We study how half-mass radii, central mass densities (Σ1), and color gradients change as galaxies evolve. We separate ∼7000 galaxies into 16 groups with similar spectral shapes; each group represents ...a different evolutionary stage. We find that different galaxy types populate different regions of both size–mass and Σ1–mass space. The nine star-forming groups lie along the integrated star-forming Σ1–mass relation. However, these star-forming groups form steep parallel relations in the size–mass plane, with slopes similar to the quiescent size–mass relation. These steep slopes can be explained as a transformation of the star-forming Σ1–mass relation and its scatter. We identify three types of transitional galaxies. Green valley and post-starburst galaxies are similarly compact at z > 1.5; however, their distinct color gradients indicate that the two populations represent different pathways to quenching. Post-starburst galaxies have flat color gradients and compact structures, consistent with a fast quenching pathway that requires structural change and operates primarily at high redshift. Green valley galaxies have negative color gradients, and are both larger and more numerous toward lower redshift. These galaxies are consistent with slow quenching without significant structural change. We find that dusty star-forming galaxies at z ≳ 2 are very compact and may represent the “burst” before post-starburst galaxies; at z ≲ 2, dusty star-forming galaxies are extended and have shallow color gradients consistent with slow quenching. Our results suggest that star-forming galaxies grow gradually up the Σ1–mass relation until (a) they naturally reach the high Σ1 values required for quiescence or (b) a compaction-type event rapidly increases their Σ1.
Abstract
Accurate models of the star formation histories (SFHs) of recently quenched galaxies can provide constraints on when and how galaxies shut down their star formation. The recent development ...of
nonparametric
SFH models promises the flexibility required to make these measurements. However, model and prior choices significantly affect derived SFHs, particularly for post-starburst galaxies (PSBs), which have sharp changes in their recent SFH. In this paper, we create mock PSBs, then use the
Prospector
SED fitting software to test how well four different SFH models recover key properties. We find that a two-component parametric model performs well for our simple mock galaxies, but is sensitive to model mismatches. The fixed- and flexible-bin nonparametric models included in
Prospector
are able to rapidly quench a major burst of star formation, but systematically underestimate the post-burst age by up to 200 Myr. We develop a custom SFH model that allows for additional flexibility in the recent SFH. Our flexible nonparametric model is able to constrain post-burst ages with no significant offset and just ∼90 Myr of scatter. Our results suggest that while standard nonparametric models are able to recover first-order quantities of the SFH (mass, SFR, average age), accurately recovering higher-order quantities (burst fraction, quenching time) requires careful consideration of model flexibility. These mock recovery tests are a critical part of future SFH studies. Finally, we show that our new, public SFH model is able to accurately recover the properties of mock star-forming and quiescent galaxies and is suitable for broader use in the SED fitting community.
https://github.com/bd-j/prospector
Abstract
We present results from MUSE spatially resolved spectroscopy of 21 post-starburst galaxies in the centers of eight clusters from
z
∼ 0.3 to
z
∼ 0.4. We measure spatially resolved star ...formation histories (SFHs), the time since quenching (
t
Q
), and the fraction of stellar mass assembled in the past 1.5 Gyr (
μ
1.5
). The SFHs display a clear enhancement of star formation prior to quenching for 16 out of 21 objects, with at least 10% (and up to >50%) of the stellar mass being assembled in the past 1.5 Gyr and
t
Q
ranging from less than 100 to ∼800 Myr. By mapping
t
Q
and
μ
1.5
, we analyze the quenching patterns of the galaxies. Most galaxies in our sample have quenched their star formation from the outside in or show a side-to-side/irregular pattern, both consistent with quenching by ram pressure stripping. Only three objects show an inside-out quenching pattern, all of which are at the high-mass end of our sample. At least two of them currently host an active galactic nucleus. In two post-starbursts, we identify tails of ionized gas indicating that these objects had their gas stripped by ram pressure very recently. Post-starburst features are also found in the stripped regions of galaxies undergoing ram pressure stripping in the same clusters, confirming the link between these classes of objects. Our results point to ram pressure stripping as the main driver of fast quenching in these environments, with active galactic nuclei playing a role at high stellar masses.
Abstract
How do galaxies transform from blue, star-forming spirals to red, quiescent early-type galaxies? To answer this question, we analyzed a set of 26 gas-rich, shocked post-starburst galaxies ...with Hubble Space Telescope (HST) imaging in
B
,
I
, and
H
bands and Sloan Digital Sky Survey (SDSS)
i
-band imaging of similar depth but lower resolution. We found that post-starbursts in our sample have intermediate morphologies between disk- and bulge-dominated (Sérsic
n
=
1.7
−
0.0
+
0.3
) and have red bulges, likely due to dust obscuration in the cores. A majority of galaxies in our sample are more morphologically disturbed than regular galaxies (88%, corresponding to >3
σ
significance) when observed with HST, with asymmetry and Sérsic residual flux fraction being the most successful measures of disturbance. Most disturbances are undetected at the lower resolution of SDSS imaging. Although ∼27% galaxies are clear merger remnants, we found that disturbances in another ∼30% of the sample are internal, caused by small-scale perturbations or dust substructures rather than tidal features, and require high-resolution imaging to detect. We found 2.8
σ
evidence that asymmetry features fade on timescales ∼200 Myr, and may vanish entirely after ∼750 Myr, so we do not rule out a possible merger origin of all post-starbursts given that asymmetric features may have already faded. This work highlights the importance of small-scale disturbances, detected only in high-resolution imaging, in understanding structural evolution of transitioning galaxies.
