arXiv subjects

Nelson Caldwell

Publications and source records attributed to Nelson Caldwell.

126 records · Page 7Linked to original sources

arXiv2004

Testing the Universality of the (U-V) Color-Magnitude Relations for Nearby Clusters of Galaxies

We present a detailed U-V CMR analysis for 3 local (z<0.06) Abell (A85, A496, A754) galaxy clusters, with 637 total spectroscopic members down to 0.1L^star. We find that these clusters have similar CMRs with a mean color of U-V=1.4 at M_V=-20+5logh, and narrow limits of intrinsic color scatter [0.047,0.079] and slope [-0.094,-0.075]. The core cluster population of red galaxies follow CMRs that are in close agreement with that of Coma. We test how sensitive the CMR uniformity is to variations in color aperture size and sample selection, and find slightly wider limits of scatter [0.047,0.112] and slope [-0.104,-0.054]. This upper limit for U-V scatter is consistent with z_form>1.2 for the bulk of the stars in red cluster galaxies, and a maximum age spread of 5.2 Gyr. We note that none of the slopes exhibit a break as claimed by Metcalf et al., and using colors from equal-light apertures does not remove the CMR slope. Our findings expand the single Coma data point and provide a much-needed z=0 base-line for comparisons to high-z cluster U-V CMRs. The CMR scatter range in low-z clusters is consistent with that found in high-z clusters, suggesting little evolution in the stars of red cluster galaxies out to z~1. To identify the most recently accreted galaxies, we divide each cluster's membership into 3 galaxy populations based on CMR-relative U-V color. Blue galaxies make up 18-23% of each cluster's galaxy counts above 0.1L^star. We test the hypothesis that CMR-relative color represents rough cluster infall times and find that blue galaxies are spatially, kinematically, and morphologically distinct from red cluster members. These distinctions provide further evidence that bluer members are indeed more recently accreted field spirals as expected in an hierarchical universe. (abridged)

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arXiv2004

GRB 021211 as a Faint Analogue of GRB 990123: Exploring the Similarities and Differences in their Optical Afterglows

We present B, V, Rc, J, H, and Ks photometry of the optical afterglow of the gamma-ray burst GRB 021211 taken at the Magellan, MMT, and WIYN observatories between 0.7 and 50 days after the burst. We find an intrinsic spectral slope at optical and near-infrared wavelengths of 0.69 +/- 0.14 at 0.87 days. The optical decay during the first day is almost identical to that of GRB 990123 except that GRB 021211's optical afterglow was intrinsically approximately 38 times fainter, and the transition from the reverse shock to the forward shock may have occurred earlier than it did for GRB 990123. We find no evidence for a jet break or the cooling break passing through optical frequencies during the first day after the burst. There is weak evidence for a break in the J-band decay between 0.89 and 1.87 days which may be due to a jet. The optical and infrared data are consistent with a relativistic fireball where the shocked electrons are in the slow cooling regime and the electron index is 2.3 +/- 0.1. The forward shock appears to have been expanding in a homogeneous ambient medium during the first day after the burst. Our analysis suggests that the jet of GRB 021211 may have a small opening angle and that the total gamma-ray energy is likely to be much less than the canonical value of 1.33E+51 erg. If this is the case then it is possible that most of the energy of the burst is in another form such as a frozen magnetic field, supernova ejecta, or a second jet component. The host galaxy of GRB 021211 is subluminous and has a star formation rate of at least one Solar mass per year.

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arXiv2004

Structural Evidence for Environment-Driven Transformation of the Blue Galaxies in Local Abell Clusters - A85, A496, and A754

We compare V-band structural properties of cluster and field galaxies, to test whether much of the current cluster membership resulted from the transformation of infalling spirals into red early types. We use 140 galaxies from 3 z<0.6 Abell clusters (A85,A496,A754 - spectroscopic membership) with blue colors, and compare them to 80 field galaxies with similar colors and luminosities. Our previous work (McIntosh et al. 2004, ApJ submitted) shows that the blue galaxies in these local clusters are a recently infalling population that has yet to encounter the dense core. We quantify galaxy internal structure from 2D B/D decompositions using GIM2D. We observe structural differences between blue galaxies in clusters, compared to the field. Most blue cluster members are physically smaller and fainter than their field counterparts. At a matched size and luminosity, recent arrivals are smoother in appearance, yet their total light is as disk-dominated as in normal field spirals. Moreover, 50% of blue cluster galaxies have blue cores or globally blue color profiles, in contrast with field spirals that have typically red color gradients. Without environment dependent evolution outside of cluster cores, we expect blue disk galaxies in clusters and the field to have similar morphologies, sizes, and color gradients. We show conclusively that galaxy properties do reflect the environment in which the galaxy is found. The data show that the transformation of accreted galaxies is not confined to the dense cluster core. The overall properties of bluer cluster galaxies are best explained by environment-driven transformation of accreted field spirals, and our results suggest that the processes that govern color and morphological evolution occur separately. (abridged)

