the influence of environment & its history on quenching ...3 theoretical models...

The influence of environment & its history

on quenching star formation

Michaela Hirschmann (OATS-INAF) with Gabriella De Lucia (OATS-INAF), Dave Wilman, Simone Weinmann,

Angela Iovino, Olga Cucciati, Stefano Zibetti, Alvaro Villalobos

1

GEE 3 - GALAXY EVOLUTION AND ENVIRONMENT12-14th November 2013, Padova

2

How star formation in a galaxy can get quenched:A) Internal processes

(nature) B) External processes

(nurture)

Motivation

best traced by isolated centrals (e.g. Hirschmann et al., 2013a)

feedback from SN &

AGN merger events

environment

How relevant is environment for quenching galaxies?( e.g. Font et al. 2008, van den Bosch et al. 2008, Kimm et al. 2009, Weinmann et al. 2009/10, Peng et al. 2010, Tinker et al. 2011, Woo et al. 2012, Cucciati et al. 2012, Wetzel et al. 2012/13, Bahe et al. 2013, Kovac et al. 2013)

2

How star formation in a galaxy can get quenched:A) Internal processes

(nature) B) External processes

(nurture)

Motivation

best traced by isolated centrals (e.g. Hirschmann et al., 2013a)

feedback from SN &

AGN merger events

environment

Our approach: analyse the environmental history!✦ Environmental effects important for centrals (super-halo scales)?✦ Typical quenching time-scales of satellites?

Method: Comparing galaxy formation models to observations

How relevant is environment for quenching galaxies?( e.g. Font et al. 2008, van den Bosch et al. 2008, Kimm et al. 2009, Weinmann et al. 2009/10, Peng et al. 2010, Tinker et al. 2011, Woo et al. 2012, Cucciati et al. 2012, Wetzel et al. 2012/13, Bahe et al. 2013, Kovac et al. 2013)

3

Theoretical models

✦ Millennium-Simulation (Springel at al. 2005): 5123 particles in a (500Mpc/h)3 box, merger trees & spatial distribution of the halos

✦ Semi-analytic model, Guo et al., 2011, assuming a gradual stripping of hot halo: Populate dark matter halos with galaxies, same selection criteria as in observations

Observational data

✦ Density catalogue of Wilman et al. (2010) using SDSS (DR8): z = 0.015-0.08, Mr<-18,

∆v = +/-1000km/s + Vmax correction

✦ Cross-correlated with Brinchmann et al. (2004) & Yang et al. (2007): Estimates for stellar masses, SFRs (Halpha emission lines), galaxy types

Density estimation: Σri,ra =Ngal

π(r2a − r2i )

ri = 0 Mpc, ra = 1 Mpcwith:see Wilman et al. 2010

Quiescent galaxies: sSFR ≡ SFR

Mstellar

<0.3

tHubble see Franx et al. 2010

Method

4

I. How well do current models (Guo) reproduce the obser-

ved quiescent fractions?

Quiescent/red fractions are dependent on both

stellar mass and density(e.g. Peng et al. 2010)

✦ Observations: overall density dependence mainly driven by satellites (Kovac et al.,2013)

similar behavior of centrals & satellites & strong density dependence (see also Cucciati et al.,2012)

4 Hirschmann et al.

Figure 2. Quiescent fraction of all (red), central (yellow) and satellite (green) galaxies versus the projected density for a 1 Mpc cylinder(top row) and for a 0.2 Mpc cylinder (bottom row) for observations (symbols) and the Guo model (lines). Different columns correspondto different stellar mass bins as indicated in the legend. At stellar masses below 1011M!, observations show a strong correlation betweenthe quiescent fraction and density of both, satellites and centrals. For stellar masses above 1010M! there is almost no difference betweenobserved centrals and satellites what is in significant contrast to the model predictions.

Solving for the hypothetical average quantities yields in:

p̄sat =

(

p̂sat −csat

1− ccenp̂cen

)

×

×1

1− csat − [(csatccen)/(1− ccen)](5)

p̄cen =

(

p̂cen −ccen

1− csatp̂sat

)

×

×1

1− ccen − [(ccencsat)/(1− csat)], (6)

When applying the above estimations to the observed av-erage quiescent central and satellite fractions and when forsimplicity assuming a contamination of centrals and satel-lites being independent of stellar mass, we find that thequiescent fractions (at a given stellar mass) remain almostunchanged. Thus, in the following analysis, we choose topresent all results uncorrected for contamination. However,we still would like the reader to keep on mind that in re-ality, the degree of contamination is rather uncertain as itdepends on stellar mass, luminosity and also on the type ofMGRS used (see Weinmann et al. 2009 for more details),why we may slightly under-estimate the difference betweenthe properties of centrals and satellites in the following anal-ysis.

4 THE DEPENDENCE OF QUIESCENTGALAXIES ON THEIR ENVIRONMENT

Having demonstrated that at a given stellar mass (mainlybelow 3× 1010M!), models fail to correctly predict the qui-escent fractions of satellites (and also the one of centrals),we now aim to investigate, where, i.e. in which environmentmodels predict a different behaviour compared to observa-tions. As definitions for environment we will consider inthe following analysis halo mass and the projected densitywithin a 1 Mpc and a 0.2 Mpc cylinder (as explained insection 2).

