arXiv subjects
Publications and source records attributed to Beverly J. Smith.
We present GALEX ultraviolet images from a survey of strongly interacting galaxy pairs, and compare with images at other wavelengths. The tidal features are particularly striking in the UV images. Numerous knots of star formation are visible throughout the disks and the tails and bridges. We also identify a possible `Taffy' galaxy in our sample, which may have been produced by a head-on collision between two disk galaxies.
We have used the GALEX ultraviolet telescope to study stellar populations and star formation morphology in a well-defined sample of more than three dozen nearby optically-selected pre-merger interacting galaxy pairs. We have combined the GALEX NUV and FUV images with broadband optical maps from the Sloan Digitized Sky Survey to investigate the ages and extinctions of the tidal features and the disks. We have identified a few new candidate tidal dwarf galaxies in this sample, as well as other interesting morphologies such as accretion tails, `beads on a string', and `hinge clumps'. In only a few cases are strong tidal features seen in HI maps but not in GALEX.
We present results from a study of proto-globular cluster candidates in the interacting galaxy system Arp 284 (NGC 7714/5). Studies of the Antennae and M51 have suggested that the majority of these star clusters dissolve within 20 Myr due to mass loss. We obtain cluster colors using archival \emph{HST} data, and estimate ages and extinctions for over 150 clusters using evolutionary synthesis models. We find that clusters in NGC 7714 are generally less than 20 Myr old, while the data in the bridge is too limited to allow good estimates for individual clusters. We also examine {H {\sc ii}} region complexes with lower-resolution \emph{GALEX} and ground-based H$α$ images. Several of these regions appear to be much older than the detected clusters within them, which may indicate the presence of an older, unresolved population of low mass or dispersed clusters.
The key to understanding the physical processes that occur during galaxy interactions is dynamical modeling, and especially the detailed matching of numerical models to specific systems. To make modeling interacting galaxies more efficient, we have constructed the `Automatic Galaxy Collision' (AGC) code, which requires less human intervention in finding good matches to data. We present some preliminary results from this code for the well-studied system Arp 284 (NGC 7714/5), and address questions of uniqueness of solutions.
We present results from a study of proto-globular cluster candidates in the interacting galaxy system Arp 284 (NGC 7714/5) using data from the Hubble Space Telescope. Previous studies of the Antennae and M51 have suggested that the majority of young massive star clusters dissolve within 20 Myr due to mass loss. We use the evolutionary synthesis code Starburst99 to estimate ages and extinctions for approximately 175 clusters visible with HST. We also use lower-resolution GALEX and ground-based H-alpha data to estimate the ages of the giant HII regions in which these clusters are found, and compare the Spitzer colours of these HII regions to those of star forming regions in other interacting systems. The ages are also used to aid in the interpretation of Chandra X-ray data. Clusters in the tidal tails of NGC 7714 are generally found to have ages less than 20 Myr, though observational limits make the significance of this result uncertain. Older clusters, though not numerous, have nearly the same spatial distribution within the imaged portion of NGC 7714 as young clusters. The cluster population in the bridge connecting the two galaxies appears to be older, but the data in this part of the system is too limited to draw firm conclusions. The ages of the giant HII regions in NGC 7714 are generally older than those of their constituent clusters, possibly indicating that the young clusters we detect are surrounded by their dispersed predecessors.
Dwarf galaxies tend to have redder [3.6 micron] - [4.5 micron] Spitzer broadband colors than spirals. To investigate this effect, for a large sample of dwarf galaxies we combine Spitzer fluxes with data at other wavelengths and compare to population synthesis models. Lower metallicity systems are found to have redder [3.6] - [4.5] colors on average, but with considerable scatter. The observed range in [3.6] - [4.5] color is too large to be accounted for solely by variations in stellar colors due to age or metallicity differences; interstellar effects must contribute as well. For the reddest systems, the 4.5 micron luminosity may not be a good tracer of stellar mass. We identify three factors that redden this color in dwarfs. First, in some systems, strong Br-alpha emission contributes significantly to the 4.5 micron emission. Second, in some cases high optical depths lead to strong reddening of the starlight in the Spitzer bands. Third, in some galaxies, the nebular continuum dominates the 4.5 micron flux, and in extreme cases, the 3.6 micron flux as well. The harder UV radiation fields in lower metallicity systems produce both more gaseous continuum in the infrared and more Br-alpha per star formation rate. The combination of these three factors can account for the 4.5 micron excess in our sample galaxies, thus it is not necessary to invoke a major contribution from hot dust to the 4.5 micron band. However, given the uncertainties, we are not able to completely rule out hot dust emission at 4.5 micron. More spectroscopic observations in the 3 - 5 micron range are needed to disentangle these effects.
