arXiv subjects

T. J. Davidge

Publications and source records attributed to T. J. Davidge.

At least 55 records · Page 3Linked to original sources

arXiv2010

Identifying Bright Stars in Crowded Environments Using Velocity Dispersion Measurements, and an Application to the Center of M32

The identification of individual stars in crowded environments using photometric information alone is confounded by source confusion. However, with the addition of spectroscopic information it is possible to distinguish between blends and areas where the light is dominated by a single star using the widths of absorption features. We describe a procedure for identifying locations in kinematically hot environments where the light is dominated by a single star, and apply this method to spectra with 0.1 arcsec angular resolution covering the 2.1 - 2.3 micron interval in the central regions of M32. Targets for detailed investigation are selected as areas of localized brightness enhancement. Three locations where at least 60% of the K-band light comes from a single bright star, and another with light that is dominated by two stars with very different velocities, are identified. The dominant stars are evolving near the tip of the asymptotic giant branch (AGB), and have M5 III spectral type. The lack of a dispersion in spectral-type suggests that the upper AGB within the central arcsec of M32 has a dispersion in J-K of only a few hundreths of a magnitude, in agreement with what is seen at larger radii. One star has weaker atomic absorption lines than the others, such that [M/H] is 0.2 dex lower. Such a difference in metallicity is consistent with the metallicity dispersion inferred from the width of the AGB in M32. The use of line width to distinguish between blends involving many relatively faint stars, none of which dominate the light output, and areas that are dominated by a single intrinsically bright star could be extended to crowded environments in other nearby galaxies.

astro-ph.GA↗

arXiv2010

Ghosts in the Attic: Mapping the Stellar Content of the S0 Galaxy NGC 5102

The spatial distribution of stars in the nearby S0 galaxy NGC 5102 is investigated using images obtained with WIRCam and MegaCam on the Canada-France-Hawaii Telescope. With the exception of gaps between detector elements, the entire galaxy is surveyed in r' and i', while the J and Ks data extend out to 6 kpc (7 disk scale lengths). A modest population of main sequence (MS) stars with M_V < -3.5 and ages 70 Myr are detected throughout the disk, with the majority located in the southern half of the galaxy. The ratio of C stars to bright M giants is consistent with an overall increase in the star formation rate within the past 1 Gyr. Star-forming activity during the interval 0.1 - 2 Gyr was more centrally concentrated than during the past 100 Myr. The structure of the disk changes near 5 kpc (5.5 disk scale lengths). RSGs and bright AGB stars are traced out to a radius of 14 kpc (15.6 scale lengths) along the southern portion of the major axis, while a tentative detection is also made of bright AGB stars at a projected distance of 16 kpc along the south east minor axis. A large clump of AGB stars that subtends an arcmin is identified to the west of the galaxy center. It is argued that this is the remnant of a companion galaxy that triggered past episodes of elevated star-forming activity.

astro-ph.CO↗

arXiv2009

The Stellar Disk of M81

Wide-field images obtained with the 3.6 meter Canada-France-Hawaii Telescope are used to investigate the spatial distribution and photometric properties of the brightest stars in the disk of M81 (NGC 3031). With the exception of the central regions of the galaxy and gaps between CCDs, the survey is spatially complete for stars with i' < 24 and major axis distances of 18 kpc. A more modest near-infrared survey detects stars with K < 20 over roughly one third of the disk. Bright main sequence (MS) stars and RSGs are traced out to galactocentric distances of at least 18 kpc. The spatial distribution of bright MS stars tracks emission at far-ultraviolet, mid- and far-infrared wavelengths, although tidal features contain bright MS stars but have little or no infrared flux. The specific frequency of bright MS stars and RSGs, normalized to K-band integrated brightness, increases with radius, indicating that during the past 30 Myr the specific star formation rate (SSFR) has increased with increasing radius. The stellar content of the M81 disk undergoes a distinct change near R ~ 14 kpc, and the luminosity-weighted mean age decreases with increasing radius in the outer regions of the M81 disk.

