Negative Color Gradients Research Articles

Color Gradients and Surface Brightness Profiles of Galaxies in the Hubble Deep Field-North

We fit elliptical isophotes to the Hubble Deep Field-North WFPC-2 and NICMOS data to study the rest-frame UV_{218}-U_{300} color profiles and rest-frame B surface brightness profiles of 33 intermediate redshift galaxies (0.5 <= z <= 1.2) with I_{814} < 25 and 50 high redshift galaxies (2.0 <= z <= 3.5) with H_{160}< 27. From the weighted least-squares fit to the color profiles we find that, at intermediate redshifts, the galaxies possess negative color gradients indicating a reddening towards the center of the profile similar to local samples whereas, at high redshifts, the galaxies possess positive color gradients. This indicates that star formation is more centrally concentrated in the distant galaxy sample which differs from the prevalent mode of extended disk star formation that we observe in the local universe. Additionally, we find that it is critical to correct for PSF effects when evaluating the surface brightness profiles since at small scale lengths and faint magnitudes, an r^{1/4} profile can be smoothed out substantially to become consistent with an exponential profile. After correcting for PSF effects, we find that at higher look-back time, the fraction of galaxies possessing exponential profiles have slightly decreased while the fraction of galaxies possessing r^{1/4} profiles have slightly increased. Our results also suggest a statistically insignificant increase in the fraction of peculiar/irregular type galaxies. We compare our results with recent semi-analytical models which treat galaxy formation and evolution following the cold dark matter hierarchical framework.

New clues to the evolution of dwarf early-type galaxies

Surface photometry of 18 Virgo cluster dwarf elliptical (dE) and dwarf lenticular (dS0) galaxies, made by Gavazzi et al. (2001) in the H-band (1.65 micron) and in the B-band (0.44 micron), shows that the ratio of the effective radii of these stellar systems in the B- and H-band, r_{e B}/r_{e H}, ranges between 0.7 and 2.2. In particular, dwarf ellipticals and lenticulars with a red total color index B-H (i.e. with 3.2 < B-H < 4) have equal effective radii in these two pass-bands. By contrast, blue (i.e. with 2.5 < B-H < 3.1) dEs and dS0s have B-band effective radii about 50% longer than the H-band ones, on average. Consistently, strong negative gradients in B-H along the galactocentric radius are found to be associated with blue total colors. This trend is not found in a sample of 29 giant E and S0 galaxies of the Coma cluster with analogous data available in the literature. These early-type giants span a broad range in r_{e B}/r_{e H} (0.2--2.2), with a mean r_{e B}/r_{e H} of about 1.1, but a narrow range in (red) color (3.3 < B-H < 4.2). In these stellar systems, color gradients are usually interpreted as due either to age/metallicity gradients along the radial coordinate or to dust attenuation, whatever the total color of the system is. Assuming each of these three distinct interpretations of the origin of color gradients, we discuss the origin of the association of strong negative color gradients with blue colors found in the early-type dwarfs under study, in relation with current scenarios of formation and evolution of dE and dS0 galaxies.

Photometric Evolution of Dusty Starburst Mergers: On the Nature of Ultraluminous Infrared Galaxies

By performing N-body simulations of chemodynamical evolution of galaxies with dusty starbursts, we investigate photometric evolution of gas-rich major mergers to explore the nature of ultraluminous infrared galaxies (ULIRGs) with the total infrared luminosity (LIR for 8-1000 μm) of ~1012 L☉. The main results are the following: (1) Global colors and absolute magnitudes during the dusty starburst of a major merger do not change with time significantly because interstellar dust heavily obscures young starburst populations that could cause rapid evolution of photometric properties of the merger. (2) Dust extinction of stellar populations in a galaxy merger with large infrared luminosity (LIR > 1011 L☉) is selective in the sense that younger stellar populations are preferentially obscured by dust more than old ones are. This is because younger populations are located in the central region where a larger amount of dusty interstellar gas can be transferred from the outer gas-rich regions of the merger. (3) Both LIR and the ratio of LIR to B-band luminosity (LB) increases as the star formation rate increases during the starburst of the present merger model, resulting in the positive correlation between LIR and LIR/LB. The dust temperature, Tdust, and the flux ratio of f60 μm/f100 μm also increase with the increase of the star formation rate because a larger number of young stars formed by massive starbursts can heat the dusty interstellar gas as the star formation becomes larger. (4) The star formation efficiency, total gas mass, the degree of dust extinction (AV), Tdust, LIR, LIR/LB, and f60 μm/f100 μm depend strongly on the separation of the two cores of the merger, which clearly reflects the fact that dynamical processes of galaxy merging play an important role in determining the photometric evolution of dusty starbursts in the merger. (5) The two-dimensional distribution of global colors (e.g., R-K) shows a negative color gradient during starburst, mainly because central young starburst populations are preferentially and very heavily obscured by dust. Furthermore, the peak of the two-dimensional distribution of f60 μm coincides with that of AV. These results clearly suggest that the spatial distribution of gas and stars is one important determinant of the two-dimensional photometric properties of major mergers. Our numerical results clearly demonstrate that dynamical processes of major galaxy merging, which can control the time evolution of the relative spatial distribution of dusty interstellar gas and young stars, play a vital role in determining photometric properties of ULIRGs.

