Spatially resolved colours and stellar population properties in early-type galaxies at z ∼ 1.5 (original) (raw)
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Colour and stellar population gradients in galaxies: correlation with mass
Monthly Notices of the Royal Astronomical Society, 2010
We analyze the color gradients (CGs) of ∼ 50 000 nearby Sloan Digital Sky Survey (SDSS) galaxies estimated by their photometrical parameters (Sérsic index, total magnitude, and effective radius). From synthetic spectral models based on a simplified star formation recipe, we derive the mean spectral properties, and explain the observed radial trends of the color as gradients of the stellar population age and metallicity. Color gradients have been correlated with color, luminosity, size, velocity dispersion, and stellar mass. Distinct behaviours are found for early-and late-type galaxies (ETGs and LTGs), pointing to slightly different physical processes at work in different morphological types and at different mass scales.
Colour gradients in normal and compact early-type galaxies at 1 < z < 2
Monthly Notices of the Royal Astronomical Society, 2011
We have derived colour gradients for a sample of 20 early-type galaxies (ETGs) at 1 < z spec < 2 selected from the GOODS-South field. The sample includes both normal ETGs (13) having effective radii comparable to the mean radius of local ones and compact ETGs (7) having effective radii from two to six times smaller. Colour gradients have been derived in the F606W-F850LP bands ( UV-U rest-frame) taking advantage of the ultradeep HST-ACS observations covering this field and providing a spatial resolution of about 0.8 kpc at the redshift of the galaxies. Despite of the narrow wavelength baseline covered (1000Å), sampling approximatively the emission dominated by the same stellar population, we detect significant radial colour variations in 50 per cent of our sample. In particular, we find five ETGs with positive colour gradients (cores bluer than the external regions), and five galaxies with negative colour gradients (cores redder than the external regions), as commonly observed in the local Universe. These results show that at 1 < z < 2, when the Universe was only 3-4 Gyr old, ETGs constituted a composite population of galaxies whose different assembly histories have generated different stellar distributions with the bluest stellar population either in the center or in the outskirts as well as throughout the whole galaxy. Moreover, we find that compact galaxies seem to preferentially show a blue cores while moving towards normal galaxies, central stellar populations become progressively redder. Nonetheless, the narrow baseline covered together with the low statistics still prevent us to be conclusive about a possible physical connection between colour gradients and the degree of compactness of high-z ETGs.
The Astrophysical Journal Supplement Series, 2007
We study the enviromental dependence and the morphological composition of the galaxy colormagnitude diagram at z ∼ 0.7, using a pilot sub-sample of the COSMOS survey. The sample includes ∼ 2000 galaxies with I AB < 24 and photometric redshift within 0.61 < z < 0.85, covering an area of 270 square-arcmin. Galaxy morphologies are estimated via a non-parametric automatic technique. The (V − z ′ ) vs. z ′ color-magnitude diagram shows a clear red-sequence dominated by early-type galaxies and also a remarkably well-defined "blue sequence" described by late-type objects. While the percentage of objects populating the two sequences is a function of environment, following a clear morphology/color-density relation also at this redshift, we establish that their normalization and slope are independent of local density. We identify and study a number of objects with "anomalous" colors, given their morphology, polluting the two sequences. Red late-type galaxies are found to be mostly highly-inclined or edge-on spiral galaxies, for which colors are dominated by internal reddening by dust. In a sample of color-selected red galaxies, these would represent a 33% contamination with respect to truly passive spheroidals. Conversely, the population of blue early-type galaxies is composed by objects of moderate luminosity and mass, concurring to only ∼ 5% of the mass in spheroidal galaxies. The majority of them (∼ 70%) occupy a position in the µ B -r 50 plane not consistent with them being precursors of current epoch elliptical galaxies. Their fraction with respect to the whole galaxy population does not depend on the environment, at variance with the general early-type class. In a color-mass diagram, color sequences are even better defined, with red galaxies covering in general a wider range of masses at nearly constant color, and blue galaxies showing a more pronounced dependence of color on mass. While the red sequence is adequately reproduced by models of passive evolution, the blue sequence is better interpreted as a specific star-formation sequence. The substantial invariance of its slope and normalization with respect to local density suggests that the overall, "secular" star formation is driven more by galaxy mass than by environment.
The evolution of the color gradients of early-type cluster galaxies
arXiv (Cornell University), 2000
We investigate the origin of color gradients in cluster early-type galaxies to probe whether pure age or pure metallicity gradients can explain the observed data in local and distant (z ≈ 0.4) samples. We measure the surface brightness profiles of the 20 brightest early-type galaxies of CL0949+44 (hereafter CL0949) at redshift z=0.35-0.38 from HST WF2 frames taken in the filters F555W, F675W, F814W. We determine the color profiles (V − R)(r), (V − I)(r), and (R − I)(r) as a function of the radial distance r in arcsec, and fit logarithmic gradients in the range −0.2 to 0.1 mag per decade. These values are similar to what is found locally for the colors (U − B), (U − V), (B − V) which approximately match the (V − R), (V − I), (R − I) at redshift ≈ 0.4. We analyse the results with up to date stellar population models. We find that passive evolution of metallicity gradients (≈ 0.2 dex per radial decade) provides a consistent explanation of the local and distant galaxies' data. Invoking pure age gradients (with fixed metallicity) to explain local color gradients produces too steep gradients at redshifts z ≈ 0.4. Pure age gradients are consistent with the data only if large present day ages ≥ 15 Gyr are assumed for the galaxy centers.