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arXiv2003

Population Synthesis in the Blue III. The Integrated Spectrum of M67 and the Spectroscopic Age of M32

We construct an integrated spectrum of the intermediate age, solar metallicity Galactic cluster M67, from individual spectra of cluster members. The integrated spectrum is used as a template to test our stellar population synthesis (SPS) models, in an age and [Fe/H] regime where such models remain largely untested. We show that our models predict a spectroscopic age of 3.5 +/- 0.5 Gyr for M67, in excellent agreement with the age we derive from the color-magnitude diagram of the cluster. The same age is obtained when using Hbeta, Hgamma or Hdelta as the age indicator. Our models predict the abundances of Fe, Mg, C and N in agreement with detailed abundance analyses of cluster stars, to within 0.1 dex. Encouraged by the high degree of consistency of our models, we apply them to the study of the integrated spectrum of the central 3 arcsec of the compact elliptical galaxy M32. The resulting luminosity-weighted age of the galaxy ranges between 2 and 3.5 Gyr, depending on the Balmer line used. According to our models, the center of M32 seems to have a super-solar light-weighted Fe abundance, ranging between [Fe/H] ~ +0.1 and +0.3, depending on the Fe line adopted. Light-weighted abundance ratios found for the center of M32 are as follows: [Mg/Fe] ~ -0.1 -- -0.2, [C/Fe] and [N/Fe] ~ 0. We find that single SPS models with a solar-scaled abundance pattern cannot fit all the Balmer and metal lines in the integrated spectrum of M32. There is a systematic trend in the sense that bluer absorption lines indicate a younger age and a higher [Fe/H]. This slight inconsistency can be due either to (unaccounted for) abundance ratio effects on blue iron and Balmer line indices, or to a spread of the ages of the stellar populations in M32. Current stellar population models cannot break this degeneracy at the level of accuracy required to address this problem.

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arXiv2003

New Low-Mass Members of the Taurus Star-Forming Region

Briceno et al. recently used optical imaging, data from the Two-Micron All-Sky Survey (2MASS), and follow-up spectroscopy to search for young low-mass stars and brown dwarfs in 8 square degrees of the Taurus star-forming region. By the end of that study, there remained candidate members of Taurus that lacked the spectroscopic observations needed to measure spectral types and determine membership. In this work, we have obtained spectroscopy of the 22 candidates that have A_V<=8, from which we find six new Taurus members with spectral types of M2.75 through M9. The new M9 source has the second latest spectral type of the known members of Taurus (~0.02 M_sun). Its spectrum contains extremely strong emission in H_alpha (W~950 A) as well as emission in He I 6678 A and the Ca II IR triplet. This is the least massive object known to exhibit emission in He I and Ca II, which together with the strong H_alpha are suggestive of intense accretion.

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arXiv2003

Star formation histories of early-type galaxies. I: Higher order Balmer lines as age indicators

(shortened) We have obtained blue integrated spectra of 175 nearby early-type galaxies, covering a wide range in galaxy velocity dispersion, and emphasizing those with sigma < 100 km/s. Galaxies have been observed both in the Virgo cluster and in lower-density environments. The main goals are the evaluation of higher order Balmer lines as age indicators, and differences in stellar populations as a function of mass, environment and morphology. In this first paper our emphasis is on presenting the evolutionary population synthesis models. Lower-sigma galaxies exhibit a substantially greater intrinsic scatter, in a variety of line strength indicators, than do higher-sigma galaxies, with the large intrinsic scatter setting in below a sigma of 100 km/s. Modeling of the observed spectral indices indicates that the strong Balmer lines found primarily among the low-sigma galaxies are caused by young age, rather than by low metallicity. Thus we find a trend between the population age and the velocity dispersion, such that low sigma galaxies have younger luminosity-weighted mean ages. We have repeated this analysis using several different Balmer lines, and find consistent results from one spectral indicator to another.

