arXiv subjects

George H. Jacoby

Publications and source records attributed to George H. Jacoby.

47 records · Page 3Linked to original sources

arXiv2002

Planetary Nebulae as Standard Candles. XII. Connecting the Population I and Population II Distance Scales

We report the results of [OIII] lambda 5007 surveys for planetary nebulae (PNe) in NGC 2403, 3115, 3351, 3627, 4258, and 5866. Using on-band/off-band [OIII] and H-alpha images, we identify samples of PNe in these galaxies and derive distances using the planetary nebula luminosity function (PNLF). We then combine these measurements with previous data to compare the PNLF, Cepheid, and surface brightness fluctuation (SBF) distance scales. We use a sample of 13 galaxies to show that the absolute magnitude of the PNLF cutoff is fainter in small, low-metallicity systems, but the trend is well modelled theoretically. When this dependence is removed, the scatter between the Cepheid and PNLF distances becomes consistent with the internal errors of the methods and independent of any obvious galaxy parameter. We then use the data to recalibrate the zero point of the PNLF distance scale. We use a sample of 28 galaxies to show that the scatter between the PNLF and SBF distance measurements agrees with that predicted from the techniques' internal errors, and that no systematic trend exists between the distance residuals and stellar population. However, we find the PNLF and SBF methods have a significant scale offset: Cepheid-calibrated PNLF distances are, on average, ~0.3 mag smaller than Cepheid-calibrated SBF distances. We discuss the possible causes of this offset, and suggest that internal extinction in the bulges of the SBF calibration galaxies is the principle cause of the discrepancy. If this is correct, the SBF-based Hubble Constant must be increased by ~7%. We use our distance to NGC 4258 to argue that the short distance scale to the LMC is correct, and that the global Hubble Constant inferred from the HST Key Project should be increased by 8 +/- 3% to H_0 = 78 +/- 7 km/s/Mpc. (abridged)

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arXiv2002

The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda VI

We have surveyed Andromeda VI, a dwarf spheroidal galaxy companion to M31, for variable stars using F450W and F555W observations obtained with the Hubble Space Telescope. A total of 118 variables were found, with 111 being RR Lyrae, 6 anomalous Cepheids, and 1 variable we were unable to classify. We find that the Andromeda VI anomalous Cepheids have properties consistent with those of anomalous Cepheids in other dwarf spheroidal galaxies. We revise the existing period-luminosity relations for these variables. Further, using these and other available data, we show that there is no clear difference between fundamental and first-overtone anomalous Cepheids in a period-amplitude diagram at shorter periods, unlike the RR Lyrae. For the Andromeda VI RR Lyrae, we find that they lie close to the Oosterhoff type I Galactic globular clusters in the period-amplitude diagram, although the mean period of the RRab stars, = 0.588 d, is slightly longer than the typical Oosterhoff type I cluster. The mean V magnitude of the RR Lyrae in Andromeda VI is 25.29+/-0.03, resulting in a distance 815+/-25 kpc on the Lee, Demarque, & Zinn distance scale. This is consistent with the distance derived from the I magnitude of the tip of the red giant branch. Similarly, the properties of the RR Lyrae indicate a mean abundance for Andromeda VI which is consistent with that derived from the mean red giant branch color.

