Elliptical Population Research Articles

Performance Analysis of Tyler's Covariance Estimator

This paper analyzes the performance of Tyler's M-estimator of the scatter matrix in elliptical populations. We focus on the non-asymptotic setting and derive the estimation error bounds depending on the number of samples n and the dimension p. We show that under quite mild conditions the squared Frobenius norm of the error of the inverse estimator decays like p^2/n with high probability.

Sailing under the Magellanic Clouds: a DECam view of the Carina dwarf

We present deep optical photometry from the DECam imager on the 4m Blanco telescope of over 12 deg$^2$ around the Carina dwarf spheroidal, with complete coverage out to 1 degree and partial coverage extending out to 2.6 degrees. Using a Poisson-based matched filter analysis to identify stars from each of the three main stellar populations, old, intermediate, and young, we confirm the previously identified radial age gradient, distance, tidal radius, stellar radial profiles, relative stellar population sizes, ellipticity, and position angle. We find an angular offset between the three main elliptical populations of Carina, and find only tentative evidence for tidal debris, suggesting that past tidal interactions could not have significantly influenced the Carina dwarf. We detect stars in the vicinity of, but distinct to, the Carina dwarf, and measure their distance to be 46$\pm$2 kpc. We determine this population to be part of the halo of the Large Magellanic Cloud at an angular radius of over 20 degrees. Due to overlap in colour-magnitude space with Magellanic stars, previously detected tidal features in the old population of Carina are likely weaker than previously thought.

Detection of galaxies withGaia

Besides its major objective tuned to the detection of the stellar galactic population the Gaia mission experiment will also observe a large number of galaxies. In this work we intend to evaluate the number and the characteristics of the galaxies that will effectively pass the onboard selection algorithm of Gaia. The detection of objects in Gaia will be performed in a section of the focal plane known as the Sky Mapper. Taking into account the Video Processing Algorithm criterion of detection and considering the known light profiles of discs and bulges galaxies we assess the number and the type of extra-galactic objects that will be observed by Gaia. We show that the stellar disk population of galaxies will be very difficult to be observed. On the contrary the spheroidal component of elliptical galaxies and bulges having higher central surface brightness and steeper brightness profile will be more easy to be detected. We estimate that most of the 20 000 elliptical population of nearby galaxies inside the local region up to 170 Mpc are in condition to be observed by Gaia. A similar number of bulges could also be observed although the low luminosity bulges should escape detection. About two thirds of the more distant objects up to 600 Mpc could also be detected increasing the total sample to half a million objects including ellipticals and bulges. The angular size of the detected objects will never exceed 4.72 arcsec which is the size of the largest transmitted windows. An heterogeneous population of elliptical galaxies and bulges will be observable by Gaia. This nearby Universe sample of galaxies should constitute a very rich and interesting sample to study their structural properties and their distribution.

Properties of galaxies reproduced by a hydrodynamic simulation.

Previous simulations of the growth of cosmic structures have broadly reproduced the 'cosmic web' of galaxies that we see in the Universe, but failed to create a mixed population of elliptical and spiral galaxies, because of numerical inaccuracies and incomplete physical models. Moreover, they were unable to track the small-scale evolution of gas and stars to the present epoch within a representative portion of the Universe. Here we report a simulation that starts 12 million years after the Big Bang, and traces 13 billion years of cosmic evolution with 12 billion resolution elements in a cube of 106.5 megaparsecs a side. It yields a reasonable population of ellipticals and spirals, reproduces the observed distribution of galaxies in clusters and characteristics of hydrogen on large scales, and at the same time matches the 'metal' and hydrogen content of galaxies on small scales.

