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| Do Young Galaxies Exist in the Local Universe? Red Giant Branch Detection in the Metal-Poor Dwarf Galaxies SBS 1415+437 |
| We present results from an HST/ACS imaging study of the metal-poor blue compact dwarf galaxy SBS 1415+437.
It has been argued previously that this is a very young galaxy that started to form stars only ~100 Myr ago.
However, we find that the optical color-magnitude diagram prominently reveals asymptotic giant branch and red
giant branch (RGB) stars. The brightness of the RGB tip yields a distance D H 13.6 Mpc. The color of the RGB
implies that its stars must be older than <1.3 Gyr, with the exact age depending on the assumed metallicity and
dust extinction. The number of RGB stars implies that most of the stellar mass resides in this evolved population.
In view of these and other HST results for metal-poor galaxies, it seems that the local universe simply may not
contain any galaxies that are currently undergoing their first burst of star formation. |
| Publication date: 20 Sep 2005 |
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| Discovery of Multiply Imaged Galaxies behind the Cluster and Lensed Quasar SDSS J1004+4112 |
| We have identified three multiply imaged galaxies in Hubble Space Telescope images of the redshift z=0.68 cluster responsible for the large-separation quadruply lensed quasar, SDSS J1004+4112. Spectroscopic redshifts have been secured for two of these systems using the Keck I 10 m telescope. The most distant lensed galaxy, at z=3.332, forms at least four images, and an Einstein ring encompassing 3.1 times more area than the Einstein ring of the lensed QSO images at z=1.74, due to the greater source distance. For a second multiply imaged galaxy, we identify Lyalpha emission at a redshift of z=2.74. The cluster mass profile can be constrained from near the center of the brightest cluster galaxy, where we observe both a radial arc and the fifth image of the lensed quasar, to the Einstein radius of the highest redshift galaxy, ~110 kpc. Our preliminary modeling indicates that the mass approximates an elliptical body, with an average projected logarithmic gradient of ~=-0.5. The system is potentially useful for a direct measurement of world models in a previously untested redshift range.
Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. |
| Publication date: 15 Aug 2005 |
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| The Stellar Content of Nearby Star-forming Galaxies (III) |
| We investigate the nature of the diffuse intracluster ultraviolet light seen in 12 local starburst galaxies, using longslit
ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble
Space Telescope (HST ). We take this faint intracluster light to be the field in each galaxy and compare its spectroscopic
signature with Starburst99 evolutionary synthesis models and with neighboring star clusters. Our main
result is that the diffuse ultraviolet light in 11 of the 12 starbursts lacks the strong O star wind features that are clearly
visible in spectra of luminous clusters in the same galaxies. The difference in stellar features dominating cluster and
field spectra indicates that the field light comes primarily from a different stellar population and not from scattering
of UV photons originating in the massive clusters. |
| Publication date: 20 Jul 2005 |
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| ST-ECF Newsletter 38 |
| Contents: HST News and Status; ST-ECF Update; Hubble's 15th Anniversary; New Development in aXe; STIS CTE Correction Science Case; MultiDrizzle in the Archive Pipelines.
