Smooth Stellar Mass Maps, and the Nature and Fate of Star-forming Clumps

“If I have seen further it is by standing on ye sholders of Giants.”  Written in 1676 and signed Sir Isaac Newton, these famous words are applicable to any scientific endeavor undertaken ever since, however ambitious or incremental.

Today’s news flash on a recently published CANDELS paper by myself and collaborators features three such metaphorical giants. The first one has an extraordinary sharp eye, but unfortunately suffers from color blindness. The second senses a rainbow of colors, from bluer than blue to redder than red, but his blurred vision reduces the most beautiful structures to a fuzzy, featureless blob. Finally, the third giant combines the virtues of the first two, but is hampered by tunnel vision, allowing him to inspect only few galaxies at a time.

For years, Giant One has been telling his brothers how, as he looked at ever more distant galaxies, they appear clumpier and more irregular in his black-and-white snapshots than their local counterparts.  This led to speculations about more abundant collisions between galaxies, as well as other processes that could wreck their appearance and may be more prevalent in the early universe.  For instance, distant galaxies are known to be more gas rich, and theory predicts that under these conditions instabilities in the gas disk can give rise to clump formation.  

Meanwhile, Giant Two developed the tools to translate his colorful yet blurry visions of distant galaxies to physical characteristics: the mass in stars hosted by the galaxy, their age, and the rate at which new stars are being formed.  Giant Three raised the concern that color variations, tell-tale signs of a diversity in stellar populations, may be present not only among a population of galaxies, but also spatially within individual galaxies.  However, he lacked the statistics to address his concerns in a systematic way.

Star-forming galaxies featuring clumpy light distributions, 

yet smooth stellar mass maps.  From left to right: color image, 

blue light distribution,  red light distribution, stellar mass map.

(credit: Stijn Wuyts)

Enter CANDELS.  With the power of Hubble’s ACS and WFC3 cameras, we combine the virtues of the giants without any of their handicaps.  In our latest study, we look 5 to 11 billion years back in time, and construct a large and complete sample of 650 massive star-forming galaxies.  For each of them, we measure the resolved light distribution in as many as seven wavebands.  Interestingly, Giant Three’s concerns seem validated: spatial color variations are indeed seen across the face of distant galaxies.  Typically, the clumps are bluer than the underlying disk, and the reddest colors are found in the center.  Applying the tools developed by Giant Two, we translate the color information of each pixel to physical quantities, and reconstruct for the first time maps of the stellar mass, rate of star formation, and age within large samples of early star-forming galaxies.

One striking trend immediately caught our attention when inspecting the mass maps: they appear more concentrated and smoother than the light distributions, particularly than those measured at short wavelengths.  The difference in concentration can be understood from the fact that dust, when present, generally obscures more light in the center than in the outskirts.  When the galaxy has old stars, contributing significantly to the mass but less so to the light, they also tend to reside in the center, leading to the same effect.  The remarkable smoothness in mass is a blow to the concept that large fractions of distant galaxies are undergoing collisions.  Such collisions would not only wreck the light but also the mass distribution.  While off-center clumps seen in light are not prominent in the mass maps, they stand out as short-lived vigorously star-forming regions, hosting younger stars than the underlying disk.   

Why only young clumps are seen at large radii remains an unsolved puzzle, and a source of heated debate among theorists and observers alike. One group of theorists advocates a scenario in which clumps migrate inward very efficiently, where they contribute to building up a central bulge. A competing group argues that supernova explosions and radiation from massive, bright stars drive strong outflows that quickly disrupt the star-forming clumps, hence limiting their lifetimes. Upcoming analyses of CANDELS data and spectral diagnostics in concert promise to untie the knot.