Transcript Does size matter (in the SFRs)?

It is well known that the Hα luminosity
is a good tracer of the star formation in a
galaxy (Kennicutt 1983 ApJ, 273, 54).
The Star Formation Rate (SFR) along the Hubble
sequence has been studied extensively
(e.g. Kennicutt 1998 ARA&A, 36, 189) and
some of the main results are:
-SFR increases for latter type galaxies a factor
of 50, being the Sm and Irr those with the
larger values.
-Moreover, the HII regions for these galaxies
are brighter (Kennicutt et al. 1989 ApJ, 435, 22)
-In addition, there is a relationship between
the disk averaged SFR per area and
the average gas density (Kennicutt 1998).
We investigated these for a sample of late-type,
small (r ≤ 5 kpc), low luminous (Mb ≥ -18) galaxies
(hereafter named as dwarf Spirals (dS)).
They were selected from the UGC catalog (Nilson 1973)
and the RC3 (de Vaucouleurs et al. 1992). For more
details about the selection of the sample see
Hidalgo-Gámez 2004 (Rev. Mex. A&A 40, 39).
Narrow-band Hα and continuum images
were obtained for a total of 18 dS galaxies at the 1.5m
telescope at San Pedro Mártir observatory
(Baja California, México). They
were reduced and analysed using the normal procedure
in MIDAS
(Hidalgo-Gámez, work in progress).
R (kpc) Log ρs (M○ pc-2)
Galaxy
type
Mb
UGC 11820
Sm
-17.1 4.96
1.3
17.1
UGC 5296
Sm
-15.1 3.02
1.2
20.3
D (Mpc)
ic and internal extincion (only for UGC 5296). No correction for NII was implemented because the nitrogen content of these galaxies is very low, from spectra take
UGC 11820
UGC 5296
1.436
M○ pc-2
0.0011
M○ pc-2
There are only 5 HII regions
with SFRs between 1.6 10-4
and 10-5 M○ pc-2. These values
are similar to those of Sa galaxies
and to of quiescent dI
(van Zee 2001, AJ 212, 2003)
SFR vs.surface density plot for normal disk galaxies.
The solid line corresponds to the correlation from
Kennicutt (98) while the slash lines show the
dispersion in such correlation. The two galaxies
presented here are plotted as stars.
The number of HII regions is
larger, 18, as well as the SFRs,
with values as large as
10-3 M○ pc-2. Now, the galaxies is
located with the Sm/Im
Why the quiescent
sented the results on the SFRs of two small spiral galaxies with very similar HI content but with different size. According to Kennicutt (1998) they both should be
galaxies
are
more
the same locations of the graph, and have very similar values of the SFRs. On the contrary of course, UGC 5296 is not the only quiescent galaxy
in the Universe, but the caveat is that 2/3 of the 18 galaxies we have observed
common among the
are quiescent. As they were selected because of their small size…..
small spirals?