Colour gradients within SDSS DR7 galaxies: hints of recent evolution
Monthly Notices of the Royal Astronomical Society, 2011
The evolutionary path followed by a galaxy shapes its internal structure, and, in particular, its internal colour variation. We present a study of the internal colour variation within galaxies from the Seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7). We statistically study the connection between the internal colour variation and global galactic properties, looking for hints of the recent galactic evolution. Considering only galaxies with good photometry and spectral measurements, we define four luminosity-threshold samples within the redshift range 0.01 < z < 0.17, each containing more than 48000 galaxies. Colour gradients are calculated for these galaxies from the surface brightness measurements provided by the SDSS DR7. Possible systematic effects in their determination have been analysed. We find that, on average, galaxies have redder cores than their external parts. We also find that it is more likely to find steep colour gradients among late-type galaxies. This result holds for a range of classifications based on both morphological and spectral characteristics. In fact, our results relate, on average, steep colour gradients to a higher presence of young stars within a galaxy. Our results also suggest that nuclear activity is a marginal driver for creating steep colour gradients in massive galaxies. We have selected pairs of interacting galaxies, with a separation of 5 arcsec, in projected radius, and a difference in redshift of 100 km/s, finding that they present steeper gradients than the average population, skewed towards bluer cores. Our analysis implies that colour gradients can be useful for selecting galaxies that have suffered a recent (minor) burst of star formation.
Early-Type Galaxies in the Sloan Digital Sky Survey. IV. Colors and Chemical Evolution
The Astronomical …, 2003
The colors and chemical abundances of early-type galaxies at redshifts z < 0.3 are studied using a sample of nearly 9000 galaxies, selected from the Sloan Digital Sky Survey using morphological and spectral criteria. In this sample, redder galaxies have larger velocity dispersions: g*Àr* / 0.26AE0.02 . Color also correlates with magnitude, g à À r à / ðÀ0:025 AE 0:003ÞM r à , and size, but these correlations are entirely due to the L-and R o -relations: the primary correlation is color-. The red light in early-type galaxies is, on average, slightly more centrally concentrated than the blue. Because of these color gradients, the strength of the colormagnitude relation depends on whether or not the colors are defined using a fixed metric aperture; the color-relation is less sensitive to this choice. Chemical evolution and star formation histories of early-type galaxies are investigated using co-added spectra of similar objects. The resulting library of co-added spectra contains spectra that represent a wide range of early-type galaxies. Chemical abundances correlate primarily with velocity dispersion: H / À0.24AE0.03 , Mg 2 / 0.20AE0.02 , Mg b / 0.32AE0.03 , and hFei / 0.11AE0.03 . At fixed , the population at z $ 0.2 had weaker Mg 2 and stronger H absorption compared to the population at z $ 0. It was also bluer. Comparison of these colors and line strengths and their evolution with single-burst stellar population models suggests a formation time of 9 Gyr ago, consistent with a fundamental plane analysis of this sample. Although the fundamental plane shows that galaxies in dense regions are slightly different from galaxies in less dense regions, the co-added spectra and color-magnitude relations show no statistically significant dependence on environment.
Monthly Notices of the Royal Astronomical Society: Letters, 2009
Predicting the colors of Luminous Red Galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS) has been a long-standing problem. The g, r, i colors of LRGs are inconsistent with stellar population models over the redshift range 0.1 < z < 0.7. The g − r colors in the models are on average redder than the data (of the order 0.1 mag) while the r − i colors in the models are bluer (by 0.05 mag) towards low redshift. Beyond redshift 0.4, the predicted r − i color becomes instead too red, while the predicted g − r agrees with the data. We provide a solution to this problem, through a combination of new astrophysics and a fundamental change to the stellar population modeling. We find that the use of the empirical library of Pickles (1998), in place of theoretical libraries based on model atmosphere calculations, modifies the evolutionary population synthesis predicted colors exactly in the way suggested by the data, i.e., gives a redder r − i color, and a bluer g − r color, in the observed frame at z = 0.1. The reason is a lower flux in the empirical libraries, with respect to the theoretical ones, in the wavelength range 5500 − 6500Å. The discrepancy increases with decreasing effective temperature independently of gravity. This result has general implications for a variety of studies from globular clusters to high-redshift galaxies. The astrophysical part of our solution regards the composition of the stellar populations of these massive Luminous Red Galaxies. We find that on top of the previous effect one needs to consider a model in which ∼ 3% of the stellar mass is in old metal-poor stars. Other solutions such as substantial blue Horizontal Branch at high metallicity or young stellar populations can be ruled out by the data. Our new model provides a better fit to the g − r and r − i colors of LRGs and gives new insight into the formation histories of these most massive galaxies. Our model will also improve the k-and evolutionary corrections for LRGs which are critical for fully exploiting present and future galaxy surveys.
