Pixel Color-magnitude Diagram Research Articles

Measuring Star Formation Histories, Distances, and Metallicities with Pixel Color–Magnitude Diagrams. II. Applications to Nearby Elliptical Galaxies

Abstract We present spatially resolved measurements of star formation histories (SFHs), metallicities, and distances in three nearby elliptical galaxies and the bulge of M31 derived using the pixel color–magnitude diagram (pCMD) technique. We compute pCMDs from archival Hubble Space Telescope photometry of M87, M49, NGC 3377, and M31, and we fit the data using the new code PCMDPy. We measure distances to each system that are accurate to ∼10%. The recovered nonparametric SFHs place reasonable (±1 dex) constraints on the recent (<2 Gyr) star formation in M31 and NGC 3377, both of which show evidence of inside-out growth. The SFHs in M87 and M49 are constrained only at the oldest ages. The pCMD technique is a promising new avenue for studying the evolutionary history of the nearby universe and is highly complementary to existing stellar population modeling techniques.

Measuring Star Formation Histories, Distances, and Metallicities with Pixel Color–Magnitude Diagrams. I. Model Definition and Mock Tests

Abstract We present a comprehensive study of the applications of the pixel color–magnitude diagram (pCMD) technique for measuring star formation histories (SFHs) and other stellar population parameters of galaxies, and we demonstrate that the technique can also constrain distances. SFHs have previously been measured through either the modeling of resolved-star CMDs or of integrated-light spectral energy distributions, yet neither approach can easily be applied to galaxies in the “semi-resolved regime.” The pCMD technique has previously been shown to have the potential to measure stellar populations and SFHs in semi-resolved galaxies. Here we present Pixel Color–Magnitude Diagrams with Python (PCMDPy), a graphics processing unit (GPU)-accelerated package that makes significant computational improvements to the original code and includes more realistic physical models. These advances include the simultaneous fitting of distance, modeling a Gaussian metallicity distribution function, and an observationally motivated dust model. GPU acceleration allows these more realistic models to be fit roughly 7× faster than the simpler models in the original code. We present results from a suite of mock tests, showing that with proper model assumptions, the code can simultaneously recover SFH, [ Fe / H ] , distance, and dust extinction. Our results suggest the code, applied to observations with Hubble Space Telescope-like resolution, should constrain these properties with high precision within 10 Mpc and can be applied to systems out to as far as 100 Mpc. pCMDs open a new window to studying the stellar populations of many galaxies that cannot be readily studied through other means.

Color Dispersion as an Indicator of Stellar Population Complexity: Insights from the Pixel Color–Magnitude Diagrams of 32 Bright Galaxies in Abell 1139 and Abell 2589

Abstract We investigate the properties of bright galaxies of various morphological types in Abell 1139 and Abell 2589, using pixel color–magnitude diagram (pCMD) analysis. The sample contains 32 galaxies brighter than M r = −21.3 mag with spectroscopic redshifts, which are deeply imaged in the g and r bands using the MegaCam mounted on the Canada–France–Hawaii Telescope. After masking contaminants with two-step procedures, we examine how the detailed properties in the pCMDs depend on galaxy morphology and infrared color. The mean g − r color as a function of surface brightness (μ r ) in the pCMD of a galaxy shows good performance in distinguishing between early- and late-type galaxies, but it is not perfect because of the similarity between elliptical galaxies and bulge-dominated spiral galaxies. On the other hand, the g − r color dispersion as a function of μ r works better. We find that the best set of parameters for galaxy classification is a combination of the minimum color dispersion at μ r ≤ 21.2 mag arcsec−2 and the maximum color dispersion at 20.0 ≤ μ r ≤ 21.0 mag arcsec−2; the latter reflects the complexity of stellar populations at the disk component in a typical spiral galaxy. Finally, the color dispersion measurements of an elliptical galaxy appear to be correlated with the Wide-field Infrared Survey Explorer infrared color ([4.6]–[12]). This indicates that the complexity of stellar populations in an elliptical galaxy is related to its recent star formation activities. From this observational evidence, we infer that gas-rich minor mergers or gas interactions may have usually occurred during the recent growth of massive elliptical galaxies.

Pixel Color–Magnitude Diagram Analysis of the Brightest Cluster Galaxies in Dynamically Young and Old Clusters Abell 1139 and Abell 2589

Abstract As a case study to understand the coevolution of Brightest Cluster Galaxies (BCGs) and their host clusters, we investigate the BCGs in dynamically young and old clusters Abell 1139 (A1139) and Abell 2589 (A2589). We analyze the pixel color–magnitude diagrams (pCMDs) using deep g- and r-band images, obtained from the Canada–France–Hawaii Telescope observations. After masking foreground/background objects and smoothing pixels in consideration of the observational seeing size, detailed pCMD features are compared between the two BCGs. (1) Although the overall shapes of the pCMDs are similar to those of typical early-type galaxies, the A2589-BCG tends to have redder mean pixel color and smaller pixel color deviation at given surface brightness than the A1139-BCG. (2) The mean pixel color distribution as a function of pixel surface brightness (pCMD backbone) indicates that the A2589-BCG formed a larger central body (∼2.0 kpc in radius) via major dry mergers at an early epoch than the A1139-BCG (a central body ∼1.3 kpc in radius), whereas they have grown commonly in subsequent minor mergers. (3) The spatial distributions of the pCMD outliers reveal that the A1139-BCG experienced considerable tidal events more recently than the A2589-BCG, whereas the A2589-BCG has an asymmetric compact core, possibly resulting from a major dry merger at an early epoch. (4) The A2589-BCG shows a very large faint-to-bright pixel number ratio, compared to early-type non-BCGs, whereas the ratio for the A1139-BCG is not distinctively large. These results are consistent with the idea that the BCG in the dynamically older cluster (A2589) formed earlier and is better relaxed.

