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HI beyond the Milky Way. Riccardo Giovanelli Cornell University. Union College, July 2005. An HI view of a section of the MW, some 2000 l.y. (700 pc) across. Credit: Dominion Radio Astronomy Observatory. Multiwavelength Milky Way. 408 MHz 2.7GHz HI (21 cm) CO FIR (IRAS) MIR (6-10m)

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Slide1 l.jpg

HI beyond the Milky Way

Riccardo Giovanelli

Cornell University

Union College, July 2005

Slide2 l.jpg

An HI view of a section of the MW, some 2000 l.y. (700 pc) across

Credit: Dominion Radio Astronomy Observatory

Slide3 l.jpg

Multiwavelength Milky Way across

408 MHz


HI (21 cm)



MIR (6-10m)

NIR (1.2-3.5 m)



Gamma (300 MeV)

Slide6 l.jpg

High Velocity Clouds across


Credit: B. Wakker

Slide7 l.jpg

The Magellanic Stream across

Discovered in 1974 by

Mathewson, Cleary & Murray

Putman et al. 2003

Slide9 l.jpg

How much of the HI stuff across

do we detect in the Universe?

Slide10 l.jpg

WMAP across

The Universe

is Flat:

W = 1

The current expansion rate is Ho = 70 km/s/Mpc

Elliptical vs spiral l.jpg
Elliptical vs Spiral across

Galaxies can be classified based on appearance

Morphology density relation l.jpg
Morphology-Density acrossRelation


The fraction of the population that is spiral decreases from the field to high density regions.



Low r

High r

[Dressler 1980]

Disk formation a primer l.jpg
Disk Formation: a primer across

  • Protogalaxies acquire angular momentum through tidal torques with nearest neighbors during the linear regime [Stromberg 1934; Hoyle 1949]

  • As self-gravity decouples the protogalaxy from the Hubble flow, [l/(d l/d t)] becomes v.large and the growth of l ceases

  • N-body simulations show that at turnaround time values of l range between 0.01 and 0.1, for halos of all masses

  • The average for halos isl = 0.05

  • Only 10% of halos havel < 0.025 or l > 0.10

  • halos achieve very

  • modest rotational support

The spin parameter l quantifies the

degree of rotational support of a system :

For E galaxies, l ~ 0.05

For S galaxies, l ~ 0.5

  • Baryons collapse dissipatively within the potential well of their halo. They lose energy through radiative losses, largely conserving mass and angular momentum

  • Thus l of disks increases, as they shrink to the inner part of the halo.

  • [Fall & Efstathiou 1980]

Angular momentum


Total Energy

  • If the galaxy retains all baryons  m_d~1/10 , and l_disk grows to ~ 0.5,

  • R_disk ~ 1/10 R_h

(mass of disk) /(total mass)

Slide17 l.jpg

Some galaxies form across

through multiple (and

often major) mergers

The orbits of their

constituent stars are

randomly oriented

Kinetic energy of random

motions largely exceeds

that of orderly, large-

scale motions such as


These galaxies have

low “spin parameter”

Elliptical galaxies

Slide18 l.jpg

Other galaxies form across

in less crowded


They accrete material

at a slower pace and

are unaffected by

major mergers for long

intervals of time

Baryonic matter (“gas”)

collapses slowly (and

dissipatively – losing

energy) within the

potential well of Dark

matter, forming a disk

Baryonic matter has

high spin parameter:

large-scale rotation

is important

Spiral Galaxy

The antennae l.jpg
The Antennae across

Toomre toomre 1972 l.jpg
Toomre & Toomre 1972 across

Restricted 3-body problem

Slide22 l.jpg

A Computer across

Simulation of the

Merger of two

Spiral galaxies

Slide24 l.jpg

Just as we use across

observations of

the orbits of

stars near the

center of our

Milky Way to

infer the presence

of a Supermassive

Black Hole…

Slide25 l.jpg

Schoedel et al across


The M(r) at the center of the Galaxy is best fitted by the combination of

- point source of 2.6+/-0.2 x 106 M_sun

- and a cluster of visible stars with a core radius of 0.34 pc and ro=3.9x106 M_sun/pc3

Slide27 l.jpg

M31 across

Effelsberg data

Roberts, Whitehurst

& Cram 1978

Slide28 l.jpg

Milky Way Rotation Curve across

 Dark Matter

is needed to

explain the

Milky Way (and

other galaxies’)


