies, and emphasize that population synthesis models with upper mass cut-offs greater than 100 M⊙ are crucial for future studies of young massive clusters.

THE VERY MASSIVE STAR CONTENT OF THE NUCLEAR STAR CLUSTERS IN NGC 5253

ABSTRACT The blue compact dwarf galaxy NGC 5253 hosts a very young starburst containing twin nuclear star clusters, separated by a projected distance of 5 pc. One cluster (#5) coincides with the peak of the Hα emission and the other (#11) with a massive ultracompact H ii region. A recent analysis of these clusters shows that they have a photometric age of 1 ± 1 Myr, in apparent contradiction with the age of 3–5 Myr inferred from the presence of Wolf-Rayet features in the cluster #5 spectrum. We examine Hubble Space Telescope ultraviolet and Very Large Telescope optical spectroscopy of #5 and show that the stellar features arise from very massive stars (VMSs), with masses greater than 100 M ⊙, at an age of 1–2 Myr. We further show that the very high ionizing flux from the nuclear clusters can only be explained if VMSs are present. We investigate the origin of the observed nitrogen enrichment in the circumcluster ionized gas and find that the excess N can be produced by massive rotating stars within the first 1 Myr. We find similarities between the NGC 5253 cluster spectrum and those of metal-poor, high-redshift galaxies. We discuss the presence of VMSs in young, star-forming galaxies at high redshift; these should be detected in rest-frame UV spectra to be obtained with the James Webb Space Telescope. We emphasize that population synthesis models with upper mass cutoffs greater than 100 M ⊙ are crucial for future studies of young massive star clusters at all redshifts.

Modern yields per stellar generation: the effect of the IMF

Gaseous and stellar metallicities in galaxies are nowadays routinely used to constrain the evolutionary processes in galaxies. This requires the knowledge of the average yield per stellar generation, $y_{\text{Z}}$, i.e. the quantity of metals that a stellar population releases into the interstellar medium (ISM), which is generally assumed to be a fixed fiducial value. Deviations of the observed metallicity from the expected value of $y_{\text{Z}}$ are used to quantify the effect of outflows or inflows of gas, or even as evidence for biased metallicity calibrations or inaccurate metallicity diagnostics. Here we show that $\rm y_{\text{Z}}$ depends significantly on the Initial Mass Function (IMF), varying by up to a factor larger than three, for the range of IMFs typically adopted in various studies. Varying the upper mass cutoff of the IMF implies a further variation of $y_{\text{Z}}$ by an additional factor that can be larger than two. These effects, along with the variation of the gas mass fraction restored into the ISM by supernovae ($R$, which also depends on the IMF), may yield to deceiving results, if not properly taken into account. In particular, metallicities that are often considered unusually high can actually be explained in terms of yield associated with commonly adopted IMFs such as the Kroupa (2001) or Chabrier (2003). We provide our results for two different sets of stellar yields (both affected by specific limitations) finding that the uncertainty introduced by this assumption can be as large as $\sim0.2$ dex. Finally, we show that $y_{\text{Z}}$ is not substantially affected by the initial stellar metallicity as long as $\text{Z}> 10^{-3}~\text{Z}_{\odot}$.

CONSTRAINTS FOR THE PROGENITOR MASSES OF 17 HISTORIC CORE-COLLAPSE SUPERNOVAE

Using resolved stellar photometry measured from archival HST imaging, we generate color-magnitude diagrams of the stars within 50 pc of the locations of historic core-collapse supernovae that took place in galaxies within 8 Mpc. We fit these color-magnitude distributions with stellar evolution models to determine the best-fit age distribution of the young population. We then translate these age distributions into probability distributions for the progenitor mass of each SNe. The measurements are anchored by the main-sequence stars surrounding the event, making them less sensitive to assumptions about binarity, post-main-sequence evolution, or circumstellar dust. We demonstrate that, in cases where the literature contains masses that have been measured from direct imaging, our measurements are consistent with (but less precise than) these measurements. Using this technique, we constrain the progenitor masses of 17 historic SNe, 11 of which have no previous estimates from direct imaging. Our measurements still allow the possibility that all SNe progenitor masses are <20 M_sun. However, the large uncertainties for the highest-mass progenitors also allow the possibility of no upper-mass cutoff.