Fig. 2 shows the quiescent fraction of all (red), cen-tral (orange) and satellite (green) galaxies versus the 1 Mpcdensity (top row) and versus the 0.2 Mpc density (bottomrow) for the Guo model (lines) and the observations (sym-bols). We have divided our central and satellite sample intodifferent stellar mass bins (different columns as indicatedin the legend) in order to disentangle the intertwined de-pendence of the quiescent fraction on stellar mass and ondensity. When first considering the observational data, itis remarkable that the quiescent fractions at a given stel-lar mass and density hardly vary for satellite and centralgalaxies, and that both fractions reveal a very strong de-pendence on density for stellar masses below 1011M!. Suchan extremely small difference between satellites and centralsin the observations may be still partly due to the difficultyin observationally distinguishing between centrals and satel-

c© 2002 RAS, MNRAS 000, 1–16

Hirschmann et al., 2013b

5

...versus density at a given stellar mass

Quiescent galaxy fractions



4 Hirschmann et al.



p̄sat =

(

p̂sat −csat

1− ccenp̂cen

)

×

×1


p̄cen =

(

p̂cen −ccen

1− csatp̂sat

)

×

×1






c© 2002 RAS, MNRAS 000, 1–16


5

✦ Models: centrals & satellites behave differently & weaker density dep.





4 Hirschmann et al.



p̄sat =

(

p̂sat −csat

1− ccenp̂cen

)

×

×1


p̄cen =

(

p̂cen −ccen

1− csatp̂sat

)

×

×1






c© 2002 RAS, MNRAS 000, 1–16


5




➱ Over-estimating quiescent satellites (e.g. Kimm et al., 2009)



4 Hirschmann et al.



p̄sat =

(

p̂sat −csat

1− ccenp̂cen

)

×

×1


p̄cen =

(

p̂cen −ccen

1− csatp̂sat

)

×

×1






c© 2002 RAS, MNRAS 000, 1–16


5





➱ Under-estimating quiescent centrals



4 Hirschmann et al.



p̄sat =

(

p̂sat −csat

1− ccenp̂cen

)

×

×1


p̄cen =

(

p̂cen −ccen

1− csatp̂sat

)

×

×1






c© 2002 RAS, MNRAS 000, 1–16


5





➱ Under-estimating quiescent centrals

How can we theoretically understand the density dependence of centrals

and satellites?


Density-halo mass relation

✦ Models and observations agree well

✦ Satellites: Halo mass is related with 1Mpc-density at all stellar masses✦ Centrals: Halo mass is unrelated with 1Mpc-density (except for the most massive galaxies >1011 M⊙)

see also Haas et al. 2012, Muldrew et al. 2012

Influence of environment and its history 5

Figure 3. Median parent halo mass versus the 1 Mpc density (top rown) and versus the 0.2 Mpc density (bottom row) in different stellarmass bins (different columns) using the Guo model (shaded areas) and observations (symbols). We also distinguish between satellites(green areas and symbols) and centrals (yellow areas and symbols). The shaded areas and the error bars of the symbols refer to the 25thand 75th percentiles, respectively. Observartions and models are in good agreement, the parent halo mass of satellites scales with theirdensity, but interestingly, in both, the halo mass of centrals correlates with density only for the most massive galaxies (right column).

lites, particularly in group environments. Note that we havetested the effect of contamination being independent of stel-lar mass and density, but in reality it is more complicatedand the contamination fractions may additionally be depen-dent on density. In contrast and basically by definition, themodeled fractions exhibit significantly larger differences be-tween the quiescent satellites and centrals and for satelliteswith stellar masses below 1010.5M! an overall weaker de-pendence on both the 0.2 Mpc and the 1 Mpc density. Forcentrals, this applies only for the 1 Mpc density (while thedependence of the quiescent central fraction on the 0.2 Mpcdensity is similarly strong in models and observations).

The agreement between observations and model pre-dictions is reasonably good for massive galaxies (Mstellar >3 × 1010M!) and for galaxies in high-density regions(log(Σ1Mpc + 1) > 1[Mpc2]). Apart from that, at a givendensity and stellar mass below Mstellar < 3 × 1010M!, theGuo model partly under-predicts the amount of quiescentcentrals and significantly over-predicts the one of quiescentsatellites. It is interesting to note that the under-estimationof quiescent centrals in the models is mainly due to galax-ies in low-mass halos (Mhalo < 1012M!). If we exclude inobservations and models galaxies with halo masses below1012M! (or with even undefined halo masses in the obser-vational data), then the quiescent fraction of model central

galaxies is larger than in observations.1 Overall, the dis-agreement between observations and model predictions forsatellite galaxies gets worse with decreasing galaxy mass anddecreasing density (see e.g. green lines and symbols in theleft column in Fig. 2).

Comparing the 1 Mpc density with the smaller scale0.2 Mpc density in observations, for both the centraland satellite quiescent fractions with stellar masses belowMstellar < 1011M! the dependence on the 0.2 Mpc densityis slightly weaker than the one on the 1 Mpc-density, as e.g.at the low density end the quiescent fractions are generallylower and at the high density end higher for the 1 Mpc den-sity than for the 0.2 Mpc density. This may indicate thatscales above 0.2 Mpc seem to have a non-negligible envi-ronmental effect on both centrals and satellites. In contrastto the observations, models show only a slightly weaker de-pendence on the 0.2 Mpc density for quiescent satellites: atthe low 0.2 Mpc density end, the fraction of satellites in themodels is higher than at the low 1 Mpc density end.

