Slide 0 - Instructions:
  • To move slides, use the arrow keys or swipe on your mobile device
  • To see the speaker notes, press "s"
  • To go to full screen, press "f"
  • To print as PDF, go to this URL: ?print-pdf, then print.
  • To get a PDF with speaker notes, add ?print-pdf&showNotes=true to the URL.

What sets stellar masses in the Galaxy's richest environments

Adam Ginsburg
Associate Professor
Department of Astronomy
University of Florida, Gainesville
  • Postdocs: Miriam Garcia Santa Maria (2024-2025), Allison Towner (2020-2023)
  • PhD: Desmond Jeff (2025), Theo Richardson (2025; postdoc), Alyssa Bulatek (2026), Nazar Budaiev (2026), Savannah Gramze (2027), Taehwa Yoo (2028)
  • Postbac: Derod Deal, Aden Dawson
  • Undergrad: Antonio Daley, Ethan Bhula
  • Supported by NSF 2008101, 2206511, CAREER 2142300, STSCI 1905, 2221, 3523, 5365, 6151, Astropy
Collaborators: John Bally, Ashley Barnes, Cara Battersby, Roberto Galván-Madrid, Jonathan Henshaw, J. M. Diederik Kruijssen, Steven Longmore, Xing Lu, Fanyi Meng, Elisabeth A.C. Mills, Juergen Ott, Justin Otter, Álvaro Sánchez-Monge, Peter Schilke, Daniel Walker, Erik Rosolowsky, Eric Koch, Ciriaco Goddi, Brett McGuire, Dick Plambeck, Melvyn Wright, Johan van der Walt, Henrik Beuther, Kei Tanaka, Yichen Zhang, the ALMA-IMF team (Timea Csengeri, Fabien Louvet, Nichol Cunningham, Frederique Motte, Patricio Sanhueza, Thomas Nony, Yohan Pouteau, Melisse Bonfand, Fernando Olguin, Sylvain Bontemps, and many others), and the ACES team
Slides available at https://keflavich.github.io/talks/colloquium_nov2025_vienna.html or from my webpage →talks

While I'm visiting, ask me about

Star formation drives the evolution of the universe

NASA/WMAP

Star Formation oversimplified

Ṁ
The star formation rate, i.e., how much gas turns to stars
L / M
The light per unit mass, i.e., how stars and stellar populations turn matter into light

High-mass stars produce photons & heavy elements
low-mass stars live practically forever

Point color shows effective temperature, point size shows luminosity (left) and mass (right)
Ekström+ 2012 models, vendian.org colors, www.tinyurl.com/HR-figure

The stellar initial mass function (IMF)

Stars are sampled from this distribution
(but it's not universal)
Hopkins 2018
https://tinyurl.com/IMF-figure
https://tinyurl.com/IMF-figure
https://tinyurl.com/IMF-figure
High-mass stars populate the stellar graveyard

Almost all of the light in star-forming galaxies is produced by high-mass stars

NASA/GALEX via wikipedia

The stars form in and from gas (as traced by dust)

Herschel

Most of what we know of star formation in detail comes from small local clouds

Herschel/Palmeirim+ 2013

Most of what we know of star formation in detail comes from small local clouds

ESO/DSS2, De Martin

Cartoon of isolated low-mass star formation

original cartoon by Cormac Purcell, modified by AG

A molecular cloud fragments

The core forms a central protostar

The protostar heats its parent core and forms a disk

It drives an outflow and consumes its core

Eventually, there is just a star-disk system

Cartoon of isolated low-mass star formation

original cartoon by Cormac Purcell, modified by AG

The isolated cartoon is wrong in several ways

Cores fragment. Massive stars cook their neighbors in hot cores.
Cores continue to grow. Protostars are flung out.

Cartoon → theory
What do we want from a star formation theory?

As observers, we want it to
  • Predict the appearance of young stellar objects (YSOs)
    • SED & morphological models
  • Predict how common each type (or phase) of YSO is
    • Accretion history, lifetime
  • ... and thereby let us infer the mass distribution and history of star formation from an observed sample
    • IMF, SFH
It's not there yet.

What do we want from a star formation theory?

