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Register a new userThe Ultraviolet-Optical Color-Magnitude Diagram III: Constraints on Evolution from the Blue to Red Sequence
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D. Christopher Martin
Publication date
2007
fields
Physics
and research's language is
English
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We introduce a new quantity, the mass flux density of galaxies evolving from the blue sequence to the red sequence. We propose a simple technique for constraining this mass flux using the volume corrected number density in the extinction-corrected UV-optical color magnitude distribution, the stellar age indexes H-delta-a and D_n(4000), and a simple prescription for spectral evolution using a quenched star formation history. We exploit the excellent separation of red and blue sequences in the NUV-r band color-magnitude diagram. The final value we measure, 0.033 M_sun yr^-1 Mpc^-3, is strictly speaking an upper limit due to the possible contributions of bursting, composite, and extincted galaxies. However, it compares favorably with estimates of the average mass flux that we make based on the red luminosity function evolution derived from the DEEPII and COMBO-17 surveys (Bell et al 2004; Faber et al. 2005), 0.034 M_sun yr^-1 Mpc^-3. We find that the blue sequence mass has remained roughly constant since z=1 (mass flux 0.01 M_sun yr^-1 Mpc^-3) but the average on-going star formation of 0.037$ M_sun yr^-1 Mpc^-3 over 0<z<1 is balanced by mass flux off the blue sequence. We explore the nature of the galaxies in the transition zone with particular attention to the frequency and impact of AGNs. The AGN fraction peaks in the transition zone. We find circumstantial, albeit weak evidence that the quench rates are higher in higher luminosity AGNs.
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Using deep NIR VLT/ISAAC and optical HST/WFPC2 imaging in the fields of the HDFS and MS1054-03, we study the rest-frame UV-to-optical colors and magnitudes of galaxies to z~3. While there is no evidence for a red sequence at z~3, there does appear to be a well-defined color-magnitude relation (CMR) for blue galaxies at all redshifts, with more luminous galaxies having redder U-V colors. The slope of the blue CMR is independent of redshift d(U-V)/dMV = -0.09 (0.01) and can be explained by a correlation of dust-reddening with luminosity. The average color at fixed luminosity reddens strongly Delta(U-V) = 0.75 from z~3 to z=0, much of which can be attributed to aging of the stars. The color scatter of the blue sequence is relatively small sigma(U-V) = 0.25 (0.03) and constant to z~3, but notably asymmetrical with a sharp blue ridge and a wing towards redder colors. We explore sets of star formation histories to study the constraints placed by the shape of the scatter at z=2-3. One particular set of models, episodic star formation, reproduces the detailed properties very well. For a two-state model with high and low star formation, the duty cycle is constrained to be > 40% and the contrast between the states must be a factor > 5 (or a scatter in log(SFR) of > 0.35 dex around the mean). However, episodic models do not explain the observed tail of very red galaxies, primarily Distant Red Galaxies (DRGs), which may have ceased star formation altogether or are more heavily obscured. Finally, the relative number density of red, luminous MV < -20.5 galaxies increases by a factor of ~ 6 from z = 2.7 to z = 0.5, as does their contribution to the total rest-frame V-band luminosity density. We are likely viewing the progressive formation of red, passively evolving galaxies.
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We have examined the outburst tracks of 40 novae in the color-magnitude diagram (intrinsic B-V color versus absolute V magnitude). After reaching the optical maximum, each nova generally evolves toward blue from the upper-right to the lower-left and then turns back toward the right. The 40 tracks are categorized into one of six templates: very fast nova V1500 Cyg; fast novae V1668 Cyg, V1974 Cyg, and LV Vul; moderately fast nova FH Ser; and very slow nova PU Vul. These templates are located from the left (blue) to the right (red) in this order, depending on the envelope mass and nova speed class. A bluer nova has a less massive envelope and faster nova speed class. In novae with multiple peaks, the track of the first decay is more red than that of the second (or third) decay, because a large part of the envelope mass had already been ejected during the first peak. Thus, our newly obtained tracks in the color-magnitude diagram provide useful information to understand the physics of classical novae. We also found that the absolute magnitude at the beginning of the nebular phase is almost similar among various novae. We are able to determine the absolute magnitude (or distance modulus) by fitting the track of a target nova to the same classification of a nova with a known distance. This method for determining nova distance has been applied to some recurrent novae and their distances have been recalculated.
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We obtained GALEX FUV, NUV, and Spitzer/IRAC 3.6$mu$m photometry for > 2000 galaxies, available for 90% of the S4G sample. We find a very tight GALEX Blue Sequence (GBS) in the (FUV-NUV) versus (NUV-[3.6]) color-color diagram which is populated by irregular and spiral galaxies, and is mainly driven by changes in the formation timescale ($tau$) and a degeneracy between $tau$ and dust reddening. The tightness of the GBS provides an unprecedented way of identifying star-forming galaxies and objects that are just evolving to (or from) what we call the GALEX Green Valley (GGV). At the red end of the GBS, at (NUV-[3.6]) > 5, we find a wider GALEX Red Sequence (GRS) mostly populated by E/S0 galaxies that has a perpendicular slope to that of the GBS and of the optical red sequence. We find no such dichotomy in terms of stellar mass (measured by $rm{M}_{[3.6]}$), since both massive ($M_{star} > 10^{11} M_{odot}$) blue and red sequence galaxies are identified. The type that is proportionally more often found in the GGV are the S0-Sas and most of these are located in high-density environments. We discuss evolutionary models of galaxies that show a rapid transition from the blue to the red sequence on timescale of $10^{8}$years.
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