1 We have also tested this result using the Guo model applied tothe Millennium-II simulation (with an increased resolution).

c© 2002 RAS, MNRAS 000, 1–16

6


Density-halo mass relation

✦ Models and observations agree well

✦ Satellites: Halo mass is related with 1Mpc-density at all stellar masses✦ Centrals: Halo mass is unrelated with 1Mpc-density (except for the most massive galaxies >1011 M⊙)

see also Haas et al. 2012, Muldrew et al. 2012


Figure 3. Median parent halo mass versus the 1 Mpc density (top rown) and versus the 0.2 Mpc density (bottom row) in different stellarmass bins (different columns) using the Guo model (shaded areas) and observations (symbols). We also distinguish between satellites(green areas and symbols) and centrals (yellow areas and symbols). The shaded areas and the error bars of the symbols refer to the 25thand 75th percentiles, respectively. Observartions and models are in good agreement, the parent halo mass of satellites scales with theirdensity, but interestingly, in both, the halo mass of centrals correlates with density only for the most massive galaxies (right column).

lites, particularly in group environments. Note that we havetested the effect of contamination being independent of stel-lar mass and density, but in reality it is more complicatedand the contamination fractions may additionally be depen-dent on density. In contrast and basically by definition, themodeled fractions exhibit significantly larger differences be-tween the quiescent satellites and centrals and for satelliteswith stellar masses below 1010.5M! an overall weaker de-pendence on both the 0.2 Mpc and the 1 Mpc density. Forcentrals, this applies only for the 1 Mpc density (while thedependence of the quiescent central fraction on the 0.2 Mpcdensity is similarly strong in models and observations).

The agreement between observations and model pre-dictions is reasonably good for massive galaxies (Mstellar >3 × 1010M!) and for galaxies in high-density regions(log(Σ1Mpc + 1) > 1[Mpc2]). Apart from that, at a givendensity and stellar mass below Mstellar < 3 × 1010M!, theGuo model partly under-predicts the amount of quiescentcentrals and significantly over-predicts the one of quiescentsatellites. It is interesting to note that the under-estimationof quiescent centrals in the models is mainly due to galax-ies in low-mass halos (Mhalo < 1012M!). If we exclude inobservations and models galaxies with halo masses below1012M! (or with even undefined halo masses in the obser-vational data), then the quiescent fraction of model central

galaxies is larger than in observations.1 Overall, the dis-agreement between observations and model predictions forsatellite galaxies gets worse with decreasing galaxy mass anddecreasing density (see e.g. green lines and symbols in theleft column in Fig. 2).

Comparing the 1 Mpc density with the smaller scale0.2 Mpc density in observations, for both the centraland satellite quiescent fractions with stellar masses belowMstellar < 1011M! the dependence on the 0.2 Mpc densityis slightly weaker than the one on the 1 Mpc-density, as e.g.at the low density end the quiescent fractions are generallylower and at the high density end higher for the 1 Mpc den-sity than for the 0.2 Mpc density. This may indicate thatscales above 0.2 Mpc seem to have a non-negligible envi-ronmental effect on both centrals and satellites. In contrastto the observations, models show only a slightly weaker de-pendence on the 0.2 Mpc density for quiescent satellites: atthe low 0.2 Mpc density end, the fraction of satellites in themodels is higher than at the low 1 Mpc density end.

1 We have also tested this result using the Guo model applied tothe Millennium-II simulation (with an increased resolution).

c© 2002 RAS, MNRAS 000, 1–16

Halo mass is a ‘theoretical’ explanation for the density dependence of satellites,

(consistent with Woo et al., 2012) but not for the one of centrals!

6

7

II. Central galaxies

Their density dependence...

... environmental effects on superhalo scales?

(as 1Mpc is larger than the size of 1012 M⊙)

✦ Backsplash population of centrals✦ Direct interaction with an extended hot halo(see also Wang et al. 2009, Knebe et al. 2011, Geha et al. 2012, Wetzel et al. 2013)

Fraction of galaxies with a hot gas fraction above 0.1

(Bahe et al. 2013)


6 Hirschmann et al.

Figure 4. Quiescent fraction of model central galaxies versus the 1 Mpc density distinguishing between centrals having always beencentrals (true centrals, dashed lines with dark red areas) and centrals having also been satellites in the past (backsplash population,dotted lines with orange shaded areas). The quiescent fraction of backsplash centrals is responsible for the overall density dependence ofquiescent centrals.

4.1 Relation between density and halo mass

A major question, we will address in this section, is howwe have to theoretically understand the observed, strong de-pendence on density (particularly on the 1 Mpc density) forboth quiescent centrals and satellites which is not capturedby models so far. We start with investigating whether thedensity dependence just reflects an overall dependence onhalo mass, i.e. we analyse how the 1 Mpc and the 0.2 Mpcdensity is related with the halo mass of centrals and theparent halo mass of satellites.