More broadly, we want it to predict
  • $L_*(t, \vec{x})$
  • $M_*(t, \vec{x})$
  • $\dot{M}_*(t, \vec{x})$
The path to better mass measurement:
YSO modeling → luminosity functions
Theo Richardson
The path to better mass measurement:
YSO modeling → luminosity functions
NSF 2008101: "How are stellar masses set?"
Theo Richardson
The path to better mass measurement:
YSO modeling → luminosity functions
Starting from Robitaille 2017 grid:
  • Models now predict mm fluxes better
  • Masses & column densities calculated for each model
  • Links from YSO radiative transfer models to accretion history models
  • [soon] Predict full populations as a function of age, accretion history, efficiency
Theo Richardson
Richardson, Ginsburg, Indebetouw, Robitaille 2024
Richardson et al 2025
Zenodo 10522816
The path to better mass measurement:
YSO modeling → luminosity functions
We can track and compare "mass" - "luminosity" histories for different accretion models
$S_{3~\mathrm{mm}}$: Envelope Mass
$S_{100~\mathrm{\mu m}}$: Total Luminosity
$\dot{M}$
The path to better mass measurement:
YSO modeling → luminosity functions
...and finally make YSO population models (the "Protostar Luminosity Function", PLF, but observable)
$\dot{M}$
Remaining major uncertainty*:
How much material is in the envelope?

$M_{\rm env}$ is measurable, but it's not part of the models.

Classic idea is 30% core→star efficiency

This is wrong for many reasons we'll explore.
Motte+ 1998
Alves+ 2007
*: there are other uncertainties I'm glossing over for now (variability, dust opacity, etc)

Most of what we know of star formation in detail comes from small local clouds

ESO/DSS2, De Martin
Briceño+ 2002
They contain only low-mass stars and do not represent star formation in general

Most stars form in dense regions

Lada & Lada 2003:
5-10% in bound clusters
in our Galaxy
Guarcello+ 2025: JWST EWOCS Wd1

In denser (parts of) galaxies, more stars form in clusters

Kruijssen 2012
Ginsburg & Kruijssen 2018
Γ is the fraction of stars forming in bound clusters
Galaxy averages

The "Bound Cluster Fraction" is higher in the CMZ

Kruijssen 2012
Ginsburg & Kruijssen 2018
Γ is the fraction of stars forming in bound clusters
Galaxy averages
CMZ prediction

The "Bound Cluster Fraction" is higher in CMZs

Kruijssen 2012
Ginsburg & Kruijssen 2018
Γ is the fraction of stars forming in bound clusters
Galaxy averages
CMZ prediction
Sgr B2 data

The "Bound Cluster Fraction" was higher in the past

Pfeffer+ 2018

Proto-cluster regions in our Galaxy are like the early universe

Mowla+ 2022
Two ALMA large programs probe these conditions:
ALMA-IMF:
  • 15 High-mass protoclusters probing young (few stars) to old (blowing out gas)
  • Two ALMA bands looking at cold gas & dust
  • Data published, analysis ongoing
ACES:
  • The whole Galactic Center, with low-SF and high-SF clouds
  • One ALMA band, biggest mosaic made.
  • Data all delivered, mostly reduced

but we see DETAILS in our Galaxy

W51e → W51 IRS2 → Wd2 → Wd1
→
→
→
.... MAKE A TITLE OR CUT THIS SLIDE ....
Are top-heavy IMFs limited to clusters?
NGC 3603 & Westerlund 1
HST
Guarcello+ 2024 EWOCS

SSCs are common in starburst nuclei and drive galactic outflows

NGC 253 protoclusters (Leroy+2018)
NGC 4945 protoclusters (Emig+2020)

Star formation drives the evolution of the universe

Most stars in most galaxies formed long ago
Madau 2014

Galaxies were smaller & denser back then

Conselice 2014
STSCI; Pappovich, Ferguson, Faber, Labbe

From stars: high-mass clusters have top-heavier IMFs

(but already IMF != PDMF)
Hosek+ 2019
https://tinyurl.com/IMF-figure

In gas, we see the process in action at $t\approx0$

Dark regions are the cold sites where new stars will be born

Infrared shows warm gas and dust and young stars

Millimeter [ALMA] shows forming and future stars

Our Galaxy

Gaia star colors via ESA

Our Galaxy

2MASS via IPAC

Our Galaxy

Planck + HI4PI via lambda.gsfc.nasa.gov

Our Galaxy

HI: "Diffuse" gas
HI4PI

Our Galaxy

Dust
Planck

Our Galaxy

CO: molecular gas
Planck

Our Galaxy

Nearby star-forming regions form few or no massive stars

The Inner Galaxy is where most stars form

The Inner Galaxy is where most stars form

ALMA-IMF: 15 high-mass star-forming regions

Orange layer shows ATLASGAL 870$\mu$m dust:
the dense gas where stars form

ALMA-IMF: 15 high-mass star-forming regions

Orange layer shows ATLASGAL 870$\mu$m dust:
the dense gas where stars form ALMA-IMF targets massive protoclusters ($10^3-10^4 \mathrm{~M}_\odot$) 2-6 kpc from the sun
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]

ALMA-IMF: CMF measurement & YSO counting

www.almaimf.com
(these papers are all now submitted or accepted, but this is a screenshot that includes everyone's photos...)