PROBING THE STAR FORMATION HISTORY AND INITIAL MASS FUNCTION OF THEz∼ 2.5 LENSED GALAXY SMM J163554.2+661225 WITHHERSCHEL

We present the analysis of Herschel SPIRE far-infrared (FIR) observations of the z = 2.515 lensed galaxy SMM J163554.2+661225. Combining new 250, 350, and 500 micron observations with existing data, we make an improved fit to the FIR spectral energy distribution (SED) of this galaxy. We find a total infrared (IR) luminosity of L(8--1000 micron) = 6.9 +/- 0.6x10^11 Lsol; a factor of 3 more precise over previous L_IR estimates for this galaxy, and one of the most accurate measurements for any galaxy at these redshifts. This FIR luminosity implies an unlensed star formation rate (SFR) for this galaxy of 119 +/- 10 Msol per yr, which is a factor of 1.9 +/- 0.35 lower than the SFR derived from the nebular Pa-alpha emission line (a 2.5-sigma discrepancy). Both SFR indicators assume identical Salpeter initial mass functions (IMF) with slope Gamma=2.35 over a mass range of 0.1 - 100 Msol, thus this discrepancy suggests that more ionizing photons may be necessary to account for the higher Pa-alpha-derived SFR. We examine a number of scenarios and find that the observations can be explained with a varying star formation history (SFH) due to an increasing star formation rate (SFR), paired with a slight flattening of the IMF. If the SFR is constant in time, then larger changes need to be made to the IMF by either increasing the upper-mass cutoff to ~ 200 Msol, or a flattening of the IMF slope to 1.9 +/- 0.15, or a combination of the two. These scenarios result in up to double the number of stars with masses above 20 Msol, which produce the requisite increase in ionizing photons over a Salpeter IMF with a constant SFH.

THE SPATIAL CLUSTERING OFROSATALL-SKY SURVEY AGNs. II. HALO OCCUPATION DISTRIBUTION MODELING OF THE CROSS-CORRELATION FUNCTION

This is the second paper of a series that reports on our investigation of the clustering properties of AGNs in the ROSAT All-Sky Survey (RASS) through cross-correlation functions (CCFs) with Sloan Digital Sky Survey (SDSS) galaxies. In this paper, we apply the Halo Occupation Distribution (HOD) model to the CCFs between the RASS Broad-line AGNs with SDSS Luminous Red Galaxies (LRGs) in the redshift range 0.16<z<0.36 that was calculated in paper I. In our HOD modeling approach, we use the known HOD of LRGs and constrain the HOD of the AGNs by a model fit to the CCF. For the first time, we are able to go beyond quoting merely a `typical' AGN host halo mass, M_h, and model the full distribution function of AGN host dark matter halos. In addition, we are able to determine the large-scale bias and the mean M_h more accurately. We explore the behavior of three simple HOD models. Our first model (Model A) is a truncated power-law HOD model in which all AGNs are satellites. With this model, we find an upper limit to the slope (\alpha) of the AGN HOD that is far below unity. The other two models have a central component, which has a step function form, where the HOD is constant above a minimum mass, without (Model B) or with (Model C) an upper mass cutoff, in addition to the truncated power-law satellite component, similar to the HOD that is found for galaxies. In these two models we find the upper limits of \alpha < 0.95 and \alpha < 0.84 for Model B and C respectively. Our analysis suggests that the satellite AGN occupation increases slower than, or may even decrease with, M_h, in contrast to the satellite's HODs of luminosity-threshold samples of galaxies, which, in contrast, grow approximately as \propto M_h^\alpha with \alpha\approx 1. These results are consistent with observations that the AGN fraction in groups and clusters decreases with richness.