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arXiv2002

Intracluster Red Giant Stars in the Virgo Cluster

We have used the WFPC2 camera of the Hubble Space Telescope to obtain deep F814W images of a blank field in the Virgo Cluster located 41 arcmin northwest of M87. We perform star counts in that field, and in another Virgo field observed by Ferguson, Tanvir & von Hippel (1998), and show that, when compared to the Hubble Deep Field North and South, the Virgo Cluster contains an excess of objects with magnitudes I > 27. We attribute this excess to a population of intracluster red-giant branch (IC-RGB) stars. By modeling the luminosity function of these stars, we show that the tip of the Virgo RGB is at I = 27.31 +0.27/-0.17 and that the cluster contains a small, but significant, excess of stars that are up to ~1 mag brighter than this tip. If this luminous component is due entirely to stars on the asymptotic giant branch (AGB), it implies an age for the population of > 2 Gyr; if foreground RGB stars contribute to the luminous tail, then the derived age for the stars is older still. The luminosity function also suggests that most of the intracluster stars are moderately metal-rich (-0.8 < [Fe/H] <-0.2), a result consistent with that expected from stars that have been tidally stripped from intermediate luminosity galaxies. Additionally, a comparison with the planetary nebulae in our field also supports this view, although the existence of a more metal-poor population (from stripped dwarfs) cannot be ruled out. Our derived average surface brightness, mu_I = 27.9 +0.3/-0.5 mag/arcsec^2 for Virgo's diffuse component suggests that intracluster stars contribute 10% to 20% of the cluster's total I-band luminosity.

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arXiv2001

A Search for Intra-group Planetary Nebulae in the M 81 Group

We present the initial results from an [O III] lambda 5007 survey for intra-group planetary nebulae in the M 81 group of galaxies. A total of 0.36 square degrees of the survey have been analyzed thus far, and a total of four intra-group candidates have been detected. These data allow us to probe the physics of galaxy interactions in small groups, and give us an upper limit for the density of intracluster starlight. We find that the M 81 group has less than 3% of its stars in an intra-group component; this is much less than the fraction found in richer galaxy clusters.

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arXiv2001

Radio Observations of New Galactic Bulge Planetary Nebulae

We observed 64 newly identified galactic bulge planetary nebulae in the radio continuum at 3 and 6 cm with the Australia Telescope Compact Array. We present their radio images, positions, flux densities, and angular sizes. The survey appears to have detected a larger ratio of more extended planetary nebulae with low surface brightness than in previous surveys. We calculated their distances according to Van de Steene & Zijlstra (1995). We find that most of the new sample is located on the near side around the galactic center and closer in than the previously known bulge PNe. Based on H-alpha images and spectroscopic data, we calculated the total H-alpha flux. We compare this flux value with the radio flux density and derive the extinction. We confirm that the distribution of the extinction values around the galactic center rises toward the center, as expected.

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arXiv1999

A Survey for Low Surface Brightness Galaxies Around M31. II. The Newly Discovered Dwarf Andromeda VI

We present B-, V-, and I-band images, as well as an H alpha image, of And VI. This is the second newly identified dwarf spheroidal (dSph) companion to M31 found using a digital filtering technique applied to the second Palomar Sky Survey for which 1550 square degrees now have been surveyed. And VI was confirmed to be a nearby dSph galaxy when it resolved into stars easily with a short 4-m V-band exposure. Sub-arcsec images taken at the Kitt Peak WIYN 3.5-m telescope provided (I,V-I) and (V,B-V) color-magnitude diagrams that yield a distance of 775 +/- 35 kpc using the tip of the red giant branch method, and a mean metallicity of [Fe/H] = -1.58 +/- 0.20 with a dispersion of approximately 0.3 dex. And VI has a galactocentric distance of approximately 271 kpc and an absolute V magnitude of -11.3. All observed properties of And VI are consistent with its classification as a dSph companion to M31. Despite the recent identification of the And V, And VI, and Cas dwarfs, the Local Group luminosity function remains highly deficient in faint galaxies relative both to CDM simulations of its formation, and to the luminosity functions for richer clusters of galaxies.