PIXEL COLOR MAGNITUDE DIAGRAMS FOR SEMI-RESOLVED STELLAR POPULATIONS: THE STAR FORMATION HISTORY OF REGIONS WITHIN THE DISK AND BULGE OF M31

ABSTRACT The analysis of stellar populations has, by and large, been developed for two limiting cases: spatially resolved stellar populations in the color–magnitude diagram, and integrated light observations of distant systems. In between these two extremes lies the semi-resolved regime, which encompasses a rich and relatively unexplored realm of observational phenomena. Here we develop the concept of pixel color–magnitude diagrams (pCMDs) as a powerful technique for analyzing stellar populations in the semi-resolved regime. pCMDs show the distribution of imaging data in the plane of pixel luminosity versus pixel color. A key feature of pCMDs is that they are sensitive to all stars, including both the evolved giants and the unevolved main sequence stars. An important variable in this regime is the mean number of stars per pixel, . Simulated pCMDs demonstrate a strong sensitivity to the star formation history (SFH) and have the potential to break degeneracies between age, metallicity and dust based on two filter data for values of up to at least 104. We extract pCMDs from Hubble Space Telescope optical imaging of M31 and derive SFHs with seven independent age bins from 106 to 1010 year for both the crowded disk and bulge regions (where ). From analyzing a small region of the disk we find a SFH that is smooth and consistent with an exponential decay timescale of 4 Gyr. The bulge SFH is also smooth and consistent with a 2 Gyr decay timescale. pCMDs will likely play an important role in maximizing the science returns from next generation ground and space-based facilities.

HUBBLE SPACE TELESCOPEPIXEL ANALYSIS OF THE INTERACTING S0 GALAXY NGC 5195 (M51B)

We report the properties of the interacting S0 galaxy NGC 5195 (M51B), revealed in a pixel analysis using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. We analyze the pixel color-magnitude diagram (pCMD) of NGC 5195, focusing on the properties of its red and blue pixel sequences and the difference from the pCMD of NGC 5194 (M51A; the spiral galaxy interacting with NGC 5195). The red pixel sequence of NGC 5195 is redder than that of NGC 5194, which corresponds to the difference in the dust optical depth of 2<\Delta\tau_V<4 at fixed age and metallicity. The blue pixel sequence of NGC 5195 is very weak and spatially corresponds to the tidal bridge between the two interacting galaxies. This implies that the blue pixel sequence is not an ordinary feature in the pCMD of an early-type galaxy, but that it is a transient feature of star formation caused by the galaxy-galaxy interaction. We also find a difference in the shapes of the red pixel sequences on the pixel color-color diagrams (pCCDs) of NGC 5194 and NGC 5195. We investigate the spatial distributions of the pCCD-based pixel stellar populations. The young population fraction in the tidal bridge area is larger than that in other areas by a factor > 15. Along the tidal bridge, young populations seem to be clumped particularly at the middle point of the bridge. On the other hand, the dusty population shows a relatively wide distribution between the tidal bridge and the NGC 5195 center.

HUBBLE SPACE TELESCOPEPIXEL ANALYSIS OF THE INTERACTING FACE-ON SPIRAL GALAXY NGC 5194 (M51A)

A pixel analysis is carried out on the interacting face-on spiral galaxy NGC 5194 (M51A), using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. After 4x4 binning of the HST/ACS images to secure a sufficient signal-to-noise ratio for each pixel, we derive several quantities describing the pixel color-magnitude diagram (pCMD) of NGC 5194: blue/red color cut, red pixel sequence parameters, blue pixel sequence parameters and blue-to-red pixel ratio. The red sequence pixels are mostly older than 1 Gyr, while the blue sequence pixels are mostly younger than 1 Gyr, in their luminosity-weighted mean stellar ages. The color variation in the red pixel sequence from V = 20 mag arcsec^(-2) to V = 17 mag arcsec^(-2) corresponds to a metallicity variation of \Delta[Fe/H] ~ 2 or an optical depth variation of \Delta\tau_V ~ 4 by dust, but the actual sequence is thought to originate from the combination of those two effects. At V < 20 mag arcsec^(-2), the color variation in the blue pixel sequence corresponds to an age variation from 5 Myr to 300 Myr under the assumption of solar metallicity and \tau_V = 1. To investigate the spatial distributions of stellar populations, we divide pixel stellar populations using the pixel color-color diagram and population synthesis models. As a result, we find that the pixel population distributions across the spiral arms agree with a compressing process by spiral density waves: dense dust \rightarrow newly-formed stars. The tidal interaction between NGC 5194 and NGC 5195 appears to enhance the star formation at the tidal bridge connecting the two galaxies. We find that the pixels corresponding to the central active galactic nucleus (AGN) area of NGC 5194 show a tight sequence at the bright-end of the pCMD, which are in the region of R ~ 100 pc and may be a photometric indicator of AGN properties.