The growth in size and mass of cluster galaxies since z = 2

We study the formation and evolution of Brightest Cluster Galaxies starting from a $z=2$ population of quiescent ellipticals and following them to $z=0$. To this end, we use a suite of nine high-resolution dark matter-only simulations of galaxy clusters in a $\Lambda$CDM universe. We develop a scheme in which simulation particles are weighted to generate realistic and dynamically stable stellar density profiles at $z=2$. Our initial conditions assign a stellar mass to every identified dark halo as expected from abundance matching; assuming there exists a one-to-one relation between the visible properties of galaxies and their host haloes. We set the sizes of the luminous components according to the observed relations for $z\sim2$ massive quiescent galaxies. We study the evolution of the mass-size relation, the fate of satellite galaxies and the mass aggregation of the cluster central. From $z=2$, these galaxies grow on average in size by a factor 5 to 10 of and in mass by 2 to 3. The stellar mass growth rate of the simulated BCGs in our sample is of 1.9 in the range $0.3<z<1.0$ consistent with observations, and of 1.5 in the range $0.0<z<0.3$. Furthermore the satellite galaxies evolve to the present day mass-size relation by $z=0$. Assuming passively evolving stellar populations, we present surface brightness profiles for our cluster centrals which resemble those observed for the cDs in similar mass clusters both at $z=0$ and at $z=1$. This demonstrates that the $\Lambda$CDM cosmology does indeed predict minor and major mergers to occur in galaxy clusters with the frequency and mass ratio distribution required to explain the observed growth in size of passive galaxies since $z=2$. Our experiment shows that Brightest Cluster Galaxies can form through dissipationless mergers of quiescent massive $z=2$ galaxies, without substantial additional star formation.

Multivariate multiple comparisons with a control in elliptical populations

Multivariate multiple comparisons with a control in elliptical populations

Hubble Space Telescope survey of the Perseus cluster - IV. Compact stellar systems in the Perseus cluster core and ultracompact dwarf formation in star-forming filaments

We present the results of the first search for Ultra Compact Dwarfs (UCDs) in the Perseus Cluster core, including the region of the cluster around the unusual Brightest Cluster Galaxy (BCG) NGC 1275. Utilising Hubble Space Telescope Advanced Camera for Surveys imaging, we identify a sample of 84 UCD candidates with half-light radii 10 pc < r_e < 57 pc out to a distance of 250 kpc from the cluster centre, covering a total survey area of ~70 armin^2. All UCDs in Perseus lie in the same size-luminosity locus seen for confirmed UCDs in other regions of the local Universe. The majority of UCDs are brighter than M_R = -10.5, and lie on an extrapolation of the red sequence followed by the Perseus Cluster dwarf elliptical population to fainter magnitudes. However, three UCD candidates in the vicinity of NGC 1275 are very blue, with colours (B-R)_0 < 0.6 implying a cessation of star formation within the past 100 Myr. Furthermore, large blue star clusters embedded in the star forming filaments are highly indicative that both proto-globular clusters (GCs) and proto-UCDs are actively forming at the present day in Perseus. We therefore suggest star forming filaments as a formation site for some UCDs, with searches necessary in other low redshift analogues of NGC 1275 necessary to test this hypothesis. We also suggest that tidal disruption of dwarf galaxies is another formation channel for UCD formation in the core of Perseus as tidal disruption is ongoing in this region as evidenced by shells around NGC 1275. Finally, UCDs may simply be massive GCs based on strong similarities in the colour trends of the two populations.

A study of the effect of kurtosis on discriminant analysis under elliptical populations

This paper is concerned with the role some parameters indexing four important families within the multivariate elliptically contoured distributions play as indicators of multivariate kurtosis. The problem is addressed for the exponential power family, for a subclass of the Kotz family and for the Pearson type II and type VII distributions. Once such a problem is analyzed, we study the effect these parameters have, as kurtosis indicators, on binary discriminant analysis by exploring their relationship with the error rate of the Bayes discriminant rule. The effect is analyzed under mild conditions on the kernel function generating the elliptical density. Some numerical examples are given in order to illustrate our theoretical insights and findings.

