We present new observations of the white dwarf sequence of the old open cluster NGC 6791. The brighter peak previously observed in the white dwarf luminosity function (WDLF) is now better delineated, and the second, fainter peak that we suggested earlier is now confirmed. A careful study suggests that we have reached the end of the white dwarf sequence. The WDs that create the two peaks in the WDLF show a significant turn to the blue in the color-magnitude diagram. The discrepancy between the age from the WDs and that from the main sequence turnoff remains, and we have an additional puzzle in the second peak in the WDLF. Canonical WD models seem to fail --at least at ~25%-level-- in reproducing the age of clusters of this metallicity. We discuss briefly possible ways of arriving at a theoretical understanding of the WDLF.
In this paper we demonstrate that the puzzling bright peak in the luminosity function of the white dwarf (WD) cooling sequence of NGC6791 can be naturally accounted for if ~34% of the observed WDs are WD+WD binary systems.
We have used deep HST WFPC2 observations of two fields in NGC 2420 to produce a cluster CMD down to V = 27. After imposing morphological selection criteria we find eight candidate white dwarfs in NGC 2420. Our completeness estimates indicate that we have found the terminus of the WD cooling sequence. We argue that the cluster distance modulus is likely to be close to 12.10 with E(B-V) = 0.04. With these parameters we find a white dwarf cooling age for NGC 2420 of 2.0 +- 0.20 (1 sigma) Gyrs. The 0.20 Gyr uncertainty includes errors in the photometry, sequence fitting, precursor time scales, and…
In the old, populous, and metal-rich open cluster NGC 6791 we have used deep HST/ACS images to track the white dwarf cooling sequence down to m_F606W~28.5. The white dwarf luminosity function shows a well defined peak at m_F606W~27.4, and a bending to the blue in the color--magnitude diagram. If this peak corresponds to the end of the white dwarf cooling sequence the comparison with theoretical isochrones provides a cluster age estimate of ~2.4 Gyr, in sharp contrast with the age of 8--9 Gyr inferred from the main-sequence turn-off. If the end is at fainter magnitudes, the peak at m_F606W~27.4…
We develop the white dwarf luminosity function (LF) of the old open cluster NGC 188 in order to determine a lower limit to the age of the cluster by using the faint end of the cooling sequence. To produce an extensive sequence of the cooling white dwarfs we imaged four contiguous HST-WFPC2 fields in the center of the cluster in the F555W and F814W filters. After imposing selection criteria on the detected objects we found a white dwarf cooling sequence (down to V ~26.5) including 28 candidate white dwarfs in the cluster. The exposures are not deep enough to reach the end of this sequence, but …
We present far-UV spectroscopy of the giant elliptical galaxy NGC 1399, obtained with the Far Ultraviolet Spectroscopic Explorer. Of all quiescent ellipticals, NGC 1399 has the strongest known ``UV upturn'' -- a sharp spectral rise shortward of 2500 A. It is now well-established that this emission comes from hot horizontal branch (HB) stars and their progeny; however, the chemical composition of these stars has been the subject of a long-standing debate. For the first time in observations of any elliptical galaxy, our spectra clearly show photospheric metallic absorption lines within the UV up…
We discuss the importance of pure hydrogen white dwarf atmosphere models with Ly-$\rm α$ far red wing opacity in the analysis of the white dwarf cooling sequence of the globular cluster NGC 6397. Our recently improved atmosphere models account for the previously missing opacity from the Ly-$\rm α$ hydrogen line broadened by collisions of the absorbing hydrogen atoms with molecular and atomic hydrogen. These models are the first that well reproduce the UV colors and spectral energy distributions of cool white dwarfs with $T_{\rm eff}<6000 \rm K$ observed in the Galactic Disk. Fitting the observ…
We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution…