Wide-field CCD surface photometry of the giant elliptical galaxy NGC 4472 in the Virgo cluster

We present deep wide field (16 ′ .4×16 ′ .4 ) Washington CT1 CCD surface photometry of the giant elliptical galaxy NGC 4472, the brightest member in the Virgo cluster. Our data cover a wider field than any previous CCD photometry as well as going deeper. Surface brightness profiles of NGC 4472 are not well fit by a single King model, but they can be fit approximately by two King models: with separate models for the inner and outer regions. Surface brightness profiles for the outer region can also be fit approximately by a deVaucouleurs law. There is clearly a negative color gradient within 3 ′ of NGC 4472, in the sense that the color gets bluer with increasing radius. The slope of the color gradient for this region is derived to be �µ(C T1)

Colors and color gradients in bulges of galaxies

view Abstract Citations (140) References (60) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Colors and Color Gradients in Bulges of Galaxies Balcells, Marc ; Peletier, Reynier F. Abstract We have obtained surface photometry in U, B, R, and I for a complete optically selected sample of 45 early-type spiral galaxies, to investigate the colors and color gradients of spiral bulges. Color profiles in U-R, B-R, U-B, and R-I have been determined in wedges opening on the semiminor axes. Based on several criteria, like the smoothness of the color profiles, the absence of dust lanes, and the central colors, we have defined a subsample of 18 objects whose colors are largely unaffected by dust. We believe such colors are suitable for inferring properties of the stellar populations of bulges. We find that the colors of bulges are predominantly bluer than those of ellipticals. This result holds even when bulges are compared to ellipticals of the same luminosity, and indicates that bulges are younger and/or more metal poor than old elliptical galaxies. Most bulges do not reach solar metallicities. Bulges show predominantly negative color gradients (bluer outward). For bright bulges M_R_^Bulge^ < -20.0), the magnitude of the gradient increases with bulge luminosity. For fainter bulges, gradients scatter around large negative values. The behavior of color gradients as a function of bulge luminosity suggests different formation mechanisms for faint and bright spheroids. For bright bulges, the scaling of gradients with luminosity suggests a formation process involving dissipation. The similarity with ellipticals suggests that the formation of the disk did not affect the stellar populations of the bulge in a major way. For small bulges (M_R_ > -20), the existence of pronounced color gradients suggests a different formation mechanism. For these objects, the presence of the disk may have severely affected the radial population distribution in the bulge. Publication: The Astronomical Journal Pub Date: January 1994 DOI: 10.1086/116839 arXiv: arXiv:astro-ph/9311005 Bibcode: 1994AJ....107..135B Keywords: Color-Color Diagram; Disk Galaxies; Galactic Bulge; Spiral Galaxies; Star Formation; Stellar Color; Astronomical Photometry; Cosmic Dust; Elliptical Galaxies; Galactic Rotation; Metallicity; Stellar Luminosity; Astrophysics; GALAXIES: SPIRAL; GALAXIES: NUCLEI; GALAXIES: FUNDAMENTAL PARAMETERS; Astrophysics E-Print: 14 pages in Latex + 4 tables + 41 figures (available through anonymous ftp at iac.es (161.72.14.2) in pub/rpelet), Kapteyn preprint 125, SLK-125, AJ, January 1994 full text sources arXiv | ADS | data products SIMBAD (32) NED (32) CDS (1)