ACTIVE AND PASSIVE GALAXIES ATz∼ 2: REST-FRAME OPTICAL MORPHOLOGIES WITH WFC3

We use the high angular resolution in the near-infrared of the WFC3 on HST to determine YHVz color-color selection criteria to identify and characterize 1.5<z<3.5 galaxies in the HUDF09 and ERS (GOODS-South) fields. The WFC3 NIR images reveal galaxies at these redshifts that were undetected in the rest-frame UV HUDF/GOODS images, as well as true centers and regular disks in galaxies classified as highly irregular in rest-frame UV light. Across the 1.5<z<2.15 redshift range, regular disks are unveiled in the WFC3 images of ~25% of both intermediate and high mass galaxies, i.e., above 10^10 Msun. Meanwhile, galaxies maintaining diffuse and/or irregular morphologies in the rest-frame optical light---i.e., not yet dynamically settled---at these epochs are almost entirely restricted to masses below 10^11 Msun. In contrast at 2.25 < z < 3.5 these diffuse and/or irregular structures overwhelmingly dominate the morphological mix in both the intermediate and high mass regimes, while no regular disks, and only a small fraction (25%) of smooth spheroids, are evident above 10^11 Msun. Strikingly, by 1.5 < z < 2.25 roughly 2 out of every 3 galaxies at the highest masses are spheroids. In our small sample, the fraction of star-forming galaxies at these mass scales decreases concurrently from ~60% to ~5%. If confirmed, this indicates that z~2 is the epoch of both the morphological transformation and quenching of star-formation which assemble the first substantial population of massive ellipticals.

What determines the fraction of elliptical galaxies in clusters?

We study the correlation between the morphological mix of cluster galaxies and the assembly history of the parent cluster by taking advantage of two independently developed semi-analytic models for galaxy formation and evolution. In our models, both the number of cluster members and that of elliptical members increase as a function of cluster mass, in such a way that the resulting elliptical fractions are approximately independent of cluster mass. The population of cluster ellipticals exhibits a marked bimodal distribution as a function of galaxy stellar mass, with a dip at masses ∼1010 M⊙. In the framework of our models, this bimodality originates from the combination of a strongly decreasing number of galaxies with increasing stellar mass, and a correspondingly increasing probability of experiencing major mergers. We show that the correlation between the measured elliptical fraction and the assembly history of the parent cluster is weak, and that it becomes stronger in models that adopt longer galaxy merger times. We argue that this results from the combined effect of a decreasing bulge production due to a reduced number of mergers, and an increasing survival probability of pre-existing ellipticals, with the latter process being more important than the former.

The properties of fossil groups of galaxies

AbstractNumerical simulations as well as optical and X‐ray observations over the last few years have shown that poor groups of galaxies can evolve to what is called a fossil group. Dynamical friction as the driving process leads to the coalescence of individual galaxies in ordinary poor groups leaving behind nothing more than a central, massive elliptical galaxy supposed to contain the merger history of the whole group. Due to merging timescales for less‐massive galaxies and gas cooling timescales of the X‐ray intragroup medium exceeding a Hubble time, a surrounding faint‐galaxy population having survived this galactic cannibalism as well as an extended X‐ray halo similar to that found in ordinary groups, is expected. Recent studies suggest that fossil groups are very abundant and could be the progenitors of brightest cluster galaxies (BCGs) in the centers of rich galaxy clusters. However, only a few objects are known to the literature. This article aims to summarize the results of observational fossil group research over the last few years and presents ongoing work by the authors. Complementary to previous research, the SDSS and RASS surveys have been cross‐correlated to identify new fossil structures yielding 34 newly detected fossil group candidates. Observations with ISIS at the 4.2 m William Herschel Telescope on La Palma have been carried out to study the stellar populations of the central ellipticals of 6 fossil groups. In addition multi‐object spectroscopy with VLTs VIMOS has been performed to study the shape of the OLF of one fossil system (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A robust morphological classification of high-redshift galaxies using support vector machines on seeing limited images