To search for Tidal Dwarf Galaxies (TDGs) and to study star formation in tidal features, we are conducting a large UV imaging survey of interacting galaxies selected from the Arp (1996) Atlas using the GALEX telescope. As part of that study, we present a GALEX UV and SDSS and SARA optical study of the gas-rich interacting galaxy pair Arp 305 (NGC 4016/7). The GALEX UV data reveal much extended diffuse UV emission and star formation outside the disks. This includes a luminous star forming region between the two galaxies, and a number of such regions in tidal tails. We have identified 45 young star forming clumps in Arp 305, including several TDG candidates. By comparing the UV and optical colors to population synthesis models, we determined that the clumps are very young, with several having ages of about 6 Myr. We do not find many intermediate age clumps in spite of the fact that the last closest encounter was about 300 Myr ago. We have used a smooth particle hydrodynamics code to model the interaction and determine the fate of the star clusters and candidate TDGs.
We present Spitzer mid-infrared imaging of a sample of 35 tidally-distorted pre-merger interacting galaxy pairs selected from the Arp Atlas. We compare their global mid-infrared properties with those of normal galaxies from the SINGS Spitzer Legacy survey, and separate the disk emission from that of the tidal features. The [8.0 micron] - [24 micron], [3.6 micron] - [24 micron], and [5.8 micron] - [8.0 micron] colors of these optically-selected interacting galaxies are redder on average than those of spirals, implying enhancements to the mass-normalized star formation rates (SFRs) of a factor of ~2. Furthermore, the 24 micron emission in the Arp galaxies is more centrally concentrated than that in the spirals, suggesting that gas is being concentrated into the inner regions and fueling central star formation. No significant differences can be discerned in the shorter wavelength Spitzer colors of the Arp galaxies compared to the spirals, thus these quantities are less sensitive to star formation enhancements. No strong trend of Spitzer color with pair separation is visible in our sample; this may be because our sample was selected to be tidally disturbed. The tidal features contribute <10% of the total Spitzer fluxes on average. The SFRs implied for the Arp galaxies by the Spitzer 24 micron luminosities are relatively modest, ~1 M(sun)/yr on average.
As part of our Spitzer Spirals, Bridges, and Tails project to help understand the effects of galaxy interactions on star formation, we analyze GALEX ultraviolet, SARA optical, and Spitzer infrared images of the interacting galaxy pair Arp 82 (NGC 2535/6) and compare to a numerical simulation of the interaction. We investigate the multiwavelength properties of several individual star forming complexes (clumps). Using optical and UV colors, EW(Halpha), and population synthesis models we constrain the ages of the clumps and find that the median clump age is about 12 Myr. The clumps have masses ranging from a few times 10^6 to 10^9 solar masses. In general, the clumps in the tidal features have similar ages to those in the spiral region, but are less massive. The 8 micron and 24 micron luminosities are used to estimate the far-infrared luminosities and the star formation rates of the clumps. The total clump star formation rate is 2.0+/-0.8 solar masses per year, while the entire Arp 82 system is forming stars at a rate of 4.9+/-2.0 solar masses per year. We find, for the first time, stars in the HI arc to the southeast of the NGC 2535 disk. Population synthesis models indicate that all of the observed populations have young to intermediate ages. We conclude that although the gas disks and some old stars may have formed early-on, the progenitors are late-type or low surface brightness and the evolution of these galaxies was halted until the recent encounter.
