arXiv subjects

Michael G. Richer

Publications and source records attributed to Michael G. Richer.

28 records · Page 2Linked to original sources

arXiv2006

The Extinction and Distance of Maffei 2 and a New View of the IC 342/Maffei Group

We have obtained spectra of HII regions in the heavily obscured spiral galaxy Maffei 2. The observations have allowed for a determination of the Galactic extinction of this galaxy using a correlation between extinction and hydrogen column density observed among spirals. The technique reveals that the optical depth of Galactic dust at 1 micron obscuring Maffei 2 is tau1 = 2.017 +/- 0.211, which implies that A_V = 5.58 +/- 0.58 mag, significantly higher than observed for the giant elliptical Maffei 1 despite its similar latitude. For comparison, we apply the same technique to IC 342, a neighbouring spiral to Maffei 2 but with more moderate obscuration by Galactic dust, owing to its higher Galactic latitude. For this galaxy, we obtain tau1 = 0.692 +/- 0.066, which agrees within errors with the value of 0.639 +/- 0.102 derived from the reddening estimate of Schlegel et al. (1998, ApJ, 500, 525). We therefore adopt the weighted mean of tau1 = 0.677 +/- 0.056 for the extinction of IC 342, which implies that A_V = 1.92 +/- 0.16 mag. A new distance estimate for Maffei 2 of 3.34 +/- 0.56 Mpc is obtained from a self-consistent Tully-Fisher relation in I adjusted to the NGC 4258 maser zero-point. With our new measurement of M_I, Maffei 2 joins Maffei 1 and IC 342 as one of three giant members of the nearby IC 342/Maffei group of galaxies. We present the revised properties of all three galaxies based on the most accurate extinction and distance estimates to date, accounting for shifts in the effective wavelengths of broadband filters as this effect can be significant for highly reddened galaxies. The revised distances are consistent with what would be suspected for the Hubble Flow, making it highly unlikely that the galaxies interacted with the Local Group since the Big Bang.

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arXiv2006

The Spectroscopic Properties of Bright Extragalactic Planetary Nebulae

The properties of bright extragalactic planetary nebulae are reviewed based upon the results of low and high resolution spectroscopy. It is argued that bright extragalactic planetary nebulae from galaxies (or subsystems) with and without star formation have different distributions of central star temperature and ionization structure. As regards the chemical compositions, oxygen and neon are generally found to be unchanged as a result of the evolution of the stellar progenitors. Nitrogen enrichment may occur as a result of the evolution of the progenitors of bright planetary nebulae in all stellar populations, though this enrichment may be (more) random in old stellar populations. Helium abundances appear to be influenced by the chemical evolution of the host galaxy, with planetary nebulae in dwarf spheroidals having systematically elevated abundances. Neither the age nor the metallicity of the progenitor stellar population has a strong effect upon the kinematics observed for nebular shells. Both the range of expansion velocites, 8-28 km/s, and the typical expansion velocity, ~18 km/s, are found to be relatively constant in all galaxies. On the other hand, bright planetary nebulae in the bulge of M31 have systematically higher expansion velocities than their counterparts in M31's disk. The expansion velocities show no trend with nebular Hbeta luminosity, apart from a lack of large expansion velocities at the highest luminosities (the youngest objects), but appear to correlate with the 5007/Hbeta ratio, at least until this ratio saturates. These results suggest a link between the evolution of the nebular shells and central stars of bright extragalactic planetary nebulae.

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arXiv2005

Iron depletion in the hot bubbles in planetary nebulae

We have searched for the emission from \fex and \fexiv that is expected from the gas emitting in diffuse X-rays in \bd, NGC 6543, NGC 7009, and NGC 7027. Neither line was detected in any object. Models that fit the X-ray spectra of these objects indicate that the \fex emission should be below our detection thresholds, but the predicted \fexiv emission exceeds our observed upper limits (one sigma) by factors of at least 3.5 to 12. The best explanation for the absence of \fexiv is that the X-ray plasma is depleted in iron. In principle, this result provides a clear chemical signature that may be used to determine the origin of the X-ray gas in either the nebular gas or the stellar wind. At present, though various lines of evidence appear to favour a nebular origin, the lack of atmospheric and nebular iron abundances in the objects studied here precludes a definitive conclusion.

