Samir Salim, Sandro Tacchella, Chandler Osborne, S. M. Faber, Janice C. Lee, Sara L. Ellison
Published 2023-10-17, 16 pages. Accepted to ApJ. Comments on content or relevant missing references welcome
The specific star formation rate (sSFR) is commonly used to describe thelevel of galaxy star formation (SF) and to select quenched galaxies. However,being a relative measure of the young-to-old population, an ambiguity in itsinterpretation may arise because a small sSFR can be either because of asubstantial previous mass build up, or because SF is low. We show, using largesamples spanning 0 < z < 2, that the normalization of SFR by the physicalextent over which SF is taking place (i.e., SFR surface density,$\Sigma_{\mathrm{SFR}}$) overcomes this ambiguity. $\Sigma_{\mathrm{SFR}}$ hasa strong physical basis, being tied to the molecular gas density and theeffectiveness of stellar feedback, so we propose $\Sigma_{\mathrm{SFR}}$-M* asan important galaxy evolution diagram to complement (s)SFR-M* diagrams. Usingthe $\Sigma_{\mathrm{SFR}}$-M* diagram we confirm the Schiminovich et al.(2007) result that the level of SF along the main sequence today is only weaklymass dependent - high-mass galaxies, despite their redder colors, are as activeas blue, low-mass ones. At higher redshift, the slope of the"$\Sigma_{\mathrm{SFR}}$ main sequence" steepens, signaling the epoch of bulgebuild-up in massive galaxies. We also find that $\Sigma_{\mathrm{SFR}}$ basedon the optical isophotal radius more cleanly selects both the starbursting andthe spheroid-dominated (early-type) galaxies than sSFR. One implication of ouranalysis is that the assessment of the inside-out vs. outside-in quenchingscenarios should consider both sSFR and $\Sigma_{\mathrm{SFR}}$ radialprofiles, because ample SF may be present in bulges with low sSFR (red color).
Junais, P. M. Weilbacher, B. Epinat, S. Boissier, G. Galaz, E. J. Johnston, T. H. Puzia, P. Amram, K. Małek
Published 2023-10-18, 12 pages, 9 figures, accepted for publication in A&A
Giant low-surface brightness (GLSB) galaxies are an extreme class of objectswith very faint and extended gas-rich disks. Malin 1 is the largest GLSB galaxyknown to date, but its formation is still poorly understood. We use VLT/MUSEIFU spectroscopic observations of Malin 1 to reveal, for the first time, thepresence of H$\alpha$ emission distributed across numerous regions along itsdisk, up to radial distances of $\sim$100 kpc. We made an estimate of the dustattenuation using the Balmer decrement and found that Malin 1 has a meanH$\alpha$ attenuation of 0.36 mag. We observe a steep decline in the starformation rate surface density ($\Sigma_{\rm SFR}$) within the inner 20 kpc,followed by a shallow decline in the extended disk. Similarly, the gas phasemetallicity we estimated shows a steep gradient in the inner 20 kpc, followedby a flattening of the metallicity in the extended disk with a relatively highvalue of $\sim$0.6 $Z_{\odot}$. We found that the normalized abundance gradientof the inner disk is similar to values found in normal galaxies but with anextreme value in the extended disk. A comparison of the star formation ratesurface density and gas surface density shows that, unlike normal disk galaxiesor other LSBs, Malin 1 exhibits a very low star formation efficiency. Owing tothe detection of emission lines over a large part of the disk of Malin 1, thiswork sheds light on the star formation processes in this unique galaxy,highlighting its extended star-forming disk, dust attenuation, almost flatmetallicity distribution in the outer disk, and exceptionally lowstar-formation efficiency. Our findings contribute to a more detailedunderstanding of the formation of the giant disk of Malin 1 and also constrainpossible proposed scenarios on the nature of GLSB galaxies in general.