<i>Context. <i/> Morphology is the most accessible tracer of galaxies physical structure, but its interpretation in the framework of galaxy evolution still remains problematic. Its quantification at high redshift requires deep high-angular resolution imaging, which is why space data (HST) are usually employed. At , the HST visible cameras however probe the UV flux, which is dominated by the emission of young stars, which could bias the estimated morphologies towards late-type systems.<i>Aims. <i/>In this paper we quantify the effects of this <i>morphological k-correction<i/> at by comparing morphologies measured in the <i>K<i/> and <i>I<i/>-bands in the COSMOS area. The <i>K<i/>s-band data indeed have the advantage of probing old stellar populations in the rest frame for , enabling determination of galaxy morphological types unaffected by recent star formation.<i>Methods. <i/>In Paper I we presented a new non-parametric method of quantifying morphologies of galaxies on seeing-limited images based on support vector machines. Here we use this method to classify ~50 000 <i>K<i/>s selected galaxies in the COSMOS area observed with WIRCam at CFHT. We use a 10-dimensional volume, including 5 morphological parameters, and other characteristics of galaxies such as luminosity and redshift. The obtained classification is used to investigate the redshift distributions and number counts per morphological type up to <i>z<i/> ~ 2 and to compare them to the results obtained with HST/ACS in the <i>I<i/>-band on the same objects. We associate to every galaxy with <i>K<i/>s < 21.5 and a probability between 0 and 1 of being late-type or early-type. We use this value to assess the accuracy of our classification as a function of physical parameters of the galaxy and to correct for classification errors.<i>Results. <i/>The classification is found to be reliable up to <i>z<i/> ~ 2. The mean probability is <i>p<i/> ~ 0.8. It decreases with redshift and with size, especially for the early-type population, but remains above <i>p<i/> ~ 0.7. The classification globally agrees with the one obtained using HST/ACS for . Above <i>z<i/> ~ 1, the <i>I<i/>-band classification tends to find less early-type galaxies than the <i>K<i/>s-band one by a factor ~1.5, which might be a consequence of morphological k-correction effects.<i>Conclusions. <i/> We argue therefore that studies based on <i>I<i/>-band HST/ACS classifications at could be underestimating the elliptical population. Using our method in a 21.5 magnitude-limited sample, we observe that the fraction of the early-type population is (21.9% <i>±<i/> 8%) at <i>z<i/> ~ 1.5-2 and (32.0% <i>±<i/> 5%) at the present time. We will discuss the evolution of the fraction of galaxies in types from volume-limited samples in a forthcoming paper.

EVOLUTION OF THE COLOR-MAGNITUDE RELATION IN GALAXY CLUSTERS ATz∼ 1 FROM THE ACS INTERMEDIATE REDSHIFT CLUSTER SURVEY

We apply detailed observations of the Color-Magnitude Relation (CMR) with the ACS/HST to study galaxy evolution in eight clusters at z~1. The early-type red sequence is well defined and elliptical and lenticular galaxies lie on similar CMRs. We analyze CMR parameters as a function of redshift, galaxy properties and cluster mass. For bright galaxies (M_B < -21mag), the CMR scatter of the elliptical population in cluster cores is smaller than that of the S0 population, although the two become similar at faint magnitudes. While the bright S0 population consistently shows larger scatter than the ellipticals, the scatter of the latter increases in the peripheral cluster regions. If we interpret these results as due to age differences, bright elliptical galaxies in cluster cores are on average older than S0 galaxies and peripheral elliptical galaxies (by about 0.5Gyr). CMR zero point, slope, and scatter in the (U-B)_z=0 rest-frame show no significant evolution out to redshift z~1.3 nor significant dependence on cluster mass. Two of our clusters display CMR zero points that are redder (by ~2sigma) than the average (U-B)_z=0 of our sample. We also analyze the fraction of morphological early-type and late-type galaxies on the red sequence. We find that, while in the majority of the clusters most (80% to 90%) of the CMR population is composed of early-type galaxies, in the highest redshift, low mass cluster of our sample, the CMR late-type/early-type fractions are similar (~50%), with most of the late-type population composed of galaxies classified as S0/a. This trend is not correlated with the cluster's X-ray luminosity, nor with its velocity dispersion, and could be a real evolution with redshift.

Testing the evolutionary link between submillimetre galaxies and quasars: CO observations of QSOs atz∼ 2

