$Sigma_{mathrm{SFR}}$-M* Diagram: A Valuable Galaxy Evolution Diagnostic to Complement (s)SFR-M* Diagrams. (arXiv:2310.11493v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Salim_S/0/1/0/all/0/1">Samir Salim</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tacchella_S/0/1/0/all/0/1">Sandro Tacchella</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Osborne_C/0/1/0/all/0/1">Chandler Osborne</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Faber_S/0/1/0/all/0/1">S. M. Faber</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_J/0/1/0/all/0/1">Janice C. Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ellison_S/0/1/0/all/0/1">Sara L. Ellison</a>
The specific star formation rate (sSFR) is commonly used to describe the
level of galaxy star formation (SF) and to select quenched galaxies. However,
being a relative measure of the young-to-old population, an ambiguity in its
interpretation may arise because a small sSFR can be either because of a
substantial previous mass build up, or because SF is low. We show, using large
samples spanning 0 < z < 2, that the normalization of SFR by the physical
extent over which SF is taking place (i.e., SFR surface density,
$Sigma_{mathrm{SFR}}$) overcomes this ambiguity. $Sigma_{mathrm{SFR}}$ has
a strong physical basis, being tied to the molecular gas density and the
effectiveness of stellar feedback, so we propose $Sigma_{mathrm{SFR}}$-M* as
an important galaxy evolution diagram to complement (s)SFR-M* diagrams. Using
the $Sigma_{mathrm{SFR}}$-M* diagram we confirm the Schiminovich et al.
(2007) result that the level of SF along the main sequence today is only weakly
mass dependent – high-mass galaxies, despite their redder colors, are as active
as blue, low-mass ones. At higher redshift, the slope of the
“$Sigma_{mathrm{SFR}}$ main sequence” steepens, signaling the epoch of bulge
build-up in massive galaxies. We also find that $Sigma_{mathrm{SFR}}$ based
on the optical isophotal radius more cleanly selects both the starbursting and
the spheroid-dominated (early-type) galaxies than sSFR. One implication of our
analysis is that the assessment of the inside-out vs. outside-in quenching
scenarios should consider both sSFR and $Sigma_{mathrm{SFR}}$ radial
profiles, because ample SF may be present in bulges with low sSFR (red color).
The specific star formation rate (sSFR) is commonly used to describe the
level of galaxy star formation (SF) and to select quenched galaxies. However,
being a relative measure of the young-to-old population, an ambiguity in its
interpretation may arise because a small sSFR can be either because of a
substantial previous mass build up, or because SF is low. We show, using large
samples spanning 0 < z < 2, that the normalization of SFR by the physical
extent over which SF is taking place (i.e., SFR surface density,
$Sigma_{mathrm{SFR}}$) overcomes this ambiguity. $Sigma_{mathrm{SFR}}$ has
a strong physical basis, being tied to the molecular gas density and the
effectiveness of stellar feedback, so we propose $Sigma_{mathrm{SFR}}$-M* as
an important galaxy evolution diagram to complement (s)SFR-M* diagrams. Using
the $Sigma_{mathrm{SFR}}$-M* diagram we confirm the Schiminovich et al.
(2007) result that the level of SF along the main sequence today is only weakly
mass dependent – high-mass galaxies, despite their redder colors, are as active
as blue, low-mass ones. At higher redshift, the slope of the
“$Sigma_{mathrm{SFR}}$ main sequence” steepens, signaling the epoch of bulge
build-up in massive galaxies. We also find that $Sigma_{mathrm{SFR}}$ based
on the optical isophotal radius more cleanly selects both the starbursting and
the spheroid-dominated (early-type) galaxies than sSFR. One implication of our
analysis is that the assessment of the inside-out vs. outside-in quenching
scenarios should consider both sSFR and $Sigma_{mathrm{SFR}}$ radial
profiles, because ample SF may be present in bulges with low sSFR (red color).
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