Cooling flow models for the hot gas in elliptical galaxies predict that gas is cooling at a rate of ~ 1 solar mass/yr, yet there is little evidence for this phenomenon beyond the X-ray waveband. If hot gas is cooling, it will pass through the 3x10^5 K regime and radiate in the OVI 1032,1038 ultraviolet lines, which can be detected with the Far Ultraviolet Spectroscopic Explorer (FUSE) and here we report on FUSE observations of the X-ray bright early-type galaxies NGC 1404 and NGC 4636. In NGC 1404, the OVI doublet is not detected, implying a cooling rate < 0.3 solar masses/yr, which is below t…
Some early-type galaxies show OVI emission, a tracer of gas at 10^5.5 K, and a predicted product of gas cooling from the X-ray emitting temperatures. We studied the spatial extent and velocity structure of this cooling gas by obtaining spectra of the OVI doublet in NGC 4636 and NGC 5846 with the Far Ultraviolet Spectroscopic Explorer. For NGC 4636, the central LWRS pointing shows that the OVI lines are double-peaked and symmetrical about the systemic velocity of the galaxy, with a separation of 210 km/s. A LWRS observation 30" from the center failed to show additional OVI emission. For NGC 584…
We present deep CCD photometry of the very rich, intermediate aged (similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-V color-magnitude diagram (CMD) for the cluster shows an extremely well populated and very tightly constrained main-sequence extending over 12 magnitudes from the turn-off. The CFH12K photometry for this cluster is faint enough (V ~ 24.5) to detect the remnants of the most massive progenitor cluster stars under the Type I SNe limit. Therefore, the CMD of the cluster also exhibits a well defined white dwarf `clump' caused by the decreased rate of cooling of these…
The central arcminute of the Perseus cooling flow galaxy, NGC 1275, has been mapped with the JCMT in 12CO(2-1) at 21 arcsec resolution, with detections out to at least 36 arcsec (12 kpc). Within the limits of the resolution and coverage, the distribution of gas appears to be roughly E-W, consistent with previous observations of CO, X-ray, H alpha, and dust emission. The total detected molecular hydrogen mass is ~ 1.6 x 10^10 solar masses, using a Galactic conversion factor. The inner central rotating disk is apparent in the data, but the overall distribution is not one of rotation. Rather, the…
We have imaged in CO(2-1) the molecular gas in NGC 1275 (Perseus A), the cD galaxy at the center of the Perseus Cluster, at a spatial resolution of $\sim$1 kpc over a central region of radius $\sim$ 10 kpc. Per A is known to contain $\sim$1.3x10$^{10}$ M$_\odot$ of molecular gas, which has been proposed to be captured from mergers with or ram-pressure stripping of gas-rich galaxies, or accreted from a X-ray cooling flow. The molecular gas detected in our image has a total mass of $\sim$4x10$^9$ M$_\odot$, and for the first time can be seen to be concentrated in three radial filaments with leng…
We have used deep high-resolution multiband images taken at the ESO Very Large Telescope to identify the optical binary companion to the millisecond pulsar (PSR J1911-5958A) located in the halo of the Galactic Globular Cluster NGC6752. The object turns out to be a blue star whose position in the Color Magnitude Diagram is consistent with the cooling sequence of a low mass (M~0.17-0.20Mo), low metallicity Helium white dwarf (He-WD) at the cluster distance. This is the second He-WD which has been found to orbit a millisecond pulsar in GGCs. Curiously both objects have been found to lie on the sa…
We present a UV CMD spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high temperatu…
We present the study of three bright Early-Type galaxies hosted in groups or poor clusters: NGC 1399, NGC 1404 and NGC 507. We used ROSAT HRI, PSPC and, when available, higher resolution Chandra data to study the structure of the X-ray halo. The halo center of NGC 1399 and NGC 507 is dominated by a bright emission peak coincident with the position of the optical galaxy. This central peak is not explained by cooling flow models and is likely to be produced by kinetically heated stellar material. Within the effective radius, the hot gas distribution is tracing the galactic potential due to lumin…
