arXiv subjects

P. Goudfrooij

Publications and source records attributed to P. Goudfrooij.

25 records · Page 2Linked to original sources

arXiv2002

Extragalactic Globular Clusters in the Near-Infrared

Visual and near-infrared colours are used to identify intermediate-age globular cluster in early-type galaxies. In NGC5846 and NGC4365 colour-colour diagrams (V-I vs. V-K) reveal a sub-population in the globular cluster systems with an age much younger than the expected old populations (13-15 Gyr). A comparison with SSP models dates the formation of this younger population 2-7 Gyr back. Further evidence for multiple star formation events is given by the age distribution in both systems and the comparison to models. Our photometric age determination was confirmed by spectroscopy for a small sample of the clusters in NGC4365.

astro-ph↗

arXiv2002

Stellar Populations in Bulges of Spiral Galaxies

We have conducted a spectroscopic analysis of the bulges of a sample of 31 spiral galaxies, spanning the Hubble sequence from S0 to Scd type, in order to study the radial distribution of their stellar population properties.

astro-ph↗

arXiv2000

Deep HST+STIS Color-Magnitude Diagrams of the Dwarf Irregular Galaxy WLM: Detection of the Horizontal Branch

We have obtained V and I-band photometry for 1886 stars down to I=27 and V=28 in the field of the dwarf irregular galaxy WLM, using deep STIS CL and LP-band images taken in the parallel mode with the Hubble Space Telescope. The photometry is used to study the horizontal branch identified in WLM. The horizontal branch, extending blueward from the red giant clump, is an unambiguous signature of an old population. We demonstrate that it is possible to reach the horizontal branch of an old population at a distance of ~1 Mpc using STIS, with relatively short exposure times. From the VI color-magnitude diagram we obtain an accurate distance modulus (m-M)_0 = 24.95 \pm 0.13 for WLM by using the V-magnitude of the horizontal branch, and by adopting E(V-I)=0.03. The implications are: 1) WLM formed stars at high redshift, and 2) the old population of WLM can be representative of a protogalactic fragment, related to those accreted to form the Milky Way halo.

astro-ph↗

arXiv2000

ISO far-infrared observations of rich galaxy clusters II. Sersic 159-03

The far-infrared emission from rich galaxy clusters is investigated. Maps have been obtained by ISO at 60, 100, 135, and 200 microns using the PHT-C camera. Ground based imaging and spectroscopy were also acquired. Here we present the results for the cooling flow cluster Sersic 159-03. An infrared source coincident with the dominant cD galaxy is found. Some off-center sources are also present, but without any obvious counterparts.

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arXiv1999

Line Strengths and Line Strength Gradients in Bulges along the Hubble Sequence

We present first results of a comprehensive survey of deep long-slit spectra along the minor axis of bulges of edge-on spiral galaxies. Our results indicate that stellar populations in bulges are fairly old and encompass a range of metallicities. The luminosity-weighted ages of bulges range from those found for cluster ellipticals to slightly ``younger'' (by up to only a few Gyr, however). Their $α$/Fe element ratio is typically supersolar, consistent with those found in giant ellipticals. The radial line-strength gradients in bulges correlate with bulge luminosity. Generally, these findings are more compatible with predictions of the ``dissipative collapse'' model than with those of the ``secular evolution'' model.

astro-ph↗

arXiv1999

The Fading Optical Counterpart of GRB~970228, Six Months and One Year Later

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope STIS CCD approximately six months after outburst and with the HST/NICMOS and Keck/NIRC approximately one year after outburst. The unresolved counterpart is detected by STIS at V=28.0 +/- 0.25, consistent with a continued power-law decline with exponent -1.14 +/- 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~0."8 and integrated magnitude V=25.8 +/- 0.25. A reanalysis of HST and NTT observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Only the extended source is visible in the NICMOS images with a magnitude of H=23.3 +/- 0.1. The Keck observations find K = 22.8 +/- 0.3. Several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with A_V = 0.75 +/- 0.2. After adjusting for Galactic extinction, we find that the size of the observed extended emission is consistent with that of galaxies of comparable magnitude found in the Hubble Deep Field (HDF) and other deep HST images. Only 2% of the sky is covered by galaxies of similar or greater surface brightness; therefore the extended source is almost certainly the host galaxy. Additionally, we find that the extinction-corrected V - H and V - K colors of the host are as blue as any galaxy of comparable or brighter magnitude in the HDF. Taken in concert with recent observations of GRB 970508, GRB 971214, and GRB 980703 our work suggests that all four GRBs with spectroscopic identification or deep multicolor broad-band imaging of the host lie in rapidly star-forming galaxies.

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arXiv1996

Globular Clusters in the Inner Regions of NGC5128 (Cen A)

We have identified 26 new globular cluster candidates in the inner 3 kpc of NGC 5128 (Cen A), the nearest known large galaxy being the probable product of a merger. The clusters are selected on the basis of their structural parameters (observed core diameters and ellipticities), as measured from archival WFPC HST images. IR photometry obtained with IRAC2B at the ESO/MPI 2.2-m telescope is combined with the optical HST photometry. Most of these clusters have normal colors typical of old globular clusters like those found in the Milky Way and M31. We estimate their metal abundances based on the R-K_0 color, confirming the existence of a metallicity gradient in the inner regions of NGC 5128. The presence of metal-rich globular clusters suggests that one of the colliding galaxies was a bulge-dominated galaxy (E or early S). A few clusters have colors and magnitudes similar to intermediate-age clusters containing carbon stars in the Magellanic Clouds. If the intermediate-age clusters were formed during a merger, then this episode must have occurred a few Gyr ago. Alternatively, we are looking at the cluster members of one of the colliding galaxies, which would then have been a late-type disk galaxy.

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