 The fractional


of the Dark

Matter to the

total mass

contained within

a given radius



The total mass

of the Galaxy

is dominated by

Dark Matter

Slide32 l.jpg

We use HI maps of galaxies to infer across

their masses, their dynamical circumstances,

to date their interactions with companions,

to infer their star formation (“fertility”) rates…

Morphological alteration mechanisms l.jpg
Morphological Alteration Mechanisms across

  • I. Environment-independent

    • Galactic winds

    • Star formation without replenishment

  • II. Environment dependent

  • a. Galaxy-galaxy interactions

    • Direct collisions

    • Tidal encounters

    • Mergers

    • Harassment

  • b. Galaxy-cluster medium

    • i. Ram pressure stripping

    • ii. Thermal evaporation

      • iii. Turbulent viscous stripping

  • Slide35 l.jpg

    Virgo Cluster across

    HI Deficiency

    HI Disk Diameter

    Arecibo data

    [Giovanelli & Haynes 1983]

    Slide36 l.jpg

    Virgo across


    VLA data

    [Cayatte, van Gorkom,

    Balkowski & Kotanyi


    Slide37 l.jpg

    VIRGO across


    Dots: galaxies w/

    measured HI

    Contours: HI deficiency

    Grey map: ROSAT

    0.4-2.4 keV

    Solanes et al. 2002

    Slide38 l.jpg

    Galaxy “harassment” across

    within a cluster


    Credit: Lake et al.


    Moore et al.

    Slide40 l.jpg

    DDO 154 across

    Arecibo map outer extent [Hoffman et al. 1993]

    Extent of



    Carignan & Beaulieu 1989 VLA D-array HI column density contours

    Slide41 l.jpg

    M(total)/M(stars across)


    Carignan &

    Beaulieu 1989

    Slide43 l.jpg

    M96 Ring across

    Schneider et al 1989 VLA map

    Arecibo map

    Schneider, Salpeter & Terzian 19

    Schneider, Helou, Salpeter &

    Terzian 1983

    Slide45 l.jpg

    Perseus-Pisces Supercluster across

    ~11,000 galaxy redshifts:

    Arecibo as a redshift machine

    Tf relation template l.jpg
    TF Relation Template across

    “Direct” slope is –7.6

    “Inverse” slope is –7.8

    SCI: cluster Sc sample

    I band, 24 clusters, 782 galaxies

    Giovanelli et al. 1997

    Measuring the hubble constant l.jpg
    Measuring the Hubble Constant across

    A TF template relation is derived

    independently on the value of H_not.

    It can be derived for, or averaged

    over, a large number of galaxies,

    regions or environments.

    When calibrators are included,

    the Hubble constant can be gauged

    over the volume sampled by the


    From a selected sample of Cepheid

    Calibrators, Giovanelli et al. (1997)


    H_not = 69+/-6 (km/s)/Mpc

    averaged over a volume of

    cz = 9500 km/s radius.

    Tf and the peculiar velocity field l.jpg
    TF and the Peculiar Velocity Field across

    • Given a TF template relation, the peculiar velocity of a galaxy can be derived from its offset Dm from that template, via

    • For a TF scatter of 0.35 mag, the error on the peculiar velocity of a single galaxy is typically ~0.16cz

    • For clusters, the error can be reduced by a factor , , if N galaxies per cluster are observed

    Slide51 l.jpg

    CMB Dipole across

    DT = 3.358 mK

    V_sun w.r.t CMB:

    369 km/s towards

    l=264o , b=+48o

    Motion of the Local Group:

    V = 627 km/s towards

    l = 276o b= +30o

    Convergence depth l.jpg
    Convergence Depth across

    Given a field of density fluctuations d(r) , an

    observer at r=0 will have a peculiar velocity:

    where W is W_mass

    The contribution to by fluctuations

    in the shell , asymptotically

    tends to zero as

    The cumulative by all fluctuations

    Within R thus exhibits the behavior :

    If the observer is the LG,

    the asymptotic matches the CMB dipole

    The dipole of the peculiar velocity field l.jpg
    The Dipole of the Peculiar Velocity Field across

    The reflex motion of the LG,

    w.r.t. field galaxies in shells of

    progressively increasing radius,

    shows :

    convergence with the CMB dipole,

    both in amplitude and direction,

    near cz ~ 5000 km/s.

    [Giovanelli et al. 1998]