Using the HST Wide Field Camera of the Advanced Camera for Surveys (ACS), we have obtained deep (V,I) photometry of an outer-halo field in NGC 5128, to a limiting magnitude of I~29. Our photometry directly reveals the core helium-burning stellar population (the ``red clump'' or horizontal branch) in a giant E/S0 galaxy for the first time. The color-magnitude diagram displays a very wide red giant branch (RGB), an asymptotic giant branch (AGB) bump, and the red clump; no noticeable population of blue HB stars is present, confirming previous suggestions that old, very metal-poor population is no…
We have analysed new R-band photometry of globular clusters in NGC 6166, the cD galaxy in the cooling flow cluster A2199. In combination with the earlier B photometry of Pritchet \& Harris (1990), we obtain B$-$R colours for $\sim$ 40 globular clusters in NGC 6166. The mean B$-$R is 1.26 $\pm$ 0.11, corresponding to a mean [Fe/H] = $-$1 $\pm$ 0.4. Given that NGC 6166 is one of the most luminous cD galaxies studied to date, our result implies significant scatter in the relationship between mean cluster [Fe/H] and parent galaxy luminosity. We obtain a globular cluster specific frequency of S$_N$…
Keck spectroscopy of 5 proto-globular cluster candidates in NGC 1275 has been combined with HST WFPC2 photometry to explore the nature and origin of these objects and discriminate between merger and cooling flow scenarios for globular cluster formation. The objects we have studied are not HII regions, but rather star clusters, yet their integrated spectral properties do not resemble young or intermediate age Magellanic Cloud clusters or Milky Way open clusters. The clusters' Balmer absorption appears to be too strong to be consistent with any of the standard Bruzual & Charlot evolutionary mode…
X-ray observations of M87 in the Virgo cluster and NGC 4874 in Coma reveal that the gas temperature beyond about 50 kpc from these cD galaxies is comparable to the virial temperature of the cluster, 3 or 9 keV respectively, but within the optical galaxy the temperature drops to the galactic virial temperature, about 1 keV. We show that these steep thermal (and density) gradients follow naturally from the usual cooling inflow assumptions without recourse to thermal conductivity. To avoid unobserved central mass concentrations, most of the gas must radiatively cool (``dropout'') before it flows …
We propose that the anomalously bright white dwarf luminosity function observed in NGC 6791 (Bedin et al 2005) is the consequence of the formation of 0.5 Msun white dwarfs with Helium cores instead of Carbon cores. This may happen if mass loss during the ascent of the Red Giant Branch is strong enough to prevent a star from reaching the Helium flash. Such a model can explain the slower white dwarf cooling (relative to standard models) and fits naturally with scenarios advanced to explain Extreme Horizontal Branch stars, a population of which are also found in this cluster.
A new Chandra observation of the northwest region of the galaxy cluster A1367 reveals four cool galaxy coronae (0.4 - 1.0 keV) embedded in the hot intracluster medium (ICM) (5 - 6 keV). While the large coronae of NGC 3842 and NGC 3837 appear symmetric and relaxed, the galaxy coronae of the $\lsim$ L* galaxies (NGC 3841 and CGCG 97090) are disturbed and being stripped. Massive galaxies, with dense cooling cores, are better able to resist ram pressure stripping and survive in rich environments than $\lsim$ L* galaxies whose galactic coronae are much less dense. The survival of these cool coronae…
The purpose of this paper is to describe a method for determining a cooling template for galaxies, using nearby galaxies, and applicable to future observations of distant galaxies. We observed two starburst galaxies (NGC 253 and Henize 2-10) with the Caltech Submillimeter Observatory in the rotational lines of carbon monoxide 12CO(J=3-2), (J=6-5) and (J=7-6) for both, and also 12CO(J=4-3) and 13CO(J=3-2) for Henize 2-10 and in the 3P2-3P1 fine-structure transitions of atomic carbon [CI] at 809 GHz for NGC 253. Some of these observations have been made previously, but the present multitransitio…
A CCD V,V-I colour-magnitude diagram (CMD) of the Galactic globular cluster NGC 2808 has been obtained with the ESO-NTT, reaching down to V ~ 24. The highly populated Main Sequence (MS) presents a significant broadening redward to the MS ridge line, larger than expected from photometric errors alone, which could be interpreted as due to binary candidates.