Fig. 3 shows the median halo mass versus the 1 Mpc-density (top row) and versus the 0.2 Mpc density (bottomrow) for centrals (yellow) and satellites (green) in models(lines) and observations (symbols). The dashed lines indi-cate the 25th and 75th percentile of halo mass. For both,centrals and satellites, we find a reasonably good agreementbetween models and observations. Note that this extendsa recent study of Haas et al. (2012), who also investigatethe relation between density (using different estimators) andhalo mass, but they do not distinguish between centrals andsatellites or different stellar mass bins as in our study. Theyfind that the density is always correlated with halo mass. Weagree with their results when we consider satellite galaxies:the parent halo mass of satellites is always strongly corre-lated with density irrespectively of the density scale and thestellar mass. The median parent halo masses have mainlyvalues above > 3×1012M! (i.e. with virial radii > 0.2 Mpc)and can obviously be traced by both the 1 Mpc and the0.2 Mpc density, even if the correlation between parent halomass and density is slightly weaker for the 0.2 Mpc scalethan for the 1 Mpc scale: the median parent halo masses atlow 0.2 Mpc densities are higher than at low 1 Mpc densi-ties. This is most likely due to the fact that the small-scale0.2 Mpc density preferentially traces “sub-halo” scales ofparent halos more massive than > 3× 1012M! (as opposedto the 1 Mpc-density). Thus, the slightly weaker correlationbetween parent halo mass and the 0.2 Mpc density may indi-cate that sub-halo scales are not sufficient to trace the wholeparent halo. This is also consistent with Fig. 2, where thedependence of the quiescent satellite fraction on the 0.2 Mpcdensity was shown to be weaker than the one on the 1 Mpc

density confirming that densities of sub-halo scales do notcapture the total effect of environment. However, overall,these results imply that the strong dependence of the qui-escent satellite fractions on the 1 Mpc density (see top rowof Fig. 2) may be - at least partly - due to their dependenceon parent halo mass. We will discuss this in more detail insection 4.3.

In contrast, the halo mass of central galaxies (indicatedin yellow in Fig. 3) is only related to the 1 Mpc density or the0.2 Mpc density for massive galaxies withMstellar > 1011M!

or Mstellar > 3 × 1010M!, respectively. In other words, acorrelation between the 1 Mpc density (resp. 0.2 Mpc den-sity) and halo mass only emerges for galaxies with a medianhalo mass above ∼ 1013M! (resp. ∼ 3 × 1012M!). Thiscorresponds roughly to virial radii larger than ∼ 300 kpc(resp. ∼ 200 kpc). This implies that the 1 Mpc densityprobes super-halo scales for haloes with Mhalo ∼ 1013M!.For smaller halos (in which low mass centrals are preferen-tially sitting, Mhalo ∼ 1011 − 1012M! with rvir ! 150 kpc),1 Mpc and 0.2 Mpc densities do not correlate with the halomasses, neither in models nore in observations. This may re-flect the averaging over too large scales (significantly largerthan the corresponding virial radii) - when computing the1 Mpc density (or the 0.2 Mpc density) - and thus, sup-pressing a correlation between halo mass and density. Whenreducing the length (i.e. z-axis) of the cylinders, a correla-tion would again emerge for all central galaxies. It may bealso interesting to point out that the uncorrelated halo massand density are not caused by any luminosity limits appliedto the neighbour and the primary galaxy samples as e.g.lowering the luminosity limit for the neighbour sample (i.e.including more faint galaxies when calculating the densities)hardly affects these results. To summarize, as a consequenceof the mainly unrelated density and halo mass for centrals(Fig. 3), the strong density dependence of quiescent cen-tral fractions, particularly those of lower mass galaxies, canhardly originate from a dependence on halo mass.

c© 2002 RAS, MNRAS 000, 1–16

Backsplash populationhave been

satellites in the past

have always been centrals

✦ True centrals: No dependence on density

✦ Backsplash centrals: Responsible for over-all density dependence ➱ environment mainly relevant for low mass centrals at high densities

Spend on average 1.5-3 Gyrs at z=0.5-2 as a satellite (depending on density & mass)

8


6 Hirschmann et al.

Figure 4. Quiescent fraction of model central galaxies versus the 1 Mpc density distinguishing between centrals having always beencentrals (true centrals, dashed lines with dark red areas) and centrals having also been satellites in the past (backsplash population,dotted lines with orange shaded areas). The quiescent fraction of backsplash centrals is responsible for the overall density dependence ofquiescent centrals.

4.1 Relation between density and halo mass

A major question, we will address in this section, is howwe have to theoretically understand the observed, strong de-pendence on density (particularly on the 1 Mpc density) forboth quiescent centrals and satellites which is not capturedby models so far. We start with investigating whether thedensity dependence just reflects an overall dependence onhalo mass, i.e. we analyse how the 1 Mpc and the 0.2 Mpcdensity is related with the halo mass of centrals and theparent halo mass of satellites.