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arXiv2008

The Identification of New Stellar Groupings in the M81 Debris Field

The stellar content of the debris field around M81 is investigated using deep images obtained with MegaCam on the Canada-France-Hawaii Telescope. Three new concentrations of bright main sequence stars and red supergiants are identified within ~ 20 kpc of M81. These systems have integrated brightnesses M_V ~ -11 and surface brightnesses ~ 27 - 28 mag per arcsec^2 in V. The main sequence turn-offs are consistent with an age of ~ 30 Myr, while the presence of a well-developed red supergiant plume indicates that their stellar content is not coeval, as they also formed stars ~ 100 Myr in the past. The photometric properties of the red supergiants indicate that the young stars in these systems have metallicities that are comparable to those of other objects in the debris field. The HI density near these groupings comfortably exceeds the threshold required to trigger star formation. Based on the close proximity of these objects to M81, coupled with their extended sizes and low inferred masses, it is argued that they will not be long-lived structures, but will probably dissipate within the next ~ 1 Gyr.

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arXiv2008

And the Rest: The Stellar Archeological Record of M82 Outside the Central Starburst

Deep images obtained with MegaCam and WIRCam on the Canada-France-Hawaii Telescope are used to probe the stellar content outside of the central star-forming regions of M82. Stars evolving on the asymptotic giant branch (AGB) are traced along the major axis out to projected distances of 12 kpc, which corresponds to 13 disk scale lengths. The specific frequency (SF) of bright AGB stars in the outer disks of M82 and the Sc galaxy NGC 2403 are identical, suggesting that the specific star formation rates (SFR) in these galaxies during intermediate epochs were similar. This similarity in stellar content, coupled with the presence of an extended stellar disk, is consistent with M82 having been a late-type disk galaxy prior to interacting with M81. Still, there is a paucity of red supergiants (RSGs) in the outer disk of M82 when compared with NGC 2403, indicating that the SFR in the outer regions of M82 during the past ~ 0.1 Gyr has declined dramatically with respect to that in isolated late-type galaxies. A mixture of bright main sequence stars, RSGs, and AGB stars are detected out to minor axis distances of 7 kpc along the outflow. It is suggested that the young and intermediate aged stars in the extraplanar regions formed in structures similar to M82 South, and that these were subsequently disrupted by the tidal action of M82.

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arXiv2008

An Arc of Young Stars in the Halo of M82

The properties of the brightest resolved stars in an arc that was originally identified by Sun et al. (2005) and is located in the extraplanar regions of M82 are discussed. The stars form an elongated structure that is traced over a projected area of 3.0 x 0.8 kpc. The integrated brightness is M_V ~ -11, while the total stellar mass is between 3 x 10^5 and 2 x 10^6 solar masses. If there is only foreground extinction then the youngest stars have a metallicity Z > 0.008 and an age log(tyears) ~ 7.75; thus, the youngest stars formed at roughly the same time as stars in tidal features that are associated with other M81 Group galaxies. If the arc is dispersing then it will deposit young, chemically enriched stars into the M82 halo.

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arXiv2008

The Stellar Content of the Post-Starburst S0 Galaxy NGC 5102

The stellar content of the S0 galaxy NGC 5102 is investigated. A modest population of bright main sequence stars and red supergiants (RSGs) is detected throughout the western portion of the disk, and the star formation rate (SFR) during the past ten million years is estimated to have been 0.02 solar masses per year. RGB stars are traced out to galactocentric distances of 10 kpc, which corresponds to ~ 14 disk scale lengths. A large population of bright asymptotic giant branch (AGB) stars are seen throughout the western portion of the disk, and it is concluded that (1) stars that formed within the past Gyr comprise ~ 20% of the total stellar disk mass, and (2) the SFR during intermediate epochs in the disk of NGC 5102 was at least 1.4 solar masses per year. It is suggested that NGC 5102 was a spiral galaxy that experienced a galaxy-wide episode of enhanced star formation that terminated a few hundred Myr in the past, and that much of its interstellar medium was ejected in an outflow.