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arXiv2002

The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda III

The Hubble Space Telescope WFPC2 camera has been used to image Andromeda III, a dwarf spheroidal companion (dSph) to M31. The resulting color-magnitude (c-m) diagrams reveal the morphology of the horizontal branch (HB) in this dwarf galaxy. We find that like Andromeda I and Andromeda II, and like most of the Galactic dSph companions, the HB morphology of And III is predominantly red, redder than that of both And I and And II despite And III having a lower mean metallicity. We interpret this red HB morphology as indicating that the bulk of the And III population is ~3 Gyr younger than the age of the majority of Galactic globular clusters. Nevertheless, the And III c-m diagram does reveal a few blue HB stars, and a number of RR Lyrae variables are also evident in the data. This indicates that And III does contain an `old' population of age comparable to that of the Galactic globular clusters. There is no evidence, however, for any young stars in And III despite a claimed association between this dSph and an HI cloud. As was the case for And II, but not And I, no radial gradient was detected in the And III HB morphology. And III is ~75 kpc from the center of M31, comparable to the Galactocentric distances of Sculptor and Draco. Comparison with standard globular cluster red giant branches indicates <[Fe/H]> = -1.88 +/-0.11, consistent with the absolute-magnitude - mean abundance relation followed by dSph galaxies. The same comparison yields an intrinsic abundance dispersion of sigma([Fe/H]) = 0.12, a low value compared to the Galactic dSphs of comparable luminosity to And III. The list of candidate variables reveals one definite and one probable Anomalous Cepheid variables.

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arXiv2001

Radial Color Gradients in K+A Galaxies in Distant Clusters of Galaxies

Galaxies in rich clusters with z $\gtrsim$ 0.3 are observed to have a higher fraction of photometrically blue galaxies than their nearby counterparts. This raises the important question of what environmental effects can cause the termination of star formation between z $\approx$ 0.3 and the present. The star formation may be truncated due to ram-pressure stripping, or the gas in the disk may be depleted by an episode of star formation caused by some external perturbation. To help resolve this issue, surface photometry was carried out for a total of 70 early-type galaxies in the cluster Cl1358+62, at z $\sim$ 0.33, using two-color images from the Hubble Archive. The galaxies were divided into two categories based on spectroscopic criteria: 24 are type K+A (e.g., strong Balmer lines, with no visible emission lines), while the remaining 46 are in the control sample with normal spectra. Radial color profiles were produced to see if the K+A galaxies show bluer nuclei in relation to their surrounding disks. Specifically, a linear gradient was fit to the radial color profile of each galaxy. We find that the K+A galaxies on average tend to have slightly bluer gradients towards the center than the normals. A Kolmogorov-Smirnov two-sample test has been applied to the two sets of color gradients. The result of the test indicates that there is only a $\sim$2% probability that the K+A and normal samples are drawn from the same parent distribution. There is a possible complication from a trend in the apparent magnitude vs. color gradient relation, but overall our results favor the centralized star formation scenario as an important process in the evolution of galaxies in dense clusters.

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arXiv2000

Starbursts versus Truncated Star Formation in Nearby Clusters of Galaxies

We present long-slit spectroscopy, B and R bandpass imaging, and 21 cm observations of a sample of early-type galaxies in nearby clusters which are known to be either in a star-forming phase or to have had star formation which recently terminated. From the long-slit spectra, obtained with the Blanco 4-m telescope, we find that emission lines in the star-forming cluster galaxies are significantly more centrally concentrated than in a sample of field galaxies. The broadband imaging reveals that two currently star-forming early-type galaxies in the Pegasus I cluster have blue nuclei, again indicating that recent star formation has been concentrated. In contrast, the two galaxies for which star formation has already ended show no central color gradient. The Pegasus I galaxy with the most evident signs of ongoing star formation (NGC7648), exhibits signatures of a tidal encounter. Neutral hydrogen observations of that galaxy with the Arecibo radiotelescope reveal the presence of ~4 x 10^8 solar masses of HI. Arecibo observations of other current or recent star-forming early-type galaxies in Pegasus I indicate smaller amounts of gas in one of them, and only upper limits in others.