Fig. 3 shows the median halo mass versus the 1 Mpc-density (top row) and versus the 0.2 Mpc density (bottomrow) for centrals (yellow) and satellites (green) in models(lines) and observations (symbols). The dashed lines indi-cate the 25th and 75th percentile of halo mass. For both,centrals and satellites, we find a reasonably good agreementbetween models and observations. Note that this extendsa recent study of Haas et al. (2012), who also investigatethe relation between density (using different estimators) andhalo mass, but they do not distinguish between centrals andsatellites or different stellar mass bins as in our study. Theyfind that the density is always correlated with halo mass. Weagree with their results when we consider satellite galaxies:the parent halo mass of satellites is always strongly corre-lated with density irrespectively of the density scale and thestellar mass. The median parent halo masses have mainlyvalues above > 3×1012M! (i.e. with virial radii > 0.2 Mpc)and can obviously be traced by both the 1 Mpc and the0.2 Mpc density, even if the correlation between parent halomass and density is slightly weaker for the 0.2 Mpc scalethan for the 1 Mpc scale: the median parent halo masses atlow 0.2 Mpc densities are higher than at low 1 Mpc densi-ties. This is most likely due to the fact that the small-scale0.2 Mpc density preferentially traces “sub-halo” scales ofparent halos more massive than > 3× 1012M! (as opposedto the 1 Mpc-density). Thus, the slightly weaker correlationbetween parent halo mass and the 0.2 Mpc density may indi-cate that sub-halo scales are not sufficient to trace the wholeparent halo. This is also consistent with Fig. 2, where thedependence of the quiescent satellite fraction on the 0.2 Mpcdensity was shown to be weaker than the one on the 1 Mpc

density confirming that densities of sub-halo scales do notcapture the total effect of environment. However, overall,these results imply that the strong dependence of the qui-escent satellite fractions on the 1 Mpc density (see top rowof Fig. 2) may be - at least partly - due to their dependenceon parent halo mass. We will discuss this in more detail insection 4.3.

In contrast, the halo mass of central galaxies (indicatedin yellow in Fig. 3) is only related to the 1 Mpc density or the0.2 Mpc density for massive galaxies withMstellar > 1011M!

or Mstellar > 3 × 1010M!, respectively. In other words, acorrelation between the 1 Mpc density (resp. 0.2 Mpc den-sity) and halo mass only emerges for galaxies with a medianhalo mass above ∼ 1013M! (resp. ∼ 3 × 1012M!). Thiscorresponds roughly to virial radii larger than ∼ 300 kpc(resp. ∼ 200 kpc). This implies that the 1 Mpc densityprobes super-halo scales for haloes with Mhalo ∼ 1013M!.For smaller halos (in which low mass centrals are preferen-tially sitting, Mhalo ∼ 1011 − 1012M! with rvir ! 150 kpc),1 Mpc and 0.2 Mpc densities do not correlate with the halomasses, neither in models nore in observations. This may re-flect the averaging over too large scales (significantly largerthan the corresponding virial radii) - when computing the1 Mpc density (or the 0.2 Mpc density) - and thus, sup-pressing a correlation between halo mass and density. Whenreducing the length (i.e. z-axis) of the cylinders, a correla-tion would again emerge for all central galaxies. It may bealso interesting to point out that the uncorrelated halo massand density are not caused by any luminosity limits appliedto the neighbour and the primary galaxy samples as e.g.lowering the luminosity limit for the neighbour sample (i.e.including more faint galaxies when calculating the densities)hardly affects these results. To summarize, as a consequenceof the mainly unrelated density and halo mass for centrals(Fig. 3), the strong density dependence of quiescent cen-tral fractions, particularly those of lower mass galaxies, canhardly originate from a dependence on halo mass.

c© 2002 RAS, MNRAS 000, 1–16

Backsplash populationhave been

satellites in the past

have always been centrals

✦ True centrals: No dependence on density

✦ Backsplash centrals: Responsible for over-all density dependence ➱ environment mainly relevant for low mass centrals at high densities

Spend on average 1.5-3 Gyrs at z=0.5-2 as a satellite (depending on density & mass)

But... dependence on density in observations seems to be stronger (out to ~1Mpc!)

➱ Missing effects?✦ Backsplash centrals continue to behave like satellites (e.g. Wetzel et al. 2013)

✦ True centrals directly interact with a hot, extended halo (e.g. Bahe et al. 2013)

Work in progress!

8

9

III. Satellite galaxies

✦ ... density correlates with parent halo mass

✦ density also correlates with the radial distance to the parent halo center

➱ Density dependence is a

superposition of both halo mass & radial distance

dependence (Woo et al. 2012)

Den

sity

log(r/rvir)

Radial distance to the halo center

✦ Observations: Environmental effects mainly affecting low-mass satellites


Residual effects of the radial distance on top of halo mass

r/rvir = <0.2r/rvir = 0.2-0.5r/rvir = 0.5-1

12 Hirschmann et al.

Figure 8. Mean radial distance versus the 1 Mpc density for satellites in different stellar mass bins (different panels) in observations(symbols) and models (lines with shaded areas). Density and radial distance are strongly correlated for densities above log(Σ+1) > 0.5.