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arXiv1998

Chemical Abundances of Planetary Nebulae in the Bulge and Disk of M31

We derive abundances and central star parameters for 15 planetary nebulae (PNe) in M31: 12 in the bulge and 3 in a disk field 14 kpc from the nucleus. No single abundance value characterizes the bulge stars: although the median abundances of the sample are similar to those seen for PNe in the LMC, the distribution of abundances is several times broader, spanning over 1 decade. None of the PNe in our sample approach the super metal-rich ([Fe/H] ~ +0.25) expectations for the bulge of M31, although a few PNe in the sample of Stasinska, Richer, & Mc~Call (1998) come close. This [O/H] vs [Fe/H] discrepancy is likely due to a combination of factors, including an inability of metal-rich stars to produce bright PNe, a luminosity selection effect, and an abundance gradient in the bulge of M31. We show that PNe that are near the bright limit of the [OIII] 5007 planetary nebula luminosity function (PNLF) span nearly a decade in oxygen abundance, and thus, support the use of the PNLF for deriving distances to galaxies (Jacoby 1996) with differing metallicities. We also identify a correlation between central star mass and PN dust formation that partially alleviates any dependence of the PNLF maximum magnitude on population age. Additionally, we identify a spatially compact group of 5 PNe having unusually high O/H; this subgroup may arise from a recent merger, but velocity information is needed to assess the true nature of the objects.

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arXiv1998

The Circumstellar Extinction of Planetary Nebulae

We analyze the dependence of circumstellar extinction on core mass for the brightest planetary nebulae (PNe) in the Magellanic Clouds and M31. We show that in all three galaxies, a statistically significant correlation exists between the two quantities, such that high core mass objects have greater extinction. We model this behavior, and show that the relation is a simple consequence of the greater mass loss and faster evolution times of high mass stars. The relation is important because it provides a natural explanation for the invariance of the [O III] 5007 planetary nebula luminosity function (PNLF) with population age: bright Population I PNe are extinguished below the cutoff of the PNLF. It also explains the counter-intuitive observation that intrinsically luminous Population I PNe often appear fainter than PNe from older, low-mass progenitors.

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arXiv1998

A Survey for Low Surface Brightness Dwarf Galaxies Around M31

By applying a digital filtering technique to 1550 square degrees of the POSS-II in the vicinity of M31, we found two previously unidentified very low surface brightness dwarf galaxies which we designate And V and VI. Follow-up imaging with the KPNO 4-m telescope resolved these into stars easily. The V- and I- band images of And V indicate a distance similar to that of M31, and <[Fe/H]> approximately -1.5. All evidence strongly supports its classification as a dwarf spheroidal companion to M31. Data for And VI are being analyzed, but preliminary indications support a similar conclusion. Our search for more dwarfs, including follow-up observations of numerous candidates found via digital filtering, is incomplete; thus, further identifications may be forthcoming.

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arXiv1998

A Survey for Low-Surface-Brightness Galaxies Around M31. I. The Newly Discovered Dwarf Andromeda V

We present images and a color-magnitude diagram for And V, a new dwarf spheroidal companion to M31 that was found using a digital filtering technique applied to 1550 square degrees of the second Palomar Sky Survey. And V resolves into stars easily in follow-up 4-m V- and I-band images, from which we deduce a distance of 810 +/- 45 kpc using the tip of the red giant branch method. Within the uncertainties, this distance is identical to the Population II distances for M31 and, combined with a projected separation of 112 kpc, provides strong support for a physical association between the two galaxies. There is no emission from And V detected in H alpha, 1.4 GHz radio continuum, or IRAS bandpasses, and there is no young population seen in the color-magnitude diagram that might suggest that And V is an irregular. Thus, the classification as a new dwarf spheroidal member of the Local Group seems secure. With an extinction-corrected central surface brightness of 25.2 V mag per square arcsec, a mean metal abundance of [Fe/H] approximately -1.5, and no evidence for upper AGB stars, And V resembles And I & III.

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arXiv1997

Intracluster Stars and the Galactic Halos of Virgo

Planetary nebulae can be useful dynamical probes of elliptical galaxy halos, but progress thus far has been hampered by the small data samples available for study. In order to obtain a large sample of planetary nebulae for dynamical analysis, we have re-observed the giant elliptical galaxy M87 in the center of the Virgo Cluster. Surprisingly, we find that the M87 sample is contaminated by a significant fraction of intracluster objects. Follow up observations of three blank fields in the cluster confirm that a significant fraction of Virgo's starlight is intracluster. We discuss the implications of intracluster stars for studies of galaxy halos in clusters.

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