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arXiv1999

The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda II

The HST WFPC2 camera has been used to image Andromeda II, a dwarf spheroidal (dSph) companion to M31. The resulting color-magnitude diagrams reveal the morphology of the horizontal branch (HB) in this dwarf galaxy. We find that like Andromeda I, and like most of the Galactic dSph companions, the HB morphology of And II is predominantly red. Unlike And I, however, there is no evidence for a radial gradient in HB morphology in the And II data. At least 50% of the total stellar population in And II is younger than the age of the globular clusters. On the other hand, the existence of blue HB and RR Lyrae variable stars in And II argues for the existence of an additional old (age > 10 Gyr) population. Thus And II has had an extended epoch of star formation like many of the Galactic dSphs. Our derived distance confirms the association of And II with M31, rather than with M33 to which And II lies closer on the sky. The true distance of And II from the center of M31 is between 160 and 230 kpc. With the current samples of dSph companions, the size of the Galaxy's and M31's dSph satellite systems are comparable, with outer radii of order 250 kpc. The And II red giant branch colors yield a mean abundance of <[Fe/H]> = -1.49 +/- 0.11 and a surprisingly large internal abundance spread, which can be characterized by sigma(int)([Fe/H]) ~ 0.36 dex, considerably larger than that derived for And I (0.21) from an identical analysis of similar data. Thus despite having very similar luminosities and mean metal abundances, these two M31 dSph companions have clearly had different chemical evolution histories. We conclude that the diversity of evolutionary histories evident among the Galactic dSph companions is now also firmly established among the dSph satellites of M31.

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arXiv1999

Surface Brightness Profiles of Three New Dwarf Spheroidal Companions to M31

CCD images of three newly discovered dwarf spheroidal companions to M31, And V, VI, and VII, are used to extract surface brightness profiles and total magnitudes. Using distance modulii provided by Armandroff et al. (1998 & 1999) and Grebel & Guhathakurta (1999), these galaxies are shown to have similar luminosities to other Local Group dwarf spheroidals; And V in particular is similar to Draco & UMi in luminosity, i.e., among the faintest known. In the luminosity-metallicity relation, And V is shown to have a significantly higher metallicity than expected.

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arXiv1998

On the Origins of Starburst and Post-Starburst Galaxies in Nearby Clusters

HST WFPC2 images in B (F450W) and I (F814W) have been obtained for three starburst (SB) and two post-starburst (PSB) galaxies in the Coma cluster, and for three such galaxies in the cluster DC2048-52. V (F555W) and I images for an additional PSB galaxy in Coma have been extracted from the archive. Seven of these galaxies were previously classified as E/S0 on the basis of ground-based images, one as Sa, and the other as an irregular. The HST images reveal these SB/PSB galaxies to be heterogeneous in morphology. Nevertheless a common theme is that many of them, especially the SB galaxies, tend to have centralized spiral structure that appears simply as a bright ``bulge''on ground-based images. In addition, while some PSB galaxies exhibit distinct spiral structure, on the whole they have smoother morphologies than the SB galaxies. The morphologies and luminosity profiles are generally consistent with substantial starbursts in the form of centralized spiral structure (the SB galaxies) which fade into smoother morphologies (the PSB galaxies), with lingering spectroscopic evidence for past central starbursts. An important point is that the PSB galaxies retain disks, i.e, they have not evolved into spheroidal systems.

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arXiv1998

The Formation History of Early-Type Galaxies: An Observational Perspective

This talk investigates the formation of early-type galaxies from a deliberately observational view point. I begin by reviewing the conclusions that can be reached by comparing the detailed properties of galaxies in present-day clusters, focusing on the colour-magnitude relation in particular. The overriding picture is one of homogeneity, implying a remarkable uniformity in the formation of these galaxies. This picture contrasts with the increasing activity seen in clusters as a function of redshift, creating an apparent paradox been the obvious diversity of star formation histories in distant cluster galaxies and their uniformity in local systems. A resolution is feasible so long as star formation occurs over an extended epoch. In addition to placing limits on variations in star formation history the existence of a tight `fundamental relations', such as the colour-magnitude relation, can be used to investigate galaxy mergers and to set limits on the degree to which present-day clusters galaxies are built by combining systems of stars formed in smaller units. The final part of this talk turns to early-type galaxies in the field, and tries to apply the same techniques that have been successful in clusters. This is an emerging field in which appropriate data-sets are only just becoming available; however, comparison of the formation histories of galaxies in a wide variety of environments is key to distinguishing between the Classical and Hierarchical models for galaxy formation.

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arXiv1997

Low-Luminosity Early-Type Galaxies in the Coma Cluster: Variations in Spectral Properties

We present spectra of nine low-luminosity early-type galaxies (M_B > -17.5) in the Coma cluster. The spectra, which were obtained with the Multiple Mirror Telescope and cover the spectral region 3300-5350 A, exhibit a large variation in Balmer line strengths. In fact the line strength variation among the Coma low-luminosity galaxies is as large as that found among a sample of low-luminosity ellipticals in the Virgo cluster and lower density environments. When compared to detailed population synthesis models, the variation in Balmer line strengths among the Coma galaxies indicates a range in luminosity-weighted mean age of from 1 Gyr to 12 Gyr. The two youngest galaxies are shown to be in a post-starburst state, i.e., they are not simply former spirals whose star formation was recently terminated. Moreover, the 1 Gyr ages of these two youngest low-luminosity galaxies are similar to those of the brighter post-starburst galaxies in Coma.