Abstract
Post-starburst (PSB), or “E + A,” galaxies represent a rapid transitional phase between major, gas-rich mergers and gas-poor, quiescent, early-type galaxies. Surprisingly, many PSBs have ...been shown to host a significant interstellar medium (ISM), despite theoretical predictions that the majority of the star-forming gas should be expelled in active galactic nuclei– or starburst-driven outflows. To date, the resolved properties of this surviving ISM have remained unknown. We present high-resolution ALMA continuum and CO(2–1) observations in six gas- and dust-rich PSBs, revealing for the first time the spatial and kinematic structure of their ISM on sub-kpc scales. We find extremely compact molecular reservoirs, with dust and gas surface densities rivaling those found in (ultra)luminous infrared galaxies. We observe spatial and kinematic disturbances in all sources, with some also displaying disk-like kinematics. Estimates of the internal turbulent pressure in the gas exceed those of normal star-forming disks by at least 2 orders of magnitude, and rival the turbulent gas found in local interacting galaxies, such as the Antennae. Though the source of this high turbulent pressure remains uncertain, we suggest that the high incidence of tidal disruption events in PSBs could play a role. The star formation in these PSBs’ turbulent central molecular reservoirs is suppressed, forming stars only 10% as efficiently as starburst galaxies with similar gas surface densities. “The fall” of star formation in these galaxies was not precipitated by complete gas expulsion or redistribution. Rather, this high-resolution view of PSBs’ ISM indicates that star formation in their remaining compact gas reservoirs is suppressed by significant turbulent heating.
Abstract
Tidal disruption events (TDEs) provide a unique opportunity to probe the stellar populations around supermassive black holes (SMBHs). By combining light-curve modeling with spectral line ...information and knowledge about the stellar populations in the host galaxies, we are able to constrain the properties of the disrupted star for three TDEs. The TDEs in our sample have UV spectra, and measurements of the UV N
iii
to C
iii
line ratios enabled estimates of the nitrogen-to-carbon abundance ratios for these events. We show that the measured nitrogen line widths are consistent with originating from the disrupted stellar material dispersed by the central SMBH. We find that these nitrogen-to-carbon abundance ratios necessitate the disruption of moderately massive stars (≳1–2
M
⊙
). We determine that these moderately massive disruptions are overrepresented by a factor of ≳10
2
when compared to the overall stellar population of the post-starburst galaxy hosts. This implies that SMBHs are preferentially disrupting higher mass stars, possibly due to ongoing top-heavy star formation in nuclear star clusters or to dynamical mechanisms that preferentially transport higher mass stars to their tidal radii.
We report two secure ( ) and one tentative (z 3.767) spectroscopic confirmations of massive and quiescent galaxies through K-band observations with Keck/MOSFIRE and Very Large Telescope/X-Shooter. ...The stellar continuum emission, absence of strong nebular emission lines, and lack of significant far-infrared detections confirm the passive nature of these objects, disfavoring the alternative solution of low-redshift dusty star-forming interlopers. We derive stellar masses of log(M /M ) ∼ 11 and ongoing star formation rates placing these galaxies 1-2 dex below the main sequence at their redshifts. The adopted parameterization of the star formation history suggests that these sources experienced a strong ( M yr−1) and short (∼50 Myr) burst of star formation, peaking ∼150-500 Myr before the time of observation, all properties reminiscent of the characteristics of submillimeter galaxies (SMGs) at z > 4. We investigate this connection by comparing the comoving number densities and the properties of these two populations. We find a fair agreement only with the deepest submillimeter surveys detecting not only the most extreme starbursts but also more normal galaxies. We support these findings by further exploring the Illustris TNG cosmological simulation, retrieving populations of both fully quenched massive galaxies at z ∼ 3-4 and SMGs at z ∼ 4−5, with number densities and properties in agreement with the observations at z ∼ 3 but in increasing tension at higher redshift. Nevertheless, as suggested by the observations, not all of the progenitors of quiescent galaxies at these redshifts shine as bright SMGs in their past, and, similarly, not all bright SMGs quench by z ∼ 3, both fractions depending on the threshold assumed to define the SMGs themselves.
Abstract
We study the size–mass relation (SMR) and recent star formation history (SFH) of post-starburst (PSB) galaxies in the local Universe using spatially resolved spectroscopy from the final data ...release of MaNGA. Our sample includes 489 PSB galaxies: 94 cPSB galaxies with central PSB regions, 85 rPSB galaxies with ringlike PSB regions, and 310 iPSB galaxies with irregular PSB regions. When compared to control galaxies of similar star formation rate, redshift, and mass, a similar SMR is found for all types of PSB samples except the cPSB galaxies, which have smaller sizes at intermediate masses (
9.5
≲
log
10
(
M
*
/
M
⊙
)
≲
10.5
). The iPSB galaxies in the star-forming sequence (iPSB-SF) show no/weak gradients in
D
n
(4000), EW(H
δ
A
), and EW(H
α
), consistent with the global star-forming status of this type of galaxy, while the quiescent iPSB (iPSB-Q) sample shows negative gradients in
D
n
(4000) and positive gradients in EW(H
δ
A
), indicating older stellar populations in the inner regions. Both the cPSB and rPSB samples show positive gradients in
D
n
(4000) and negative gradients in EW(H
δ
A
), indicating younger stellar populations in the inner regions. These results imply that the four types of PSB galaxies can be broadly divided into two distinct categories in terms of evolutionary pathway: (1) iPSB-SF and iPSB-Q, which have SMRs and SFHs similar to control galaxies, preferring an inside-out quenching process, and (2) rPSB and cPSB, which appear to be different stages of the same event and likely to follow the outside-in quenching process driven by disruption events such as mergers that result in a more compact structure as quenching proceeds.
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