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| Publication date: 15 Jul 2005 |
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| MULTISPEC: Crowded-Field Slitless Spectroscopy with HST |
| I have developed an automated slitless-spectrum extraction software package that allows for the simultaneous obtention of hundreds of point-source spectra within a field. MULTISPEC is an spectroscopic analog to crowded-field PSF-fitting photometry packages such as DAOPHOT or HSTPHOT in the sense that it fits spatial profiles to the spectrum of each star simultaneously. The code has been written in IDL, and implemented and tested for the STIS NUV-MAMA objective-prism. It could be rather easily adapted to observations with other spectral elements with slitless capabilities (objective prisms, grisms, and gratings) in HST or in other telescopes. I discuss the techniques employed and I show the first results obtained with MULTISPEC. |
| Publication date: 01 Jun 2005 |
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| The stellar IMF of Galactic clusters and its evolution |
| We show that one can obtain a good fit to the measured main sequence mass function (MF) of a large sample of Galactic clusters (young and old) with a tapered Salpeter power law distribution function with an exponential truncation. The average value of the power law index is very close to Salpeter (~2.3), whereas the characteristic mass is in the range 0.1 - 0.5 Msolar and does not seem to vary in a systematic way with the present cluster parameters such as metal abundance and central concentration. However, a remarkable correlation with age is seen, in that the peak mass of young clusters increases with it. This trend does not extend to globular clusters, whose peak mass is firmly at ~0.35 Msolar. This correlation is due to the onset of mass segregation following early dynamical interactions in the loose cluster cores. Differences between globular and younger clusters may depend on the initial environment of star formation, which in turn affects their total mass. |
| Publication date: 01 Jun 2005 |
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| Metals in the Neutral Interstellar Medium of Starburst Galaxies |
| Thanks to their proximity, local starbursts are perfectly suited for high-resolution
and sensitivity multiwavelength observations aimed to test our ideas about star
formation, evolution of massive stars, physics and chemical evolution of the
interstellar medium (ISM). High-resolution UV spectroscopy with FUSE and
STIS has recently given the possibility to characterize in great detail the neutral
ISM in local starbursts thanks to the presence in this spectral range of many
absorption lines from ions of the most common heavy elements. Here we present
the results for two nearby starburst galaxies, I Zw 18 and NGC 1705, and show
how these results relate to the star-formation (SF) history and evolutionary state
of these stellar systems. |
| Publication date: 12 May 2005 |
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| The Complex Star Formation History of NGC 1569 |
| We present new results on the star formation history of the dwarf irregular galaxy NGC 1569. The data were obtained
with the Hubble Space TelescopeNICMOS/NIC2 in the F110W (J ) and F160W(H) near-infrared (NIR) filters
and interpreted with the synthetic color-magnitude diagram method. The galaxy has experienced complex star formation
(SF) activity. The best fit to the data is found by assuming three episodes of activity in the last 1-2 Gyr. The
most recent and strong episode constrained by these NIR data started ~3.7x107 yr ago and ended ~1:3 x 107 yr ago,
although we cannot exclude the possibility that up to three SF episodes occurred in this time interval. The average star
formation rate (SFR) of the episode is ~3.2 MSun yr-1 kpc-2, in agreement with literature data. A previous episode
produced stars between ~1.5x108 and ~4x107 yr ago, with a mean SFR about two-thirds lower than the mean SFR
of the youngest episode. An older SF episode occurred about 1x109 yr ago. All these SFRs are 2-3 orders of
magnitude higher than those derived for late-type dwarfs of the Local Group. In all cases an initial mass function
similar to Salpeter's allows for a good reproduction of the data, but we cannot exclude flatter mass functions. These
results have been obtained adopting a distance of 2.2 Mpc and a reddening E(B - V )=0.56. A larger distance would
require younger episodes and higher SFRs.We have explored some possible scenarios using the astrated mass in the
best-fit model, in order to constrain the past star formation history. We cannot rule out a low past SFR, but we can
safely conclude that the last 1-2 Gyr have been peculiar.
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| Publication date: 10 May 2005 |
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| Hubble rescue mission gears up despite shuttle setback |
| The space shuttle Discovery will have to wait at least another seven weeks before returning to duty, NASA decided last week. But preparations for an astronaut servicing mission to the Hubble Space Telescope will resume anyway - another sign that new NASA administrator Michael Griffin may reverse his predecessor's decision to abandon the telescope later in the decade.
