GALAXIES 626
Lecture 15:
Galaxy morphology and environment
Why classify galaxies?
• The Hubble system gives us our basic description of galaxies.
•
• The sequence of galaxy types may reflect an underlying physical and evolutionary sequence. – provides an overview of integrated properties
– reproducing the variation in these properties along the Hubble sequence is a major (unsolved) challenge for galaxy formation/evolution theory
What we want from a system
• Bring order to diversity of galaxy forms
• Include nearly all galaxies
• Use unambiguous easily identified criteria
•
•
• Hopefully relate to important physical properties and provide insight into internal processes, formation and evolution of galaxies
Hubble Classification System
Basic elements
•
• 4 basic components used in the classification: – Spheroid, disk, bar and arms
– Presence and absence and relative strength of these components define classes
• Principle criteria for spiral stage:
– Openness of spiral arms
– Bulge/disk ratio
– Degree of resolution of arms into HII regions
Caveats
• Based on limited number of nearby galaxies, in particular, high surface brightness galaxies, because they are easier to find
• Most criteria are descriptive, I.e., very difficult to quantify and develop automatic procedures
• Does not contain information about the size, luminosity or kinematic information of the galaxy
• Based mostly on photographic images taken in the BLUE
– Emphasizes star formation rather than mass distribution
– Appearance can vary greatly with wave­band
• Difficult to compare with high­redshift galaxies, which are mostly observed in the rest­frame UV
• Requires reasonably good spatial resolution, difficult for galaxies at z>0.1 from the ground
visible
24 µm •
Elliptical galaxies
– smooth structure, elliptical light distribution
– relatively little evidence of gas, dust
– subtypes defined by projected flattening
E0 ­ E7 where n = 10(a­b)/a
– n is not fully intrinsic because of spatial projection
– Few have n>6
– Deviations from pure ellipse small  concepts of disky and boxy Es SO galaxies (lenticulars)
•
–
Structureless
–
Not elliptical, with disk/bulge structure
–
No spiral structure
–
Difficult to classify
–
In many cases, we just say E/S0 for early type galaxies as a whole
•
Spiral galaxies
– flattened disk + central bulge (usually)
– two major subclasses: normal and barred
– subtypes Sa, Sb, Sc distinguished by 3 criteria
• bulge/disk luminosity ratio
– B/D ranges from >1 (Sa) to <0.2 (Sc)
• spiral arm pitch angle
– ranges from 1­7o (Sa) to 10­35o (Sc)
• “resolution” of disk into knots, HII regions, stars
– these three criteria are not necessarily consistent!
– each reflects an underlying physical variable
• B/D ratio ­­­> spheroid/disk mass fractions
• pitch angle ­­­> rotation curve of disk, mass concentration
• resolution ­­­> star formation rate Grand Design vs. Patchy (flocculent) Spirals
Irregulars: I
•
•
•
•
Very late, no nucleus, low luminosity, often dwarfs
Labelled Irr I by Hubble
Magellenic Clouds type Labelled Sm, SBm, Im, Ibm by de Vaucouleurs and by Sandage later
Irregular II
•
•
•
•
•
probably mergers, amorphous appearance
Labelled Irr II by Holmberg, Hubble
Labelled I0 by de Vaucouleurs
M82 type; starburst
Labelled Am by Sandage
Dwarf Irregulars (dIrr)
•
No clear disk or spirals or nucleus
•
Patchy star formation on fainter old population
•
Often HI rich
•
Extreme examples are BCD, blue compact dwarfs with very strong star formation
Dwarf Elliptials (dE) and Dwarf Spheroidals (dSph)
• Very small, 0.1­1 kpc
• Higher/lower surface brightness corresponds to dE/dSph
• Morphology similar to Es
• Light profile similar to Ss
• Do not follow fundamental law for Es  different origin
• Most common kind of galaxy in the Universe
•
Unclassifiable galaxies?
– ~2% of galaxies cannot be classified as E, S, Irr
– predominantly disturbed or interacting systems
– At high­redshift, ~30% galaxies are peculiar
NGC 5128 = Cen A NGC 4038/9 = “Antennae”
Correlation with Hubble type
Surface brightness
size
luminosity
mass
M/L
Surface mass density
Surface HI density
Quantitative Classification
• Motivation
– automated classification is needed for very large imaging or spectroscopic surveys (e.g., Sloan Digital Sky Survey = SDSS)
– can obtain objective measures that are less susceptible to systematic or subjective effects
– the current morphological sequence may not be representative of galaxies at earlier cosmic epochs – since many physical and spectral properties of galaxies correlate with type, a physical classification system can be created
– parametric classifications provide information on the dimensionality of the galaxy parameter space
• Example of quantitative image classification
(Abraham)
– simple 2­parameter system
• concentration index C ­­> ratio of fluxes in two isophotal regions
• asymmetry index A ­­> flip image, subtract from initial image, measure fraction of residual flux
•
• Form of LF at faint luminosities still uncertain,
• LF probably is dependent on galaxy environment
Galaxies and their Environments
•How are galaxies affected by their environment?
•What impact do galaxies have on their environment?
Why Study Environment? ...
Galaxies in dense environments form fewer star
As Cosmic time progresses, more and more galaxies are bound into groups and clusters
Does environment drive the decline in theStar Formation rate?
Or is it that the formation of the structure is paralleling the drop in the SFR?
Outline:
• The properties of galaxies are a function of their environment:
•
– Contrast clusters and “field”
The recent – Variation in properties a function of local density
universe
–
– The mechanisms at work:
–
– Mergers and tidal effects
– Ram­pressure stripping
– “suffocation”
The young universe
Clusters of Galaxies are most extreme environments
●
- lots of galaxies in one place
●
- extreme environment makes great laboratory
●
About 5% of galaxies are in clusters
The Perseus Cluster
cluster galaxies have E or S0 morphology (few disks with spiral arms)
cluster galaxies have red colour
On Hubble’s tuning fork diagram cluster galaxies are almost all “early” type Cluster galaxies
early
late
•
• Form of LF at faint luminosities still uncertain,
• LF probably is dependent on galaxy environment
The colour­magnitude relation
 In contrast to the field, cluster galaxies have a very well defined relation between magnitude and colour.
 Brighter galaxies are redder following a well defined sequence  The sequence appears the same in all clusters
 The cloud of points at faint magnitudes are background field galaxies
 The CMR tells us a lot about how cluster galaxies formed Terlevich et al 2002
Groups
Make up ~60% of local population; abundance evolves strongly with redshift
Much harder to do because contrast with background is lower. Individual groups have few members.
How do we measure environment?
• To go beyond studying clusters, we need a measure of the surroundings of a galaxy
• Two approaches:
– Measure mass of the dark matter halo that the galaxy is embedded in (lensing, dynamics)
• Easy to compare with theory
– Measure local number of companions
• Easy to measure observationally
•
The APM Galaxy Survey ( 1/10 th of the sky)
2dF GRS slices in wedge diagram
Measuring the 3­D Environment
What is required?
 min number of galaxies
 low shot noise
 insensitive to peculiar velocities