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arXiv1997

Dwarf Elliptical Galaxies in the M81 Group: The Structure and Stellar Populations of BK5N and F8D1

We have obtained HST WFPC2 images through the F555W and F814W filters of two M81 group dE's: BK5N and a new system, designated F8D1. The resulting color-magnitude diagrams show the upper two magnitudes of the red giant branch. Surface brightness and total magnitude measurements indicate that BK5N and F8D1 have similar central surface brightness (24.5 and 25.4 mag/arcsec^2 in V, respectively), but F8D1's larger length scale results in it being 3 magnitudes more luminous than BK5N. BK5N lies on the relation between central surface brightness and absolute magnitude defined by Local Group dwarf ellipticals, but F8D1 does not. F8D1 is more luminous for its central surface brightness than the relation predicts, similar to the large low surface brightness dwarf galaxies found in, for example, Virgo. The mean color of the giant branch is used to establish the mean abundance of each galaxy. F8D1, the more luminous galaxy, is significantly more metal rich ([Fe/H] ~ -1.0) than BK5N ([Fe/H] ~ -1.7). Both BK5N and F8D1 lie on the relation between absolute magnitude and metal abundance defined by Local Group dwarf ellipticals. However, as regards the relation between central surface brightness and metal abundance, BK5N again follows the Local Group dwarfs, while F8D1 deviates significantly from this relation. This suggests that the total amount of luminous matter is more fundamental in controlling metal enrichment than the surface density of luminous matter. We also discuss the intermediate age stars, and note 1 globular cluster in F8D1. (edited)

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arXiv1996

The Butcher-Oemler Effect at Low Redshift: Spectroscopy of Five Nearby Clusters of Galaxies

We present multi-fiber spectroscopy and broadband imaging of early-type galaxies in five nearby rich clusters of galaxies. The main purpose was to look for ``abnormal'' spectrum galaxies (i.e., post-starburst galaxies which have strong Balmer absorption lines and emission line galaxies) in nearby clusters that are similar to those found by Caldwell et al. (1993) in the Coma cluster. Our primary conclusion is that 15% of the early-type galaxies in these nearby rich clusters have signs of ongoing or recent star formation. Thus activity similar to that seen in distant clusters is still ongoing, at a reduced level, in present-epoch rich clusters. The frequency of such galaxies appears to be enhanced significantly over that seen in field galaxies. Most of the new post-starburst galaxies are disk galaxies; three E galaxies in one cluster and one in another cluster have current star formation. We also find evidence in the spatial and kinematic structure of several of the clusters that subclusters have recently passed through the main clusters and are now emerging out the other side. If, as we suspect, the subclusters passed through the main clusters more than 1 Gyr ago, then the post-starburst timescales of 1 Gyr imply that the star formation bursts are only triggered during (or after) the subcluster passage through the center of the main cluster.

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arXiv1996

The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda I

Images have been obtained with the Hubble Space Telescope WFPC2 camera of Andromeda I, a dwarf spheroidal (dSph) galaxy that lies in the outer halo of M31. The resulting color-magnitude diagrams reveal for the first time the morphology of the horizontal branch in this system. We find that, in a similar fashion to many of the galactic dSph companions, the horizontal branch (HB) of And~I is predominantly red. Combined with the metal abundance of this dSph, this red HB morphology indicates that And I can be classified as a ``second parameter'' system in the outer halo of M31. This result then supports the hypothesis that the outer halo of M31 formed in the same extended chaotic manner as is postulated for the outer halo of the Galaxy.

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arXiv1995

Spatial Distribution of the Starbursts in Post-Starburst Coma Cluster Galaxies

We present long slit spectra and multi-color CCD images which demonstrate that the strong star formation episodes that occurred in the post-starburst galaxies in the Coma Cluster and two field galaxies were not restricted to the central regions of the galaxies. Rather, the remnant young stars from the starbursts are found to be distributed over a large radius, though changes in the relative strength of the old and young components with radius are evident in a few cases. The Coma galaxies are shown to have exponential disk profiles, and the spectra provide further kinematical evidence that the galaxies are rotating systems, indicating that the galaxies are not ellipticals. The starburst material also appears to be distributed in disks. Such information places constraints on models for the starbursts that involve mergers. Some emission line galaxies are also discussed.

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