Figure 9. Quiescent fractions of satellite galaxies divided into bins of their radial distance to the halo centre (different colours asindicated in the legend) are plotted versus the parent halo mass for observations (symbols) and the Guo model (lines with shaded areas).Different panels correspond to different stellar mass bins. Interestingly, observations mainly reveal a dependence on the radial distancefor lower mass satellites.

sity, models reveal such a trend only at low densities for thelow and the intermediate stellar mass bins considered. Thetrend in the model is, however, less pronounced than in theobservations. For high densities and more massive galaxiesmodels do not predict any residual trend – in contrast withobservations.

In the middle and bottom row of Fig. 7, we again splitthe central galaxy sample into backsplash and true centrals,respectively, to investigate their contribution to the overallresidual effects shown in the top row. As before, the back-splash and true central quiescent fraction are estimated withrespect to the total amount of central galaxies (see equation5). This clearly shows that any residual density effect onscales above 0.2 Mpc in the model is caused by the backsplashpopulation.

Low-mass, true central model galaxies do not reveal anyresidual effect of the large-scale density on top of a givensmall-scale density. For more massive true centrals, an anti-correlation between their quiescent fraction and the large-scale density emerges: at a given small scale density, thequiescent fraction of true centrals increases with decreas-ing large-scale density. This behaviour can be understoodas follows: the quiescent fraction of true centrals is givenby equation 7, which is the product of the quiescent, truecentral fraction with respect to all true centrals ntrue andthe true fraction of all centrals ncent. The former is not de-

pendent on the (large-scale) density, while the latter (thetrue fraction of centrals) increases with decreasing large-scale density (see bottom row in Fig. 4). This means thatthe more isolated central galaxies are (lower density), thehigher is the true fraction of centrals and thus, the prob-ability that they have always been central galaxies. As aconsequence, the plotted fraction nqu,true/ncent will increasewith decreasing large-scale density (simply because the truecentral fraction is increasing).

The discrepancy between observations and models withrespect to the density dependence of quiescent centrals onsuper-halo scales between 0.2 - 1 Mpc indicates that modelsmight miss environmental effects working on centrals. Pos-sible solutions will be discussed in section 6.2.2.

4.4 Satellite galaxies

In this section, we turn to quiescent satellite galaxies andexplore the physical origin of the density dependence of qui-escent satellites. We have already pointed out in section 4.2that their density dependence is partly caused by the parenthalo masses which strongly correlate with the 1 Mpc densityin both observations and models. We now examine the roleof the projected radial distance of satellites to their parenthalo centre.

c© 2002 RAS, MNRAS 000, 1–22




for the quiescent fraction of centrals versus the small-scale0.2 Mpc density, binned in different large-scale 0.2-1 Mpc an-nulus densities. Different panels show different stellar massbins. While observations show the typical, strong residualeffects for all stellar mass bins on top of the 0.2 Mpc den-sity, models reveal such a trend only at low densities for thelow and the intermediate stellar mass bins considered. Thetrend in the model is, however, less pronounced than in theobservations. For high densities and more massive galaxiesmodels do not predict any residual trend – in contrast withobservations.


Low-mass, true central model galaxies do not reveal anyresidual effect of the large-scale density on top of a givensmall-scale density. For more massive true centrals, an anti-correlation between their quiescent fraction and the large-scale density emerges: at a given small scale density, thequiescent fraction of true centrals increases with decreas-

ing large-scale density. This behaviour can be understoodas follows: the quiescent fraction of true centrals is givenby equation 7, which is the product of the quiescent, truecentral fraction with respect to all true centrals ntrue andthe true fraction of all centrals ncent. The former is not de-pendent on the (large-scale) density, while the latter (thetrue fraction of centrals) increases with decreasing large-scale density (see bottom row in Fig. 4). This means thatthe more isolated central galaxies are (lower density), thehigher is the true fraction of centrals and thus, the prob-ability that they have always been central galaxies. As aconsequence, the plotted fraction nqu,true/ncent will increasewith decreasing large-scale density (simply because the truecentral fraction is increasing).



In this section, we turn to quiescent satellite galaxies andexplore the physical origin of the density dependence of thequiescent fraction of satellites. We have already pointed outin section 4.2 that their density dependence is partly caused

c© 2002 RAS, MNRAS 000, 1–22

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c© 2002 RAS, MNRAS 000, 1–22

✦ Models: Much stronger residual dependence at all masses Quiescent fractions over-estimated, particularly in the innermost regions











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c© 2002 RAS, MNRAS 000, 1–22

✦ Models: Much stronger residual dependence at all masses Quiescent fractions over-estimated, particularly in the innermost regions











c© 2002 RAS, MNRAS 000, 1–22

The Guo model is stripping the hot gas too efficiently!

➱ Too short quenching time-scales

10

✦ Using galaxy merger trees✦ How long has a satellite galaxy been living in a parent halo of a certain mass?

CentralType 1 satelliteType II satellite

galaxy became the first time a

satellite galaxy

galaxy was accreted onto

the final parent halo

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Can we gain any information on the typical time-scale of star formation quenching in satellites?

Investigate the environmental history of galaxies...