LIFTING THE VEIL OF DUST FROM NGC 0959: THE IMPORTANCE OF A PIXEL-BASED TWO-DIMENSIONAL EXTINCTION CORRECTION

We present the results of a study of the late-type spiral galaxy NGC 0959, before and after application of the pixel-based dust extinction correction described in Tamura et al. 2009 (Paper I). Galaxy Evolution Explorer (GALEX) far-UV (FUV) and near-UV (NUV), ground-based Vatican Advanced Technology Telescope (VATT) UBVR, and Spitzer/Infrared Array Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 micron images are studied through pixel Color-Magnitude Diagrams (pCMDs) and pixel Color-Color Diagrams (pCCDs). We define groups of pixels based on their distribution in a pCCD of (B - 3.6 micron) versus (FUV - U) colors after extinction correction. In the same pCCD, we trace their locations before the extinction correction was applied. This shows that selecting pixel groups is not meaningful when using colors uncorrected for dust. We also trace the distribution of the pixel groups on a pixel coordinate map of the galaxy. We find that the pixel-based (two-dimensional) extinction correction is crucial to reveal the spatial variations in the dominant stellar population, averaged over each resolution element. Different types and mixtures of stellar populations, and galaxy structures such as a previously unrecognized bar, become readily discernible in the extinction-corrected pCCD and as coherent spatial structures in the pixel coordinate map.

Is the (J-Ks) excess in the nuclear region of NGC 1241 due to dust or to C-stars?

NGC 1241 is a Sy 2 galaxy with a 1.4 kpc circumnuclear ring (CNR) of star formation revealed in Paα. A 0.3 kpc long Paα emitting bar centered on the nucleus is present, apparently without associated absorption features. GEMINI (+QUIRC+Hokupa) IR pixel-photometry reveals instead an azimuthally symmetric (J−Ks) color which is redder at the nucleus than at the CNR. This property may well be due to the increasing importance of dust when going from the ring inward into the nucleus. Nevertheless the (V−H) color does not indicate special absorption conditions in the nucleus with respect to the CNR, and no absorption features are evident, as normally expected near emitting bars. Then, we propose as an alternative explanation an excess of C-stars in the nuclear region which decreases outwards until reaching the CNR and its colors. We have compared the pixel color-magnitude diagram with the 2-MASS (J−Ks) vs. Ks diagram for the Large Magellanic Cloud: about 5×102 C-stars and 2.5×104 AGB Oxygen-rich stars inside r∼50 pc, are enough to reproduce the observed nuclear tip in the diagram. This stars would release gas that, gravitationally bounded, pollutes the nuclear environment and could amount 10−2 to 10−1 M. This scenario may also explain the observed increased strength of the CN-bands in the stellar populations of Sy 2 nuclei, and the recent claim of a significant contribution of intermediate age stars to the optical continuum of low luminosity AGNs.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Ultraviolet‐Optical Pixel Maps of Face‐on Spiral Galaxies: Clues for Dynamics and Star Formation Histories

Ultraviolet and optical images of the face-on spiral galaxies NGC 6753 and NGC 6782 reveal regions of strong ongoing star formation that are associated with structures traced by the old stellar populations. We use these images to construct NUV-(NUV-I814) pixel color-magnitude diagrams (pCMDs) that reveal plumes of pixels with strongly varying near-ultraviolet (NUV) surface brightness and nearly constant I814 surface brightness. The plumes correspond to sharply bounded radial ranges, with (NUV-I814) at a given NUV surface brightness being bluer at larger radii. The plumes are parallel to both the reddening vector and simple model mixtures of young and old populations, thus neither reddening nor the fraction of the young population can produce the observed separation between the plumes. The images and radial surface brightness and color plots indicate that the separate plumes are caused by sharp declines in the surface densities of the old populations at radii corresponding to disk resonances. The maximum surface brightness of the NUV light remains essentially constant with radius, while the maximum I814 surface brightness declines sharply with radius. A mid-ultraviolet (MUV) image of NGC 6782 shows emission from the nuclear ring. The distribution of points in an (MUV-NUV)-(NUV-I814) pixel color-color diagram is broadly consistent with the simple mixture model but shows a residual trend that the bluest pixels in (MUV-NUV) are the reddest pixels in (NUV-I814). This may be due to a combination of red continuum from late-type supergiants and [S III] emission lines associated with H II regions in active star-forming regions. We have shown that pixel mapping is a powerful tool for studying the distribution and strength of ongoing star formation in galaxies. Deep, multicolor imaging can extend this to studies of extinction and the ages and metallicities of composite stellar populations in nearby galaxies.