One example:

The Σ5 density estimate
 Volume to the 5th nearest neighbour
 In a slice of +­ 1000 km/s
 Express as density
 A good measure of 3d density
Allocate galaxies to groups
Friends­of­
friends algorithm
Connect galaxy to
neighbors lying within
fixed spacing and
velocity.
Then connect these
to their neighbors
and continue till no
more objects to
join
How do colors and spectra depend on environment?
Morphology has a strong dependence on the density environment…but is morphology secondary information? does it just result from changes to the star formation rate? The important point...
●
The distribution of galaxy star formation rates is bimodal
there two distinct galaxy populations:
Star Forming galaxies (“late types”)
Passive galaxies (“early types”)
This has been recognised in clusters for a long time, but its also true of galaxies in the “field”
●
●
●
●
Hα distribution
Hα distribution shows a bimodality: mean/median of whole distribution can be misleading
Isolate star­forming galaxies with W(Hα)
>4 Å
The colour­magnitude diagram
Red sequence
Note the
bimodality of
galaxy colours
blue sequence
Sloan DSS data
The colour­magnitude diagram
Dependence on
environment
Bright
Analysis of colours in SDSS data:
Colour distribution in 0.5 mag bins can be fit with two Gaussians
Mean and dispersion of each distribution depends strongly on luminosity
Dispersion includes variation in dust, metallicity, SF history, and photometric errors
Faint
The color­magnitude diagram
 Fraction of star forming galaxies suppressed in dense environments – but it’s a continuous trend
 Local density is more important than halo mass  Luminosity is more important than environment
 Even isolated regions contain “passive” galaxies
Fraction of red galaxies depends strongly on density. This is the primary influence of environment on the color distribution. Density­dependence stronger than luminosity. Bright and faint galaxies show trend with density
Observations ­Summary
• There are distinct populations of red (“early”) and blue (“late”) galaxies
• The mean colors and spread of the two populations depends only weakly on environment
• But the relative abundance of the populations is a strong function of environment.
• Blue galaxies move quickly into the red population as their environment changes – Why?
Mechanisms
Why should galaxy properties depend on the environment?
 Collisions / harassment
 "Strangulation"
 Ram­pressure (actual effects)

 Different history...
End