Environmental history

De Lucia et al. 2012

12

Lines: ‘Environmental fractions’ vs the z=0 density = fraction of model satellite galaxies having been residing in halos more massive

than 1e12, 5e12 or 1e13 M⊙ for a longer time than 3, 5 or 7 Gyr


9.5 < logMgal < 10

hierarchical structure formation

12

Lines: ‘Environmental fractions’ vs the z=0 density = fraction of model satellite galaxies having been residing in halos more massive

than 1e12, 5e12 or 1e13 M⊙ for a longer time than 3, 5 or 7 Gyr


9.5 < logMgal < 10

hierarchical structure formation

ftrans =fqu,sat(z = 0)− fqu,cent(tinf)

fsf,cent(tinf)

Observed transition fraction (van den Bosch et al., 2008 and Peng et al., 2011)

at fixed stellar mass, i.e. ”the probability of satellites getting

quenched after being accreted”:

compare with


Lines: ‘Environmental’ fractions of model satellite galaxiesSymbols: Transition fractions from observations

13

Quenching time-scales


Figure 10. Fraction of model satellite galaxies having spent more time than Y in halos more massive than X (different line styles) versusthe present-day 1 Mpc density for different stellar mass bins (different rows). Different columns correspond to different values of timeY. The cyan squares illustrate the observed transition fraction a la “vdB”, while the cyan and purple circles show the refined transitionfractions derived from the observations and the Guo model, respectively. Comparing the observed refined transition fractions with modelpredictions suggests long quenching time scales of ca 5-7 Gyr the lowes stellar mass bin which are decreasing with increasing stellar mass.

choice may be justified by Tinker &Wetzel (2010) who foundno evolution in the quiescent fraction for satellites at fixedmagnitude at z ! 1 based on halo occupation modeling.

The vdB and the refined transition fractions are illus-trated by the cyan squares and circles, respectively, in Fig.10. Comparing vdB transition fractions with the environ-mental fractions suggests that galaxies should be quenchedafter 5 − 7 Gyr in ∼ 1013M!-mass halos which is nearlyindependent of the stellar mass bin. For low mass satel-lites < 1010M!, this is in agreement with the predic-tions from the refined transition fractions. However, to-

wards more massive satellites, the differences between thevdB and the refined transition fractions become larger, sug-gesting quenching time scales of 3 − 5 Gyr for satelliteswith 1010 < Mstellar < 3 × 1010M! and relatively shortquenching time scales of ∼ 3 Gyr for massive satellites with3 × 1010 < Mstellar < 1011M!. Overall, when consideringthe refined transition fractions the quenching time scalesare longer for low mass satellites than for massive ones.

To test the estimation of the refined transition fractionsin the observational data, we additionally calculate the re-fined transition fractions for the Guo model in the same

c© 2002 RAS, MNRAS 000, 1–16


Lines: ‘Environmental’ fractions of model satellite galaxiesSymbols: Transition fractions from observations

13

Quenching time-scales

Very gentle mode of strangulation/stripping

Conclusion: Long quenching timescales for low-mass satellites: 5-7 Gyr


Figure 10. Fraction of model satellite galaxies having spent more time than Y in halos more massive than X (different line styles) versusthe present-day 1 Mpc density for different stellar mass bins (different rows). Different columns correspond to different values of timeY. The cyan squares illustrate the observed transition fraction a la “vdB”, while the cyan and purple circles show the refined transitionfractions derived from the observations and the Guo model, respectively. Comparing the observed refined transition fractions with modelpredictions suggests long quenching time scales of ca 5-7 Gyr the lowes stellar mass bin which are decreasing with increasing stellar mass.

choice may be justified by Tinker &Wetzel (2010) who foundno evolution in the quiescent fraction for satellites at fixedmagnitude at z ! 1 based on halo occupation modeling.

The vdB and the refined transition fractions are illus-trated by the cyan squares and circles, respectively, in Fig.10. Comparing vdB transition fractions with the environ-mental fractions suggests that galaxies should be quenchedafter 5 − 7 Gyr in ∼ 1013M!-mass halos which is nearlyindependent of the stellar mass bin. For low mass satel-lites < 1010M!, this is in agreement with the predic-tions from the refined transition fractions. However, to-

wards more massive satellites, the differences between thevdB and the refined transition fractions become larger, sug-gesting quenching time scales of 3 − 5 Gyr for satelliteswith 1010 < Mstellar < 3 × 1010M! and relatively shortquenching time scales of ∼ 3 Gyr for massive satellites with3 × 1010 < Mstellar < 1011M!. Overall, when consideringthe refined transition fractions the quenching time scalesare longer for low mass satellites than for massive ones.

To test the estimation of the refined transition fractionsin the observational data, we additionally calculate the re-fined transition fractions for the Guo model in the same

c© 2002 RAS, MNRAS 000, 1–16

Circles: Increasing quenching time-scales with decreasing stellar mass

✦ stronger internal quenching effects for massive satellites

✦ longer dynamical friction time-scales for less massive satellites

14

Stellar mass dependence

For low mass satellites consistent with De Lucia et al. 2012Fully consistent with Wetzel et al. 2012 (HOD)


Influence of the environmental history 19

Figure 12. Quenching timescales estimated from transition frac-tions (vdB-method: blue squares, refined method: blue circles)and directly from the Guo model (purple circles) versus stellarmasses. The quenching times scales estimated from the refinedtransition fractions are in rough agreement with the results fromWetzel et al. (2013a) (grey stars) and are increasing with decreas-ing stellar mass.

of a recent study of Wetzel et al. (2013a) using a completelydifferent approach based on an empirical HOD model (seegrey stars in Fig. 12). In their model, they find a “delayed-then-rapid” quenching scenario to reproduce the observedquiescent satellite fraction and the bi-modality of the SFRs.In their proposed scenario, satellite SFRs evolve unaffectedfor the first several Gyr after infall, after which the star for-mation quenches rapidly (assuming an exponential decline).