To search for phase lags in the optical-infrared light curves of asymptotic giant branch stars, we have compared infrared data from the COBE DIRBE satellite with optical light curves from the AAVSO and other sources. We found 17 examples of phase lags in the time of maximum in the infrared vs. that in the optical, and 4 stars with no observed lags. There is a clear difference between the Mira variables and the semi-regulars in the sample, with the maximum in the optical preceding that in the near-infrared in the Miras, while in most of the semi-regulars no lags are observed. Comparison to published theoretical models indicates that the phase lags in the Miras are due to strong titanium oxide absorption in the visual at stellar maximum, and suggests that Miras pulsate in the fundamental mode, while at least some semi-regulars are first overtone pulsators. There is a clear optical-near-infrared phase lag in the carbon-rich Mira V CrB; this is likely due to C2 and CN absorption variations in the optical.
We present Spitzer infrared imaging of the peculiar galaxy pair Arp 107, and compare with an optical H-alpha map and a numerical model of the interaction. The [3.6 micron] - [4.5 micron] colors of clumps in the galaxy do not vary around the ring-like primary spiral arm and are consistent with those of stars, thus these bands are dominated by starlight. In contrast, the [5.8 micron] - [8.0 micron] colors are consistent with those of interstellar dust, and vary by about 0.2 magnitudes around the ring/spiral, with redder colors associated with regions with stronger star formation as indicated by H-alpha and mid-infrared luminosity. The [4.5 micron] - [5.8 micron] colors for clumps in this arm are bluer than dust and redder than stars, and vary by 1.3 magnitudes around the arm. This color is therefore a measure of the relative number of young stars to old stars, with a redder color indicating a higher proportion of young stars. There is a clear azimuthal sequence in the [4.5] - [5.8] color around the arm, indicating a sequence in average stellar age. The L(H-alpha)/L(8.0 micron) ratio varies around the arm by a factor of ~7; this variation may be due to extinction or to PAH excitation by non-ionizing photons. Our model of Arp 107 accounts for the general morphology of the system, and explains the age variation along the arm as the result of differences in the time of maximum compression in the arm. Using Spitzer colors, we are able to distinguish background quasars and foreground stars from star forming regions associated with Arp 107.
We present Chandra X-ray images for the interacting galaxy pair NGC 7714/5. In addition to the unresolved starburst nucleus, a variable point source with L(X) ~ 10^40 erg/s was detected 1.5" (270 pc) northwest of the nucleus, coincident with a blue, extremely optically-luminous (M(V) ~ -14.1) point source on HST images. Eleven more candidate point-like ultraluminous X-ray sources (ULXs) are seen, two >= 10^40 erg/s. Ten of these are associated with interaction-induced features but only two with star formation. We found diffuse emission with L(X) ~ 3 X 10^40 erg/s extending 11" (1.9 kpc) to the north of the nucleus. Its spectrum can be fit with a 2-temperature Mekal function (0.6/8 keV) or a 0.6 keV Mekal function plus a power law. The hard component may be due to high mass X-ray binaries (HMXBs) with contributions from inverse Compton radiation, while the soft component is likely from a superwind. We also detected extended X-ray emission from four extra-nuclear HII region complexes. This emission may be due to HMXBs or to diffuse gas heated by winds from supernovae, if the X-ray production efficiency L(X)/L(mech) is high (~5%). To estimate L(X)/L(mech), we collected published data for well-studied HII regions and superbubbles in nearby galaxies. For young HII regions (<3.5 Myrs), the median L(X)/L(mech) ~ 0.02%, while for older regions, L(X)/L(mech) ~ 0.2-7%. Thus gas heating by supernovae may be sufficient to account for the HII region emission. In galaxies much more distant than NGC 7714, for example, the Cartwheel galaxy, HII region complexes similar to those in NGC 7714 will be unresolved by Chandra and will mimick ULXs.