ALMA-IMF gas flows → Panta-Rei, EMERGE

Gas flows in N2H+ filaments (Álvarez-Gutiérrez+ 2024, Sandoval-Garrido+ 2025, Salinas+ 2025 )
Collapse is slower on larger scales, but fast enough to matter.
Panta-Rei will explore this for a larger sample.
With Alvaro, we'll go to larger scales

ALMA-IMF: Continuum Data → core catalogs

www.almaimf.com/data.html
Pouteau+ 2022 W43-MM2/3

ALMA-IMF "Core" Mass CDF

The CMF in protoclusters is shallower than the IMF
(i.e., more massive objects than expected)
Louvet+ 2024
Context: Recent CMF measurements
Many! Differ in: resolution, algorithm, selection.
Nearby surveys are only-starless; more distant are starless + protostellar

Publication Distance Ncores Mmin Mmax Resoln [au] Slope "stage" Figure
Louvet+ 2024 2-6 330 1.64 200 2000 0.97 HM
Zhang+ 2024 0.4 927 0.3 20 8000 1.4 LM
Armante+ 2024 2.4 80 0.03 13.2 2000 1.44 HII
Cheng+2024 4.5 183 0.5 10 3000 1.15 HM
Pouteau+ 2023 6 205 0.8 70 2500 0.93 HM
Li+ 2023 0.7 570 4 200 15000 1.35 LM
Suárez+ 2021 7.1 40 2.0 59 1000 1.11 HII
Könyves+ 2020 0.4 292 0.2 20 8000 1.33 LM
...

Assuming the classic model, we end up with a shallow IMF

Alves+ 2007

...but the naïve version doesn't work

Offner+ 2014
However, a core is not a core.
≠
Cores change states
Offner+ 2022
How does the CMF become the IMF?
Many cores are protostellar - so there's a mass correction
Brighter cores are systematically warmer, therefore less massive for a fixed flux
however, the correction depends on the underlying distribution
How does the CMF become the IMF?
Cores fragment as they form YSOs
2000 AU → 200 AU
(no YSO, 1 YSO, >1 YSO) = (25%, 30%, 45%) W51-E, (53%, 39%, 8%) IRS2

Taehwa Yoo+: fragmentation
toward W51
Cores
Fragments
(no YSO, 1 YSO, >1 YSO) = (25%, 30%, 45%) W51-E, (53%, 39%, 8%) IRS2

Taehwa Yoo+: fragmentation
toward W51
20% of YSOs in W51 lie within the $>100$ K contour
Cores
Fragments
(no YSO, 1 YSO, >1 YSO) = (25%, 30%, 45%) W51-E, (53%, 39%, 8%) IRS2

Taehwa Yoo+: fragmentation
toward W51
5000 AU → 500 AU
(no YSO, 1 YSO, >1 YSO) = (17%, 37%, 45%) Sgr B2 ("cores" are bigger)

Budaiev+ 2024: fragmentation in Sgr B2

Trend: More massive cores fragment more.



Yoo+ 2025: W51
Budaiev+ 2024: Sgr B2
Suggestive Trend: More massive cores make more massive single stars


Yoo+ 2025: W51
Budaiev+ 2024: Sgr B2

The number of detectable fragments goes up, but the mass gets more concentrated into the most massive object.
Fragmentation summary:
  • N(YSO) > N(core) [maybe obvious, but now empirical]
  • More massive cores contain more fragments
  • ...but more massive fragments dominate their cores more
  • 20% of YSOs reside in hot cores: exposed to hot core chemistry even if they didn't form there

Several different scenarios: mix of mechanisms

Offner+ 2014
We aimed to measure the CMF with ALMA.
Protostars exist within most cores; the PMF is the useful observable
Richardson et al 2025

(this cartoon is super wrong)

JWST peers into Super Star Clusters

W51e → W51 IRS2 → Wd2 → Wd1
→
→
→

Super Star Cluster formation in the Galactic Center

Super Star Cluster formation in the Galactic Center

Is star formation different in other environments?
In the Galactic Center?

Is star formation different in other environments?
In the Galactic Center?

Is star formation different in other environments?
In the Galactic Center?