We present the first detailed X-ray observations, using Chandra, of NGC 6482 - the nearest known `fossil group'. The group is dominated by an optically luminous giant elliptical galaxy and all other known group members are at least two magnitudes fainter. The global X-ray properties (luminosity, temperature, extent) of NGC 6482 fall within the range of other groups, but the detailed properties show interesting differences. We derive the gas temperature and total mass profiles for the central 30 kpc using ACIS spatially resolved spectroscopy. The temperature profile shows a continuous decrease …
We present {\it Hubble Space Telescope} {\it V,I} photometry of the central region of the LMC cluster NGC 1866, reaching magnitudes as faint as V=27 mag. We find evidence that the cluster luminosity function shows a strong dependence on the distance from the cluster center, with a clear deficiency of low luminosity stars in the inner region. We discuss a {\it global} cluster luminosity function as obtained from stars in all the investigated region, which appears in impressive agreement with the prediction from a Salpeter mass distribution. We also revisit the use of NGC 1866 as a probe for det…
We present a deep color magnitude diagram for the galactic globular cluster NGC 2808 (C0911-646), reaching 3 magnitudes below the main sequence turnoff. A set of local photometric standards are provided, and the mean cluster reddening and metallicity are constrained using several photometric techniques. The unusual horizontal branch morphology discovered by Sosin et al. (1997, ApJ, 480, L35) is confirmed. No significant radial population gradients are identified to within the limits of the data. We investigate the redwards color spread on the main sequemce found by Ferraro et al. (1997, A&A, 3…
Deep BV photometry for a large field covering the outer-halo globular cluster NGC 6229 is presented. For the first time, a colour-magnitude diagram (CMD) reaching below the main sequence turnoff has been obtained for this cluster. Previous results regarding the overall morphology of the horizontal and giant branches are confirmed. In addition, nine possible extreme horizontal-branch stars have been identified in our deep images, as well as thirty-three candidate blue stragglers. We also find the latter to be more centrally concentrated than subgiant branch stars covering the same range in V ma…
We present results from a 25 ksec CHANDRA ACIS-S observation of galaxies NGC4438 and NGC4435 in the Virgo Cluster. X-ray emission in NGC4438 is observed in a ~700 pc nuclear region, a 2.3 kpc spherical bulge, and a network of filaments extending 4-10 kpc to the W and SW of the galaxy. The X-ray emission in all 3 regions is highly correlated to similar features observed in Halpha. Spectra of the filaments and bulge are well represented by a 0.4 keV MEKAL model with combined 0.3-2 keV intrinsic luminosity of 1.24x10^{40}erg/s, electron densities ~ 0.02-0.04 cm^{-3}, cooling times of 400-700 Myr …
We report on the identification of the optical counterpart of the binary millisecond pulsar PSR J1911-5958A, located in the outskirts of the globular cluster NGC 6752. At the position of the pulsar we find an object with V=22.08, B-V=0.38, U-B=-0.49. The object is blue with respect to the cluster main sequence by 0.8 magnitudes in B-V. We argue that the object is the white dwarf companion of the pulsar. Comparison with white dwarf cooling models shows that this magnitude and colors are consistent with a low-mass white dwarf at the distance of NGC 6752. If associated with NGC 6752, the white dw…
We present VLT (FORS1) photometry of the lower main sequence (MS) of the Galactic Globular Cluster (GGC) NGC 6397, for stars located in 2 fields extending from a region near the cluster center out to ~ 10'. The obtained CMD shows a narrow MS extending down to V ~ 27 (figure c), much deeper than any previous ground based study and comparable with previous HST photometry (Cool et al. 1996). The comparison between observed MS Luminosity Functions (LFs) derived for 2 annuli at different radial distance from the center of the cluster shows a clear-cut correlation between their slope before reaching…
We present CCD photometric and mass function study of 9 young Large Magellanic Cloud star clusters namely NGC 1767, NGC 1994, NGC 2002, NGC 2003, NGC 2006, SL 538, NGC 2011, NGC 2098 and NGC 2136. The BVRI data reaching down to V ~ 21 mag, are collected from 3.5-meter NTT/EFOSC2 in sub-arcsec seeing conditions. For NGC 1767, NGC 1994, NGC 2002, NGC 2003, NGC 2011 and NGC 2136, broad band photometric CCD data are presented for the first time. Seven of the 9 clusters have ages between 16 to 25 Myr while remaining two clusters have ages $32\pm4$ Myr (NGC 2098) and $90\pm10$ Myr (NGC 2136). For 7 …
We analyze ultraviolet (~1500 A) images of the old open clusters M67, NGC 188, and NGC 6791 obtained with Ultraviolet Imaging Telescope (UIT) during the second flight of the Astro observatory in March 1995. Twenty stars are detected on the UIT image of M67, including 11 blue stragglers, seven white dwarf candidates, and the yellow giant -- white dwarf binary S1040. The ultraviolet photometry of the blue stragglers F90 (S975) and F131 (S1082) suggests that these stars have hot subluminous companions. We present a semi-empirical integrated ultraviolet spectrum of M67, and show that the blue stra…
We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ 0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS spectra, suggests that most of these stars are undermassive, <M> = 0.43 +/- 0.06 Msun, and therefore could not have formed from canonical stella…