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| Publication date: 05 May 2005 |
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| Masses of Star Cluster in the Nuclei of Bulgeless Spiral Galaxies |
| In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (106108 LSun) but compact (re ~ 5 pc) nuclear star clusters
in very late type spirals with the Ultraviolet and Visual Echelle Spectrograph at the VLT. We find that the velocity dispersions of the nine clusters in our sample range from 13 to 34 km s-1. Using photometric data from the Hubble
Space Telescope WFPC2 and spherically symmetric dynamical models, we determine masses between 8 x 105 and 6 x 107 MSun. The mass-to-light ratios range from 0.2 to 1.5 in the I band. This indicates a young mean age for most clusters, in agreement with previous studies. |
| Publication date: 05 Jan 2005 |
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| The Global Mass Function of M15 |
| Data obtained with the NICMOS instrument on board the Hubble Space Telescope (HST) have been used to determine the H-band luminosity function (LF) and mass function (MF) of three stellar fields in the globular cluster M15, located ~7' from the cluster centre. The data confirm that the cluster MF has a characteristic mass of ~0.3 Msolar, as obtained by Paresce & De Marchi (2000) for a stellar field at 4.6' from the centre. By combining the present data with those published by other authors for various radial distances (near the centre, at 20" and at 4.6'), we have studied the radial variation of the LF due to the effects of mass segregation and derived the global mass function (GMF) using the Michie-King approach. |
| Publication date: 14 Dec 2004 |
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| ST-ECF Newsletter 37 |
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| Publication date: 02 Nov 2004 |
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| The binary progenitor of Tycho Brahe's 1572 supernova |
| The brightness of type Ia supernovae, and their homogeneity as a class, makes them powerful tools in cosmology, yet little is known about the progenitor systems of these explosions. They are thought to arise when a white dwarf accretes matter from a companion star, is compressed and undergoes a thermonuclear explosion. Unless the companion star is another white dwarf (in which case it should be destroyed by the mass-transfer process itself), it should survive and show distinguishing properties. Tycho's supernova is one of only two type Ia supernovae observed in our Galaxy, and so provides an opportunity to address observationally the identification of the surviving companion. Here we report a survey of the central region of its remnant, around the position of the explosion, which excludes red giants as the mass donor of the exploding white dwarf. We found a type G0-G2 star, similar to our Sun in surface temperature and luminosity (but lower surface gravity), moving at more than three times the mean velocity of the stars at that distance, which appears to be the surviving companion of the supernova. |
| Publication date: 28 Oct 2004 |
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| First determination of the dynamical mass of a binary L dwarf |
| We present here the results of astrometric, photometric and spectroscopic observations leading to the determination of the orbit and dynamical masses of the binary L dwarf 2MASSW J0746425+2000321. High angular resolution observations spread over almost 4 years and obtained with the Hubble Space Telescope (HST), the ESO Very Large Telescope (VLT), and a the W. M. Keck Observatory (Keck) allow us to cover ~36% of the period, corresponding to 60% of the orbit, and, for the first time, to derive a precise estimate of the total and individual masses of such a late-type object. We find an orbital period of 3850.9+904-767 days. The corresponding total mass is 0.146+0.016-0.006 MSun with uncertainties depending on the distance. Spatially resolved low resolution optical (550-1025 nm) spectra have been obtained with HST/STIS, allowing us to measure the spectral types of the two components (L0± 0.5 for the primary and L1.5 ±0.5 for the secondary). We also present precise photometry of the individual components measured on the high angular resolution images obtained with HST/ACS and WFPC2 (visible), VLT/NACO (J, H and KS bands) and Keck I (KS band). These spectral and photometric measurements enable us to estimate their effective temperatures and mass ratio, and to place the object accurately in a H-R diagram. The binary system is most likely formed by a primary with a mass of 0.085±0.010 MSun and a secondary with a mass of 0.066±0.006 MSun, thus clearly substellar, for an age of approximately 300±150 Myr. H-alpha variability indicates chromospheric and/or magnetic activity. |
| Publication date: 15 Aug 2004 |
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| ESA SP-1276: ESA's Report to the 35th COSPAR Meeting |
Scientific editor: A. Gimenez
Editor: A. Wilson
The report for the 35th COSPAR Meeting covers, as in previous issues, the missions
of the Scientific Programme of ESA in the areas of astronomy, Solar System
exploration and fundamental physics. This year's COSPAR Meeting occurs only
weeks after the Saturn-orbit insertion of the Cassini spacecraft - carrying Europe's
Huygens probe to explore the atmosphere of Titan - and at the same time as the
launch of the second satellite of the Double Star project. |