The stellar mass dependence of the quenching timescales appears counter-intuitive at first sight as environmen-tal processes are expected to more easily affect low-masssatellites due to their low internal binding energies. Thephysical origin of this mass dependence is likely twofold.

First, internal quenching processes such as AGN feed-back are working more efficiently on more massive satel-lites than on less massive ones, reducing their quenchingtime scales after infall in addition to the environmental ef-fects they are experiencing. In other words, for more mas-sive satellites, environmental quenching is not the main/onlyquenching effect. This means that the galaxy status doesnot matter for massive galaxies to first order. Such an (ad-ditionally) internally quenched star formation would also beconsistent with the independence of the quiescent fractionsof massive satellites on the radial distance as seen in Fig.9. Our approach based on the “vdB” or “refined” transitionfractions cannot separate such additional internal processesafter accretion. In the (extreme) case that satellites wouldexperience the same amount of internal quenching as cen-tral galaxies (at fixed stellar mass), the “vdB” transitionfractions (showing no stellar mass dependence) would prob-ably more correctly express the “real” transition fractions.In reality we may expect the solution to be somewhere be-tween the “vdB” and the refined transition fractions: satel-lite galaxies with their own sub-halo probably still experi-ence some (possibly reduced) internal-like effects similar tocentral galaxies.

Second, different orbits and dynamical friction timescales for satellites of different masses may also influence thequenching time scales. The dynamical friction time scales arestrongly dependent on the ratio between satellite and hosthalo mass (e.g. Boylan-Kolchin et al. 2008; Villalobos et al.2013), i.e. the smaller the satellite galaxy (at a given hosthalo mass), the longer is its dynamical friction time scale.This implies a longer spiralling around in the outer, low-density regions for low-mass satellites before they get intothe denser, inner regions where environmental processes be-come more efficient. The rate at which gas is removed froma satellite is most likely related to how often it has crossedthe higher density medium along its orbit. This means thata satellite could retain more gas if it is exposed less often tothe high density of the central regions of the host halo. Forthe stellar component, this is shown by Villalobos et al. (inprep.) and it is plausible to assume that the gas will behavesimilarly.

This would also provide a physical explanation for the“delayed-then-rapid” quenching scenario of the study ofWetzel et al. (2013a), where the rapid (exponentially de-creasing) quenching may only start when the satellite hasreached the dense inner regions of the halo. In contrast,massive satellites have short dynamical friction time scalesand reach the central, dense regions rapidly so that theirgas gets stripped efficiently in a short time. In addition,massive galaxies will more rapidly merge with the centralgalaxy and then drop out of the satellite population. Thesewill be the ones which have been in the parent halo longestand so have also been most likely quenched, but are lostby merging. This means that massive galaxies cannot havelong quenching times because otherwise we would not seeany difference between the massive central (infall) and mas-sive satellite quiescent fractions as there would be no timefor environmental quenching before they merge away.

The relative contribution of the two scenarios (internalquenching and orbits/dynamical friction time scales) on themass dependence of quenching satellites is unclear. We planto address this issue in a future work.

6.4 Satellite quenching time scales at higher

redshifts

The analysis in this paper only focuses on the present-dayUniverse. At higher redshifts, however, the Universe is ap-parently not old enough for such long quenching timescalesas we infer at low redshifts. One explanation for this ap-

parent disagreement is that the currently used models donot properly account for the internal quenching processes.Another, possibly complimentary option is that it is verylikely that the quenching timescale scales vary with the dy-namical time, i.e. depend (in addition to halo mass, galaxymass and density) on redshift and are thus, shorter at higerredshift. The latter possibility is in agreement with resultsfrom the work by Mok et al. (submitted to MNRAS) whoprovide constraints for quenching time-scales using observa-tions of groups at z ∼ 0.4 and z ∼ 0.9. They find that theobserved fractions are best matched with a model that in-cludes a delay (when quenching starts) that is proportionalto the dynamical time followed by a rapid quenching timescale of ∼ 0.25 Gyr.

c© 2002 RAS, MNRAS 000, 1–22

as De Lucia et al. 2012

Wetzel al. 2012

This work

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Challenges:Current models (with similar prescriptions as the Guo model) cannot predict observed environmental trends

More continuous transition between centrals & satellites!

1. Environmental effects important for low mass Centrals out to ~1Mpc

➱ Non-negligible fraction of backsplash centrals➱ Additional effects in the models?

2. Satellites: Environmental history indicates:long quenching time-scales (5-7Gyrs) in low-mass satellites

➱ modifying recipes for internal & environmental processes (dep. on dynamical friction time-scales?)

Conclusion

Thank you

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GEE 3 - GALAXY EVOLUTION AND ENVIRONMENT12-14th November 2013, Padova

the influence of environment & its history on quenching ...3 theoretical models...

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