Cold molecular gas has been recently detected in several cooling flow clusters of galaxies where huge optical nebulosities often stand. These optical filaments are tightly linked to the cooling flow and to the related phenomena, like the rising bubbles of relativistic plasma, fed by the radio jets. We present here a map in the CO(2-1) rotational line of the cold molecular gas associated with some Halpha filaments surrounding the central galaxy of the Perseus cluster: NGC 1275. The map, extending to about 50 kpc (135 arcsec) from the center of the galaxy, has been made with the 18-receiver arra…
We present the results of a deep photometric study of the outer halo of NGC 1275, the highly active cD galaxy at the center of the Perseus cluster. We find a modest excess of faint ($R > 22.5$) starlike objects in its halo, indicating a population of old-halo globular clusters. However, the total estimated cluster population corresponds to a specific frequency of $S_N = 4.4 \pm 1.2$, no larger than that of normal giant ellipticals and three times lower than that of other central cD galaxies such as M87. We discuss several ideas for the origin of this galaxy. Our results reinforce the view that…
The NGC 1407 Group stands out among nearby structures by its properties that suggest it is massive and evolved. It shares properties with entities that have been called fossil groups: the 1.4 magnitude differential between the dominant elliptical galaxy and the second brightest galaxy comes close to satisfying the definition that has been used to define the fossil class. There are few intermediate luminosity galaxies, but a large number of dwarfs in the group. We estimate there are 250 group members to the depth of our survey. The slope of the faint end of the luminosity function (reaching M_R…
Here we present a self-consistent stationary solution for spherically symmetric winds driven by massive star clusters under the impact of radiative cooling. We demonstrate that cooling may modify drastically the distribution of temperature if the rate of injected energy approaches a critical value. We also prove that the stationary wind solution does not exist whenever the energy radiated away at the star cluster center exceeds ~ 30% of the energy deposition rate. Finally we thoroughly discuss the expected appearance of super-star cluster winds in the X-ray and visible line regimes. The three …
We report new HST NICMOS and WFPC2 imaging of emission-line nebulae in the central galaxies of 3 clusters of galaxies purported to host massive cooling flows, NGC1275, A2597, and PKS0745. The spectral signature of vibrationally- excited molecular hydrogen (VEMH) has been seen in every galaxy searched so far in a cluster cooling flow with an optical emission line nebula. We have discovered that the VEMH gas extends several kpc from the centers of A2597 and PKS0745, while the vibrationally-excited molecular hydrogen in NGC1275 appears to be mostly confined to its nucleus, with some extended emis…
We continue our study of rich Galactic clusters by presenting deep CCD observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22 and NGC 2323 contains approximately 2100 stars brighter than our photometric limit of V = 23) and young (age of NGC 2168 = 180 Myrs, age of NGC 2323 = 130 Myrs). The color-magnitude diagrams for the clusters exhibit clear main sequences stretching over 14 magnitudes in the V, B-V plane. Comparing these long main sequences with those of earlier clusters in the survey, as well …
We present an analysis of optical and ultraviolet Hubble Space Telescope photometry for evolved stars in the core of the distant massive globular cluster NGC 2419. We characterize the horizontal branch (HB) population in detail including corrections for incompleteness on the long blue tail. We present a method for removing (to first order) lifetime effects from the distribution of HB stars to facilitate more accurate measurements of helium abundance for clusters with blue HBs and to clarify the distribution of stars reaching the zero-age HB. The population ratio R = N_HB / N_RGB implies there …
We present the CMD from deep HST imaging in the globular cluster NGC 6397. The ACS was used for 126 orbits to image a single field in two colors (F814W, F606W) 5 arcmin SE of the cluster center. The field observed overlaps that of archival WFPC2 data from 1994 and 1997 which were used to proper motion (PM) clean the data. Applying the PM corrections produces a remarkably clean CMD which reveals a number of features never seen before in a globular cluster CMD. In our field, the main sequence stars appeared to terminate close to the location in the CMD of the hydrogen-burning limit predicted by …
We present the analysis of a deep colour-magnitude diagram (CMD) of NGC 1831, a rich star cluster in the LMC. The data were obtained with HST/WFPC2 in the F555W (~V) and F814W (~I) filters, reaching m_555 ~ 25. We discuss and apply a method of correcting the CMD for sampling incompleteness and field star contamination. Efficient use of the CMD data was made by means of direct comparisons of the observed to model CMDs. The model CMDs are built by an algorithm that generates artificial stars from a single stellar population, characterized by an age, a metallicity, a distance, a reddening value, …
We analyze two time-dependent cluster cooling flow models in spherical symmetry. The first assumes that the intracluster gas resides in a static external potential, and includes the effects of optically thin radiative cooling and mass deposition. This corresponds to previous steady-state cooling flow models calculated by White & Sarazin (1987). Detailed agreement is found between steady-state models and time-dependent models at fixed times in the simulations. The mass accretion rate is found either to increase or remain nearly constant once flows reach a steady state. The time rate of change o…