We have used the IRAM Plateau de Bure millimeter interferometer and the UKIRT 1– 5µm Imager Spectrometer (UIST) to test the connection between the major phases of spheroid growth and nuclear accretion by mapping CO emission in nine submillimetredetected QSOs at z = 1.7–2.6 with black hole (BH) masses derived from near-infrared spectroscopy. When combined with one QSO obtained from the literature, we present sensitive CO(3–2) or CO(2–1) observations of 10 submillimetre-detected QSOs selected at the epoch of peak activity in both QSOs and submillimetre (submm) galaxies (SMGs). CO is detected in 5/6 very optically luminous (MB ∼ −28) submm-detected QSOs with BH masses MBH ≃ 10 9 –10 10 M⊙, confirming the presence of large gas reservoirs of Mgas ≃ 3.4 × 10 10 M⊙. Our BH masses and dynamical mass constraints on the host spheroids suggest, at face value, that these optically luminous QSOs at z = 2 lie about an order of magnitude above the local BH-spheroid relation, MBH/Msph, although this result is dependent on the size and inclination of the CO-emitting region. However, we find that their BH masses are ∼ 30 times too large and their surface density is ∼ 300 times too small to be related to typical SMGs in an evolutionary sequence. Conversely, we measure weaker CO emission in four fainter (MB ∼ −25) submm-detected QSOs with properties, BH masses (MBH ≃ 5 ×10 8 M⊙), and surface densities similar to SMGs. These QSOs appear to lie near the local MBH/Msph relation, making them plausible ‘transition objects’ in the proposed evolutionary sequence linking QSOs to the formation of massive young galaxies and BHs at high-redshift. We show that SMGs have a higher incidence of bimodal CO line profiles than seen in our QSO sample, which we interpret as an effect of their relative inclinations, with the QSOs seen more face-on. Finally, we find that the gas masses of the four fainter submm-detected QSOs imply that their star formation episodes could be sustained for ∼ 10Myr, and are consistent with representing a phase in the formation of massive galaxies which overlaps a preceding SMG starburst phase, before subsequently evolving into a population of present-day massive ellipticals.

Optimal rank-based tests for homogeneity of scatter

We propose a class of locally and asymptotically optimal tests, based on multivariate ranks and signs for the homogeneity of scatter matrices in m elliptical populations. Contrary to the existing parametric procedures, these tests remain valid without any moment assumptions, and thus are perfectly robust against heavy-tailed distributions (validity robustness). Nevertheless, they reach semiparametric efficiency bounds at correctly specified elliptical densities and maintain high powers under all (efficiency robustness). In particular, their normal-score version outperforms traditional Gaussian likelihood ratio tests and their pseudo-Gaussian robustifications under a very broad range of non-Gaussian densities including, for instance, all multivariate Student and power-exponential distributions.

Short Gamma‐Ray Bursts and Binary Mergers in Spiral and Elliptical Galaxies: Redshift Distribution and Hosts

To test whether the short GRB rates, redshift distribution and host galaxies are consistent with current theoretical predictions, we use avery large database of population synthesis calculations to examine BH-NS and NS-NS merger rates in the universe, factoring in (i) the star formation history of the universe, (ii) a heterogeneous population of star-forming galaxies, including spirals and ellipticals, and (iii) a simple flux-limited selection model for short GRB detection. When we require our models reproduce the known short GRB rates and redshift measurements (and, for NS-NS, the merger rates extrapolated from binary pulsars in the Galaxy), a small fraction of models reproduce all observations, both when we assume a NS-NS and a BH-NS origin for bursts. Most commonly models produce mergers preferentially in spiral galaxies if short GRBs arise from NS-NS mergers alone. Model universes where present-day binary mergers occur preferentially in elliptical galaxies necessarily include a significant fraction of binaries with long delay times between birth and merger (often $O(10{\rm Gyr})$). Though long delays occur, almost all of our models predict that a higher proportion of short GRBs should occur at moderate to high redshift (e.g., $z>1$) than has presently been observed, in agreement with recent observations which suggest a selection bias towards successful follow-up of low-redshift short GRBs. Finally, if only a fraction of BH-NS mergers have the right combination of masses and spins to make GRBs, then at best only a small fraction of BH-NS models could be consistent with all {\em current} available data. (Abridged)

The Efficiency of the Asymptotic Expansion of the Distribution of the Canonical Vector under Nonnormality

In canonical correlation analysis, canonical vectors are used in the interpretation of the canonical variables. We are interested in the asymptotic representation of the expectation, the variance and the distribution of the canonical vector. In this study, we derive the asymptotic distribution of the canonical vector under nonnormality. To obtain the asymptotic expansion of the canonical vector, we use a perturbation method. In addition, as an example, we show the asymptotic distribution with an elliptical population. In multivariate statistical analysis, the distributions of latent roots and latent vectors of certain symmetric matrices constructed from the sample covariance matrix are important in some cases and have been studied by many authors. These studies can be used as the basis for the canonical correlation analysis, which is an approach that characterizes the correlation structure between two sets of variables. Considering the distribution of the canonical correlation with the assumption of the multivariate normal population, asymptotic expansions of the distributions were studied by Sugiura (1976), Fujikoshi (1977, 1978), Muirhead (1978) and others. The distributions of a function of latent roots of the sample covariance matrix in nonnormal populations were studied by Fujikoshi (1980), Muirhead and Waternaux (1980), Fang and Krishinaiah (1982), Siotani et al. (1985), Seo et al. (1994) and others. The distribution of the canonical vector was studied by Eaton and Tyler (1994), Boik (1998), Anderson (1999), Taskinen et al. (2006) and others. This paper deals with the asymptotic expansion of the canonical vector under nonnormality. Let us denote x =( x � 1, x � 2) � as p + q dimensional variables with mean µ and