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arXiv2005

Infrared Properties of Star Forming Dwarf Galaxies: I. Dwarf Irregular Galaxies in the Local Volume

A sample of 34 dwarf irregular galaxies in the Local Volume, most nearer than 5 Mpc, has been imaged in the near-infrared (NIR) in J and K_s at CFHT in Hawaii and OAN-SPM in Mexico. In CFHT images, stars brighter than M_Ks=-7.5 were resolved. In most cases, the resolved population represents less than 5% of the total NIR flux in K_s. Although exponential at large radii, surface brightness profiles for the unresolved component flatten in the centres. They can be fitted across the whole range of radii with a hyperbolic secant (sech) defined as a function of two parameters: the central surface brightness and the scale length of the exponential. Isophotal, total, and fitted sech magnitudes have been calculated for all galaxies for which the unresolved component was detected, along with semimajor axes. The scale length and the semimajor axes correlate linearly with absolute isophotal magnitude. The same is true for colors and the central brightness. More luminous dIs tend to be larger, redder and brighter in the centre. The Tully-Fisher relation shows considerable scatter, but residuals are tied to surface brightness. The galaxies appear to lie in a ``fundamental plane'' defined by the sech absolute magnitude, the sech central surface brightness, and the HI line-width. The rms of residuals in M_K is only 0.4 mag, which implies that the plane can be used to evaluate the distances of star-forming dwarfs. Color-magnitude diagrams are presented for 29 galaxies in which stars were resolved. Most show a finger centered around J-K_s=1 mag. In some cases, there is a red tail extending to J-K_s=2.5 mag. (Abstract slightly shortened).

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arXiv2004

Planetary Nebulae as Probes of the Chemical Evolution of Dynamically Hot Systems

The measurement of chemical abundances in planetary nebulae in nearby galaxies is now relatively straightforward. The challenge is to use these chemical abundances to infer the chemical evolution of their host galaxies. At this point, our understanding of chemical evolution based upon planetary nebulae in galaxies without star formation is strongly coupled to our understanding of the relationship between the chemical abundances in stars and planetary nebulae in the Milky Way bulge. Supposing that the same relation holds in all systems where star formation ceased long ago, these systems follow a metallicity-luminosity relation that is displaced to higher chemical abundances compared to that found for dwarf irregular galaxies. A more efficient enrichment process appears to be required as part of the explanation for this shift, in addition to the inevitable fading of these galaxies.

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arXiv2003

Uncovering Additional Clues to Galaxy Evolution. II. The Environmental Impact of the Virgo Cluster on the Evolution of Dwarf Irregular Galaxies

The impact of the cluster environment on the evolution of dwarf galaxies is investigated by comparing the properties of a sample of dwarf irregulars (dIs) in the Virgo Cluster with a control sample of nearby ("field") dIs having oxygen abundances derived from [O III]4363 measurements and measured distances from resolved stellar constituents. Spectroscopic data are obtained for H II regions in 11 Virgo dIs distributed in the central and outer regions of the cluster. To ensure that oxygen abundances are derived in a homogeneous manner, oxygen abundances for field and Virgo dIs are computed using the bright-line method and compared with abundances directly obtained from [O III]4363, where available. They are found to agree to within about 0.2 dex with no systematic offset. At a given optical luminosity, there is no systematic difference in oxygen abundance between the sample of Virgo dIs and the sample of nearby dIs. However, five of the eleven Virgo dIs exhibit much lower baryonic gas fractions than field dIs at comparable oxygen abundances. Using field dIs as a reference, a gas-deficiency index for dIs is constructed, making it possible quantitatively to identify which galaxies have lost gas. For the Virgo sample, some of the dwarfs are gas-deficient by a factor of 30. The gas-deficiency correlates roughly with the X-ray surface brightness of the intracluster gas. Ram-pressure stripping can best explain the observed gas-poor dIs in the cluster sample. Together with the lack of significant fading and reddening of the gas-poor dIs compared to gas-normal dIs, these observations suggest that the gas-poor dIs in Virgo have recently encountered the intracluster medium for the first time. Faded remnants of gas-poor dIs in Virgo will resemble bright dwarf ellipticals presently seen in the cluster core.