We present the COBE DIRBE Point Source Catalog, an all-sky catalog containing infrared photometry in 10 bands from 1.25 microns to 240 microns for 11,788 of the brightest near and mid-infrared point sources in the sky. Since DIRBE had excellent temporal coverage (100 - 1900 independent measurements per object during the 10 month cryogenic mission), the Catalog also contains information about variability at each wavelength, including amplitudes of variation observed during the mission. Since the DIRBE spatial resolution is relatively poor (0.7 degrees), we have carefully investigated the question of confusion, and have flagged sources with infrared-bright companions within the DIRBE beam. In addition, we filtered the DIRBE light curves for data points affected by companions outside of the main DIRBE beam but within the `sky' portion of the scan. At high Galactic latitudes (|b| > 5 degrees), the Catalog contains essentially all of the unconfused sources with flux densities greater than 90, 60, 60, 50, 90, and 165 Jy at 1.25, 2.2, 3.5, 4.9, 12, and 25 microns, respectively, corresponding to magnitude limits of approximately 3.1, 2.6, 1.7, 1.3, -1.3, and -3.5. At longer wavelengths and in the Galactic Plane, the completeness is less certain because of the large DIRBE beam and possible contributions from extended emission. The Catalog also contains the names of the sources in other catalogs, their spectral types, variability types, and whether or not the sources are known OH/IR stars. We discuss a few remarkable objects in the Catalog. [abridged]
We present high resolution X-ray imaging data from the ACIS-S instrument on the Chandra telescope of the nearby interacting galaxy group NGC 4410. Four galaxies in the inner portion of this group are clearly detected by Chandra, including the peculiar low luminosity radio galaxy NGC 4410A. In addition to a nuclear point source, NGC 4410A contains diffuse X-ray emission, including an X-ray ridge extending out to about 12" (6 kpc) to the northwest of the nucleus. This ridge is coincident with an arc of optical emission-line gas, which has previously been shown to have optical line ratios consistent with shock ionization. This structure may be due to an expanding superbubble of hot gas caused by supernovae and stellar winds or by the active nucleus. The Chandra observations also show four or five possible compact ultra-luminous X-ray (ULX) sources (L(x) >= 10^39 erg/s) associated with NGC 4410A. At least one of these candidate ULXs appears to have a radio counterpart, suggesting that it may be due to an X-ray binary with a stellar-mass black hole, rather than an intermediate mass black hole. In addition, a faint diffuse intragroup X-ray component has been detected between the galaxies (L(x) ~ 10^41 erg/s). This supports the hypothesis that the NGC 4410 group is in the process of evolving via mergers from a spiral-dominated group (which typically have no X-ray-emitting intragroup gas) to an elliptical-dominated group (which often have a substantial intragroup medium).
For a complete 12 micron flux-limited sample of 207 IRAS sources, we have extracted light curves in seven infrared bands between 1.25 - 60 microns using the database of the DIRBE instrument on the COBE satellite. Using previous infrared surveys, we filtered these light curves to remove datapoints affected by nearby companions. In the time-averaged DIRBE color-color plots, we find clear segregation of semi-regulars, Mira variables, carbon stars, OH/IR stars, and red giants without circumstellar dust and with little or no visual variation. The DIRBE 1.25 - 25 micron colors become progressively redder and the variability in the DIRBE database increases along the oxygen-rich sequence non-dusty slightly varying red giants -> SRb/Lb -> SRa -> Mira -> OH/IR and the carbon-rich SRb/Lb -> Mira sequence. This supports previous assertions that these are evolutionary sequences involving the continued production and ejection of dust. Carbon stars are redder than oxygen-rich stars for the same variability type, except in the F(12)/F(25) ratio. Of the 28 sources in the sample not previous noted to be variable, 18 are clearly variable in the DIRBE data, with amplitudes of variation of ~0.9 magnitudes at 4.9 microns and ~0.6 magnitudes at 12 microns, consistent with them being very dusty Mira-like variables. The light curves of the semi-regular variable L2 Pup are particularly remarkable. The maxima at 1.25, 2.2, and 3.5 microns occur 10 - 20 days before those at 4.9 and 12 microns, and, at 4.9 and 12 microns, another maximum is seen between the two near-infrared maxima.