The CMZ

$\sim10^8$ M$_\odot$ of gas in $\sim200$ pc, 10% of Galactic star formation
Image: NRAO, Ginsburg
Ginsburg+ 2018a
$>\frac{1}{3}$ of CMZ SF is in bound clusters (3-8$\times$ local)
$\sim$50% of CMZ SF occurs in the Sgr B2 cloud.

The Central Molecular Zone of the Galaxy represents one extreme of star forming conditions in the Galaxy

PPVII review: Henshaw, Barnes, Battersby, Ginsburg, Sormani, Walker
Sgr B2 with JWST/ NIRCam MIRI on MEERKAT: Ionized gas shows layers of the cloud
Budaiev+ 2025
MIRI reveals deeply embedded, feedback-heated gas
Budaiev+ 2025
MIRI 25 micron shows the outflow:
first IR light from within Sgr B2 N
Budaiev+ 2025
Sgr B2 N Accretion + Outflow: the most deeply embedded protocluster
Budaiev+ 2025: H$_2$O masers & SiO outflow
Schwörer+ 2019: Accretion along filaments: \(\sim0.1~\mathrm{M}_\odot \mathrm{yr}^{-1}\)
JWST found new HII regions:
the SFR is higher than estimated from radio
(remember Sgr B2 already accounts for $\sim$50% of the CMZ SFR)
R=770
G=(480-[410-405])
B=Brα
■ New HII regions
Budaiev+ 2025
There is an overall asymmetry in the star formation seen both in JWST (HII regions) and ALMA (embedded YSOs)
JWST shows the transition is sharp
What ALMA sees, JWST doesn't: 3/700 point source matches
(HII regions, outflow cavities do match)
What does this mean in context?
TBD, but it hints at an ongoing compression event
Budaiev+ 2025

The CMZ: ACES

The first complete survey of the CMZ with 2.4" resolution between ~2 microns and 10 cm. (previous best was ~15")
https://sites.google.com/view/aces-cmz/home
First five data papers submitted July 2025, refereed, resub in Nov 2025.
Six early result papers out: https://sites.google.com/view/aces-cmz/publications
Ask if you want to know more about the MUBLO

Galactic Plane Massive Clusters: W51

480 405 187
182 162 140
360 335 210
480 410 405
770 560 480
335/2100 480-360 410-405
PAH Hot dust? Scattered?
Colors show physical mechanism
Interesting feature highlight reel






starformation.astro.ufl.edu/Aladin_tours/w51_tour.html
Sharp dust filaments (low column density)
The most extreme massive stars still forming.
IRS2E saturates at long wavelengths - it's a massive star with a hot disk
Giant proplyds? EGGs?
The HII region is criss-crossed by overdensities and dust lanes
A sharp bar analogous to the Orion Bar
The extinction-producing layers seen at short wavelengths are not the ALMA star-forming gas: this hints at edge-on filamentary (or planar) structures
A gigantic bow-shock-like bubble, far from the cluster
Objects classified as YSOs w/Spitzer - they are massive stars lighting up leftover dust
Symmetric HII region
Symmetric HII region - but more embedded
Paintbrush-stroke filaments
HII regions with PAHs
20/200 ALMA sources have JWST matches
20/200 ALMA sources have JWST matches

Probably disks (hot dust) with small but still significant envelopes

JWST - ALMA mismatches:

In high-mass star-forming regions, JWST detects (almost) only later-stage YSOs
This is different from local clouds: the more massive cloud is responsible.
Consequence: YSO counts have environmentally different meanings
→ we measure (recent) star formation history w/JWST,
not ongoing, current star formation
  • YSO model grid is testing accretion models
    Theo Richardson, 2024 & 2025
  • Sgr B2 is forming & fragmenting YSOs - asymmetrically. Large-scale triggering.
    Nazar Budaiev, 2024 & 2025
  • W51 fragments are dominated by most massive. Colorful feedback to be understood.
    Taehwa Yoo, 2025
The Galactic Center Dust ridge...
...has a foreground cloud in front
We measure ice via stellar absorption
(I'll talk more about this with Joao's group Thursday)
  • Smith+ 2025: 27 data points
  • YSO model grid is testing accretion models
    Theo Richardson, 2024 & 2025
  • Sgr B2 is forming & fragmenting YSOs - asymmetrically. Large-scale triggering.
    Nazar Budaiev, 2024 & 2025
  • W51 fragments are dominated by most massive. Colorful feedback to be understood.
    Taehwa Yoo, 2025
  • JWST measures ice abundance across the galaxy.
    Savannah Gramze, 2025
  • We can measure metallicity with ice