Keck spectroscopy of the faint dwarf elliptical galaxy population in the Perseus Cluster core: mixed stellar populations and a flat luminosity function

We present the result of a photometric and Keck-LRIS spectroscopic study of dwarf galaxies in the core of the Perseus Cluster, down to a magnitude of M_B = -12.5. Spectra were obtained for twenty-three dwarf-galaxy candidates, from which we measure radial velocities and stellar population characteristics from absorption line indices. From radial velocities obtained using these spectra we confirm twelve systems as cluster members, with the remaining eleven as non-members. Using these newly confirmed cluster members, we are able to extend the confirmed colour-magnitude relation for the Perseus Cluster down to M_B = -12.5. We confirm an increase in the scatter about the colour magnitude relationship below M_B = -15.5, but reject the hypothesis that very red dwarfs are cluster members. We measure the faint-end slope of the luminosity function between M_B = -18 and M_B = -12.5, finding alpha = -1.26 \pm 0.06, which is similar to that of the field. This implies that an overabundance of dwarf galaxies does not exist in the core of the Perseus Cluster. By comparing metal and Balmer absorption line indices with alpha-enhanced single stellar population models, we derive ages and metallicities for these newly confirmed cluster members. We find two distinct dwarf elliptical populations: an old, metal poor population with ages ~ 8 Gyr and metallicities [Fe/H] -0.33. Dwarf galaxies in the Perseus Cluster are therefore not a simple homogeneous population, but rather exhibit a range in age and metallicity.

Large sample approximations for the LR statistic for equality of the smallest eigenvalues of a covariance matrix under elliptical population

This paper is concerned with large sample approximations of the LR statistic for testing the hypothesis that the smallest eigenvalues of a covariance matrix are equal. Under a normal population Lawley [1956. Tests of significance for the latent roots of covariance and correlation matrices. Biometrika 43, 128–136.] and Fujikoshi [1977. An asymptotic expansion for the distributions of the latent roots of the Wishart matrix with multiple population roots. Ann. Inst. Statist. Math. 29, 379–387.] obtained a Bartlett-correction factor and an asymptotic expansion for the LR statistic, respectively, when the sample size is large. In this paper we extend the Bartlett correction factor to an elliptical population. The accuracy of our approximations is examined through simulation experiments.

Observational Evidence for the Coevolution of Galaxy Mergers, Quasars, and the Blue/Red Galaxy Transition

We compile a number of observations to estimate the time-averaged rate of formation or buildup of red sequence galaxies, as a function of mass and redshift. Comparing this with the mass functions of mergers and quasar hosts, and independently comparing their clustering properties as a function of redshift, we find that these populations trace the same mass distribution, with similar evolution, at redshifts 0 < z ≲ 1.5. Knowing one of the quasar, merger, or elliptical mass/luminosity functions, it is possible to predict the others. Allowing for greater model dependence, we compare the rate of early-type buildup with the implied merger and quasar triggering rates as a function of mass and redshift and find agreement. Over this redshift range, observed merger fractions can account for the entire bright quasar luminosity function and buildup of the red sequence at all but the highest masses at low redshift (≳1011 M☉ at z ≲ 0.3) where "dry" mergers appear to dominate. This supports a necessary prediction of theories where mergers between gas-rich galaxies produce ellipticals with an associated phase of quasar activity, after which the remnant becomes red. These populations trace a similar characteristic transition mass, possibly reflecting the mass above which the elliptical population is mostly (≳50%) assembled at a given redshift, which increases with redshift over the observed range in a manner consistent with suggestions that cosmic downsizing may apply to red galaxy assembly as well as star formation. These mass distributions as a function of redshift do not uniformly trace the all/red/blue galaxy population, ruling out models in which quasar activity is generically associated with star formation or is long lived in "old" systems.