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arXiv2008

Probing the Central Regions of Nearby Compact Elliptical Galaxies

K-band spectroscopic observations recorded with NIFS+ALTAIR on Gemini North are used to probe the central arcsec of the compact elliptical galaxies NGC 4486B, NGC 5846A, and M32. The angular resolution of these data is ~0.1 arcsec FWHM. The central stellar contents of NGC 4486B and NGC 5846A are similar, in the sense that they occupy the same regions of the (Ca I, 12CO), (Na I, 12CO) and (13CO, 12CO) diagrams. The NGC 4486B and NGC 5846A observations depart from the sequence defined by solar neighborhood giants in the (Na I, 12CO) diagram, in a sense that is consistent with both galaxies having non-solar chemical mixtures. For comparison, the M32 data is consistent with a chemical enrichment history like that in the Galactic disk; M32 could not have formed from the stripping of a larger elliptical galaxy. The behaviour of the near-infrared line indices as a function of radius is also investigated. The radial gradients that are present in NGC 4486B and NGC 5846A at large radii break down or reverse within the central few tenths of an arcsec. Evidence is presented that the nuclear regions of NGC 4486B and NGC 5846A harbour intermediate age populations.

astro-ph↗

arXiv2007

NGC 5253 and ESO 269-G058: Dwarf Galaxies with a Past

Deep r' and i' images obtained with GMOS on Gemini South are used to probe the bright stellar content in the outer regions of NGC 5253 and ESO269-G058. Red giant branch (RGB) stars are traced out to a distance of 8 kpc along the major axis of NGC 5253, and 6 kpc in ESO269-G058. The outer regions of both galaxies are metal-poor; RGB stars located between projected major axis distances of 2 and 4 kpc in NGC 5253 have [Fe/H] ~ -1, whereas RGB stars in the corresponding portion of ESO269-G058 have [Fe/H] ~ -1.8. Stars with metallicities that differ from the mean by more than a few tenths of a dex make only a modest contribution to the stellar content in the outer regions of both galaxies. A population of bright asymptotic giant branch (AGB) stars is seen in both galaxies. Roughly 1 - 10% of the stellar mass of NGC 5253 may have formed during the past few hundred million years, and it is suggested that the progenitors of the two recent SN Ia in this galaxy may have formed at this time. It is argued that the current episodes of star formation in NGC 5253 and ESO269-G058 may have been triggered up to ~ 1 Gyr in the past. Finally, a distance modulus is computed for each galaxy based on the brightness of the RGB-tip, and the results are 27.48 +/- 0.14 for NGC 5253, and 27.93 +/- 0.18 for ESO269-G058.

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arXiv2007

The Disk and Extraplanar Regions of NGC 2403

Wide field images obtained with WIRCam and MegaCam on the Canada-France-Hawaii Telescope are used to probe the spatial distribution of young stars in the Sc galaxy NGC 2403. Bright main sequence stars and blue supergiants are detected out to projected galactocentric distances of ~14 kpc (~7 disk scale lengths) along the major axis. The star formation rate (SFR) in the disk of NGC 2403 during the past 10 Myr has been 1 solar masses per year based on the number of bright main sequence stars. The radially-averaged number density of red supergiants (RSGs) per unit r-band surface brightness is constant throughout the disk, indicating that (1) RSGs follow the integrated r-band light, and (2) the SFR per unit mass has been constant with radius when averaged over time scales of a few tens of millions of years. The mean color of RSGs varies with galactocentric distance, suggesting that there is a metallicity gradient among recently formed stars. A comparison of the radially-averaged number density of bright main sequence stars also indicates that the SFR per unit stellar mass in NGC 2403 has been ~3 times higher than in NGC 247 during recent epochs, and this is in rough agreement with what would be predicted from the far infrared fluxes of these galaxies. Finally, the data are used to investigate the extraplanar regions of NGC 2403. A population of M giants with peak brightness M_K = -8 is detected at projected distances between 12 and 14 kpc above the disk plane, and six new globular cluster candidates are identified.

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arXiv2006

The Brightest Stars in M32: Comparing Predictions from Spectra with the Resolved Stellar Content

Broad- and narrow-band images covering the 1 - 4um wavelength interval are used to investigate the properties of the brightest AGB stars in the Local Group galaxy M32. The brightest AGB stars near the center of M32 have peak M_L' brightnesses and K-L' colors that are similar to those of luminous AGB stars in the Galactic disk, while the density of bright AGB stars per unit visible and near-infrared surface brightness is found to be constant out to projected major axis distances of 1 kpc. The J-K color distribution of bright AGB stars throughout much of the galaxy is consistent with that of a single population of AGB stars, the majority of which are long period variables, having a common metallicity and age. Thus, these data do not support spectroscopic studies that find an age gradient in M32. The well-mixed AGB content of M32 is consistent with tidal stirring. The stellar content of M32 is compared with that of the M31 bulge. While the peak K-band brightnesses of AGB stars in the two systems agree to within a few tenths of a magnitude, M32 contains more bright AGB stars per unit integrated brightness than the outer bulge of M31.