Internal colour gradients for E/S0 galaxies in Abell 2218
Monthly Notices of the Royal Astronomical Society, 2005
We determine colour gradients of −0.15 ± 0.08 magnitudes per decade in radius in F450W−F606W and −0.07±0.06 magnitudes per decade in radius in F606W−F814W for a sample of 22 E/S0 galaxies in Abell 2218. These gradients are consistent with the existence of a mild (∼ −0.3 dex per decade in radius) gradient in metal abundance, (cf. previous work at lower and higher redshift for field and cluster galaxies). The size of the observed gradients is found to be independent of luminosity over a range spanning M * − 1 to M * + 1.5 and also to be independent of morphological type. These results suggest a fundamental similarity in the distributions of stellar populations in ellipticals and the bulges of lenticular galaxies. These results are not consistent with simple models of either monolithic collapse or hierarchical mergers.
The Role of Galaxy Stellar Mass in the Colour–Density Relation up to z ~ 1
Astrophysics and Space Science Proceedings, 2011
Aims. With the first ∼10 000 spectra of the flux limited zCOSMOS sample (I AB ≤ 22.5) we want to study the evolution of environmental effects on galaxy properties since z ∼ 1.0, and to disentangle the dependence among galaxy colour, stellar mass and local density. Methods. We use our previously derived 3D local density contrast δ, computed with the 5th nearest neighbour approach, to study the evolution with z of the environmental effects on galaxy U-B colour, D4000 Å break and [OII]λ3727 equivalent width (EW[OII]). We also analyze the implications due to the use of different galaxy selections, using luminosity or stellar mass, and we disentangle the relations among colour, stellar mass and δ studying the colour-density relation in narrow mass bins. Results. We confirm that within a luminosity-limited sample (M B ≤ −20.5 − z) the fraction of red (U − B ≥ 1) galaxies depends on δ at least up to z ∼ 1, with red galaxies residing mainly in high densities. This trend becomes weaker for increasing redshifts, and it is mirrored by the behaviour of the fraction of galaxies with D4000 Å break ≥1.4. We also find that up to z ∼ 1 the fraction of galaxies with log(EW[OII]) ≥ 1.15 is higher for lower δ, and also this dependence weakens for increasing z. Given the triple dependence among galaxy colours, stellar mass and δ, the colour-δ relation that we find in the luminosity-selected sample can be due to the broad range of stellar masses embedded in the sample. Thus, we study the colour-δ relation in narrow mass bins within mass complete subsamples, defining red galaxies with a colour threshold roughly parallel to the red sequence in the colour-mass plane. We find that once mass is fixed the colour-δ relation is globally flat up to z ∼ 1 for galaxies with log(M/M) 10.7. This means that for these masses any colour-δ relation found within a luminosity-selected sample is the result of the combined colour-mass and mass-δ relations. On the contrary, even at fixed mass we observe that within 0.1 ≤ z ≤ 0.5 the fraction of red galaxies with log(M/M) 10.7 depends on δ. For these mass and redshift ranges, environment affects directly also galaxy colours. Conclusions. We suggest a scenario in which the colour depends primarily on stellar mass, but for an intermediate mass regime (10.2 log(M/M) 10.7) the local density modulates this dependence. These relatively low mass galaxies formed more recently, in an epoch when more evolved structures were already in place, and their longer SFH allowed environment-driven physical processes to operate during longer periods of time.
Monthly Notices of the Royal Astronomical Society, 2013
We combine UV/optical/near-IR/mid-IR data on a sample of ∼240 S0 galaxies to examine various star formation related processes in them. We split the sample into bright and faint S0 galaxies based on their K-band luminosity. Comparing the far-ultraviolet (FUV)−nearultraviolet (NUV) versus NUV−K colour-colour diagram with a simple stellar population model shows that ellipticals and bright S0 galaxies are dominated by a stellar population of age >10 9 yr while faint S0 galaxies may contain stars as young as 10 8 yr, providing evidence for relatively recent star formation activity. The strength of the 4000 Å break is also systematically higher in brighter S0 galaxies, again indicating the presence of an old stellar population. Their mid-IR colours indicate that bright S0 colours are like those of ellipticals while faint S0 colours are more like spirals. All these observations are consistent with a scenario in which low-luminosity S0 galaxies likely formed by the stripping of gas from the discs of late-type spiral galaxies, which in turn formed their pseudo-bulges through secular evolution processes, possibly involving multiple episodes of star formation. On the other hand, more luminous S0 galaxies likely formed the bulk of their stars at early epochs, similar to the star formation in elliptical galaxies, and are characterized by an old coeval stellar population and classical bulges.