Evidence for a Nonuniversal Stellar Initial Mass Function from the Integrated Properties of SDSS Galaxies

This paper revisits the classical Kennicutt method for inferring the stellar initial mass function (IMF) from the integrated light properties of galaxies. The large size, uniform high quality data set from the Sloan Digital Sky Survey DR4 is combined with more in depth modeling and quantitative statistical analysis to search for systematic IMF variations as a function of galaxy luminosity. Galaxy H alpha equivalent widths are compared to a broadband color index to constrain the IMF. This parameter space is useful for breaking degeneracies which are traditionally problematic. Age and dust corrections are largely orthogonal to IMF variations. In addition the effects of metallicity and smooth star formation history e-folding times are small compared to IMF variations. We find that for the sample as a whole the best fitting IMF slope above 0.5 M_sun is Gamma = 1.4535 with a negligible random error of +/- 0.0004 and a systematic error of +/- 0.1. Galaxies brighter than around M_r,0.1 = -20 (including galaxies like the Milky Way which has M_r,0.1 ~ -21) are well fit by a universal Gamma ~ 1.4 IMF, similar to Salpeter, and smooth, exponential star formation histories (SFH). Fainter galaxies prefer steeper IMFs and the quality of the fits reveal that for these galaxies a universal IMF with smooth SFHs is actually a poor assumption. Several sources of sample bias are ruled out as the cause of these luminosity dependent IMF variations. Analysis of bursting SFH models shows that an implausible coordination of burst times is required to fit a universal IMF to the M_r,0.1 = -17 galaxies. This leads to the conclusions that the IMF in low luminosity galaxies has fewer massive stars, either by steeper slope or lower upper mass cutoff, and is not universal.

The Extraordinary Infrared Spectrum of NGC 1222 (Markarian 603)

The infrared spectra of starburst galaxies are dominated by low-excitation lines of [Ne II] and [S III], and the stellar populations deduced from these spectra appear to lack stars larger than about 35 M⊙. The only exceptions to this result until now were low-metallicity dwarf galaxies. We report our analysis of the mid-infrared spectra obtained with the Infrared Spectrograph on Spitzer of the starburst galaxy NGC 1222 (Mrk 603). NGC 1222 is a large spheroidal galaxy with a starburst nucleus that is a compact radio and infrared source, and its infrared emission is dominated by the [Ne III] line. This is the first starburst of solar or near-solar metallicity known to us that is dominated by high-excitation lines and is a likely host of high-mass stars. We model the emission with several different assumptions as to the spatial distribution of high- and low-excitation lines and find that the upper mass cutoff in this galaxy is 40-100 M⊙.

The Cool ISM in Elliptical Galaxies. I. A Survey of Molecular Gas

We present preliminary results from a survey of CO emission from members of a volume-limited sample of noncluster elliptical galaxies. Our intent is to compare the gas properties of these ellipticals to a sample of lenticulars selected using similar criteria. The data, although still sparse, suggest that the cool gas in ellipticals shows the same puzzling upper mass cutoff found in the lenticular galaxies. We find, however, significantly lower detection rates and possibly much lower H2/Himass ratios in the ellipticals. The detection rate is higher among the lower mass galaxies, ashasbeenfoundpreviously.Thisseemspuzzlinggiventhat thedeeperpotentialwells of thelargergalaxies oughtto make gas retention easier, but perhaps that effect is overwhelmed by feedback from the central supermassive black hole. As we have observed � 40% of our full sample, the conclusions are necessarily tentative at this time. Subject headingg galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: ISM

The Cool ISM in S0 Galaxies. II. A Survey of Atomic Gas

The place of lenticular galaxies within the range of types of galaxies remains unclear. We previously reported the mass of molecular hydrogen for a volume-limited sample of lenticular galaxies, where we saw that the amount of gas was less than that predicted by the return of stellar mass to the interstellar medium. Here we report observations of atomic hydrogen (H I) for the same sample. Detections in several galaxies make more compelling the case presented in our earlier paper that the mass of cool gas in S0 galaxies cuts off at ~10% of what is expected from current models of gas return from stellar evolution. The molecular and atomic phases of the gas in our sample galaxies appear to be separate and distinct, both spatially and in velocity space. We propose that the molecular gas arises mostly from the stellar mass returned to the galaxy, while the atomic hydrogen is mainly accumulated from external sources (infall, captured dwarfs, etc.). While this proposal fits most of the observations, it makes the presence of the upper mass cutoff even more mysterious.