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arXiv2002

Optical Line profile variability of the B1+neutron star binary system LS I+65 010 = 2S 0114+650

We present high resolution spectroscopic observations of the massive X-ray binary system LS I+65 010=2S 0114+650 in the optical wavelength region. A correlation between equivalent width and radial velocity of photospheric absorption lines is found. The systemic velocity, inferred from the weaker lines is v_helio = -31 +/- 5 km s^-1, which, if attributed solely to the Galactic rotation curve, implies that LS I+65 010 lies within 3 kpc from the Sun. The ISM Na I D lines display 2 resolved high velocity components at v_helio = -70, -48 km s^-1, possibly associated with gas surrounding the binary system, in addition to the -24, -8 km s^-1 ISM features due to the Orion and the Perseus arm regions. Strong photospheric line profile variability is present on a night to night timescale, with He I 5875 A displaying an additional blue-shifted absorption in some of the spectra, similar to what is observed in the optical counterpart of Vela X-1. A connection between the extended blue wing and X-ray maximum is suggested. Short timescale variations in line profiles are detected on only two nights, but the evidence that these variations occur on the 2.78 hour X-ray flaring period is marginal.

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arXiv1999

The Dwarf Irregular Galaxy UGC 7636 Exposed: Stripping At Work In The Virgo Cluster

We present the results of optical spectroscopy of a newly discovered H II region residing in the H I gas cloud located between the dwarf irregular galaxy UGC 7636 and the giant elliptical galaxy NGC 4472 in the Virgo Cluster. By comparing UGC 7636 with dwarf irregular galaxies in the field, we show that the H I cloud must have originated from UGC 7636 because (1) the oxygen abundance of the cloud agrees with that expected for a galaxy with the blue luminosity of UGC 7636, and (2) M_{H I}/L_B for UGC 7636 becomes consistent with the measured oxygen abundance of the cloud if the H I mass of the cloud is added back into UGC 7636. It is likely that tides from NGC 4472 first loosened the H I gas, after which ram-pressure stripping removed the gas from UGC 7636.

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arXiv1998

The chemical evolution of dynamically hot galaxies

We investigate the chemical properties of M32, the bulges of M31 and the Milky Way, and the dwarf spheroidal galaxies NGC 205, NGC 185, Sagittarius, and Fornax using previously published oxygen abundances for their planetary nebulae. Our principal result is that the mean stellar oxygen abundances for all of these galaxies correlate very well with their mean velocity dispersions. This implies that the balance between energy input from type II supernovae and the gravitational potential energy controls how far chemical evolution proceeds in bulges, ellipticals, and dwarf spheroidals. It appears that chemical evolution ceases once supernovae have injected sufficient energy that a galactic wind develops. All of the galaxies follow a single relation between oxygen abundance and luminosity, but the dwarf spheroidals have systematically higher [O/Fe] ratios than the other galaxies. Consequently, dynamically hot galaxies do not share a common star formation history nor need they share a common chemical evolution, despite attaining similar mean stellar oxygen abundances when forming similar stellar masses. The oxygen abundances support previous indications that the stars in higher luminosity ellipticals and bulges were formed on a shorter time scale than their counterparts in less luminous systems. We calibrate the Mg$_2$ index for dynamically hot galaxies as a function of observed age and oxygen abundance using the mean oxygen abundances from the planetary nebulae and the best age estimates available for the stars in M32 and the bulges of M31 and the Milky Way.

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arXiv1998

Planetary nebulae in M32 and the bulge of M31: Line intensities and oxygen abundances

We present spectroscopy of planetary nebulae in M32 and in the bulge of M31 that we obtained with the MOS spectrograph at the Canada-France-Hawaii Telescope. Our sample includes 30 planetary nebulae in M31 and 9 planetary nebulae in M32. We also observed one H II region in the disk of M31. We detected [O III]$λ$4363 in 18 of the planetary nebulae, 4 in M32 and 14 in the bulge of M31. We use our line intensities to derive electron temperatures and oxygen abundances for the planetary nebulae.

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