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arXiv2006

Red Supergiants in the Disk of M81: Tracing the Spatial Distribution of Star Formation 25 Million Years in the Past

Near-infrared images are used to investigate the brightest red stars in the disk of the nearby spiral galaxy M81. Red supergiants (RSGs) form a well-defined sequence on the color-magnitude diagrams (CMDs) that peaks near M_K = -11.5; RSGs with this peak brightness are seen throughout all fields that were studied, indicating that star formation occured over a large part of the M81 disk only ~ 10 Myr in the past. The number of RSGs per unit integrated K-band light is compared at various locations in the disk. We conclude that star-forming activity in M81 during the past 10 - 25 Myr (1) was distributed over a larger fraction of the disk than it is at the present day, and (2) was not restricted to a given radial interval, but was distributed in a manner that closely followed the stellar mass profile. Star counts indicate that the mean SFR of M81 between 10 and 25 Myr in the past was ~ 0.1 solar masses per year, which is not greatly different from the present day SFR estimated from Halpha and FUV emission.

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arXiv2006

The Central Regions of M31 in the 3 - 5 micron Wavelength Region

Images obtained with NIRI on the Gemini North telescope are used to investigate the photometric properties of the central regions of M31 in the 3 - 5 micron wavelength range. The light distribution in the central arcsecond differs from what is seen in the near-infrared in the sense that the difference in peak brigh tness between P1 and P2 is larger in M' than in K'; no obvious signature of P3 is dete cted in M'. These results can be explained if there is a source of emission that contributes ~ 20% of the peak M' light of P1 and has an effective temperature of no more than a few hundred K that is located between P1 and P2. Based on the red K-M' color of this source, it is suggested that the emission originates in a circumstellar dust shell surrounding a single bright AGB star. A similar bright source that is ~ 8 arcsec from the center of the galaxy is also detected in M'. Finally, the (L', K-L') color-magnitude diagram of unblended stars shows a domin ant AGB population with photometric characteristics that are similar to those of the most luminous M giants in the Galactic bulge.

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arXiv2006

The Disk and Extraplanar Environment of NGC 247

The stellar content of the spiral galaxy NGC 247 is investigated. The main sequence turn-off (MSTO) in the inner 12 kpc of the disk corresponds to an age of 6 Myr. A mean star formation rate (SFR) of 0.1 solar masses per year during the past 16 Myr is computed from star counts. The color of the red supergiant plume does not change with radius, suggesting that the mean metallicity of young stars does not vary by more than 0.1 dex. The number of bright main sequence stars per local stellar mass density climbs towards larger radii out to a distance of 12 kpc; the scale lengths that characterize the radial distributions of young and old stars in the disk thus differ. The density of bright main sequence stars with respect to projected HI mass gradually drops with increasing radius. The population of very young stars disappears in the outer disk; the MSTO at galactocentric radii between 12 and 15 kpc corresponds to 16 Myr, while between 15 and 18 kpc the age is > 40 Myr. Red giant branch (RGB) stars are resolved at a projected minor axis galactocentric distance of 12 kpc. There is a broad spread in metallicity among the RGB stars, with a mean [M/H] = -1.2. The RGB-tip occurs at i' = 24.5 +/- 0.1, indicating that the distance modulus is 27.9 +/- 0.1. Luminous AGB stars with an age 3 Gyr are also seen in this field.