(abridged) We present new N-body, hydrodynamical simulations of the interaction between the starburst galaxy NGC 7714 and its post-starburst companion NGC 7715, focusing on the formation of the collisional features, including: 1) the gas-rich star forming bridge, 2) the large gaseous loop (and stellar tails) to the west of the system, 3) the very extended HI tail to the west and north of NGC 7714, and 4) the partial stellar ring in NGC 7714. Our simulations confirm the results of earlier work that an off-center inclined collision between two disk galaxies is almost certainly responsible for the peculiar morphologies of this system. However, we have explored a wider set of initial galaxy and collisional encounter parameters than previously, and have found a relatively narrow range of parameters that reproduce all the major morphologies of this system. The simulations suggest specific mechanisms for the development of several unusual structures. We find that the complex gas bridge has up to four distinct components, with gas contributed from two sides of NGC 7715, as well as from NGC 7714. The observed gas-star offset in this bridge is accounted for in the simulations by the dissipative evolution of the gas. The models also indicate that the low surface brightness HI tail to the far west of NGC 7714 is the end of the NGC 7715 countertail, curved behind the two galaxies. Spectral evolutionary models of the NGC 7714 core by Lançon et al. suggest the possibility of multiple starbursts in the last 300 Myr. Our hydrodynamic models suggest that bursts could be triggered by induced ring-like waves, and a post-collision buildup of gas in the core of the galaxy.
The NGC4410 group of galaxies provides us a rare opportunity to study a nearby (97 h75^-1 Mpc) example of a radio galaxy (NGC4410A) embedded in an extended X-ray source, with evidence for star formation that can be readily spatially distinguished from regions dominated by the AGN and shocks. We present broadband and narrowband optical images along with optical and IUE ultraviolet spectroscopy for the radio galaxy NGC 4410A and its companion NGC 4410B. Our H-alpha+[NII] images reveal six luminous HII regions (L_H-alpha ~ 1e40 erg/s distributed in an arc near NGC 4410A. Partially completing the ring is a prominent stellar loop containing diffuse ionized gas. This filamentary gas, in contrast to the H II regions, shows spectroscopic signatures of shock ionization. The star formation in this system may have been triggered by a collision or interaction between the two galaxies, perhaps by an expanding density wave, as in classical models of ring galaxies. Alternatively, the star formation may have been induced by the impact of a radio jet on the interstellar matter. Extended Ly-alpha is detected in the ultraviolet IUE spectrum. The ultraviolet continuum, which is presumably radiated by the nucleus of NGC4410A, is not extended. NGC4410A appears to be interacting with its neighbors in the NGC4410 group, and could be an example of a spiral galaxy transforming into an elliptical.
Using HST WFPC2 images, we have obtained U, B, V, I, and H-alpha photometry for 76 star clusters in the nuclear star-forming ring of the barred spiral galaxy NGC 4314. These clusters are likely associated with an inner Inner Lindblad Resonance, or IILR. The blue colors and H-alpha emission for most of these clusters imply very young ages of 1-15 Myr. Age estimates based on several reddening-free parameters indicate that the present epoch of star formation has lasted at least 30 Myr. By estimating the masses of stars in the clusters and comparing with the H-alpha luminosity, we conclude that a significant fraction of ongoing star formation in the nuclear ring of NGC 4314 occurs in clusters. The cluster masses identify these as young open clusters, not young globular clusters. Further out in the galaxy, just exterior to the ring of young stars, previous ground-based observations revealed two symmetric stellar spiral arms which may be associated with an outer Inner Lindblad Resonance, or OILR. With our HST data, we have revealed part of this structure and its colors in more detail. The spiral arm colors are consistent with stellar ages between 40 and 200 Myr. The age difference between the inner ring of young stars (IILR) and the larger oval-like feature containing the blue arms (OILR) supports an interpretation of the morphology of the nuclear region of NGC 4314 that requires a reservoir of gas that becomes more compact over time. We speculate that as the gas distribution becomes more centrally concentrated, it interacts with these two resonances. Each resonance triggers star formation, resulting in two distinct epochs of star formation.