| Publication date: 15 Jun 2004 |
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| A Probable z~7 Galaxy Strongly Lensed by the Rich Cluster A2218: Exploring the Dark Ages |
| We discuss the observational properties of a remarkably faint triply imaged galaxy revealed in a deep z'-band Advanced Camera for Surveys observation of the lensing cluster A2218 (z=0.175). A well-constrained mass model for the cluster, which incorporates the outcome of recent Keck spectroscopic campaigns, suggests that the triple system arises via a high-redshift (z>6) source viewed at high magnification (~=25 times). Optical and infrared photometry from Hubble Space Telescope and the Keck Observatory confirms the lensing hypothesis and suggests that a significant discontinuity occurs in the spectral energy distribution within the wavelength interval 9250-9850 Å. If this break is associated with Gunn-Peterson absorption from neutral hydrogen, a redshift of 6.6 < z < 7.1 is inferred. Deep Keck spectroscopy conducted using both optical and infrared spectrographs fails to reveal any prominent emission lines in this region. However, an infrared stellar continuum is detected whose decline below 9800 Å suggests a spectroscopic redshift toward the upper end of the range constrained photometrically, i.e., z~=7. Regardless of the precise redshift, the source is remarkably compact (<~1 h-170 kpc) and faint (zF850LP=28.0) yet is undergoing vigorous star formation at a rate ~=2.6 Msolar yr-1. An intriguing property is the steep slope of the ultraviolet continuum implied by the photometry, which may suggest that the source is representative of an early population of galaxies responsible for cosmic reionization. Independent verification of these results is highly desirable, but our attempts highlight the difficulty of studying such sources with present facilities and the challenges faced in pushing back the frontiers of the observable universe beyond z~6.5. |
| Publication date: 01 Jun 2004 |
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| Spatium 12: Ten Years Hubble Space Telescope |
| In the 1970s, NASA and ESA took up the idea of a space-based telescope. Funding began to flow in 1977. Later, it was decided to name the telescope after Edwin Hubble. Although the Hubble Space Telescope (HST) was downsized later to a 2.4 m primary mirror diameter from the initial 3 m, the project started to attract significant attention from astronomers.
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| Publication date: 01 Jun 2004 |
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| ST-ECF Newsletter 36 |
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| Publication date: 15 May 2004 |
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| Hubble Space Telescope Imaging of HD 44179, The Red Rectangle |
| We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 and deep ground-based images of the Red Rectangle (RR), a bipolar proto-planetary nebula associated with the post-AGB binary system HD 44179. The high-resolution HST images reveal complex new structures, many of them unique to this object. The RR nebula is dominated by a discontinuous "bicone,'' whose bright, sharp linear edges give the nebula an overall X-shaped appearance. The edges of the bicone are connected by a remarkable series of linear features elongated perpendicular to the radius vector, giving the object a ladder-like structure. The "rungs'' of the ladder structure show a quasi-periodic spacing, suggesting that they have arisen from discrete episodes of mass loss from the central star, separated by a few hundred years. The total timescale over which mass has been shed into the visible nebula is of order 14 000 yr. Outside the X-shaped bicone, parabolas curl inward, resembling wineglasses, which terminate on the bicone edges in large, limb-brightened vortices. The central object is bisected by a dark band, indicating that the star is not seen directly but is instead obscured by a surrounding opaque dust disk. |
| Publication date: 15 Apr 2004 |
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| Rings in the haloes of planetary nebulae |
| We present a search for rings or arcs in the haloes of planetary nebulae (PNe). We discovered such structures in eight PNe, tripling the sample of PNe with known rings. This shows that, contrary to what was believed to date, the occurrence of mass loss fluctuations with timescales of 102-10³ yrs at the end of the asymptotic giant branch phase (AGB) is common. We estimate a lower limit of the occurrence rate of rings in PN haloes to be ~35%. Using these new detections and the cases previously known, we discuss the statistical properties of ring systems in PNe haloes. We estimate that the mass modulation producing the rings takes place during the last 10 000 or 20 000 yrs of AGB evolution. In PNe, the spacing between rings ranges from <0.01 pc to 0.06 pc, significantly larger than those seen in proto-PNe. This, together with the finding of a possible positive correlation of spacing with the post-AGB age of the nebulae, suggests that the spacing of the rings increases with time. These properties, as well as the modest surface brightness amplitudes of rings, are consistent with the predictions of the dust-driven wind instability model explored by Meijerink et al., but do not immediately exclude other proposed models. |
| Publication date: 15 Apr 2004 |
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