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arXiv2005

The Evolved Stellar Content of NGC 147, NGC 185, and NGC 205

Broad-band near-infrared images are used to probe the photometric properties of the brightest asymptotic giant branch (AGB) stars within 2 arcminutes of the centers of the dwarf elliptical galaxies NGC 147, NGC 185, and NGC 205. Based on the peak brightness and color of the M giant sequences, ages of 1 Gyr and 0.1 Gyr are predicted for the most recent significant star forming events in NGC 185 and NGC 205, respectively. The brightest AGB stars in NGC 147 indicate that the most recent significant star-forming activity in that galaxy occured \~3 Gyr in the past. The specific frequency of C stars outside of the areas of most recent star formation is found to agree in all three galaxies. The fractional contribution made by C stars to the total AGB light in the K-band is found to be highest in NGC 147 and lowest in the central regions of NGC 205, which is qualitatively consistent with model predictions. The fractional contribution that C stars make to the total K-band light is found to be constant both within and between galaxies, with C stars contributing 2% of the total K-band light. It is concluded that, when averaged over timescales of a few Gyr, these galaxies have turned similar fractions of gas and dust, normalised according to total galaxy mass, into stars.

astro-ph↗

arXiv2005

The Globular Cluster Content of Maffei 1

Near-infrared images have been used to search for bright globular clusters near the center of the elliptical galaxy Maffei 1. The number of objects in various radial intervals from the center of Maffei 1 are compared with the number density of sources in two control fields, and we find an excess of 31 +/- 11 objects with K between 16.6 and 18.0 in an annulus between 20 and 90 arcsec from the galaxy center. At radii in excess of 90 arcsec the ability to find clusters is frustrated by statistical noise in the foreground star population. It is demonstrated that the globular clusters located within 90 arcsec of the center of Maffei 1 span a range of near-infrared colors that are at least as great as that which is seen among M31 clusters; some of the clusters appear to have colors that are consistent with them being very young. The specific globular cluster frequency within 90 arcsec of the center of Maffei 1 is S_N = 1.3 +/- 0.6 if the distance modulus is 28.1 +/- 0.2 and A_V = 4.7 +/- 0.2. The specific cluster frequency of Maffei 1 is thus similar to that of other nearby field elliptical galaxies.

astro-ph↗

arXiv2005

The Disk and Extraplanar Regions of NGC 55

The stellar content of the nearby SB(s)m galaxy NGC 55 is investigated using images covering the visible and near-infrared wavelength regions. A well-defined plume, which stellar evolution models suggest contains stars with masses near the RSG - AGB transition, is detected in CMDs of the disk, and it is concluded that star formation in the thin disk of NGC 55 has ocurred at a significant rate for at least the past 0.1 - 0.2 Gyr. The disk also contains a large population of old (log(t_yr) ~ 10) stars, and it is argued that a stable disk has been in place in NGC 55 for a significant fraction of the age of the Universe. At projected distances in excess of 2 kpc off of the disk plane the brightest AGB stars have ages 10(+3)(-2) Gyr. Thus, despite indications that dust and gas are present in the envelope surrounding the NGC 55 disk, the AGB content suggests that recently formed stars do not occur in large numbers in the extraplanar region. The mean metallicity of extraplanar RGB stars is in excellent agreement with that measured in the extraplanar HII regions EHR 1 and 2, suggesting that the age-metallicity relation in this part of NGC 55 has been flat for at least a few Gyr. Finally, the RGB-tip occurs near i' = 23.1 in the extraplanar region, and a distance modulus of 26.5 is computed from this feature.

astro-ph↗

arXiv2004

Deep ALTAIR + NIRI Imaging of the Disk and Bulge of M31

Deep J, H, and K' images, recorded with the ALTAIR adaptive optics system and NIRI imager on Gemini North, are used to probe the stellar content of the disk and bulge of the Local Group galaxy M31. With FWHM near 0.08 arcsec in K, these are the highest angular resolution near-infrared images yet obtained of this galaxy. Four fields that sample M31 at galactocentric radii of 62, 9, 4, and 2 arcmin were observed. The RGB-tip occurs between K = 17.0 and 17.2, and the color of the RGB in the field closest to the center of M31 is consistent with that of NGC 6528. After accounting for random photometric errors, the upper RGB in each field has a width on the (K, J-K) CMD that is consistent with a +/- 0.5 dex dispersion in [Fe/H], in rough agreement with what is seen in other disk and spheroid fields in M31. A population of very bright red stars, which we identify as C stars, are seen in the three fields that are closest to the center of M31. The spatial distribution of these objects suggests that they are well mixed throughout this part of M31, and so likely did not form in a compact region near the galactic nucleus, but more probably formed in the inner disk. We speculate that these C stars may be the most luminous members of the intermediate age population that has been detected previously in studies of the integrated spectrum of the central regions of M31.

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